Nondestructive testing bond inspection

Nondestructive Testing. Nondestmctive inspection of an explosion-welded composite is almost totally restricted to ultrasonic and visual inspection. Radiographic inspection is appHcable only to special types of composites consisting of two metals having a significant mismatch in density and a large wave pattern in the bond interface (see Nondestructive evaluation). 

The use of probability-density-function analysis, an important topic in statistical analysis, is mentioned with respect to its utility in nondestructive testing for inspectability, damage analysis, and F-map generation. In addition to the mathematical concepts, several sample problems in composite material and adhesive bond inspection are discussed. A feature map (or F map) is introduced as a new procedure that gives us a new way to examine composite materials and bonded structures. Results of several feasibility studies on aluminum-to-alumi-num bond inspection, along with results of color graphics display samples will be presented. 

Bond Inspection. After the adhesive or sealant is cured, the joint area can be inspected to detect gross flaws or defects. This inspection procedure can be either destructive or nondestructive. The nondestructive type of tests can be visual or use advanced analytical tests. These types of bond inspections are described below. 

Nondestructive testing (NDT) is far more economical, and every assembly can be tested if desired. However, there is no single nondestructive test or technique that will provide the user with a quantitative estimate of bond strength. There are several ultrasonic test methods that provide qualitative values. However, a trained eye can detect a surprising number of faulty joints by close inspection of the adhesive around the bonded area. Table 7.16 lists the characteristics of faulty joints that can be detected visually. The most difficult defects to be found by any method are those related to improper curing and surface treatments. Therefore, great care and control must be given to surface-preparation procedures and shop cleanliness. 

Possible nondestructive testing techniques for adhesively bonded structures and composite materials will be introduced along with a literature survey of successes and applications to date. Emphasis on ultrasonic inspection will also be highlighted, including such topics as ultrasonic wave generation, wave velocity, dispersion, reflection factor, wave refraction, attenuation, ultrasonic field analysis, resolution, thickness and defect location measurement, and C-scan testing. 

The most popular ultrasonic nondestructive testing application has been associated with thickness measurement of a test object and defect location within the particular test object. Most of the applications to date have been associated with the testing of homogeneous isotropic materials. Recent work has extended the basic ultrasonic test philosophy to the field of composite materials and adhesive bonding inspection. Unfortunately, many difficulties occur because of the inhomogeneous and anisotropic characteristic of a composite material. This section includes a review of the physical principles associated with ultrasonic testing and the particular items that must receive special attention when inspecting composite materials or adhesively bonded sections of a structure. 

Nondestructive Testing (NDT, NDE, NDi, NDC) -Techniques used to examine the quality of adhesive bonds without causing damage to the item or bond. The terms NDT (Testing), NDE (Evaluation), NDI (inspection) and NDC (Characterisation) tend to be Interchangeable. Nondestructive inspection systems may be manual or automated to some extent. Whilst effective for finding defects in the bond, no method currently exists which allows a reliable, quantitative measure of bond strength. 

Tests, nondestructive Inspection tests for the evaluation of bond quality without damaging the assembly, for example, ultrasonics and visual inspection. 

Industrial processing of adhesives has made considerable progress from the crude processes of the past [1]. Unfortunately, one of the disadvantages of adhesive bonding as an assembly method is that a bond area cannot be inspected visually. Inspection must be carried out by two methods destructive and nondestructive. Destructive inspection may be carried out on process-control test specimens prepared from the same adherend and adhesive materials as used for the production parts. The process-control specimen, as the name implies, accompanies the production parts throughout the stages of cleaning, assembly, and cure. The adhesives and adherends are all assembled at the same time and cured in the same press or autoclave. 

As an additional control, each part may be designed with an expendable tab as an integral part of the assembly. After the cure, the tab is removed and subjected to the same tests as the control test specimens. The results are checked against the specification requirements and the part is accepted or rejected based on these results. The rejected parts may subsequently be inspected nondestructively for final acceptance or rejection. Final rejection would result in systematic destruction to learn how good or bad the parts really were. In initial production of critical parts, such as primary bonded stmctures for aircraft, where human lives are dependent on reliability, a sampling and destructive analysis of actual production parts may be included in the test program [2]. 

After the adhesive is cured, the joint area can be inspected to detect gross flaws or defects. This inspection procedure can be either destmctive or nondestructive, as discussed in Section 13.1. Destmctive testing generally involves placing samples of the production mn in simulated or accelerated service and determining if it has properties similar to a specimen that is known to have a good bond and adequate service performance. The causes and remedies for a number of faults revealed by such mechanical tests are described in Table 13.1. Destmctive (mechanical) tests that can be carried out on adhesive bonds are Usted in Section 13.2.1.1 (Adhesives mechanical properties). Nondestmctive tests are far more economical, and many more assemblies can be tested. 

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