Estimating Residual Strength in Filament Wound Casings from Nondestructive Evaluation of Impact Damage
The purpose of this study is to improve the ability to detect hidden impact damage in thick composites caused by low velocity impact and to predict the remaining strength of those materials. An impact study has been undertaken on filament wound graphite/epoxy casings, such as those proposed for NASA’s space shuttle solid fuel rocket boosters. In thick composite materials, low-velocity impact damage may not be visually evident, depending on the impacter shape; yet the damage may compromise the composite’s ultimate strength. A model of a filament wound casing was fabricated with one fifth of the diameter (30 inches) but with the full thickness (1.4inches) of the full rocket motor (12 feet and 1.4 inches, respectively). It was impacted with various masses and energy levels using a one inch diameter ball as the indenter. This casing was subsequently cut into coupons of 2 in. width by 12 in. length. These samples were nondestructively examined for the degree of damage. Next, these samples were loaded in tension until failure. Efforts to accurately detect the damage with dye penetrants and x-ray methods have proven unsatisfactory in the samples that displayed no visible damage. In spite of the high attenuation of this material, ultrasonic phase velocity and attenuation images show promise in predicting the residual strength of the coupons. Predictions of the damage profile, and therefore the cross-section of the damage in the direction of loading, were obtained by assuming an “effective” value for the attenuation of the damaged part of the filament wound casing material (15 dB/MHz-cm) and an “effective” value for the velocity of the damaged part of the filament wound casing material (2250 m/s). These estimates were based partially on measurements made on impact damaged thin composite material. The remaining strength predictions from these ultrasonic data showed a significant improvement over the x-ray predictions of remaining strength and the method may be usable for predictions of remaining strength of full scale rocket motors that may have suffered impact damage.
KeywordsResidual Strength Impact Damage Rocket Motor Break Fiber Fiber Breakage
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