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19.2 Nondestructive Examination

David Colameco, M.Ed.

Basics of Nondestructive Examination

Nondestructive examination describes a set of examination techniques that do not destroy the fabrication being inspected. Destructive tests provide information such as the tensile and yield strength of a weld that can not be determined by nondestructive tests. However if we only performed destructive tests on our weldment, we wouldn’t have any fabrications to place in-service because they would all be destroyed. The basic premise of a nondestructive test is to provide the inspector with information about the quality of the weldment without damaging or altering the weldment.

Uses of Nondestructive Examination in industry today

As a welder in school learning to weld, you visually inspect your welds and your classmates’ welds all the time; whenever you get a chance. It is highly recommended as a welder learning to weld that you take pictures of your welds each week so you can see the progress you have made with time.

In industry, especially at fabrication shops with quality assurance programs, there will be a weld sample board with examples for welders to see of what acceptable and unacceptable welds look like. The purpose of these examples is to not only provide you with a means to determine if you are doing a good job welding, but to also provide an example weld to strive for so your welds meet a minimum quality standard that will more likely pass the official quality inspection.

Inspectors, such as American Welding Society (AWS) Certified Welding Inspectors (CWI) will examine your welds using the Nondestructive Examination (NDE) techniques described in this chapter. It is important to note that NDE and Nondestructive Testing (NDT) are terms that are commonly used interchangeably but they have different meanings. NDT refers to the testing itself, while NDE refers to the testing and the interpretation of the results.

The more critical the application of your weldment, the more stringent the inspection requirements will be depending on the code used. NDE is used by industry to ensure that quality weldments are being made to meet both safety and quality requirements and expectations that customers have. Let’s discuss some of the more common NDE methods used by industry.

Visual Inspection (VT)

Visual inspection (VT) is the most common inspection method used today because it is relatively easy to do and welders and inspectors are expected to have a minimum level of visual acuity, meaning the ability to see minor details, to inspect their welds. Visual discontinuities that visual inspection is intended to find are:

The list of discontinuities above are common discontinuities found on or near welds. A discontinuity becomes a defect if its size or number of occurrences exceeds the criteria in a welding code or standard being used for inspection.

Overlap

Figure 19.1 below shows an example of overlap. Overlap occurs when the molten weld pool does not fuse into the base material. The weld pool falls on top of the base material and is not solidly connected to it. Other terms used for overlap are cold roll and cold lap.

Overlap welding defect shown on a weld bead. The weld bead has a slight gap between the bottom piece of the fillet weld and the bead.
Figure 19.1. Overlap / Photo Credit: United States Department of Transportation, Federal Highway Administration, PD

Porosity

Porosity occurs when gasses produced during welding can not escape the molten weld pool before it solidifies. The gas occupies volume that the solidifying weld pool can not occupy. If the welding process involves slag, the slag could be what blocked the gas from escaping. Figure 18.2 shows an example of porosity in a solidified weld bead.

Porosity welding defect shown on a weld bead joining plates together. The weld bead has many small holes in it. Two white arrows drawn in chalk on the image point to the holes in the weld bead.
Figure 19.2. Porosity / Photo Credit: United States Department of Transportation, Federal Highway Administration, PD

Welding codes for steel will typically not allow porosity or limit it to a low number of pores for welder qualification tests. Aluminum, by nature of the process, has more porosity than steel. Porosity weakens the weld because there isn’t solid material to take up the load applied to the weldment. Fortunately, porosity is circular or rounded, which means that each pore will not create a devastating stress riser by itself.

Stress risers are areas where lines of stress concentrate. If too much stress exists in one area, the yield strength of the material may be exceeded which will cause plastic deformation of the material and possible failure of the weldment.

Slag Inclusions

Slag is a very important piece of the welding processes that form slag. Slag forms when certain elements in the fluxes of the consumables react with elements in the weld that are impurities that must be removed to ensure a quality weld. The designers of the consumables, whether that be the flux coatings on Shielded Metal Arc Welding (SMAW) electrodes, the flux within Flux Cored Arc Welding (FCAW) wires, or even the granular flux used in Submerged Arc Welding (SAW), design the consumable to remove impurities. Gas Metal Arc Welding (GMAW) and Gas Tungsten Arc Welding (GTAW) are processes that do not produce a slag.

During the design process, if the welds suffered defects during the testing phase of development, the chemistry of the flux would be changed. This change is done much like a baker or cook changes a recipe if the end product doesn’t have the taste, texture, or color desired. The impurities that must be removed bond with the elements in the flux that were added to bond to the impurities, and they float to the surface of the weld pool to form slag.

Slag not only removes impurities from the weld pool, it also helps control the cooling rate of the weld pool by slowing down how fast the weld pool solidifies. Once a weld bead has been put in place, the slag must be removed by mechanical means before the next weld bead is put in place. This can be done using a chipping hammer, wire brush or a wire wheel for example.

