X-RAY STRESS ANALYSIS OF DAMAGE EVOLUTION IN Ti-SiC UNIDIRECTIONAL FIBER COMPOSITES

A composite’s local response to initial damage under stress is the primary micromechanical process determining its fracture toughness, strength, and lifetime. Through the use of high energy X-ray microdiffraction, the elastic lattice strains of both phases in a Ti-SiC composite were revealed providing the in-situ load transfer under applied tensile stress at the scale of the microstructure. To understand the damage evolution, the measured strains were compared to those predicted by a modified shear lag model. Comparisons between the model and the data demonstrated the importance of accounting for the matrix axial and shear stiffness, provided an optimal stiffness ratio for load transfer and planar interpretation of the geometry in the composite, showed the matrix within and around the damage zone sustained axial load, and highlighted matrix yielding observed in the composite. It was also shown that an area detector is essential in such a study as it provides multiaxial strain data and helps eliminate the ‘graininess’ problem. INTRODUCTION Load transfer from a broken fiber to the rest of a composite is one of the fundamental micromechanical processes determining composite strength, lifetime and fracture toughness. It is a complex process that depends on fiber/matrix interface properties, the constitutive behavior of matrix and fibers, the geometric arrangement of fibers, fiber volume fraction, and fiber strength distribution. This process is further complicated since the in-situ mechanical properties of the constituents are significantly different from those of their monolithic forms [1-4]. In composites, the macroscopic stress-strain curves obtained by conventional means result from the co-deformation of the individual phases making it impossible to determine the phase-specific insitu constitutive behavior. Typical composite deformation includes: collective nucleation and evolution of damage, fiber fractures, matrix fractures and plasticity, as well as interface separation and sliding. To predict the strength and lifetime of a fiber composite, the load transfer from broken fibers to the surrounding intact material must be understood. This requires accurate determination of stress-strain evolution at the scale of microstructure usually on the order of the fiber diameter. In special cases, this has been achieved using optical methods such as micro-Raman and piezospectroscopy [4-8]. These studies provided valuable insight about fiber strains in damaged composites at length scales approaching several μm. However, in most of these studies either the matrix could not be characterized, or only shallow surface regions were investigated. Copyright©JCPDS International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume 46. 136