Thursday, September 10, 2009

Distortion Control: Tips and Tricks to reduce or prevent heat distortion when welding.

Distortion Control: Tips and Tricks to reduce or prevent heat distortion when welding.

If you weld or design weldments you will experience shrinkage and distortion. Anticipating what will happen is half the battle.

Distortion Control

The effect of heat on weldments can cause all kinds of distortions problems. Many of these problems can be eliminated by planning the sequence, size, and location of the welds. Some general rules that have been outlined in many of the texts published by the Lincoln Electric Company are listed below.

1) Do not overweld

2) Use intermittent welding.

3) Use as few weld passes as possible.

4) Place welds near the neutral axis.

5) Balance the welds about the neutral axis.

6) Use backstep welding

7) Anticipate the shrinkage forces.

8) Plan the welding sequence.

9) Remove weld shrinkage forces after welding.

10)Minimize welding time.

An excellent text for reference is The Procedure Handbook from which the above list was taken. The Lincoln Electric Company, 22801 St. Clair Ave. Clevland, Ohio

The Welding Institute has an informative website:

The following discussion shows some examples for problems encountered and possible solutions.

Distortion of a simple Tee joint

The first example is very common and has several possible solutions. The tee joint using plate is a common subject of examples in text and has some problems that many people fail to visualize.

In example A the bracing prevents movement and forces the weld to stretch or yield as it cools. There will be some tension on the bead faces . It will be the amount of tension that would match the yield strength of the weld metal. Some minor distortion will be evident when the braces are released but it usually is within acceptable tolerances.

In Example B the brace will ensure that the relationship between the right side and the upright will stay close to ninety degrees however the left side is free to shrink and distort. Often the braces will be placed as in this example because braces on the left interfere with the welder's free access to that side.

Example C is the result of welding equally on both sides. The upright remains vertical but the base piece is bowed.

Example D is the best solution to prevent C however it requires that the assembly be pre-bent against the brace or strongback.


Overwelding is a common error which increases the chance of distortion.. Sometimes it is caused by simple ignorance and other times it is a combination of people and factors. Typically the engineer calls for a 3/8 fillet five inches long. The detail draftsman rounds it up to say six inches. . . after all a six inch bead was called for for all the other parts of the job. The fabricator in the shop assembles the unit and marks out the welds for the welder. He wants to make sure that the marks are at least six inches apart and in his haste some of the marks are seven inches apart. The welder, seeing that he must use stitch welds goes one half inch past the marks left by the fitter.

Now the weldment is completed. The welds are certainly strong enough however the item has been overwelded by 3/5 or sixty percent. If there was potential a distortion problem it will now rear its ugly head. Now is the time everyone will try to point fingers. It is of no use because the deed is done. Care in following instructions would have prevented the problem before it occurred.

An example is the welding done on a small aluminum tool tote. The material that it was made of was 3mm thick aluminum The bottom and sides were one piece formed into a "U" with end pieces welded on. The welds were all corner joints done from the outside to form a nice radius. The owner had not time to run fillets on the inside of the box. He expressed the intention of doing those inside welds when he had time so that the box would be much stronger. It was pointed out to him that the only likely place of weakness was at the top edge of the corner joint and a one inch long bead on the inside at the top would be more than adequate. If he welded the complete inside corner he would be the owner of a twisted and buckled tool tote that would not be any stronger.

The example is shown below. For most cases an open corner joint will produce more than enough strength when it is welded from one side only. If there is a serious strength concern then stitch the inside at critical points only.


Welding About Central Axis

This concept along with proper sequencing can significantly reduce unwanted distortion. The objective is to make the shrinkage forces exert their influence against each other in order to balance out. In most cases the forces created by shrinking weld metal will act thru the central axis as a fulcrum. The diagram shows this effect. The left side of the piece is welded which causes the shrinking force as shown by the arrows. The left side is placed under tension and stretches so that the unwelded side becomes the long side and the welded side becomes the short side. In the example on the right the forces of the two welds balance out and the assembly remains straight. Keep in mind however that the overall length will be slightly reduced.

There are two main shrinkage forces that change the shape of a welded assembly as the weld cools. The transverse shrinkage is the one of main concern to most situations. When splicing plates or using long weld beads the longtitudinal shrinkage forces must also be considered. The diagram below illustates their action. These are not the only forces that are considered by engineers but for the average application they are the primary forces.

Identifying the central axis can be difficult in some applications. Generally the central axis will run through the center of gravity. If one imagines hanging the structure from a string at two single points the string will intersect the center of gravity.

A square block of metal is a simple problem that has an obvious solution . In the case of the angle iron shown the central axis is in the air between the two legs of the angle. Shrinkage from the heat of welding will act though the central axis as a fulcrum.

In the first example below the addition of a flat bar to the edge of a channel will not only pull the channel upwards but also sideways. With a three inch channel and a two inch wide flat bar welded on to the corner this bending would not be significant until the length is over two feet. One of the quick solutions is to skip weld. Often continuous welds are not required for strength.

Reinforcements added to beams can cause unwanted bending if consideration is not given to how the assembly will distort from weld shrinkage.

Often a flat bar doubler will be added to the top or bottom surface of a beam continuously welding will result in a pronounced camber. If the camber is desired the beam can be left in that condition and loading will cause it to straighten.

If the camber is not wanted then planning is required. The example below will cause the beam to camber. It is important to use a staggered sequence starting from the center and alternately moving towards the ends of the beam. Welding one side only will pull the beam sideways.
Removing the camber can be done two ways. The first is to weld "false" welds or heats with a torch on the opposite flange so that there is an equal shrinkage force on both flanges above and below the central axis. The second method would be to bend the beam opposite to the direction it will distort by an amount equal to the expected distortion. The diagram illustrates this application to beams however this method can be used for any assembly that has to be welded on one side ot the central axis and distortion is expected once the welds cool. This is not unlike prepositioning a Tee joint so that when the weld pulls the tee becomes ninety degrees.
Distortion Control of Sheets

Joining plates or other items along a straight seam not only creates problems with transverse shrinkage but also longtitudinal shrinkage. The two main tactics are skip welding and back stepping. Skip welding reduces the effect of distortion simply by reducing the total amount of heat put into the work. Often there is no need for continuous weld beads particularly if the item welded on is acting as a stiffener. Back stepping reduces distortion by locking up the joint ahead of where the main part of the weld will occur thereby preventing movement of the joint as the heat builds up in the weldment. Prepositioning of large plates can help in some situations. An example is the longtitudinal welds on the side of a truck dump box. Below is drawn a single side with an insert strip that creates the inside of the box

Left and right sides are placed against each other with a spacer block in the center and clamps on the ends creating a bow on each piece. When the weld is completed and cooled the clamps are released and the sidewalls spring back to a flat shape.

In the previous example skip welding and then welding in the remaining sections or a backstepping procedure would still be prudent. When trying to control distortion one must use all available tactics in combination for maximum effect.

Splicing plates together can result in unwanted buckling. When initial fitting the plates there are several approaches. It is wise to lay the edges together and see how they meet. Often for some reason the edges are not exactly straight. This can be used to the advantage of the fitter or can create future problems both for the fitter and for the welder. Generally when butting two plates together edge to edge there will be shrinkage as the edges are tack welded together. The point that closest contact is made is often the best location for the first tack weld. If the edges are absolutely true then it is wise to set the plates almost touching at one end and having a gap at the other end of the joint. The first tack is placed at the point of contact and successive tack welds will pull the plates together as the joint is lined up and tacked. If the joint is closing up too rapidly then the tacks sould be made in a backstepping direction and kept small in size. It the gap needs to be closed then the tacks should be larger and made in the direction of the successive tacks. One can place a finger across the weld joint ahead of where the tack is being made. As the tack weld is made one can feel the initial spreading of the joint and then the closing of the gap as the cooling tack weld shrinks. The movement is so slight that the eye often will not detect it.