Figure 19.3 below shows slag inclusions that are pointed to with red arrows.

Slag Inclusions shown on a weld bead that has been cut into, to expose the slag. Three rough spots appear on the edge of a piece of metal. Three red areas point to the rough spots.
Figure 19.3. Slag Inclusions / Photo Credit: United States Department of Transportation, Federal Highway Administration, PD

Cracks

Cracks are never acceptable because they will spread overtime, especially when they are exposed to a cyclical load. When describing cracks in welds, there are generally transverse cracks which occur perpendicular to the direction of travel of the weld bead, longitudinal cracks which occur parallel to the direction of travel of the weld bead, and crater cracks which usually occur in a depression or crater at the end of a weld bead and radiate from the center of the crater out.

Weld Cracks shown on a groove joint with a weld. This illustration shows a V-Groove weld. The weld bead is portrayed as a series of even, horizontal bumps. At the front of the illustration, where the edge of the weld is seen is a crack labeled “Longitudinal Cracks.” It runs perpendicular to the bumps, but follows the length of the weld bead. Toward the back edge of the illustration are two cracks labeled “Transverse Cracks.” These cracks run parallel to the bumps in the weld bead, but are perpendicular to the weld bead itself.
Figure 19.4. Metal Weld Cracks / Photo Credit: Nicholas Malara, CC BY 4.0

Figure 19.4 shows an illustration of transverse longitudinal cracks, while Figure 19.5 shows an example of a crater crack that typically forms in an aluminum weld bead when a crater is not filled in at the end of a weld bead.

Crater Crack at the end of a weld bead in a common location. Crater cracks and how they appear is described in the preceding text.
Figure 19.5. Crater Crack / Photo Credit: United States Department of Transportation, Federal Highway Administration, PD

Spatter

Spatter, see Figure 19.6 below, by itself is generally not detrimental to a weldment, however it does show poor workmanship if the welding process used normally doesn’t have spatter. Spatter is usually removed by mechanical means such as with a grinder.

Spatter shown on a weld joint that is painted grey. The spatter appears as little bumps beneath the paint coating.
Figure 19.6. Spatter / Photo Credit: United States Department of Transportation, Federal Highway Administration, PD

Spatter does interfere with Magnetic Particle Testing and must be removed. Ultrasonic testing, which is discussed later in this chapter, may use a transducer which is slid across the surface of a weldment to send ultrasonic sound waves into the material at a predetermined angle. If spatter exists on the surface of the weldment, the angle of the ultrasonic sound waves would not be correct if the spatter interfered with the placement of the transducer relative to the surface of the weldment being inspected.

If your blueprint shows a welding symbol with MT or UT in the tail, it is best to know whether or not spatter needs to be removed after welding has been performed.

Arc Strikes

When welding, welders should always strike an arc within the weld joint that is being welded. The reason for this is more than just cosmetic or for avoiding poor workmanship. When steels are cooled too quickly, they can form martensite which is brittle.

One discussion that comes up regularly amongst welders is whether quenching mild steel in a quench tank will form martensite. Typically, the amount of base material that exists retains enough heat that your weldment does not form martensite when quenched. However this is not the case when the base material is at room temperature, and a small amount of base material is melted and rapidly cooled following an arc strike. Martensite, depending on the steel, will form a brittle spot which will likely be the location for crack formation. This is why if you ever accidentally arc strike a gas bottle, that bottle is immediately taken out of service, because it could fail. As a welder, if you arc strike outside of the weld joint, your employer or the code may require you to gouge out the arc strike and fill the removed material with a weld.

So why aren’t arc strikes in the weld joint a bad idea? Fortunately, the arc strikes in the weld joint melt when the weld is placed into the joint. The weld will penetrate into the base material by melting and mixing with the base material before solidifying. Any martensite that was formed in the arc strike is destroyed when it melts.

Arc Strikes shown on metal that appear as burn marks
Figure 19.7. Arc Strikes / Photo Credit: United States Department of Transportation, Federal Highway Administration, PD

Liquid Penetrant Testing (PT)

Liquid Penetrant Testing (PT) is a multistep process that is used to highlight cracks, porosity, and even arc strikes on a weldment. This method works by applying a very viscous liquid that easily penetrates into the cracks and pores on the surface, which are later highlighted by a developer. Let’s look into this process more thoroughly.

The following general steps are followed when performing liquid penetrant testing:

  • Thoroughly clean the surface of the weldment being examined
  • Apply the liquid penetrant
  • Allow the liquid penetrant to remain for the prescribed dwell time
  • Remove the excess liquid penetrant from the surface with carefully cleaning
  • Apply the developer
  • Wait for any indications to appear through the developer

The steps described above would be performed by a trained and qualified inspector for code inspections. PT is available to the public wishing to perform the tests at home.

