Volume 3, Number 2
April 2004
FYI Practical Contact Ultrasonics - Angle Beam Inspection
by Jim Houf*
Angle beam inspection uses shear (transverse) waves to interrogate a part as opposed to the longitudinal waves used in straight beam testing. Properly used, highly effective and accurate angle beam evaluations can determine the soundness of the part being inspected.In the simplest terms, angle beam inspection combines the basic operating principle of a fish finder with a bank shot in a game of pool or billiards. The sound beam is sent through the part at a known angle that is created by attaching a piezoelectric transducer to an acoustically transparent wedge at a predetermined angle. The sound beam reflects from the back surface of the part and returns to the inspection surface some distance away from the transducer. The direction the sound takes is called the sound path (Fig. 1). For purposes of illustration, only the centerline of the sound beam is shown. The sound beam between the transducer and the back surface of the material is called the first leg or node. Sound that reflects back up from the back surface is called the second leg. The total distance down the sound path from the entry point to the point where the beam again hits the top surface, the sum of the legs, is called the skip distance. As the transducer is moved back and forth over the surface of the part, the sound beam travels through the part ahead of the transducer. If the sound beam does not hit a reflector (discontinuity) while traveling through the part, it will simply reflect back down into the part, traveling outward until the sound attenuates or dissipates.
This is typical behavior of the angle beam sound path in a piece of flat material. The next step is to determine precisely where the sound is going. To do this, it is necessary to know the angle of the sound beam in the part in relation to a line drawn vertically through the material thickness. This is called the refracted angle and is determined by the angle at which the transducer is mounted on the wedge in the transducer assembly or probe. The most commonly used search angles in contact UT work are 45, 60 and 70 degrees as referenced to steel. The refracted angle in other materials will not be the angle marked on the probe. Figure 2 illustrates a 45 degree probe.
On the side of most commercial wedges is a mark called the exit point that denotes the point at which the center of the sound beam leaves the base of the transducer. When using a 45 degree probe, the distance from the exit point to the point directly above an internal reflector is the same as the depth to that reflector. Another use of the exit point is to determine the location of the probe with respect to a fixed point on the part such as a weld centerline. This distance should be recorded on the inspection report form as the surface distance and as long as the reference point is also recorded, the inspection can be accurately repeated should the need arise.
Probe Selection
In many cases, the governing code or specification will specify the angle to be used for a given inspection. However, operators may find that for some inspections no wedge angle, probe size or frequency is specified and the operator will be required to determine what combination of equipment will be needed to perform a valid inspection. In this situation, the factors to be considered are material thickness, length of the transducer's near field, type and possible orientation of discontinuities and geometry of the part.
Material thickness will define the inspection angle required to adequately cover the full volume of the area to be inspected. For example, on thinner materials, if a wedge with a steep angle such as 45 degrees is used, the second leg of the sound beam may come back up under the front edge of the wedge, making it impossible to measure the surface distance. If a shallower angle is used such as 70 degrees, the distance the probe must be backed away from the area of interest may be excessive and can exceed the part size or geometry. Generally, to ensure a complete inspection, the operator must be able to back the transducer away from the nearest edge of the area of interest by at least the full skip distance for the angle being used plus the length of the transducer. This will permit the sound beam to interrogate the part using both legs of the sound beam (Fig. 3). The scanning surface must be free of weld spatter, dirt, loose scale and other foreign matter to allow proper coupling of probe and base metal. Part size may limit choice of search angle if there is limited scanning space next to the area of interest.
The near field length of a transducer varies depending on the diameter and frequency of the crystal. As discussed in the first article of this series, inspections performed using the near field are not reliable. If uncertain of the length of the near field for a given transducer, the operator should either calculate it or have the Level III do so to confirm the proper frequency/diameter combination. Some near field will always exist, but if made short enough to be kept within the wedge material, an accurate inspection can be performed.