These principles are universal and don't really change whether one is joining half inch thick steel plate or autobody sheetmetal. Big job or small, the mistakes in the fitting are just as embarrassing. In addition to paying attention to the gap along the joint the thicknesses of the two plates must be aligned as the tacks are made. The following situations show the approach to take when panels are not cut absolutely straight. The sequence of tack welds are indicated. Similarly the sequence of weld should follow a similar sequence. The welder, if he takes a few moments, can reduce problems of buckling by monitoring plate movement and changing the placement and direction of his welding weld bead as he notices problems.

The example below is more realisitic. In this case one will have to start the tack welding sequence at the center of the seam line. Note that tacks are alternated to prevent the plates from hinging and opening more at one end than the other. The welding would follow a similar sequence however the welder would decide if he need to pull the joint together more with his welding or not. If the joint is closing up too much he would back step. If no problems are indicated the proper backstepping procedure would be: 2 to 1 then 3 to 1 then 4 to 2 then 5 to 3 then 6 to 4 then 7 to 5 and finally edge to 7 and edge to 6.

A true insert is always a problem because one cannot simply "chase" the shrinkage out to the edges of the plate. In the diagram below one can see that the transverse shrinkage will pull the plate in several directions. The likely result is buckling at the turn of the seam. This oil can effect on the plate can be reduced by strongbacking across the seam before welding . In addition it would be wise to follow a sequence. The fit at "b" and "c" is closed and will pull the sheet to the right. The area at "d" should not open up so can be left as the last location to tack weld. The welding will also have to follow a similar sequence. Likely b-a , c-b, a-d, edge to c and finally edge to d would offer the least distortion. The sequence might vary as the welder notes the nature of the shrinkage after each weld. Usually the curve of the seam will control or minimize the longtitudinal shrinkage. The transverse shrinkage now becomes the concern. Buckling or oil canning is a problem with this type of insert.

Distortion Control of Frameworks Frames are the easiest to control because they are inherently braced. A plan of attack by the welder will prevent most problems. Solving the problem afterwards is usually the most difficult since the framework is locked in position.

A basic tip is to start at the centre of the framework and work outward.

Hand railings often show a kink at the stanchions. The amount of bending may not be important in an industrial setting. In a residence or shopping mall it would not be acceptable.

The standard measures to control this effect are:
  • Minimize gaps. Excessively large gaps will require more weld metal to fill and will increase the distortion.
  • Minimize size of a weld bead.
  • Preheat top of rail. Even heating top surface to a blue with a torch before or during welding the underside of the rail will reduce the shrinkage.
  • Use temporary bracing on all free ends. Free ends such as the lower part of a stanchion or the extension of the handrail will pull unless a temporary brace not tacked into position and removed after the weldment has cooled. A full length brace is another option.
  • The diagonal and the full length are not both required.
Placing the railing horizontal on saw horses makes welding more convenient however all the welding on one side must not be completed before the railing is turned over otherwise the railing will come out bent to one side. If one joint is going to be completely welded before the next then it is important to start in the middle of the railing and work out from the centre as should be done for a truss.

When tubes meet at right angles to form a tee, the welds that join will apply heat on one side of the long tube.

It is wise to regularly check the straightness of members using a two foot straightedge. A camber over 1/8 inch in two feet becomes easily noticeable.

A similar thing happens with permanent braces placed to strengthen and hold a corner connection at ninety degrees. The diagram below illustrates the problem. Applying that correct heats to the red areas will pull the tubes back.

One simple solution is to duplicate the welds placed on the inner corners of the tubing with simple weld beads placed on the free corners of the tubing. Heating with a torch to a red heat and letting cool will have the same effect. One must be careful not to apply too much heat. Doing it in two or three stages is prudent until one is sure what amount of heat is required.

Some framing such as a canopy that attaches to a wall over a store front entrance requires additional bracing before welding and sometimes requires further straightening after welding is completed. The blue indicates the forces and resulting distortion. The red indicates the location of the corrective heats. Bracing would reduce or eliminate this problem.

The photos show actual examples of framing. Note that shrinking heats were applied in order to straighten the assembly after it had distorted from welding. The major problem was caused by a little angle that had to be continuously welded on the inside of the tubing.

Small trailer frames are usually built on a single plane. By using a sequence one can keep the frame square and straight. If desired, a positive camber between the axle and hitch can be welded in and a negative camber created on the overhang behind the axle supports. If not successful at causing the frame to take a camber after welding is completed, the camber can be created afterward using heat from a torch. The blue lines indicate a desired trailer camber. This camber is slight in the order of 1/8 in a three-foot span.

Distortion is always a problem when welding. The suggestions outlined here are by no means the only solutions. If one keeps in mind that he may face a distortion problem whenever he welds there is a likelihood that the problem will not become impossible to control.

Flame Shrinking or "The Hot Side Becomes The Short Side"
The process seems to be a black art but really is based on some basic principles. Autobody repairers are masters at shrinking light guage panels. My experience is primarily with material over 1/4 thick. Even with my knowledge I still envy those guys working with light guage because I am not familiar with all the particular tricks they use. For me they are the magicians/wizards.

The procedure goes something like this:
  • A small area is heated until it becomes red hot. The torch is then removed.
  • The heat expands outward against the surrounding cold metal.
  • The red area is plastic and soft like putty. As a result it "upsets". The metal cannot push anymore outward so it expands by swelling in thickness.
  • This thickened area now starts to cool. As it contracts it pulls at the cold surrounding metal. Since there is not enough metal to replace the thickened area the cooling spot pulls even more causing the metal around to move towards the area that was originally red.

The term "Hot side is the short side." is a quick way to remember which way the steel will move after it is cool. Credit is due to a man named John Adolph who has extensive experience in flame shrinkage

The pictures show an example of shrinking to repair a set of four frames. The frames had developed a pronounced bend after welding was completed. The Rosebud torch can be seen on the steel horse.

What was required was a series of heats along the flanges of the large channel. The lower flange needed more heats since it was opposite the majority of the weld beads. After cooling the string showed a bow of 1/8 inch which was "good enough".

In this case wet rags were used to cool the assembly down rather than a siphon gun which might have been quicker. Two frames were shrunk at the same time, alternating heats between. Because of all the locked in shrinkage from the welding, heats every six inches along the lower flange had to be applied. Usually four heats evenly spaced over the complete length were done then let cool before applying more heats.

This large rack for holding parts was overwelded. the vertical support tubes bent resulting in all the shelf arms sloping downward before they were even loaded. The following picture shows the location s of heat applied to straighten the vertical tubes. The shelves were emptied before the shrinking began. The burned paint indicates the pie shaped locations that were heated.

It is important to understand that the air and water mist is used to increase the gradient of temperature across the heated area. It is not used for direct cooling on the red hot metal. Quenching the red area with the water spray will not aid in shrinking the metal and can affect the strength and ductility in some cases.