A hand wearing a blue latex-style glove uses an aerosol spray can to spray Red Liquid Penetrant Application on a flat weldment. The weldment is sitting on a paper towel.
Figure 19.8. Apply Liquid Penetrant / Photo Credit: United States Department of Transportation, Federal Highway Administration, PD

Figure 19.8 above shows the application of liquid penetrant to the weldment being inspected. The viscous liquid flows very easily into tight spaces through capillary action. Capillary action allows liquids to move against the force of gravity in trees from the roots to the top of the tree, or in a kitchen sponge stood on end in a shallow bowl of water. Over time you would see the sponge absorb the water into areas above the top level of the water in the bowl. This is capillary action in action.

Through capillary action and the viscous nature of the liquid penetrant, the penetrant will seep into the cracks and porosity that is exposed to the surface of the weldment. Capillary action is a relatively slow process and the dwell time enables the penetrant enough time to seep into these surface discontinuities.

A close-up of two hands, wearing latex-style gloves using a paper towel to clean the surface of a weldment covered in red liquid penetrant.
Figure 19.9. Clean Surface / Photo Credit: United States Department of Transportation, Federal Highway Administration, PD

Figure 19.9 above shows the inspector carefully cleaning the excess liquid penetrant from the weldment. It is important to remove the liquid penetrant from the surface so there aren’t any false indications once the developer is put on the surface. Once cleaned, only the liquid penetrant that has seeped into the surface discontinuities will be drawn back out to react with the developer.

Wearing gloves during this process is very important, otherwise the liquid penetrant will seep into your porous skin and stain them. There are likely also other health hazards from liquid penetrant that would be discussed on the Safety Data Sheet (SDS) from the manufacturer of the penetrant.

Applying Developer to the metal and allowing time for the penetrant to show indications of possible defects. This weldment shows a bright red edge right along the weld bead where the liquid penetrant is concentrated on the weld.
Figure 19.10. Apply Developer / Photo Credit: United States Department of Transportation, Federal Highway Administration, PD

Figure 19.10 shows the developer being applied to the weldment after the surface has been cleaned of any excess penetrant. As an example of this inspection process, Figure 19.10 provides some great indications on the surface of this weldment. The certified liquid penetrant inspector will know what the indications on the surface mean.

For example, it appears that there may be spatter located on the left hand side of the picture below the weld bead. Spatter is a discontinuity but not a defect. If you are performing a liquid penetrant test at home to try it out, it is important to make note of surface features such as spatter so you don’t mistake them for defects later after the developer has reacted with any remaining liquid penetrant.

Additionally, some liquid penetrant may have remained in the lower parts of the weld bead contours. These contours may be acceptable resulting in another set of false indications.

Crack shown on a weld sample that has been tested using liquid penetrant testing. The crack is labeled and is indicated by a thin line about two inches long at the base of the weld bead.
Figure 19.11. Crack Indication / Photo Credit: United States Department of Transportation, Federal Highway Administration, PD

Figure 19.11 shows a crack that was identified by the liquid penetrant inspector. Figure 19.10 and 19.11 show that training is important when interpreting the results from liquid penetrant testing.

A close up of a weldment with an uneven red line about 2 inches above the weld bead and red surrounding two bolt holes. This is a crack seen on a sample that has been liquid penetrant tested. The crack is visible because the penetrant is red.
Figure 19.12. Crack Indication / Photo Credit: United States Department of Transportation, Federal Highway Administration, PD

Liquid penetrant testing is used for many different parts of all different sizes. Figure 19.12 below shows possible crack indications on a part around the bolt holes in the image. If this is a cast part or a part with a rough surface, the cracks might be hard to see with the naked eye. The liquid penetrant and the developer provide the contrast that is needed to identify cracks and other discontinuities.

Two gloved hands holding a small rectangular weldment with four holes in it. The background is very dark and the piece is lit up with a blacklight. Fluorescent liquid penetrant is showing up as green on the sample being tested.
Figure 19.13. Fluorescent Liquid Penetrant / Photo Credit: U.S. Department of Energy, Office of Energy, Efficiency and Renewable Energy, National Renewable Energy Laboratory, PD

Sometimes even more contrast is required to detect defects in fabrications. Fluorescent liquid penetrant testing provides additional contrast as shown in Figure 19.13. A black light is used in a dark room to react with the fluorescent liquid penetrant to provide the bright indications for the inspector to interpret. Again, note the gloves being used to protect the inspector from the chemicals used; the liquid penetrant would easily dye the inspector’s hands and would likely be difficult to remove.

Leak Testing (LT)

Leak testing (LT) is a process that pressurizes the system, typically a piping system, tank, or other vessel, for a given period of time to detect any leaks in the system. Figure 19.17 shows hoses being attached to a piping system to test for leaks prior to the pipes being buried underground.