Probe size is often dictated by the near field effect, but geometry of the part should also be considered when selecting a probe. In some conditions, the physical size of the probe can affect the inspection. For example, when inspecting a girth weld on small diameter pipe, a large transducer may not sit flat on the scanning surface. It may have a tendency to rock from side-to-side while scanning the part. When this occurs, the probe is not coupled properly to the part and some of the sound can be lost. The amount of sound entering the part is less than the amount of sound used to calibrate the system. This results in less sound striking the potential discontinuity and negates the value of the inspection. Changing to a smaller width probe reduces lateral rocking and more nearly matches the calibration conditions thereby providing better inspection results. In some situations, it may even be necessary to use a wedge contoured to fit the inspection surface.
The distance from the probe exit point to the front end or nose of the wedge is another consideration. If too small a search angle is used, the nose may hit the toe or edge of a weld crown before the sound beam reaches the root or bottom of the weld. In this instance, the root will not be interrogated in the first leg and the probability of missing a root indication is greatly increased. If too steep an angle is used on thinner materials, the sound path may remain totally under the wedge and no sound will enter the area of interest.
Orientation of discontinuities should also be considered when selecting a wedge angle. The greatest amount of sound will be reflected back from a discontinuity if the sound beam strikes it perpendicularly to the major surface of the discontinuity. As an illustration, visualize a blade held in a stream of water. If parallel to the flow, no water bounces off the blade but just flows past it. However, if held perpendicular to the flow, water hits the flat side of the blade and bounces back. The same effect occurs with sound and a discontinuity. The best results are obtained when the sound beam is perpendicular to the largest surface of the discontinuity.
It should be noted that if the backwall of the material being inspected is not parallel to the scanning surface, the angle of the second leg will change, and reflectors will display a screen signal at an improper location. This can occur at pipe-to-fitting welds where the fitting may have an internal bevel.
Equipment Setup
Once material thickness is determined and the correct probe combination is selected, the next step is to set up the equipment. Selection of screen width greatly affects the ability of the operator to discriminate between vertical screen traces or signals that appear when sound is reflected back to the transducer. The term screen width refers to the distance that the baseline of the screen represents. Operators must inspect parts requiring sound paths of various lengths. It is necessary to adjust the screen face to represent the distance that best displays the image of the sound reflecting back from the part.
To select optimum screen width, the length of the sound path for a full skip distance in the thickness of material to be tested must be determined. As thickness increases, the length of a full skip also increases, and at some point can require that a wider screen width be used. The screen width must be able to display the full skip. If not, indications generated at the far end of the second leg may not appear on the screen.
Commonly used screen widths for general weld inspections are 5 and 10 in. (13 and 25 cm). This means that the width of the screen is set to represent either a 5 or 10 in. sound path. If the screen is set at 5 in., each major graticule (numbered left to right) represents 0.5 in. (1.3 cm) of sound path with minor graticules equal to 0.1 in. (0.25 cm). For a 10.0 in. screen, major graticules represent 1 in. and minor graticules equal 0.2 in. (0.5 cm). Figure 4 shows the sound path and screen presentation for a sound beam striking a reflector at 1 in. and a back-wall at 5 in. Each major vertical graticule is shown, with each representing 0.5 in. of sound path. In an angle beam inspection, different reference blocks would be used. However, for demonstration purposes, Fig. 4 shows the relationship between the transducer, sound beam and screen presentation.
When using a 70 degree probe on 1.5 in. (3.8 cm) thick material, the sound beam reaches the back surface at a distance (sound path) of approximately 4.25 in. (11 cm) with a full skip distance of approximately 8.5 in. (22 cm). Therefore, if a 5 in. screen is used, 3.5 in. of the sound path is not shown on the screen and any discontinuities covered by that segment of the beam will be missed. For this example then, it would be necessary to use a 10.0 in. screen (Fig. 5). As can be seen, the point where the sound enters the part under the probe exit point, shows a strong signal at the extreme left of the screen, and the sound reflecting from the hole shows a screen signal at an 8 in. (20 cm) sound path. No signal is seen where sound reflects from the back wall at 4.25 in. because all sound is reflected away from the transducer.
