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Long-term Stress Rupture Limitations of Unidirectionasl High Strain Composites in Bending



The efficiency of any deployable structure for aerospace applications is directly correlated to the mass/stiffness ratio and the volume compaction ratio. While High Strain Composite (HSC) materials have a large mass/stiffness ratio the volume compaction ratio is a direct function of the maximum safe stowage strain-limit of the HSC. Previous works have shown that HSC materials are capable of achieving surface strains in excess of the tensile and compressive allowable strains for the constituent fiber. However, it has also been noted that large surface strains exhibit a time-dependent failure behavior. The purpose of this paper was to begin an investigation of the stressstrain- time dependent behavior of an IM7 unidirectional TP-HSC utilizing a novel adaptation of the Column Bend Test method for Long-Term testing (CBT-LT). Ultimate failure curvature testing was conducted on the IM7 laminate to determine long-term test curvature levels corresponding to 80%, 85%, and 90% of the ultimate value. Out of the ten stress rupture/relaxation tests conducted, five were observed to reach stress rupture within six-months of accelerated effective time. All five of the ruptured samples were held at surface strains in excess of 1.7%, corresponding to 89.5% of the reported ultimate tensile failure strain of an IM7 fiber and 91.8% of the ultimate failure surface strain of the IM7 laminate determined in this study. While the initial stress relaxation/rupture trends observed using the CBT-LT method is promising, future work is needed to improve the fidelity of the test method.


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