Leak Test of a pipeline showing a hose pressurizing the pipe. A worker sits on top a large box-shaped van or trailer and watches as a hose attached to the van and to the pipe operates the test. Another two workers stand on the ground.
Figure 19.14. Leak Testing / Photo Credit: U.S. Department of Transportation, PD

Finding leaks prior to burial makes repairs much easier because the dirt does not have to be removed, and it is also safer because the pump that is pressurizing the system has a much lower flow rate than the normal flow within the piping system at in-service conditions, meaning that a weld failure during leak testing won’t leak as much volume of liquid. Think of your garden hose that you turn off at the valve on the side of a building; the hose is still pressurized but when you open the other end of the hose only a small burst of water comes out.

Magnetic Particle (MT)

Magnetic Particle (MT) is a process that uses a magnetic field and fine magnetic particles that will orient themselves in that magnetic field to make discontinuities visible. Figure 19.15 below shows different types of prods inducing magnetic fields around a weld.

Two examples of Magnetic Testing Prods and the magnetic fields that they produce on two flat weldments. On the left, are two magnetic testing prods that look like two toothpicks mounted on holders with electrical cords are pointed into the weld. Red arrows oriented in circles around the toothpick-sized rods demonstrate how a magnetic field would form around the magnetic testing prods. On the right, a rectangular shaped yoke that looks a little like a mini-goal post for soccer minus the net, with a cord leading to it is placed so that the two sides of the yoke straddle the weld. Red markings show how a magnetic field is created in the space below the prod.
Figure 19.15. Magnetic Testing Prods (L) Yoke (R) / Photo Credit: Nicholas Malara, CC BY 4.0

Magnetic particle testing can only be performed on magnetic materials to be effective. Because defects within magnetic materials change the direction of the magnetic field, the magnetic particles located on the surface of the weldment will move and align themselves in a particular pattern due to the shape of the discontinuity. Only discontinuities that are not parallel to the magnetic field are detectable, see the right side of Figure 19.16.

Magnetic Flow through a part without flaws and one with flaws. Each part is marked S on the right and N on the left, orienting the magnetic field as moving from south to north. The part without flaws shows straight red arrows that are flowing easily from south to north. In the part with flaws all arrows flow from south to north, but on the lower right of the illustration two arrows arc around a discontinuity that interrupts the magnetic field. In about the center of the image and toward the left edge, a crack marked flux leakage also shows an arrow have to arc around the discontinuity rather than travel a straight path.
Figure 19.16. Magnetic Testing Particle Accumulation / Photo Credit: Nicholas Malara, CC BY 4.0

In practice the magnetic particle inspector will apply the magnetic particles from a bottle, similar to applying baby powder from a bottle. Figure 19.17 below shows the inspector actively applying the iron powder from a red bottle in their right hand while holding the yoke in their left hand. The massive fillet weld along the T-Joint that is being illuminated by the flash light in the picture is likely being inspected.

Magnetic Particle Testing of a very large weldment which is explained in the text preceding the image.
Figure 19.17. Magnetic Testing / Photo Credit: United States Department of Transportation, Federal Highway Administration, PD

The magnetic particle inspector will take great care to gently blow the excess magnetic particles with the puffer bulb. If the inspector is too aggressive with the puffer bulb, they will blow too many magnetic particles away and possibly miss an indication of a discontinuity. If they don’t blow away enough particles, then the indication of a discontinuity will be masked.

Eddy Current Testing (ECT)

Eddy Current Testing (ECT) is a process that induces currents into the weldment being inspected. Figure 19.18 shows an electric coil with an alternating current (AC) signal applied to it. Alternating current is current that changes direction regularly. The electricity that is supplied from the wall outlets in your home or at school are 60 Hz AC, meaning the signal has 60 cycles per second with two changes in direction per cycle for 120 changes per second.

You may have heard the term electromagnetism before. This term refers to the fact that electricity and magnetism are forces that act together. To be super technical about it, electricity and magnetism are two effects that result from an electric charge; magnetism results from a moving charge while electricity can occur with both a moving and non moving charge. For moving charges think of electricity in a wire and for non-moving charges think static electricity or a disconnected charged battery.

Eddy Current Testing and the magnetic fields that are created. The image is described in the text following the image.
Figure 19.18. Eddy Current Testing / Photo Credit: Nicholas Malara, CC BY 4.0

Figure 19.18 shows the magnetic field from the coil when it is too far away from the conductive material to induce a magnetic field. In the middle of Figure 19.18 the coil is close enough to induce a magnetic field with a uniform field because there are no discontinuities, while the position on the right has a disturbed magnetic field.