Siphon gun

A siphon gun is simply an air gun that sucks up liquid into the air stream and propels the mixture out the nozzle end. This section will explain how to make your own and the techniques to use when shrinking. Basic parts required:
  • a quick disconnect fitting to match the air line.
  • a valve to shut off or control air flow. (ball valve preffered)
  • short sections of pipe to match the air fitting and valve.
    rubber hose or plastic tubing.
  • three inches of brake line or similar tubing to match the hose.
100 cfm is required for good effect. The siphon gun in this case would be using a 3/4 inch pipe nozzle. For smaller applications a siphon gun using only 1/8 pipe and using a 10 cfm air supply is adequate.

Metal can be shrunk without the application of water to cool the surrounding area however using some sort of cooling increases the effect. Partial measures include a plastic squirt bottle of water or even a bucket and a wet rag The picture shows a homemade unit using discarded welding hose for the tubing.
The unit shown was made from scrap pieces of pipe and a ball valve purchased at a local plumbing supply house. A short length of small diameter steel tube is welded to the pipe.

A hole was drilled in the side of the 1/2 inch pipe. I made it large enough so that the small steel tube would slide in easily and could be wiggled around . Strike the pipe in the area of the hole so that it is narrowed creating a venturi. Slip the small tube into the pipe while air is blasting through. By trial and error the "sweet" spot can be located. Tack weld the tube in position and then check before welding up solid.
If the draw of water is excessive then make up some sort of clamp to squeeze the rubber hose in order to restrict the flow of water. Ideally you want just enough water to increase the cooling effect of the air blast without dumping tons of water that will end up as puddles on the floor.

The main thing to understand when using a siphon gun when shrinking is not to quench the red area. The goal is to keep the area surrounding the red as cool as possible. Slowly move the blast around the perimeter, moving in toward the red as the metal turns black.
Don't quench the red area with the air blast!!

Listed below are tricks and tools that might be useful for the welder/fabricator

Guide bars are very helpful when you want to make a short straight cut. The pictures illustrate the variations. The design is limited by your imagination.

To help the torch tip ride smoothly along the side of the bar rub the bar with soapstone. It works like a high temperature lubricant. A very gentle side ways pressure is used to hold the torch agains the guide. The user then only has to contend with height of the torch off the work and travel speed.

Some people design their guide bars so that they can ride the shoulder of the torch tip nut on an edge so that height is predetermined.

If one has to cut many slots or belvels that are all identical then some sort of guide will increase the quality of cut as well as increasing the speed. The picture below shows a Tee shaped guide used to cut flanges on beams when they are laying on their side.

There are several other guides hanging wating for another opportuninty to save time and trouble.

Running "buggo" or track burners can save considerable time and produce long quality cuts. The picture below is Tony's (workmate) invention. He used these little brackets to support the track when having to split beams into tees.


Large turnbuckles are used for a variety of rigging jobs but also very handing when building frames. Locally surplus turnbuckles with a length of over 18 inches can be purchased used for under twenty dollars Canadian. The short example shown was picked up for twelve.

If the turnbuckle is not long enough to span across a frame simply tack some bar to it. The photo shows a turnbuckle that is in regular service and paid for itself many times over. Scrap bar is tacked on the ends from the last time it was used.

When tack welded across the corner of a frame the turnbuckle can be used to push or pull the corner square. Keep in mind that the corner must be lightly tacked so that it will hinge.
Once square welding can commence and the turnbuckle can be removed after everyting is cooled down.

Flame Cutting

There are several adjustments on a torch that control your success. They can be sorted into two groups according to whether you can do the adjustment while cutting or you have to do it prior. They are:

1) size of tip
2) oxy pressure ( fuel pressure is generally left constant)

3) amount of preheat (second group)

4) angle of torch (material under 1/4 inch)

5) cutting speed
6) height above work ( critical for acetylene)

Also remember the number one cause of frustration is a dirty tip.

The variables listed are not hard and fast for each thickness and application.

For example if the tip is too big then the preheat will be too big and the plate will overheat creating a lot of melted slag hanging on the bottom edge. The solution is to pickup the speed.

Excessive speed is indicated by sweeping lines on the edge when cutting thicker materials. If there is a need to increase the speed of the cut to save time and sacrifice quality then larger tips and more preheat is the solution

Freehand cuts need getting used to. Steady shallow breathing will control movements. A person holding their breathe cannot keep steady on longer cuts.

If you are right handed your left hand should be formed into a fist and resting on the plate. The right hand should be holding the torch so that the forward part rests on your fist which becomes a fulcrum. If you roll your fist the torch will move forward and back about an inch. You swing the torch with your right hand pivoting the torch on your fist creating a left to right motion that is a slight arc. You remove the arc by rolling your left fist. This will give you a cut about ten inches long before you must reposition.

I don't believe in using any hardware on my torch, however some people swear by their circle cutters. I have better success with my freehand approach.

I do use a guide bar that is simply a piece of heavy flat bar 3/4 by 1 ½ inch about twenty inches long. I usually have a short one also about ten inches or less for cutting flanges on beams. The edge that I use to slide the torch along is rubbed with soapstone. The stuff makes an excellent high temperature lube so that your torch tip glides smoothly along. Always make cuts towards yourself when using the bar. You must be careful however because the slag will be dropping in front of you and it could get into your boots if you stand too close.

When you start a cut the important thing to remember is to heat the edge to kindling temperature then move your torch off the edge, depress the lever and travel into your cut. If you press the lever while you are over the plate edge the metal erupts with slag flying up at you. The same thing applies if you are restarting. Heat the end of the cut until red, move back into the open kerf, depress your lever and move into the plate again.
" Piercing "

If you must pierce plate heat a spot until you have it a red colour then slightly raise your torch and tilt it slightly. Gently depress the lever and the slag should erupt and fly up vertically but slightly off in another direction. If you have the torch vertical the slag will blow up into the tip and your tip is instantly fouled. If you still want to have a few friends be careful which way you tilt the torch. The slag will fly up and out until you manage to blow completely through.

Sunday, August 2, 2009

Panduan Solat Qasar Dan Jama'

Penerangan solat qasar dan jama’ adalah seperti berikut:

1.0 Solat Qasar

1.1 Takrifan

Solat Qasar adalah mengqasarkan (memendekkan) solat yang empat rakaat iaitu Zuhur, Asar dan Isya’ kepada dua rakaat sementara solat Maghrib dan solat Subuh tidak boleh diqasarkan.
1.2 Hukum Qasar

Harus berdasarkan kepada dalil berikut:

i.Al-Quran surah al-Nisa’ ayat 101 yang bermaksud;
“Dan apabila kamu musafir di muka bumi, maka kamu tidaklah berdosa mengqasarkan
(memendekkan) solat jika kamu takut diserang oleh orang kafir…”

ii. Al-Sunnah yang bermaksud;

‘Ya’la bin Umayyah pernah bertanya kepada Umar bin al- Khattab, “Kenapa kita perlu
mengqasarkan solat sedangkan kita berada dalam keamanan?” Lalu beliau menjawab, “Aku telah bertanya kepada Nabi s.a.w., dan Baginda bersabda,solat qasar itu adalah sedekah yang
Tuhan telah bersedekah kepadamu, maka terimalah sekalian kamu akan sedekah- Nya”(Riwayat
al-Nasa’i dan Abu Daud)

1.3 Jarak Dibenarkan Qasar

Dua marhalah iaitu empat burudin di mana setiap burudin mempunyai empat farsakh dan setiap
farsakh terdiri daripada 5514 meter mengikut kiraan moden. Dari itu, dua marhalah adalah
bersamaan 89 kilometer.

