STIFFNESS DEGRADATION OF SIC TI TUBES SUBJECTED TO BIAXIAL LOADING/

This paper presents the results of a combined experimental and theoret ical investigation of the stiffness degradation of unidirectional [0] and angle-ply [+/-45]s SiC/Ti tubes subjected to axial, torsional and combined axial/torsional loadings. The test program consisted of a ser ies of axial and torsion tests, to sequentially higher load levels, on tubes fabricated using SCS6 silicon carbide fibers and Ti-15-3 titani um matrix. At each load level, both the axial and torsional stiffnesse s were measured. This procedure provided the opportunity to study the axial and torsional response at different levels of interfacial damage , matrix plasticity and combined load states. The experimentally measu red axial and shear moduli, and initial yield stresses were compared w ith micromechanics predictions generated with different degrees of imp erfect fiber/matrix bonding. No evidence of moduli degradation in the [0(4)] tubes was present under tensile loading, whereas under shear lo ading, shear modulus degradation was observed only after occurrence of damage accompanied by pronounced plastic flow. Substantial degradatio n of both axial and shear moduli was observed in the [+/-45]s tubes un der axial loading. The results suggest that the degradation of the int erfacial bond in the [+/-45]s tubes accelerated yielding of the matrix which, in turn, contributed to further degradation in the case of the shear modulus. The results of this investigation point to the importa nce of damage-induced stiffness degradation and damage-accelerated yie lding in SiC/Ti laminated tubes.