A distinct advantage of MT over PT is the speed with which MT can be performed. Further, MT can be performed while the part is warm. This is particularly helpful when MT is used to determine the extent of cracking. The crack can be removed by grinding or air arc gouging and immediately in- spected with MT to see if the full crack has been removed.MT is used for inspection of weld access holes in heavy plate or shapes when required by AISC Specification. While either PT or MT is permitted, MT is typically preferred as it is quicker, simpler, and requires less cleanup afterwards. RADIOGRAPHIC INSPECTION (RT)Radiographic inspection uses X rays or gamma rays that are passed through the weld and expose a radiographic film on the opposite side of the joint (see Figure 9–3). X rays are produced by high-voltage generators, while gamma rays are produced by atomic disintegration of radioactive isotopes.Whenever radiography is used, precautions must be taken to protect workers from exposure to excessive radia- tion. Safety measures dictated by the Occupational Safety and Health Administration (OSHA), the National Electrical Manufacturer’s Association (NEMA), the Nuclear Regu- latory Commission (NRC), the American Society of Nondestructive Testing (ASNT), and other agencies should be carefully followed when radiographic inspection is con- ducted.Radiographic testing relies on the ability of the material to allow some of the radiation to pass through, while absorbing part of this energy within the material. Different materials have different absorption rates. Thin materials will absorb less radiation than thick materials. The higher the density of the material, the greater is the absorption rate. As differentFigure 9–3. Radiographic inspection. levels of radiation are passed through the materials, portions of the film are exposed to a greater or lesser degree than the rest.Since the source of radiation must be on one side of the part, and the film on the other, access to both sides of the joint is required for RT.When the film is developed, the resulting radiograph will bear the image of the plan view of the part, including its internal structure. A radiograph is actually a film negative in which the darkest regions are those that were most exposed, since the material being inspected absorbed the least amount of radiation. Thin parts will be darkest on the radiograph. Porosity will be revealed as small, dark, round circles. Slag is also generally dark, and may look similar to porosity, but will be irregular in its shape. Cracks that lie parallel to the source of radiation appear as dark lines. Incomplete fusion and underfill will show up as darker areas. Weld reinforce- ment will result in a lighter region.Radiographic testing is most effective for detecting volu- metric discontinuities such as slag and porosity. When cracks are oriented perpendicular to the direction of the radiation source (e.g., parallel to the film), they may be missed with the RT method. Cracks that are parallel to the radiation path are the most detectable, although tight cracks have gone un- detected by RT at times.Radiographic testing has the advantage of generating a permanent record for future reference. With a “picture” to look at, many people are more confident that the interpreta- tion of weld quality is meaningful. However, reading a ra- diograph and interpreting the results requires stringent train- ing, so the effectiveness of radiographic inspection depends to a great degree on the skill of the technician.
Radiographic testing is ideally suited for inspection of complete joint penetration (CJP) groove welds in butt joints. It is not suitable for inspection of partial joint penetration (PJP) groove welds or fillet welds. When applied to T- and corner joints, the geometric constraints of the applications make RT inspection difficult, and interpretation of the results is highly debatable.
ULTRASONIC INSPECTION (UT)
Ultrasonic inspection relies on the transmission of high frequency sound waves through materials (see Figure 9–4). Solid, discontinuity-free materials will transmit the sound throughout a part uninterrupted. A receiver “hears” the sound reflected off of the back surface of the part being inspected. If a discontinuity is contained between the transmitter and the backside of the part, an intermediate signal will be sent to the receiver, indicating the presence of this discontinuity. The pulses are read on a display screen.
The magnitude of the signal received from the disconti- nuity is proportional to the amount of reflected sound. This
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