1.4 Syarat Solat Qasar

i. Hendaklah pelayaran atau perjalanan itu diharuskan oleh syara’.
ii.Jauh pelayaran atau perjalanan itu tidak kurang daripada dua marhalah (89 kilometer / 56
iii.Berniat qasar di dalam takbiratul ihram.
iv.Tidak berimam dengan orang yang solat tamam (sempurna).
v.Hendaklah solat yang diqasarkan itu terdiri dari solat empat rakaat.

1.5 Contoh Niat
أصلي فرض الظهر ركعتين قصرا لله تعالى
(Sahaja aku solat fardhu Zuhur dua rakaat Qasar kerana Allah Taala)

2.0 Solat Jama'

2.1 Takrifan

Solat Jama’ terbahagi kepada :
i. Jama’ Taqdim iaitu mengerjakan solat Zuhur dan solat Asar dalam waktu Zuhur atau solat
Maghrib dan solat Isya’ dalam waktu Maghrib.
ii.Jama’ Ta’khir iaitu mengerjakan solat Zuhur dan solat Asar dalam waktu Asar atau solat
Maghrib dan solat Isya’ dalam waktu Isya’.

2.2 Syarat Solat Jama'

2.2.1.Solat Jama’ Taqdim
i.Hendaklah bersolat secara tertib iaitu mendahulukan solat Zuhur daripada solat Asar dan solat
Maghrib daripada solat Isya'.
ii.Hendaklah niat Jama’ Taqdim di dalam solat pertama sebelum salam tetapi yang afdhalnya di
dalam takbiratul ihram.
iii.Berturut-turut di antara solat yang dijama’kan itu.
iv.Berkekalan pelayaran atau perjalanan itu hingga takbiratul ihram solat yang kedua.
v.Berkeyakinan adanya waktu solat yang pertama (Zuhur atau Maghrib) berkekalan sehingga
didirikan solat yang kedua (Asar atau Isya’).

2.2.2. Solat Jama’ Ta’khir
i.Hendaklah berniat jama’ dalam waktu pertama iaitu Zuhur atau Maghrib.
ii.Berkekalan pelayaran atau perjalanan hingga memberi salam solat yang kedua.

2.3 Contoh Niat

2.3.1 Jama’ Taqdim

i.Jama’ Taqdim Empat Rakaat
أصلي فرض الظهر اربع ركعات مجموعا إليه العصر اداء لله تعالى
(Sahaja aku solat fardhu Zuhur empat rakaat dihimpunkan kepadanya Asar tunai kerana Allah Taala)

ii.Niat Jama’ Taqdim Beserta Qasar
أصلي فرض الظهر ركعتين قصرا مجموعا إليه العصر اداء لله تعالى
(Sahaja aku solat fardhu Zuhur dua rakaat dihimpunkan kepadanya Asar tunai kerana Allah Taala)

2.3.2 Jama’ Ta’khir

i.Jama’ Ta’khir Empat Rakaat
أصلي فرض الظهر اربع ركعات مجموعا إلى العصر اداء لله تعالى
(Sahaja aku solat fardhu Zuhur empat rakaat dihimpunkan kepada Asar tunai kerana Allah Taala)

ii. Niat Jama’ Ta’khir Beserta Qasar
أصلي فرض الظهر ركعتين قصرا مجموعا إلى العصر اداء لله تعالى
(Sahaja aku solat fardhu Zuhur dua rakaat dihimpunkan kepada Asar tunai kerana Allah Taala)

Friday, July 31, 2009

Characteristics of Piezoelectric Transducers

The transducer is a very important part of the ultrasonic instrumentation system. As discussed on the previous page, the transducer incorporates a piezoelectric element, which converts electrical signals into mechanical vibrations (transmit mode) and mechanical vibrations into electrical signals (receive mode). Many factors, including material, mechanical and electrical construction, and the external mechanical and electrical load conditions, influence the behavior of a transducer. Mechanical construction includes parameters such as the radiation surface area, mechanical damping, housing, connector type and other variables of physical construction. As of this writing, transducer manufacturers are hard pressed when constructing two transducers that have identical performance characteristics.
A cut away of a typical contact transducer is shown above. It was previously learned that the piezoelectric element is cut to 1/2 the desired wavelength. To get as much energy out of the transducer as possible, an impedance matching is placed between the active element and the face of the transducer. Optimal impedance matching is achieved by sizing the matching layer so that its thickness is 1/4 of the desired wavelength. This keeps waves that were reflected within the matching layer in phase when they exit the layer (as illustrated in the image to the right). For contact transducers, the matching layer is made from a material that has an acoustical impedance between the active element and steel.Immersion transducers have a matching layer with an acoustical impedance between the active element and water. Contact transducers also incorporate a wear plate to protect the matching layer and active element from scratching.

The backing material supporting the crystal has a great influence on the damping characteristics of a transducer. Using a backing material with an impedance similar to that of the active element will produce the most effective damping. Such a transducer will have a wider bandwidth resulting in higher sensitivity. As the mismatch in impedance between the active element and the backing material increases, material penetration increases but transducer sensitivity is reduced.

Transducer Efficiency, Bandwidth and Frequency

Some transducers are specially fabricated to be more efficient transmitters and others to be more efficient receivers. A transducer that performs well in one application will not always produce the desired results in a different application. For example, sensitivity to small defects is proportional to the product of the efficiency of the transducer as a transmitter and a receiver. Resolution, the ability to locate defects near the surface or in close proximity in the material, requires a highly damped transducer.
It is also important to understand the concept of bandwidth, or range of frequencies, associated with a transducer. The frequency noted on a transducer is the central or center frequency and depends primarily on the backing material. Highly damped transducers will respond to frequencies above and below the central frequency. The broad frequency range provides a transducer with high resolving power. Less damped transducers will exhibit a narrower frequency range and poorer resolving power, but greater penetration. The central frequency will also define the capabilities of a transducer. Lower frequencies (0.5MHz-2.25MHz) provide greater energy and penetration in a material, while high frequency crystals (15.0MHz-25.0MHz) provide reduced penetration but greater sensitivity to small discontinuities. High frequency transducers, when used with the proper instrumentation, can improve flaw resolution and thickness measurement capabilities dramatically. Broadband transducers with frequencies up to 150 MHz are commercially available.

Transducers are constructed to withstand some abuse, but they should be handled carefully. Misuse, such as dropping, can cause cracking of the wear plate, element, or the backing material. Damage to a transducer is often noted on the A-scan presentation as an enlargement of the initial pulse.

UT on weldments (Welded Joints)

The most commonly occurring defects in welded joints are porosity, slag inclusions, lack of side-wall fusion, lack of inter-run fusion, lack of root penetration, undercutting, and longitudinal or transverse cracks.
With the exception of single gas pores all the defects listed are usually well detectable by ultrasonics. Most applications are on low-alloy construction quality steels, however, welds in aluminum can also be tested. Ultrasonic flaw detection has long been the preferred method for nondestructive testing in welding applications. This safe, accurate, and simple technique has pushed ultrasonics to the forefront of inspection technology.
Ultrasonic weld inspections are typically performed using a straight beam transducer in conjunction with an angle beam transducer and wedge. A straight beam transducer, producing a longitudinal wave at normal incidence into the test piece, is first used to locate any laminations in or near the heat-affected zone. This is important because an angle beam transducer may not be able to provide a return signal from a laminar flaw.
The second step in the inspection involves using an angle beam transducer to inspect the actual weld. Angle beam transducers use the principles of refraction and mode conversion to produce refracted shear or longitudinal waves in the test material. [Note: Many AWS inspections are performed using refracted shear waves. However, material having a large grain structure, such as stainless steel may require refracted longitudinal waves for successful inspections.] This inspection may include the root, sidewall, crown, and heat-affected zones of a weld. The process involves scanning the surface of the material around the weldment with the transducer. This refracted sound wave will bounce off a reflector (discontinuity) in the path of the sound beam. With proper angle beam techniques, echoes returned from the weld zone may allow the operator to determine the location and type of discontinuity.

