This report describes an ultrasonic‐acoustic technique used to measure a “stress wave factor” in order to nondestructively assess the quality of composite structures. In a prior study this factor was found effective in evaluating the interlaminar shear strength of fiber‐reinforced composites. Details of the method used to measure the stress wave factor are described. In addition, frequency spectra of the stress waves are analyzed in order to indicate the nature of the wave phenomena involved. The stress wave factor can be measured with simple contact probes requiring only one‐side access to a part. This is beneficial in nondestructive evaluations because the waves can run parallel to fiber directions and thus, in many cases, measure material properties in directions assumed by actual loads. Moreover, the technique can be applied where conventional through transmission techniques are impractical or where more quantitative data are required. The stress wave factor was measured for a series of graphite polyimide composite panels and results obtained are compared with through transmission immersion ultrasonic scans.

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A heuristic mathematical basis is proposed for the experimental correlations found between ultrasonic propagation factors and fracture toughness factors in metallic materials. A crack extension model is proposed wherein spontaneous stress (elastic) waves produced during mic roc racking are instru-" mental in promoting the onset of unstable crack extension. Material microstructural factors involved in this process are measurable by ultrasonic oo probing. Experimental results indicate that ultrasonic attenuation and velocity n°* measurements will produce significant correlations with fracture toughness properties and also -yield strength.