Similar to the magnetic particle testing in the previous section, eddy current testing can only detect discontinuities that are not parallel to the lines of the magnetic field. Discontinuities that are parallel, or mostly parallel to the lines of the magnetic field do not produce a strong enough signal to detect. Figure 19.19 shows the disturbed lines of magnetic force due to the presence of the discontinuities. If the discontinuities did not exist in Figure 19.19, then the lines of magnetic force would be concentric circles. Notice in the Figure that the crack that is perpendicular to the magnetic lines interrupts the magnetic lines by pushing them further apart as they go around the crack, and the crack parallel to the magnetic lines does not affect the magnetic lines enough to notice.

Eddy Current flow visualization shown as rings that get deformed into ovals by defects.
Figure 19.19. Eddy Currents and Flows / Photo Credit: Nicholas Malara, CC BY 4.0

Figure 19.20 illustrates a crack that exists on a small piece of a railroad rail. Figure 19.21 is a composite of multiple pictures that show the indications of the cracks on the screen and pictures of the rail and close up pictures of the cracks that are being displayed on the screen.

Crack shown in a railroad rail. The rail is labeled head, at the top where the train wheels would fit around the rail. Web, the thin part below the head. Finally, foot is labeled, which is the wide flat bottom that helps to hold the rail stead and where spikes would be driven to secure the rail. A crack is shown below the head on the web of the rail.
Figure 19.20. Crack Location in a Rail / Photo Credit: Nicholas Malara, CC BY 4.0

Before we look at Figure 19.21 in detail let’s explain why there are two screens shown, one labeled “Real part of Impedance” and the other “Imaginary part of Impedance.” Mathematically, electromagnetism has real and imaginary values when measuring it. We are all familiar with real numbers such as whole numbers, decimals and fractions. We use them in welding settings on our machines such as 95 Amps, or 24 Volts. For engineers and technicians working with electromagnetism it is easier to describe electromagnetism with a complex number, meaning a number with a real and imaginary part than to describe it using amperage, voltage, capacitance, and inductance.

UT Crack Indications shown on a UT screen with photos of the cracks underneath.
Figure 19.21. UT Crack Indications / Photo Credit: United States Department of Transportation, Federal Highway Administration, PD

An in depth discussion of complex numbers is beyond the scope of this chapter, but it was worth mentioning in case you were curious about the “Imaginary part of impedance.” capacitance is the resistance to a change in voltage while inductance is the resistance to a change in current. Inductance is a setting on your welding machine to control spatter when welding GMAW in the short circuit transfer mode. By increasing the inductance you reduce the amount of spatter that occurs when the wire short circuits with the workpiece. Turning up the inductance too much will cause stubbing where the wire hits the workpiece and pushes your welding gun away. Inductance will be discussed more in the GMAW and/or welding machine chapter.

Figure 19.22 shows another example of a digital readout of an eddy current test with a flaw being indicated on the screen (Left) and an eddy current probe (Right). Below that Figure 19.23 shows another type of eddy current probe that you may see in the field.

Eddy Current Equipment showing the main controller on the left and the probe to the right. The probe is a black cylinder about two inches long with a wider metal knob on the top and a cord at the back.
Figure 19.22. Eddy Current Equipment / Photo Credit: Research and Special Programs Administration and the Department of Transportation, PD
Eddy Current Probe sitting on a desk. The probe has a large red base with a smaller black cylinder on top of that. There is a cord leading out of the black cylinder.
Figure 19.23. Eddy Current Probe / Photo Credit: NASA White Sands Test Facility, PD

Radiographic Testing (RT)

Radiographic Testing (RT) uses X-rays to penetrate weldments so we can see into what is normally not visible to the naked eye. If you have ever had an x-ray for medical reason or even had a friend showing off an x-ray of a broken bone you are familiar with some of the images in Figure 19.24. If you haven’t seen an x-ray of the human body, Figure 19.24 shows the powerful imagery it can provide.

Medical X-Rays of various body parts and the bones within shown in a 3 by 3 array
Figure 19.24. Medical X-Rays / Photo Credit: U.S. Department of Health and Human Seervices, Centers for Disease Control and Prevention, PD

An X-Ray works by positioning your weldment between the X-Ray source and a piece of film or digital sensor.

As times change so does Radiography. With the market for analog film being mostly medical, once the medical field goes to all digital radiography the cost of film would skyrocket and welding inspectors will switch to digital radiography also. Figure 19.25 shows film under the weldment, but in digital radiography a digital sensor would replace the film.

X-Ray Tube shown above a weld joint to illustrate how it is used. The X-Ray tube hovers over the weldment and film under the weldment. Labels reading slag and lack of fusion are also present with red arrows pointing to these discontinuities.
Figure 19.25. Radiographic Testing / Photo Credit: Nicholas Malara, CC BY 4.0

An actual image of a similar X-Ray tube used in Figure 19.25 is found in Figure 19.26 below.

