Лако-красочные материалы — производство

The pulse echo technique is the simplest method for detecting defects in an adhesive joint. The sample is placed into a water bath, as is the probe that supplies ultrasound to the sample; the system is shown schematically in Figure 7.19. The amplitude of the measured signal allows conclusions to be drawn with regard to the sound transmis­sion ofthe wetting surfaces and the adhesive. Good wetting and sound transmission within the adhesive produce a high amplitude, whereas the ultrasound wave of the back-wall echo is attenuated or totally shadowed in the case of defects within the adhesive bond. Consequently, there is higher amplitude of the reflector in flawless regions than in areas with defects. This can also be detected by means of shorter echo times.

Low testing frequencies (at 5 MHz) reduce the lateral resolution. However, due to the integral evaluation of the total structure it is not possible to map the defect along the thickness of the adhesive joint. The depth of a defect can be estimated on the basis of the signal attenuation. With C-scans, the defects appear in color, and can be detected by means of any color inhomogeneities, although neither the size nor the form of the defect can be determined precisely. The defects are displayed with a larger size than reality due to the funnel-shaped geometry of the sound beam; the higher the parallel alignment, the better the resolution.

As the test results may be impaired by background noise, this method is not an all­purpose technique. In relation to the wavelength, the thickness ofthe components to be bonded must be relatively large in order to obtain a good resolution of the echo of each interface. This method, among others, is very suitable for the testing of metal-to — metal bonds. Less satisfactory results have been obtained when testing bonded nonmetals due to the higher attenuation coefficient.

The major disadvantage of this method is that it is possible to evaluate only the first interface — the detection of cohesive defects within the adhesive or detachment on the opposite interface is not possible. In addition, each component must be placed into a water bath for testing. Finally, as this is a scanning technique it is relatively time­consuming.