Adhesive nondestructive testing

Thermal imaging is sensitive to infrared radiation that detects temperature changes over the surface of a part when heat has been applied. Thermal diffusion in a solid is affected by variation in composition or by the presence of cracks, voids, delaminations, etc the effects are detected by surface temperature changes. Defects cannot be detected if their depth below the surface is more than two to three times their diameter. Nondestructive testing has been primarily used for composites and analysis of adhesive bonds or welds. Several studies are documented in the literature (322—327). 

Nondestructive Testing. Nondestructive testing (NDT) is far more economical than destructive test methods, and every assembly can be tested if desired. Several nondestructive test methods are used to check the appearance and quality of structures made with adhesives or sealants. The main methods are simple ones such as visual inspection, tap, proof, and more advanced physical monitoring such as ultrasonic or radiographic inspection. The most difficult defects to find are those related to improper curing and surface treatments. Therefore, great care and control must be exercised in surface preparation procedures and shop cleanliness. 

Each individual process step is of decisive importance for the total quality of the bonded product. In this context, it may be permitted to define an adhesive as a process material. This demand derives decisively from the fact that nondestructive testing for bonded joints is only possible to a limited extent and/or their realization involves a great metrological effort. 

The interfaces in bonded structures influence the mechanical behavior of components significantly. Therefore, an important task in nondestructive testing (NOT) is the investigation of the interaction forces in adhesive joints and the development of techniques to evaluate the bond quality. The load capacity of such joints is often limited by regions of weak bonding. As in all materials, the... 

Nondestructive Testing of Adhesive Curing in Class-Metal Compounds by Unilateral NMR... 

D.H. Buckley, "Effect of Sulfur, Oxygen, and Hydrogen Sulfide Surface Films on the Adhesion of Clean Iron," Int. J. Nondestructive Testing, 2, 171 (1970). 

G. J. Curtis, Adhesively bonded structure nondestructive testing with acoustic methods, in Ultrasonic Testing, J. Szilard, Ed., John Wiley, 1980. 

One problem, always present in the use of structural adhesives, is how to determine the reliability of the bonded structure. This may be at the time of manufacture, to ensure that a good joint has been produced, or during service, to monitor behavior under operating conditions. It is often impractical, or impossible, to use the types of test methods employed to determine mechanical properties and interest has centered on nondestructive test methods."" " ... 

Nondestructive Testing of Adhesive Joints. Many commercially available adhesives are based on polymers. NDT of adhesive joints (180) poses challenges because of (2) the small size of possibly deleterious defects (eg, porosity), (2) the thinness of the adhesive layers, and (3) sometimes, the size, shape, and material of the adherends. Acousto-ultrasonics (181) and various types of ultrasonics (182,183) are applicable to adhesive joints. Ultrasonics can be combined with noncontact optical methods (184) and pulsed thermography (185). However, the assessment of the quality or of the quantitative strength of the adhesive bond based on the NDT data often proves difficult. 

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 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. 

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. 

E. Segal and J. L. Rose, Nondestructive testing of adhesive bond joints, in Research Techniques in Nondestructive Testing, Vol. IV (R. S. Sharpe, ed.), Chapter 8, Academic Press, London (1980). 

R. J. Schliekelmann, Nondestructive testing of adhesive bonded joints, Fokker-VFW Technical Report, NTIS (N81-28190), 38 pp. (April, 1982). 

G. J. Curtis, Nondestructive testing of adhesively bonded structures with acoustic methods. Ultrasonic Testing Non-Conventional Testing Techniques, pp. 495-554 (1982). 

Filimonov, S.A., Resonance method as a means for inspecting the quality of adhesive joints. Sov. J. Nondestruct. Test., 26, 853-860(1991). 

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. 

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