148 GMAW/FCAW Handbook Copyright Goodheart-Willcox Co., Inc. Penetrant inspection does not reveal low welds or undercut. A variation of this inspection technique uses a black light and fl uorescent dye. Penetrant fl processes can be used in any position. Magnetic Particle Inspection Magnetic particle inspection is a nondestructive method of detecting cracks, seams, inclusions, segrega- tions, porosity, lack of fusion, and similar discontinuities in magnetic materials. When a magnetic field is estab- fi lished in a ferromagnetic material (iron-based with c magnetic properties) that contains one or more defects in the path of the magnetic flux, poles are developed at fl fl the defects. These poles have a stronger attraction for iron particles than the surrounding material does. The magnetic fi eld is established by applying electric fi fi current to the work, and then a fl uid containing iron parti- fl fl cles is applied. Any defects are shown by the pattern of the iron particles. See Figure 17-9 and Figure 17-10. Magnetic particle inspection is used mainly for locating defects on the surface of the material. However, this process can locate some defects up to 1/4″ 4 4 below the surface. ″ Ultrasonic Inspection Ultrasonic inspection is a nondestructive method of detecting the presence of internal cracks, inclusions, porosity, lack of fusion, and similar discontinuities in metals. High-frequency sound waves are transmitted through the part. The sound waves return to the sender and appear on a cathode ray tube (CRT). See Figure 17-11. Skilled technicians interpret the test results. Recent developments in phased-array ultrasonic Goodheart-Willcox Publisher Figure 17-7. A liquid penetrant test was completed on this fillet weld. The red indications show porosity and undercut. Step 4 Inspection Step 3 Development Step 2 Rinse Step 1 Penetration Goodheart-Willcox Publisher Figure 17-8. Liquid penetrant test sequence.