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Application of lock-in thermography in nondestructive evaluation of adhesively-bonded aluminum joints

A major problem in industrial applications of structural adhesives is the quality assurance of manufactured joints. At present, for lack of a suitable nondestructive technique, production standards for adhesively-bonded aluminum joints are established on the basis of destructive tests and statistical inference. An experimental study was carried out to assess if lock-in thermography (LT) could be used as a tool for nondestructive evaluation of adhesively-bonded aluminum joints. Several samples were fabricated by varying the governing parameters such as nature of aluminum alloy, substrate thickness, surface treatment, adhesive type and bondline thickness. The effects of surface treatments on the loading capability of lap joints were evaluated through both destructive tensile tests and nondestructive evaluation with infrared LT. Tensile tests showed that the joint performance was not affected by the nature of the aluminum alloy but by the substrate thickness, the adhesive type and the bondline thickness. LT was capable of detecting imperfections such as scratches on substrates and foreign inclusions in the adhesive layer.