Лако-красочные материалы — производство

The establishment of special requirements for the nondestructive testing of bonded assemblies helps to define suitable criteria for nondestructive testing methods in

general. These refer to the applicability of a specific method for the nondestructive testing of bonded joints, the test equipment required, the time needed to test the component, the safety at work, the ease of use, and the provisions to be taken when preparing the samples. At present, with regards to general applicability, the following four methods have particular potential for the detection of defects in adhesive — bonded joints [7]:

• neutron radiography

• pulse echo

• shearography

• lockin thermography

Investigations have shown that, when using neutron radiography, accurate infor­mation can be obtained with regards to the position, the nature, and the depth of defects. However, as extensive equipment is needed for these measurements, and extensive provisions must be taken, for example with regards to sample geometry or to the nature or accuracy of the adhesive layer, an industrial application is out of the question. In the same way, when using the pulse echo method, one exclusion criterion for industrial application is the fact that a couplant is needed. The applicability of the method for serial production depends on the development of high-performance, air-coupled probes.

Shearography, on the other hand, could be perfectly suited because it does not require any extensive technical equipment, although the existing sensors were not adapted to the testing of adhesive-bonded joints. To date, no satisfactory results have been obtained with regards to the detection of flaws.

Lockin thermography, and particularly ultrasonic lockin thermography, requires only moderately technical equipment and provisions, yet yields very good results with regards to the detection of flaws. Furthermore, it is considered a robust and very rapid testing method that is perfectly suited to integration into serial production [8].

An example of ultrasonic lockin thermography is shown in Figure 7.26, where two sections of the flange joint, into which defects were introduced when the door was manufactured, are shown in selective enlargements. Within these areas, modified signals are detected. With regards to bonded joints, this technique is still at the research stage; however, this complex example illustrates a potential future applica­tion of ultrasound thermography as a nondestructive testing method for bonded components.

7.7