Radiographic X-Ray Generator shown positioned in its storage and transport case.
Figure 19.26. Radiographic X-Ray Generator / Photo Credit: Idaho National Laboratory, PD

For safety the X-Ray generator has a lead plug to protect against the X-Rays when not in use. These generators should only be handled by qualified personnel or personnel being trained under the supervision of a qualified inspector because they can generate a lethal x-ray dose. Any time radiation is involved as an occupational hazard, the term ALARA is used which stands for As Low As Reasonably Achievable. ALARA is achievable through decreased exposure time and increased distance to/from the radiation source.

In practice this means that only those personnel who have to be near the X-Ray generator will be near it for as short a time as possible. Otherwise personnel will make as much distance between themselves and the source of radiation as possible. This is why when you get a medical x-ray the technician leaves the room; you getting a handful of X-Rays in your lifetime is much much different from the technician getting exposed to a dozen X-rays per working day.

Radiographic X-Ray generator and the solid lead plug removed. A large cylindrical device with a hole in the top part of the cylinder that is labeled X-Ray port. A hand holds the solid lead plug, which is also labeled. The plug caps the X-Ray port when the rodiographic x-ray generator is not in use.
Figure 19.27. Radiographic X-Ray Generator / Photo Credit: Idaho National Laboratory, PD

Figure 19.27 shows the plug removed with the X-Ray source behind it. Figure 19.28 shows a radiographic inspection of a weldment. In this Figure you can see the weldment is in between the X-Ray source and a plate of film which is underneath the weld. As part of the inspection process Image Quality Indicators (IQIs) are used to ensure that the radiograph can detect a minimum required detail. If the letters of the IQI are not readable, the radiographic inspection is not acceptable and must be redone. These IQI’s can be seen in Figure 19.29.

Radiographic Testing showing an picture of a source above the weld joint
Figure 19.28. Radiographic Testing Source Setup / Photo Credit: U.S. Department of Transportation, Federal Highway Administration, PD
Radiograph Image Quality Indicators shown on a x-ray image. The image shows a straight line across the center of the x-ray. At the top there are a series of letters and numbers which represent different sized objects on the image and the IQI or penetrameter number of 15. Toward the bottom is another set of numbers and letters, all of which are used to ensure that the image is in focus enough to see the detail of the letters and numbers well. If you can clearly see the lettering and numbers then you can see defects of a similar size and nature. The numbers 3 and 4, are also shown which are exposure numbers.
Figure 19.29. Radiographic Testing Film Image / Photo Credit: U.S. Department of Transportation, Federal Highway Administration, PD
Radiographic Testing showing defects on an x-ray image facsimile. Both defects are a darker shade of grey than the rest of the image. Lack of fusion on the top left of this illustration appears as a thin squiggly line. Slag, which is on the lower right, appears as a large bumpy shape.
Figure 19.30. Radiographic Testing / Photo Credit: Nicholas Malara, CC BY 4.0

Figure 19.30 and Figure 19.31 show illustrations of what welding discontinuities would look like. Trained welding inspectors know what to look for when they are inspecting weldments.

Cracks shown on an x-ray image. The weld appears as a light grey strip across a dark black background. The weld cracks are darker grey lines. This x-ray includes two transverse cracks and one longitudinal crack.
Figure 19.31. Metal Weld Cracks / Photo Credit: NASA, PD

Ultrasonic Testing (UT)

Ultrasonic Testing (UT) is a technology that was adapted from the medical field similarly to Radiographic Testing which was discussed in the previous section. Instead of using radiation, such as X-Rays, ultrasonic testing uses sound waves. The transducer shown in Figure 19.32 and 19.33 both sends and receives the sound waves.

From left to right: A chart labeled A-Scan (from display) shows a sine wave running across a chart. This is the display from the UT Machine, which is pictured to the right. It is a small square box with a stand to prop it up. It includes two buttons and a display screen. A cord runs from the UT Machine to the transducer. The transducer is placed on the right side of the weldment. The couplant, which is usually a gel rubbed beneath the transducer is labeled as well. A dashed red line, representing how sound would travel across the weldment, forms a series of long zig zags through the weldment. The sound path is labeled 1st leg for when the sound leaves the transducer and goes to the bottom of the weldment and reaches a point marked a, then bounces up in the 2nd leg, which moves up to marker b, to bounce back down to point c, and then rise again. The weldment is labeled Face A, the top of the weldment, and Face B, the bottom of the weldment.
Figure 19.32. Ultrasonic Testing Butt Joint Schematic / Photo Credit: Nicholas Malara, CC BY 4.0

There are many different types of UT, for the discussion in this chapter we will stick to the UT depicted in Figure 19.32. Other forms of UT are immersed UT where the specimen being inspected is submerged in a pool of water and the transducer is located away from the surface of the weldment. There is also UT where there is an air gap between the transducer and the weldment being inspected. These two forms of UT are beyond the scope of this chapter and will not be discussed.

