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Electronic tap hammer for composite damage assessment

The percentage of composite materials used on new aircraft continues to rise, creating a greater need for NDI methods that can be used on-aircraft and are effective in identifying degradation and damage in composite structures. A particular type of degradation (delamination due to impact), can produce significant strength or stiffness reduction without being visible to the naked eye. Methods such as pulse echo ultrasonics, shearography, and thermography are being applied to this problem, but are costly to implement. The traditional coin tap or tap hammer method, is low cost, but is inherently subjective, and operator dependent. Boeing has developed a low cost instrumented tap hammer that provides a quantitative measure of the hammer/composite impulse time that can be correlated to delaminations in the structure. The instrumented tap hammer supplements the tonal discrimination of the operator with a numeric readout that can readily be related to local part quality. The effect of background noise and operator differences on the inspection results can be eliminated. An increased sensitivity is also shown over the audible tap test method. This paper describes the theoretical basis for the impulse measurement, the tap hammer design, test methods, and test results which validate the device.