Fatigue lifetime assessment of adhesive joints by ultrasonic and thermal wave imaging

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Abstract
Various nondestructive evaluation (NDE) methods are frequently employed to inspect the adhesive bonds of aircraft structures in service. The literature on the capability of various NDE techniques reveals a deficiency in linking NDE test parameter characteristics of the frequency or size of defects to critical failure properties such as the lifetime and the strength of adhesive bonds. In this study an attempt has been made to develop such correlations. A specimen geometry was employed so as to permit cleavage-type debonding under fatigue loading. This geometry and loading configuration provide for a simple fatigue testing program and simple analytical methods. Damage by flexural fatigue aging of these adhesively bonded specimens was induced at different intervals of their fatigue lifetime. The specimens were composed of materials that were commonly used in actual aircraft production during the 1970s. Pulse-echo ultrasonic C-scanning and thermal wave imaging were performed to inspect the adhesive joints at various percentages of the fatigue lifetime. A novel low-frequency ultrasonic method was used for making the C-scans; this technique was immune to signal amplitude changes due to interference phenomena caused by bond thickness variation. A direct correlation of the ultrasonic parameter (size of the debonded area) with the percentage lifetime of the adhesive joints was tentatively established. It was also found that this correlation was consistent when the scanning was conducted from either the top surface or the bottom surface of the adhesive joints. A similar correlation between the size of the debonded area and the percentage of fatigue lifetime of the adhesive joint was found using thennal wave imaging. Thus, it appears that the measurements obtained from both techniques are consistent.

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