The sound beam is not a single solid line like a laser beam as shown in most illustrations but is cone shaped and more like the beam of a flashlight that spreads as it travels farther from the source. This is called beam spread. As the transducer moves forward toward a reflector, the leading edge of the sound cone strikes the reflector first. The sound beam is less intense at this location and as a result, less sound is reflected back. This gives a low amplitude signal at a longer screen distance or sound path (Fig. 6a).
As the transducer continues to move toward the reflector, the centerline of the sound beam strikes the base of the notch where maximum reflection will occur (Fig. 6b), resulting in a higher signal amplitude at a shorter sound path than was seen in Fig. 6a. As the back portion of the sound beam travels over the notch (Fig. 6c), the majority of the sound beam has already passed over the notch. Thus a low amplitude screen signal is seen at an even shorter sound path. The amplitude of this signal may be higher than that of Fig. 6a because the shorter sound path results in more sound being reflected back to the transducer.
Scanning Patterns
In order to ensure that the full volume of the area of interest is inspected, several standard scanning patterns are often required by the governing code or specification. The more common patterns are described here and shown in Fig. 7. Proper transducer manipulation is required to ensure full coverage, and with practice and some dexterity the motions will become second nature to the operator.
The primary scan pattern requires that the operator move the transducer toward and away from the area of interest for at least a full skip distance back from that area (Fig. 7a). On each successive scan, the transducer is moved slightly to the right, so that the path the transducer follows overlaps the previous scan. The percentage of overlap is usually spelled out in the governing documents. At the same time as the transducer is being moved forward and back, it also needs to be oscillated sideways over a range of approximately ±15 degrees as in Fig. 7c. Again, the actual range of oscillation should be set by the code or specification. The weld (in this example) should be inspected from both sides to ensure no possible indications are missed.
The scan pattern shown in Fig. 7b is used to detect transverse discontinuities. The transducer is again oscillated as before but is guided along the side of the weld with the transducer point slightly in toward the weld centerline so that the full width of the weld is interrogated. As shown, the weld should be scanned from both ends and from both sides of the weld.
The need for scanning from both sides of the weld is demonstrated in the following example. Figure 8 shows a welded plate with a planar discontinuity oriented parallel to the original weld groove, which is typical of sidewall lack of fusion.
When scanning from the left side of the weld, as the transducer is moved toward the weld the nose of the transducer bumps into the weld crown at position A1. At that point the sound beam has not yet moved forward far enough to reflect off of the discontinuity. As the transducer is moved back away from the weld, the sound will reflect from the root reinforcement, and when it does start reflecting from the base metal back-wall, the second leg is above the discontinuity and it is missed again. Therefore, if the weld is only scanned from the left side, this discontinuity would not be found. Because of beam spread, it is likely that some signal would be seen on the screen caused by the sound at the edges of the sound beam hitting the discontinuity, but it is entirely possible that the reflected sound would not cause a signal amplitude high enough to be rejectable.
If the weld is scanned from the right side also, the operator must make sure that the transducer is moved back far enough to ensure full coverage of the weld volume. At position B1, the orientation of the discontinuity is such that the reflected sound would most likely reflect back and down to the back-wall, reflecting from there back up to a point behind the transducer, so the discontinuity would again be missed. Only when the transducer is moved back to position B2 would the main portion of the sound beam hit the discontinuity at a near-perpendicular angle, giving a solid signal.
This is not an unusual example, for this condition occurs much more frequently than expected. The condition can be further aggravated if the weld joint has poor initial fit up. Then, the weld crown can be excessively wide, making the odds of seeing a far-side planar discontinuity even more difficult. However, experienced UT operators should realize that if fit-up is poor, greater diligence is required when they see a weld crown that is too wide for the material thickness. TNT
*Jim Houf is Senior Manager of ASNT’s Technical Services Department and administers all ASNT certification programs. Involved in NDT since 1972, he has been an ASNT Level NDT Level III since 1984 and currently holds ASNT NDT and ACCP Professional Level III certificates in four NDT methods. He’s an AWS Senior Welding Inspector and an ASQ Certified Quality Auditor. (800) 222-2768 X212, (614) 274-6899 fax, <jhouf@asnt.org>.
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