To determine the proper scanning area for the weld, the inspector must first calculate the location of the sound beam in the test material. Using the refracted angle, beam index point and material thickness, the V-path and skip distance of the sound beam is found. Once they have been calculated, the inspector can identify the transducer locations on the surface of the material corresponding to the crown, sidewall, and root of the weld.

Motivasi Kerja

Aku ibarat lilin (the defensive system to ensure continues integrity of a pipeline system).
A sacrificial anode, or sacrificial rod, is a metallic anode used in cathodic protection where it is intended to be dissolved to protect other metallic components.
The more active metal corrodes first (hence the term "sacrificial") and generally must oxidize nearly completely before the less active metal will corrode, thus acting as a barrier against corrosion for the protected metal.
More scientifically, a sacrificial anode can be defined as a metal that is more easily oxidized than the protected metal. Electrons are stripped from the anode and conducted to the protected metal, which becomes the cathode. The cathode is protected from corroding, i.e., oxidizing, because reduction rather than oxidation takes place on its surface.
Tuntutan Berbuat Baik & Bersabar Di Atas Perilaku Manusia

(Riwayat Muslim)

Seorang lelaki bertanya:

“Wahai Rasulullah sesungguhnya aku mempunyai beberapa orang kerabat, yang hubungan aku erat dengan mereka tetapi mereka memutuskannya. Aku berbuat baik kepada mereka tetapi mereka berperangai buruk kepadaku. Aku bersabar menghadapi sikap mereka itu tetapi mereka menganggap sikapku itu adalah bodoh.”

Kemudian Baginda menjawab:

”Sekiranya benar sepertimana yang engkau katakan itu, maka seolah-olah engkau menyuap mereka dengan abu yang panas, sedangkan engkau sentiasa mendapat pertolongan daripada Allah Taala selama mana engkau berada di dalam kebenaran atas perkara yang demikian itu”.


1. Islam mengajar umatnya supaya banyak bersabar di dalam semua keadaan sama ada ketika senang atau pun susah, kerana ia merupakan pengukur darjat keimanan seseorang. Allah sentiasa memberi pertolongan kepada orang yang sentiasa sabar dan benar dalam bertindak.

2. Islam mengajar umatnya supaya jangan membalas keburukan orang sebaliknya menambahkan lagi kebaikan terhadap mereka. Mudah-mudahan dengan itu, hati mereka menjadi lembut dan baik.
3.Setiap umat Islam wajib menghubungkan silaturrahim dan haram memutuskannya.

Ada orang datang ke pejabat dengan perasaan penuh gembira dan ceria, ada datang dengan perasaan 'biasa' dan ada datang dengan perasaan serba tak kena. Ingatlah! sesiapa yang datang hanya dengan perasaan 'biasa' saja, hasilnya adalah 'biasa' saja. Sesiapa datang dengan ceria, hasilnya akan lebih daripada biasa ataupun luar biasa. Bekerjalah dengan ceria agar menghasilkan produktiviti yang luar biasa yang akan menggembirakan orang di sekeliling kita. Semoga hasil itu akan mendapat keberkatan, InsyaAllah.

Jadi renugilah…
1. Ada antara kita datang ke pejabat hanya memenuhi tanggungjawab DATANG KERJA tapi hampeh, hasilnya macam kita tak datang kerja.
2. Ada kala kita rasa kita BUSY gila, rupanya kita 'kelam kabut'.
3. Adakala kita rasa PRIHATIN, tapi rupanya kita 'busy body'.
4. Adakalanya kita rasa kita OPEN MINDED dan OUTSPOKEN tapi rupanya kita 'kurang pengajaran'.
5. Adakala kita rasa kita berpemikiran KRITIS rupanya kita hanya lebih kepada 'kritik ' yang mencipta 'krisis'.
6. Adakala kita rasa kita ingin menjadi LEBIH MESRA tapi rupanya kita di lihat lebih 'mengada'.
7. Adakala kita suka bertanya "KENAPA DIA NIE MACAM TAK ADA KERJA?", adalah lebi baik kita tanya "Apa lagi kerja yang boleh aku buat?"
8. Adakala kita rasa kita ni pekerja yang SEMPURNA,BAIK DAN BERDEDIKASI tapi cuba tengok dalam-dalam,selagi hati kita berdengki, jatuhkan reputasi sesama rakan sekerja (report kat bos kawan kita tak bagus dari kita) dan tak amanah (mencuri dan tak siapkan kerja),kita sebenarnya patut terima hakikat betapa kita lebih teruk dari anggapan kita itu sendiri.

Pejam mata dan renunglah diri, kalau kita perlu melakukkan anjakan paradigma maka lakukanlah SEGERA. Tetapi manusia tetap manusia, sukar untuk berubah kerana kita selalu beranggapan kita lebih baik. Adakah dengan merasakan itu kita sememangnya terbaik?

Maka untuk itu, mari kita mula senyum, ceria, mesra sesama kita dan tingkat kerjasama dalam kerja, tak rugi kita semai rasa 'kekeluargaan' dalam tugasan kita. Kalau kita kurang kerja, cari la kerja dengan membantu teman-teman lain.

Tak dapat gaji lebih pun tak apa sebab pahala dapat. Kita dapat di akhirat nanti. Kerja adalah satu ibadah. Tapi kalau kita asyik dengki mendengki bukan pahala yang kita dapat, tapi dosa. Jadi, kejarlah pahala free ini.

“Mengkritik tidak bererti menentang. Menyetujui tidak semestinya menyokong. Menegur tidak bermakna membenci dan berbeza pendapat adalah kawan berfikir yang baik.”

Renungilah, berapa orang kawan kita dan berapa orang lawan kita?.

Thursday, July 30, 2009

Kawan Makan Kawan

Sudah lama kumengenalinya .Dan aku menganggap dialah saudara kerja.
Walaupun apa saja yang aku mampu .Aku membantunya apa yang perlu .Sehingga aku tersisih. Berbalah dengan boss. Kau kata teman sejati.
Lebih sukar dicari. Memang benar orng berkata, Harus awas dan waspada. Pada yang manis dibibir .Aku tersingkir.
Kawan yang ku benar harap-harapkan. Bukan jujur seperti yang kusangkakan. Kawan kini makan kawan .Ku dalam keadaan teraniaya. Kawan makan kawan.
Kini hilang segala yang ku ada. Ku hilang kerja, maruah dan muka.Kau racunkan fikiranku dan nya. Kau ambil kesempatan merapatinya boss
Aku dah terkena dengan hasutannya. Dan rupa-rupanya ada agendanya. Kerja ku pergi kerna batu api, Kawan ku sendiri membakar ku ini.Setelah ku berikan kepercayaan .Tak terlintas di fikiran kau buat bukan-bukan. Semua tipu helah tak akan terjadi lagi .Tanyalah diri apa yang sudah jadi. Kini kau tahu bagaimana kurasa ,Menahan peritnya bisa.
Semua ini tidak kuduga. Kawan makan kawan.

