Estimation of interfacial properties from hysteretic energy loss in unidirectional ceramic matrix composites

When ceramic matrix composites are subjected to fatigue loading levels suff icient Co initiate microstructural damage to the constituents, the mechanic al response of the laminate, e.g. the residual strength, stiffness and life of the composite, is governed by the physical state of the fiber/matrix in terface. During loading, the chemical bonds, which develop between fiber an d matrix during processing, are broken. This 'debonding' results in a signi ficant decline in load transfer between the two constituents and leads to a measurable increase in laminate compliance. With continued cyclic loading, the interface debonds grow in length which f urther degenerates the composite strength. Moreover, within the debonded re gions, frictional sliding between the fiber and matrix is permitted and lea ds to surface wear of the constituents [1]. Ultimately, the progression of this damage mode leads to a further decline in the interfacial shear stress and load transfer between the constituents. Knowledge of the progression of both damage mechanisms, debonding and the r eduction in interfacial shear strength, is critical to characterize ceramic composites since these mechanisms govern, in large part, the degradation i n laminate properties. Unfortunately, experimental observation of these kin ds of damage is not an easy task. However, attempts to measure these proper ties experimentally using single fiber and microcomposite tests have been c onducted [2, 3]. Moreover, several techniques for estimating interfacial pr operties computationally using various models have been presented in the li terature [4-8]. As in this study, several micromechanics models use hystere sis measurements to gain insight into the state of the fiber/matrix interfa ce [4-6, 8]. The authors use the hysteresis data for a myriad of purposes r anging from the derivation of empirical constants to validation of simplifi ed failure criterion. The current study is unique in that assumptions are n ot made regarding either the failure of the interface (debonding), nor the associated degradation in shear resistance during fatigue. The present stud y attempts to infer a logical progression of both mechanisms without specif ic failure criteria. Rather, the analysis is a 'what must they be' comparis on between the experimental measurements of hysteretic energy loss within a given fatigue cycle and the numerical predictions from the one-dimensional shear-lag analysis. The strength of the model and its application as prese nted herein resides in its simplicity allowing the validated approach to be incorporated into more rigorous micromechanics models which more accuratel y model the instantaneous state of stress within the laminate as has been t he evolution of the early shear-lag models.