The selection of the ultrasonic flaw detection ultrasonic flaw detection conditions includes the thickness of the coupling layer, the selection of the coupling material, the consideration of various compensations, the detection frequency, the detection mode and the selection of relevant parameters, and the adjustment of the detection sensitivity. For different workpieces and technical requirements, different detection conditions can be selected, which will be further introduced in the specific workpiece inspection. The contents of this section are only general principles and general methods. First, the coupling and surface compensation coupling is the only way to achieve the transmission of acoustic energy. The coupling agent is the sound transmission medium required to realize the acoustic energy transmission between the two sources of the probe sound source and the workpiece, and to ensure the soft contact. It has the function of eliminating air and filling uneven pits and gaps, and has the functions of anti-wear and convenient movement. 1. Coupling loss In addition to the material properties of the couplant, the coupling loss is related to the thickness d of the coupling layer and the wavelength l of the ultrasonic wave in the coupling layer. The thickness of the coupling layer of the direct contact ultrasonic testing is generally thin, d<l/4. Figure 3–35 shows the relationship between the thickness of the coupling layer and the height of the echo when the spindle oil is used as a coupling agent for the 2.25MHz straight probe (the shape of the curve obtained by different coupling conditions is similar). As can be seen from the figure, in d<l In the range of /4, as the d/l increases, the coupling loss increases, and the coupling loss is maximum at d=l/4. It can also be seen from the figure that at the time of coupling loss, the acoustic energy transmission rate is small, and at that time, the acoustic energy transmittance is large and the coupling loss is small. The sound transmission performance of the coupling layer material itself and the roughness of the detection surface are also factors that affect the coupling loss. Figure 3–36 shows the effect of surface roughness on the height of the echo when using different acoustic impedance couplants. The abscissa is the average height RZ of the surface roughness, which determines the thickness of the coupling layer, ie d=RZ. It can be seen from the figure that the larger the acoustic impedance of the coupling agent is, the closer it is to the acoustic impedance of the wafer (or protective film) and the workpiece. The smaller the surface roughness of the flaw detector is, the higher the finish is, the smaller the coupling loss is, and the better the sound transmission performance is. it is good. In addition, the coupling loss is also related to the detection frequency and the properties of the protective film.
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