"Kawan"! - Sebuah Puisi... " - Oleh burung ciok umoh!

Untuk apa ertinya berkawan... kalau kawan tak boleh nak tolong kawan! Kita kawan-kawan tak mau lawan! Terus kerja sampai ajal!

Kalau ada kawan yg suka makan kawan... Tak perlulah adanya kawan! Kawan-kawan bukan haiwan! So jangan ikut perasaan ok!

Ada kawan suka melihat kegembiraan kawan yg lain... Ada kawan yg suka melihat tangisan kawan yg lain! Kalau begitu maknanya kawan... Memang sukar menilai ertinya kawan!

Ada kawan hanya bersama ketika senang... Ada kawan hanya melihat saja kawan yg sedang kesusahan! Ada pula kawan ygsuka melihat kejatuhan kawan-kawan! Malah ada juga kawan yg sentiasa berusaha menikam kawan dari belakang!

Untuk apa ertinya berkawan kalau bukan sebagai kawan! Untuk apa ertinya berkawan kalau tak mampu menjadi kawan! Untuk apa ertinya berkawan kalau masing-masing suka berlawan! Untuk apa ertinya berkawan kalau kawan suka makan kawan!

Kawan yg baik adalah kawan yg mahu berada bersama kawan ketika susah dan senang! Kawan yg baik adalah kawan yg sentiasa sporting! "

Berkawan dengan prinsip hasad dengki lama-kelamaan membawa mati!

Kawan tanpa kawalan memang menyeronokan! Namun awas akan kebatilan seorang kawan yg mungkin akan membahayakan!

Apa gunanya erti berkawan kalau kawan tak mahu memahami hati seorang kawan! Apa gunanya erti berkawan kalau kawan hanya mementingkan diri sendiri sehingga melupakan kawan!

Apa gunanya erti berkawan kalau ada kawan yg tak sanggup menangis kerana kawan! Kawan adalah kawan!

Tanpa kawan akan kosonglah ertinya berkawan!

Untuk berkawan memanglah mudah! Namun utk menjaga hubungan antara kawan adaah sesuatu yg maha sukar! Berkawan memerlukan pengorbanan! Berkawan memerlukan kesefahaman! Berkawan memerlukan kesesuaian!

Tanpa kawan dalam kehidupan... akan suramlah perjalanan! Untuk apa ertinya berkawan kalau bukan sebagai sebuah sandaran!

Untuk apa ertinya berkawan kalau bukan sebagai sebuah platform utk meluahkan perasaan! Untuk itu...

sama-samalah menjaga kawan! Jangan sampai kawan makan hati kerana bengang dengan sikap kawan! Jangan sampai kawan jadi serba-salah apabila terpaksa berhadapan dengan karenah seorang kawan!

Hormati kawan umpama menghormati diri sendiri! Nescaya akan terbitlah makna sebenar "Untuk Apa Ertinya Berkawan"

KIta kawan-kawan tak mahu lawan! aku caya sama lu! nie puisi khas untuk kau... bernama kawan!!

Mangsa tikam belakang dan kaki ampu..... burung ciok umoh

Wednesday, July 29, 2009


Bintulu Crude Oil Terminal

Sarawak Shell Bintulu Plant (SSBP) formerly known as Bintulu Crude Oil Terminal (BCOT) was the first major industrial project to go off the ground Tg. Kidurong in 1979.

The Project comprises three crude oil storage tanks, each with a capacity of 410,000 barrels. Located on the western boundary of the MLNG site, the plant coprises 3 areas of operations namely:

1.Crude Oil Operations (BCOT)

2.Condensate Stabilisation (BSTAB)

3.Gas Sales Facilities (BAGSF)

Daily Crude Production nett is 60,000 barrels per day. Daily Condemate Production is about 80,000 barrerls per day. Daily Gas Sales to downstream customers such as SMDS, ABF, SESCO and Petronas Gas Berhad is about 190 MMSCF per day. The Crude Oil and Condenate from the plant is either exported locally or to outside customers.


SMDS Plant

Shell MDS (Malaysia) Sdn. Bhd. a joint-venture company between Shell Gas, Petronas, Mitsubishi Corporation and the Sarawak State Government was formed in 1986. The company owns and operates the Shell Middle Distillate Synthesis (SMDS) plant, the world's first commercial gas to liquid plant, in Bintulu Sarawak.

The plant converts natural gas into high quality synthetic oil products and specialty chemicals which are paraffnic and colourless. Some 100 million standard cubic feet per day of natural gas are converted into 470,000 tonnes per annum of middle distillates (gasoil, kerosene, naphtha) and specialty products (detergent feedstocks, solvent feedstocks, various grades of waxes). The plant started operations in May 1993 and its products are sold globally. For more information, please contact 086-252211 / 292405. Email:

Monday, July 27, 2009

SapuraCrest awarded RM3B Shell Gumusut-Kakap Offshore Field Contract

Kuala Lumpur, March 17, 2009 - Charting another milestone in its involvement in deepwater technology, leading regional oil and gas services provider SapuraCrest Petroleum Berhad will be undertaking a contract worth RM3 billion for offshore installation works at the Gumusut-Kakap Field, operated by Sabah Shell Petroleum Company Limited (Shell). This follows the recent award of a lump sum contract to SapuraCrest’s wholly owned subsidiary, TL Offshore Sdn Bhd. The contract is expected to be executed over a three year period beginning this year.
SapuraCrest will deliver the contract through its joint venture company Sapura Acergy Sdn Bhd (SASB) where a major part of the work will be executed using the Sapura 3000, its state-of-the-art dynamically positioned heavy lift and deepwater pipelay vessel. Engineering and procurement work will commence with immediate effect and offshore installation will begin in 2010. The Gumusut-Kakap field is located about 200 km offshore Sabah with a water depth of 1,200 meters.

The field is the first deepwater development for Shell in Malaysia. However, it presents SapuraCrest its second and most advanced deepwater construction work to date, having earlier completed the Kikeh gas pipeline in Sarawak at a water depth of 1,400 meters. This project will involve more Malaysian engineers and managers, giving them opportunities to further develop in this highly-specialised field.

"The award reflects Shell’s and Petronas’ confidence in our capability and reaffirms the position of SapuraCrest as a leading provider of technologically superior and high quality services in the region. We have invested heavily in strategic assets and resources to the tune of RM1 billion over the last 5 years, and this has helped us position SapuraCrest to undertake projects of this magnitude and complexity. It is an honour to be selected by Shell, one of the biggest players in the global oil and gas industry today," said Datuk Shahril Shamsuddin, Executive Vice-Chairman of SapuraCrest Petroleum.

"This contract award is a tribute to the extensive experience and expertise of our engineers as it involves advanced and complex engineering design in the execution of deepwater works and highlights the capabilities of our latest advanced vessel, the Sapura 3000. The project will give us the opportunity to develop Malaysian engineers and managers in this new frontier and will allow Malaysians to undertake future deepwater engineering, construction and installation in Malaysia and across the region. This will move Malaysia higher up in the value chain in the ability to execute jobs with increasing complexities," Datuk Shahril added.

