Copyright Goodheart-Willcox Co., Inc. 270 Gas Tungsten Arc Welding Handbook used in the test. The color selected should provide good contrast with the material being tested. The test can be modifi ed by adding fl uorescent dye to the particles. In this method, an ultraviolet light is used to illuminate fl uorescent dye on the iron particles, allowing the inspector to clearly see and interpret the formation of the particles at the defect. As with fl uores- cent penetrant testing, it may be necessary to examine the weldment in a darkened area. Accurate interpretation of magnetic particle tests requires training. Discontinuities revealed by the test pattern can be misleading to the untrained eye and may have no consequence on the weld’s acceptability. If the size of the discontinuity falls within allowable limits, the weld is still acceptable. If the size of the discontinuity is larger than the allowable limit, the weld is rejectable. Ultrasonic Test Ultrasonic testing (UT) is a nondestructive method of detecting the presence of internal cracks, inclusions, segregations, porosity, lack of fusion, and similar discontinuities in all types of metals. It can be used as the sole type of inspection, or it can be used with other types of testing. UT is often used in conjunc- tion with radiographic testing because it determines the depth of the defect from the test surface. In ultrasonic testing, very-high-frequency sound waves are transmitted through the part to be tested. The sound waves then return to the sender and are displayed as a graph on a monitoring screen for interpretation. Since very-high-frequency sound waves travel only short distances in air, the test must be done with the part (signal sender) and the transducer (receiver) immersed in water or with the transducer coupled to the workpiece by a thin liquid fi lm. These two methods are shown in Figure 18-8. UT inspection techniques include several different patterns and techniques. The technique used depends on the material, weld thick- ness, welding process, and inspection criteria being used. Where tests are required out-of-perpendicular with the transducer, a wedge or angle block is placed under the transducer at the desired angle to properly scan the material, as shown in Figure 18-9. Ultrasonic testing is portable and nonhazardous. In addition, UT inspection has the following advantages: • Great penetration power allows the testing of thick materials. • High sensitivity allows detection of small discontinuities in a short period of time. • Inspection can be done from one surface. The major disadvantage of ultrasonic testing is the advanced skill required to properly interpret the results. Weld design, location of the defect, internal structure, and complexity of the weldment affect the interpretation of the ultrasonic signal. In order to achieve the desired results, calibration blocks and reference weld samples are used to cali- brate the equipment prior to making the test. With the proper calibration, the operator can then interpret the results to the inspection specifi cation. Radiographic Test A radiographic test (RT) is a nondestructive method that reveals the presence and nature of discon- tinuities in the interior of welds. This test makes use Transducer Water Immersion Test Contact Test Coupling fluid Transducer Cathode ray tube (CRT) Figure 18-8. Ultrasonic tests are made with the part and the transducer submerged in water. If this is not practical, the transducer is coupled (connected) to the test area by a thin layer of liquid. Cathode ray tube (CRT) Transducer Angle Test Plastic mount block machined to fit transducer at desired angle. Figure 18-9. Wedges or angle blocks are used to send and receive ultrasonic signals in areas where signal transmission could be blocked in a straight-line plane.