Ultrasonic testing is a non-destructive testing method used to find both surface and sub-surface defects. It uses sound waves in ultrasonic frequency ranges to accomplish this. There are 3 main types of sound waves used to locate defects. They are Longitudinal waves, Shear waves, and Surface waves.
Longitudinal waves (or Compression waves) are sound waves that enter the part perpendicular to it's surface (or before the first critical angle). One thing these types of waves are great for is thickness measurements. The wave enters the part, travels in a straight line to the back side of the part, and reflects straight back to where it entered. Based on the velocity of the sound wave in the material, the time that it takes the wave to complete the round trip, you can measure the thickness of the part you are testing. Most of the equipment today has a built in feature that does all the calculations for the operator. When I was going to school to learn UT, the equipment that we used didn't have this feature, so we had to do the calculations ourselves.
Shear waves (or Transverse waves) are sound waves that enter the part beyond the first critical angle of the part. The critical angle is based upon the material being tested. For example, the sound enters the part at a 40 degree angle and then refracts to 45 degrees. This type of testing is commonly used in weld inspections. I have also used it to inspect the links for wing-to-fuselage attachments for cracks. To calculate where cracks (or other defects) are, the operator uses basic trigonometry formulas to calculate the distance forward from where the sound enters the part, and the distance down from where the sound enters the part. Again, most equipment today already does these calculations for the operator, but when I learned this, we had to do them on our own. To show how accurate these formulas are, we lost 1 point for every mm we were off on our calculations. Out of 12 different defects, I lost a total of 1 point.
The last type of sound wave that is commonly used is a Surface (or Lamb) wave. These are sound waves that enter the surface of the part beyond the second critical angle for the material being tested. For example, they may enter the part at 70 degrees, and refract to 80 degrees. They travel along the surface (or just under the surface) of the part. When they run into a defect, they reflect back. Wheels are inspected using this method. One thing that is good about surface wave, is that they can travel along a radius of a part.
There are now 3 main types of displays (equipment) used to interpret these ultrasonic waves. They are the A-Scan, B-Scan, and C-Scan presentations. Currently, other types of displays can be used too, but they are for specific applications and I will go into more detail about them in another thread when I tell about other methods.
An A-Scan display is a display on a CRT that consists of a time-base line on a horizontal grid. When the sound enters the part on an A-Scan display, you see a vertical spike near the front (left) side of the grid. The spike should be relatively high in amplitude - maybe even go completely off the screen - due to a lot of sound energy entering the part. Toward the right side of the time-base line, another spike will go vertical. This is the back-wall of the part being tested. Most codes want the back wall set to 80% full screen height. This is the representation of a longitudinal wave on an A-Scan presentation. Any change in the thickness of the part will cause the back-wall to move left if the part is getting thinner, or right if the part is getting thicker. Also, if a spike appears between the initial pulse and the back-wall, that is usually a defect that needs to be evaluated. For shear and surface waves, on an A-Scan, all you will see is the initial pulse on the left, and a flat time-base line until a defect reflects the sound wave back. At that point you will see a vertical spike.
A B-Scan display is pretty much a profile display of the part being tested. If the technician is monitoring corrosion, for example, they will see a fairly solid block on a CRT until the sound reflects the signals from the corrosion. When that happens, the block starts to get thinner and back to the original thickness - almost looking like a mountain range on the horizon. A B-Scan is also the type of display that is used to find a baby in the doctors office. You can definately see why you have to read these displays with a very experienced technician for quite a long time before being able to interpret this display with any degree of reliability. B-Scans are mostly used for in-service type inspections
A C-Scan display shows a plane view of the part being tested, printed on a sheet of paper (or saved electronically). What this type of display maps out is the amount of sound that is (or isn't) being reflected back to the transducer. C-Scans are mostly used in the manufacturing industry.
There is a lot more info. that I can go into, but I don't want to make this thread too confusing so I will stop here. Any questions that arise, I will be happy to answer, and if anyone else is involved with UT, please feel free to answer too.