The contract entails project management, procurement, engineering, transportation and installation services, and works for an oil export pipeline and catenary riser, including the shore approach. It will also involve the installation of flowlines, jumpers, Steel Catenary Risers (SCRs), Pipeline End Terminations (PLET) and flowline inline structures. SASB will also be required to supply and install mooring wires, chains and piles for a semi-submersible Floating Production System (FPS) including towing and installation of the FPS at offshore location. More than 50 percent of the project will be carried out by local manpower and resources.

“We are delighted to be part of Malaysia’s long term growth plans and are pleased to be building solid relationships, creating additional localised support for all our clients in the region. This project will also help promote growth in the oil and gas industry by fostering technology transfer and local talent development, creating new employment opportunities and maximizing local procurement in Malaysia. We also hope that this project will in some small way assist local industries during these trying economic times," said Datuk Shahril.
The Gumusut-Kakap contract will propel SapuraCrest to the next stage of its expansion plans into higher technology services for the oil and gas industry. The contract is also expected to significantly boost current and future revenues.

* Exchange rate of USD1 = RM3.6 used
About SapuraCrest Petroleum Berhad

SapuraCrest Petroleum Berhad, a public-listed subsidiary of the Sapura Group of Companies, is one of the largest local oil and gas integrated service providers in Malaysia.

Through its subsidiaries, SapuraCrest Petroleum is involved in marine installation and construction, offshore drilling and marine services in Malaysia, Asia Pacific and South Asian regions. In particular, SapuraCrest Petroleum services cover the provision of accommodation and support vessels and tender assisted drilling rigs, installation of offshore pipelines and structures, provision of offshore hook-up, commissioning and topside maintenance services, provision of underwater services and provision of offshore geotechnical and geophysical services.

For more information, please visit
For Media Enquiries, please contact:
Norliza Kamaruddin, Sapura Group Corporate Communications
Tel: +603 8949 7727

FPSO Kikeh

Malaysia has completed the FPSO Kikeh, its first deepwater floating production storage and offloading (FPSO) facility, a significant event in the nation's endeavour to develop world class deepwater engineering and construction capability.

FPSO Kikeh, completed in 26 months, is also notable in that it is the largest such facility to be constructed in Malaysia. Built by Malaysia Marine & Heavy Engineering (MMHE), a subsidiary of MISC, at its yard in Pasir Gudang, Johor, peninsular Malaysia, the floating production unit was converted from the 337m long 279,000dwt VLCC SS Atlas. FPSO Kikeh, owned and operated by Malaysia Deepwater Terminal, a joint venture between MISC and SBM Offshore, is now on location in block K offshore Sabah, East Malaysia. Leased to Murphy Sabah Oil, the production sharing partner of Petronas Carigali, the FPSO is contracted for an initial period of eight years with options for five three-year follow-on extensions.

Moored in a water depth of 1320m about 120km northwest of Labuan Island, the FPSO will unload its cargo of oil to shuttle tankers every ten days, produced from Malaysia's first deepwater discovery using subsea wells connected to the FPSO by pipelines on the seabed and flexible risers.

Murphy Oil estimates the Kikeh field had proved reserves of 47.5mmbo and 74.6bcf of gas as at year-end 2006. Initial oil production, scheduled for startup in the second half of this year, is expected to be 40,000b/d of oil with a one-year ramp up to a plateau of 120,000b/d.

The turret

FPSO Kikeh's external turret, at around 2300t, is the heaviest ever designed by SBM. It affords permanent mooring, achieved with 10 anchor legs in a 4-3-3 configuration consisting of 127mm studless chain and 98mm wire rope, and acts as a support for the production, injection and utilities lines.

Product, water, power and communication data will be transferred between FPSO Kikeh and the anchored Kikeh DTU (dry tree unit) truss spar by way of fluid transfer lines (FTL) that utilise SBM's Gravity Actuated Pipe (GAP) system.

The turret provides fluid transfer and control functions to and from the vessel to the seabed and the DTU. Flexible subsea risers suspended from the turret feed commingled production fluids from the wells into the turret manifold. Jumpers connected to the GAP take care of fluid transfer to and from the DTU.

Produced fluids, a mixture of oil, gas, water and solid particles, flow to the topsides process modules via the turret swivels.

Treated water will pass through the turret into injection wells and produced gas not used for power generation on board will be initially injected, then exported by pipeline to Labuan Island, about a year after field production starts up.

Wellheads and DTU are controlled from the FPSO through the turret swivels and umbilicals to the seabed and DTU.

Malaysia rising

Through the transfer of technology resulting from the joint effort between MISC and SBM, the FPSO Kikeh project team from MISC and MMHE were able to develop expertise in deepwater construction. This collaboration also supported the growth and development of local vendors by providing business opportunities to more than 80 Malaysian subcontractors and service suppliers.

Marking this achievement the FPSO Kikeh naming ceremony was held at the MMHE yard on 29 March in the presence of Mohd Hassan Marican, MISC chairman and Petronas president and CEO, Claiborne Deming, Murphy Oil president and CEO, Didier Keller, SBM president, and Shamsul Azhar Abbas, MISC president and CEO.

Saturday, July 25, 2009

Basic Principles of Ultrasonic Testing

Basic Principles of Ultrasonic Testing

Ultrasonic Testing (UT) uses high frequency sound energy to conduct examinations and make measurements. Ultrasonic inspection can be used for flaw detection/evaluation, dimensional measurements, material characterization, and more. To illustrate the general inspection principle, a typical pulse/echo inspection configuration as illustrated below will be used.

A typical UT inspection system consists of several functional units, such as the pulser/receiver, transducer, and display devices. A pulser/receiver is an electronic device that can produce high voltage electrical pulses. Driven by the pulser, the transducer generates high frequency ultrasonic energy. The sound energy is introduced and propagates through the materials in the form of waves. When there is a discontinuity (such as a crack) in the wave path, part of the energy will be reflected back from the flaw surface. The reflected wave signal is transformed into an electrical signal by the transducer and is displayed on a screen. In the applet below, the reflected signal strength is displayed versus the time from signal generation to when a echo was received. Signal travel time can be directly related to the distance that the signal traveled. From the signal, information about the reflector location, size, orientation and other features can sometimes be gained.

Ultrasonic Inspection is a very useful and versatile NDT method. Some of the advantages of ultrasonic inspection that are often cited include:

* It is sensitive to both surface and subsurface discontinuities.
* The depth of penetration for flaw detection or measurement is superior to other NDT methods.
* Only single-sided access is needed when the pulse-echo technique is used.
* It is highly accurate in determining reflector position and estimating size and shape.
* Minimal part preparation is required.
* Electronic equipment provides instantaneous results.
* Detailed images can be produced with automated systems.
* It has other uses, such as thickness measurement, in addition to flaw detection.

As with all NDT methods, ultrasonic inspection also has its limitations, which include:

* Surface must be accessible to transmit ultrasound.
* Skill and training is more extensive than with some other methods.
* It normally requires a coupling medium to promote the transfer of sound energy into the test specimen.
* Materials that are rough, irregular in shape, very small, exceptionally thin or not homogeneous are difficult to inspect.
* Cast iron and other coarse grained materials are difficult to inspect due to low sound transmission and high signal noise.
* Linear defects oriented parallel to the sound beam may go undetected.
* Reference standards are required for both equipment calibration and the characterization of flaws.