In Figure 19.32 there is a couplant which is usually in the form of a gel that is applied to the surface of the weldment. This couplant ensures that the transducer is “coupled” to the weldment in a way that allows sound waves to pass through the transducer and into the weldment. If an air gap existed between the transducer and the weldment due to a non-level surface, the air gap would interfere with the results of the UT inspection.

The inspector then slides the transducer into position. This sliding action requires a smooth surface generally free of spatter. Remove spatter prior to UT if requested or required.

As we saw in the first Chapter image, and in Figure 19.37, the inspector holds the transducer with their fingers and moves it around while watching the display. If you have watched a tv show, commercial, or been in a doctors office when an ultrasound was being performed you are familiar with the technician moving the transducer around on the patient’s body as they watch the image on the screen. You may even remember the gel couplant that was applied to the skin.

70 degree transducer showing the sound’s path through the metal to the weld discontinuity.
Figure 19.33. Ultrasonic Testing with a 70° Transducer / Photo Credit: Nicholas Malara, CC BY 4.0

The distance between the weldment and the transducer allows for the depth of the discontinuity to be calculated. In reality the time it takes for the ultrasonic wave to return determines the distance, and the angle of the transducer allows for the depth to be calculated from the distance the sound traveled. The spikes seen in Figure 19.32 are interpreted by the trained technician. Some spikes are expected, such as the initial bang which represents sound bouncing back from the surface of the material directly next to the transducer, spikes from surface reflectivity as shown in Figure 19.32, and discontinuities.

Ultrasonic Testing setup showing the sound waves from the transducer and the surface distance to the discontinuity.
Figure 19.34. Ultrasonic Testing Discontinuity Distance / Photo Credit: Nicholas Malara, CC BY 4.0

Figure 19.35 below shows two transducers in the figure. This image is showing how the position of the transducer will detect different discontinuities at different depths into the weldment based upon the transducer’s position. The closer the transducer is to the discontinuity, the shallower it is.

Ultrasonic testing setup showing how moving the transducer detects at different depths in the weld
Figure 19.35. Ultrasonic Testing / Photo Credit: Nicholas Malara, CC BY 4.0
Phased Array visualization showing how phased arrays can detect over a larger depth of the weld
Figure 19.36. Phased Array Ultrasonic Testing / Photo Credit: Nicholas Malara, CC BY 4.0

An improvement upon the single transducer previously discussed is Phased Array UT (PAUT). With a phased array, multiple transducers within a single element transmit ultrasonic sound waves into the weldment being examined. This allows for a more complete picture to be formed on the screen. See the images in Figure 19.37 and Figure 19.38.

Phased Array Ultrasonic Testing in use by a technician, the PAUT screen shows a wider swathe of sound path.
Figure 19.37. Phased Array Ultrasonic Testing / Photo Credit: U.S. Department of Transportation, Federal Highway Administration, PD
Ultrasonic Test Results as shown on the display of a UT machine shown as 2D visual representations of a 3D sound wave propagating through a material.
Figure 19.38. Phased Array Ultrasonic Testing / Photo Credit: U.S. Department of Transportation, Federal Highway Administration, PD

The image shown in section D of Figure 19.38 depicts discontinuities encountered in a weldment being inspected. Again, these images are interpreted by a trained inspector and not by welders. That being said, it is still important for the welder to be familiar with the inspection techniques used so the proper preparation of the weldment can be performed prior to the inspection.

