A longitudinal beam is also known as a straight beam, or compression wave. Some people think that NDT technicians who are certified as Level II ultrasonic testing (UT) thickness can also do longitudinal beam inspections. That may or may not be true, depending on what kind of training they have had. For instance, do they know how to use a flaw detector or is their experience limited to just using thickness gage instruments? With that in mind, I recommend using a Level II UT shear wave technician to do this inspection due to the experience that comes from doing shear wave inspections with a flaw detector where one learns to recognize the differences between good and bad indications. Shear wave testing, also known as angle beam testing, is different than straight beam testing, and is used to examine welds. Shear wave testing technicians must be knowledgeable of special codes and standards and as such have a specialized skill set for certain applications.

One of the first things a UT technician should ask for is the available reference standard. Ultrasonic testing procedures have acceptance criteria relative to a reference standard. One of the most common reference standards is an International Institute of Welding (IIW) block. It has a serial number and can be traced back to the manufacturer who can certify that it meets all applicable material and dimensional specifications.

What the UT technician might actually be asking for is a “special” reference standard. It could be a shaft that is the same shape and size as what needs to be inspected. It would have machined notches to represent cracks. It should have a serial number and be traceable back to the machine shop that made it and can certify what material was used to make it plus the length, depth, and width of the notches. If it cannot be certified, it would be on the responsible nondestructive testing (NDT) Level III to decide whether it is acceptable for use as a “special” reference standard.

When there is no “special” reference standard available, the inspection can be done with what is available. When it comes to longitudinal beam inspections, the IIW block can be used with the ultrasonic instrument to calibrate for the length of the item being inspected. This can be done using an IIW block as a thickness calibration block.

A drawing of the item to be inspected should be made available to the UT technician. This drawing should show how long the item is and how far below the surface are any radiuses, keyways, or the start of a taper in the diameter of a shaft. Any of these might be where a crack may originate and/or be the source of a non-relevant geometry indication. With anchor bolts, it helps to know if it is a hook end, chisel point end, or if it has a flat head. Often the customer will want to know if a straight or threaded rod was used. If it is a hook end, odds are that you will not see a back reflection due to the geometry of the hook end. But if it is a flat end, you might see the back reflection.

The ultrasonic testing procedure needs to be specific to the item being inspected. The procedure should state what percentage of full screen height (FSH) is needed. Some procedures say 80 to 100% FSH from the first back reflection; another procedure may say the same thing but to add an additional 20 dB of gain to the 100%. Another procedure may say 80 to 100% FSH from the second back reflection.

Most ultrasonic testing procedures specify a single-element contact longitudinal beam transducer that produces a 0° beam. This is different than dual-element transducers, which produce a V-shaped beam. Dual-element transducers are typically used with thickness gage instruments; single-element transducers are typically used with flaw detectors. Some procedures use transducers with a refracted longitudinal beam.

The transducer frequency and element size should be specified in the UT procedure. Some UT procedures specify only one transducer; other UT procedures specify doing the inspection using two different transducers with different frequencies. But when it comes to doing longitudinal beam ultrasonic inspections, the effects of a wide beam divergence angle are often overlooked. Beam divergence is reduced with a higher frequency and/or a larger element size transducer. Beam divergence should be kept to a minimum, but if the specific procedure requires using a transducer with a wide beam divergence, its side effects should be known.

A transducer with a wide beam divergence can accurately measure the length of the item being inspected, but the wide beam divergence can generate mode conversion indications. Mode conversion indications can be a reflection from a change in geometry or refraction from a tight fit with a bushing. If you are inspecting the same area with two different transducers with different beam divergence angles, you may see mode conversion indications from the same change in geometry or crack appearing at different locations on the screen.

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