The above introduction provides a simplified introduction to the NDT method of ultrasonic testing. However, to effectively perform an inspection using ultrasonics, much more about the method needs to be known. The following pages present information on the science involved in ultrasonic inspection, the equipment that is commonly used, some of the measurement techniques used, as well as other information

Physics of Ultrasound

Refraction and Snell's Law

When an ultrasounic wave passes through an interface between two materials at an oblique angle, and the materials have different indices of refraction, both reflected and refracted waves are produced. This also occurs with light, which is why objects seen across an interface appear to be shifted relative to where they really are. For example, if you look straight down at an object at the bottom of a glass of water, it looks closer than it really is. A good way to visualize how light and sound refract is to shine a flashlight into a bowl of slightly cloudy water noting the refraction angle with respect to the incident angle.

Refraction takes place at an interface due to the different velocities of the acoustic waves within the two materials. The velocity of sound in each material is determined by the material properties (elastic modulus and density) for that material. In the animation below, a series of plane waves are shown traveling in one material and entering a second material that has a higher acoustic velocity. Therefore, when the wave encounters the interface between these two materials, the portion of the wave in the second material is moving faster than the portion of the wave in the first material. It can be seen that this causes the wave to bend.

Snell's Law describes the relationship between the angles and the velocities of the waves. Snell's law equates the ratio of material velocities V1 and V2 to the ratio of the sine's of incident (Q1) and refracted (Q2) angles, as shown in the following equation.

VL1 is the longitudinal wave velocity in material 1.
VL2 is the longitudinal wave velocity in material 2.

Note that in the diagram, there is a reflected longitudinal wave (VL1' ) shown. This wave is reflected at the same angle as the incident wave because the two waves are traveling in the same material, and hence have the same velocities. This reflected wave is unimportant in our explanation of Snell's Law, but it should be remembered that some of the wave energy is reflected at the interface. In the applet below, only the incident and refracted longitudinal waves are shown. The angle of either wave can be adjusted by clicking and dragging the mouse in the region of the arrows. Values for the angles or acoustic velocities can also be entered in the dialog boxes so the that applet can be used as a Snell's Law calculator.
When a longitudinal wave moves from a slower to a faster material, there is an incident angle that makes the angle of refraction for the wave 90o. This is know as the first critical angle. The first critical angle can be found from Snell's law by putting in an angle of 90° for the angle of the refracted ray. At the critical angle of incidence, much of the acoustic energy is in the form of an inhomogeneous compression wave, which travels along the interface and decays exponentially with depth from the interface. This wave is sometimes referred to as a "creep wave." Because of their inhomogeneous nature and the fact that they decay rapidly, creep waves are not used as extensively as Rayleigh surface waves in NDT. However, creep waves are sometimes more useful than Rayleigh waves because they suffer less from surface irregularities and coarse material microstructure due to their longer wavelengths.

Wave Propagation
Ultrasonic testing is based on time-varying deformations or vibrations in materials, which is generally referred to as acoustics. All material substances are comprised of atoms, which may be forced into vibrational motion about their equilibrium positions. Many different patterns of vibrational motion exist at the atomic level, however, most are irrelevant to acoustics and ultrasonic testing. Acoustics is focused on particles that contain many atoms that move in unison to produce a mechanical wave. When a material is not stressed in tension or compression beyond its elastic limit, its individual particles perform elastic oscillations. When the particles of a medium are displaced from their equilibrium positions, internal (electrostatic) restoration forces arise. It is these elastic restoring forces between particles, combined with inertia of the particles, that leads to the oscillatory motions of the medium.
In solids, sound waves can propagate in four principle modes that are based on the way the particles oscillate. Sound can propagate as longitudinal waves, shear waves, surface waves, and in thin materials as plate waves. Longitudinal and shear waves are the two modes of propagation most widely used in ultrasonic testing. The particle movement responsible for the propagation of longitudinal and shear waves is illustrated below.

In longitudinal waves, the oscillations occur in the longitudinal direction or the direction of wave propagation. Since compressional and dilational forces are active in these waves, they are also called pressure or compressional waves. They are also sometimes called density waves because their particle density fluctuates as they move. Compression waves can be generated in liquids, as well as solids because the energy travels through the atomic structure by a series of comparison and expansion (rarefaction) movements.

In the transverse or shear wave, the particles oscillate at a right angle or transverse to the direction of propagation. Shear waves require an acoustically solid material for effective propagation, and therefore, are not effectively propagated in materials such as liquids or gasses. Shear waves are relatively weak when compared to longitudinal waves. In fact, shear waves are usually generated in materials using some of the energy from longitudinal waves.

Precision Velocity Measurements

Changes in ultrasonic wave propagation speed, along with energy losses, from interactions with a materials microstructures are often used to nondestructively gain information about a material's properties. Measurements of sound velocity and ultrasonic wave attenuation can be related to the elastic properties that can be used to characterize the texture of polycrystalline metals. These measurements enable industry to replace destructive microscopic inspections with nondestructive methods.

Of interest in velocity measurements are longitudinal wave, which propagate in gases, liquids, and solids. In solids, also of interest are transverse (shear) waves. The longitudinal velocity is independent of sample geometry when the dimensions at right angles to the beam are large compared to the beam area and wavelength. The transverse velocity is affected little by the physical dimensions of the sample.

Pulse-Echo and Pulse-Echo-Overlap Methods

Rough ultrasonic velocity measurements are as simple as measuring the time it takes for a pulse of ultrasound to travel from one transducer to another (pitch-catch) or return to the same transducer (pulse-echo). Another method is to compare the phase of the detected sound wave with a reference signal: slight changes in the transducer separation are seen as slight phase changes, from which the sound velocity can be calculated. These methods are suitable for estimating acoustic velocity to about 1 part in 100. Standard practice for measuring velocity in materials is detailed in ASTM E494.

Precision Velocity Measurements (using EMATs)

Electromagnetic-acoustic transducers (EMAT) generate ultrasound in the material being investigated. When a wire or coil is placed near to the surface of an electrically conducting object and is driven by a current at the desired ultrasonic frequency, eddy currents will be induced in a near surface region. If a static magnetic field is also present, these currents will experience Lorentz forces of the form

F = J x B

where F is a body force per unit volume, J is the induced dynamic current density, and B is the static magnetic induction.

The most important application of EMATs has been in nondestructive evaluation (NDE) applications such as flaw detection or material property characterization. Couplant free transduction allows operation without contact at elevated temperatures and in remote locations. The coil and magnet structure can also be designed to excite complex wave patterns and polarizations that would be difficult to realize with fluid coupled piezoelectric probes. In the inference of material properties from precise velocity or attenuation measurements, use of EMATs can eliminate errors associated with couplant variation, particularly in contact measurements.
Differential velocity is measured using a T1-T2---R fixed array of EMAT transducers at 0, 45°, 90° or 0°, 90° relative rotational directions depending on device configuration:

EMAT Driver Frequency: 450-600 KHz (nominal)

Sampling Period: 100 ns

Time Measurement Accuracy:

Resolution 0.1 ns

Accuracy required for less than 2 KSI Stress Measurements: Variance 2.47 ns

Accuracy required for texture: Variance 10.0 Ns




Time Measurement Technique

Fourier Transform-Phase-Slope determination of delta time between received RF bursts (T2-R) - (T1-R), where T2 and T1 EMATs are driven in series to eliminate differential phase shift due to probe liftoff.

Slope of the phase is determined by linear regression of weighted data points within the signal bandwidth and a weighted y-intercept. The accuracy obtained with this method can exceed one part in one hundred thousand (1:100,000).