Carbon fiber reinforced epoxy (CFRP) coupons previously damaged by low velocity impact were subjected to flexural loading and the microscopic progress of damage was monitored by acoustic emission (AE). Although the compression-after-impact test is more or less a standardized method for the of the residual strength estimation of damaged composites, it is rather a complicated and expensive test method. The flexural loading, therefore, was employed to induce both the tensile and the compressive stresses in the same coupons and the AE behavior was utilized to correlate the residual strength with the degree of damage. Test coupons were cut from autoclave-cured laminates from the high strength carbon/epoxy prepreg in two stacking sequences of [0/90/±45]2S and [0/90]4S then subjected to impact velocities ranged from 1.05 to 3.50 m/s, which introduced the amount of damage rated as 10%, 30%, and 50% with reference to the total absorbed energy at fracture. Other means such as ultrasonic C-scan and scanning electron microscopy were also utilized before or after flexural tests to correlate the degree of impact damage with the AE activity by the progressive failure in microscopic scale. Special attention was paid to determine optimal AE parameters to characterize the microscopic fracture process and to predict the residual strength of composite laminates. AE rms voltage during the initial stage of loading was found an effective parameter to quantify the degree of impact damage. The activity during this stage of loading is known as the friction generated AE. With the increasing degree of impact damage, the time for initiating AE activity was considerably shortened and the activity prior to the major ply failures was also increased. From the result of analysing AE activity during this period, a new parameter named as the integrity ratio (IR) instead of the traditional felicity ratio (FR) has been proposed for an efficient index for the impact damage assessment of composite laminates. The parameter was so named as the ratio decreases rapidly with the degree of damage. Both IR and FR were found to be closely related to the stacking sequence of the CFRP laminates.
Publication Source: Trends in NDE Science & Technology; Proceedings of the 14th World Conference on Non-Destructive Testing, New Delhi, 8-13 December 1996.Vol. 4, pages 2433 - 2436 Publisher:Ashgate Publishing Company
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