Compressive Strength Analysis for High Performance Fibers with Different Modulus in Tension and Compression

The assumption of equal tensile and compressive modulus necessary to determine single fiber axial compressive strength from the elastica loop test is relaxed by deriving a compressive strength equation based on the analysis of the flexural response of a fiber with different modulus in tension and compression. Previously determined tensile (E-1t) and compressive (E-1c) modulus values for different high performance organic fibers with varying degrees of lateral molecular interactions are used to determine fiber compressive strength. The importance of using the bi-moduli equation becomes evident in the case of fibers that lack strong intermolecular interactions, since calculations done with the original loop test equation can result in an overestimation of the compressive strength on the order of 80%, as seen for the poly-(p-phenylene benzobisoxazole) fiber PBO. Kink band angles measured from looped fiber specimens are documented and correlated to the calculated compressive strength values.