Attributions

  1. Figure 19.1: Overlap in a fillet weld by United States Department of Transportation, Federal Highway Administration in the Public Domain; United States government work
  2. Figure 19.2: Example of porosity by United States Department of Transportation, Federal Highway Administration in the Public Domain; United States government work
  3. Figure 19.3: Example of slag trapped in a CJP weld by United States Department of Transportation, Federal Highway Administration in the Public Domain; United States government work
  4. Figure 19.4: Metal Weld Cracks by Nicholas Malara, for WA Open ProfTech, © SBCTC, CC BY 4.0
  5. Figure 19.5: Example of crack in underfilled weld crater by United States Department of Transportation, Federal Highway Administration in the Public Domain; United States government work
  6. Figure 19.6: Example of weld spatter by United States Department of Transportation, Federal Highway Administration in the Public Domain; United States government work
  7. Figure 19.7: Examples of Arc Strikes by United States Department of Transportation, Federal Highway Administration in the Public Domain; United States government work
  8. Figure 19.8: Applying Dye Penetrant to Surface by United States Department of Transportation, Federal Highway Administration in the Public Domain; United States government work
  9. Figure 19.9: Cleaning Surface to Remove Dye by United States Department of Transportation, Federal Highway Administration in the Public Domain; United States government work
  10. Figure 19.10: Applying Developer to Provide Contrast with Dye Emerging from a Crack by United States Department of Transportation, Federal Highway Administration in the Public Domain; United States government work
  11. Figure 19.11: Crack Indication Formed by Dye Emerging from Crack by United States Department of Transportation, Federal Highway Administration in the Public Domain; United States government work
  12. Figure 19.12: Peening evidence on Specimen 1U3 by United States Department of Transportation, Federal Highway Administration in the Public Domain; United States government work
  13. Figure 19.13: Part Inspection/Evaluation by U.S. Department of Energy, Office of Energy, Efficiency and Renewable Energy, National Renewable Energy Laboratory in the Public Domain; United States government work
  14. Figure 19.14: Test Pressure by U.S. Department of Transportation in the Public Domain; United States government work
  15. Figure 19.15: Magnetic Testing Prods (L) Yoke (R) by Nicholas Malara, for WA Open ProfTech, © SBCTC, CC BY 4.0
  16. Figure 19.16: Magnetic Testing Particle Accumulation by Nicholas Malara, for WA Open ProfTech, © SBCTC, CC BY 4.0
  17. Figure 19.17: Magnetic Testing by United States Department of Transportation, Federal Highway Administration in the Public Domain; United States government work
  18. Figure 19.18: Eddy Current Testing by Nicholas Malara, for WA Open ProfTech, © SBCTC, CC BY 4.0
  19. Figure 19.19: Eddy Currents and Flows by Nicholas Malara, for WA Open ProfTech, © SBCTC, CC BY 4.0
  20. Figure 19.20: Crack Location in a Rail by Nicholas Malara, for WA Open ProfTech, © SBCTC, CC BY 4.0
  21. Figure 19.21: Magnetic Testing by United States Department of Transportation, Federal Highway Administration in the Public Domain; United States government work
  22. Figure 19.22: Left: Eddy Current Equipment used in the Project. This is one of several systems that are commercially available for eddy current inspection of thread regions. The authors, the RSPA and the DOT do not endorse any specific system manufacturer. Right: Closeup of an Eddy Current Probe Threaded onto the Calibration Block by Research and Special Programs Administration and the Department of Transportation in the Public Domain; United States government work
  23. Figure 19.23: An eddy current probe fixture is used to evaluate a thruster center body diffuser nozzle. by NASA White Sands Test Facility in the Public Domain; United States government work
  24. Figure 19.24: Radiation in Healthcare: X-Rays by U.S. Department of Health and Human Seervices, Centers for Disease Control and Prevention in the Public Domain; United States government work
  25. Figure 19.25: Radiographic Testing by Nicholas Malara, for WA Open ProfTech, © SBCTC, CC BY 4.0
  26. Figure 19.26: X-ray generator shipping box by Idaho National Laboratory in the Public Domain; United States government work
  27. Figure 19.27: Installing solid plug by Idaho National Laboratory in the Public Domain; United States government work
  28. Figure 19.28: Radiography in a bridge shop by U.S. Department of Transportation, Federal Highway Administration in the Public Domain; United States government work
  29. Figure 19.29: Radiography in a bridge shop by U.S. Department of Transportation, Federal Highway Administration in the Public Domain; United States government work
  30. Figure 19.30: Radiographic Testing by Nicholas Malara, for WA Open ProfTech, © SBCTC, CC BY 4.0
  31. Figure 19.31: Illustration of notional radiographic inspection of metal weld cracks by NASA in the Public Domain; United States government work
  32. Figure 19.32: Ultrasonic Testing Butt Joint Schematic by Nicholas Malara, for WA Open ProfTech, © SBCTC, CC BY 4.0
  33. Figure 19.33: Ultrasonic Testing with a 70° Transducer by Nicholas Malara, for WA Open ProfTech, © SBCTC, CC BY 4.0
  34. Figure 19.34: Ultrasonic Testing Discontinuity Distance by Nicholas Malara, for WA Open ProfTech, © SBCTC, CC BY 4.0
  35. Figure 19.35: Ultrasonic Testing by Nicholas Malara, for WA Open ProfTech, © SBCTC, CC BY 4.0
  36. Figure 19.36: Phased Array Ultrasonic Testing by Nicholas Malara, for WA Open ProfTech, © SBCTC, CC BY 4.0
  37. Figure 19.37: Use of PAUT for an unusual CJP bridge weld by U.S. Department of Transportation, Federal Highway Administration in the Public Domain; United States government work
  38. Figure 19.38: PAUT examination output by U.S. Department of Transportation, Federal Highway Administration in the Public Domain; United States government work
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Introduction to Welding Copyright © by Washington State Board for Community and Technical Colleges is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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