Stress Rupture in Ceramic‐Matrix Composites: Theory and Experiment

Abstract

A micromechanically based model for the deformation, strength, and stress-rupture life of a ceramic-matrix composite is developed for materials that do not degrade by oxidative attack. The rupture model for a unidirectional composite incorporates fiber-strength statistics, fiber degradation with time at temperature and load, the state of matrix damage, and the effects of fiber pullout, within a global load sharing model. The constituent material parameters that are required to predict the deformation and lifetime can all be obtained independent of stress-rupture testing through quasi-static tension tests and tests on the individual composite constituents. The model predicts the tertiary creep, the remaining composite strength, and the rupture life, all of which are dependent critically on the underlying fiber-strength degradation. Sensitivity of the rupture life to various micromechanical parameters is studied parametrically. To complement the model, an extensive experimental study of stress rupture in a Nextel 610/alumina-yttria composite at temperatures of 950°and 1050°C is reported. The Larson-Miller and Monkman-Grant life-prediction methods are inadequate to explain the current data. Constituent parameters for this material system are derived from quasi-static tests and literature data, and the micromechanical model predictions are compared with measured behavior. For a slow-crack-growth model of fiberstrength degradation, the lifetime predictions are shorter by two orders of magnitude. When the rupture life is fitted with one parameter, however, the model prediction of the tertiary creep and residual strength at 1050°C agrees well with the experimental results. For a more complex degradation model, the rupture life and tertiary creep at 1050°C can be predicted quite well; however, the spread in residual strength is not, and the lifetimes at 950°C are greatly overpredicted. Thus, the micromechanical model can be successful quantitatively but clearly shows that the rupture life of the composite is extremely sensitive to the detailed mechanisms of fiber degradation. The model has practical applications for extrapolating laboratory lifetime data and predicting life in components with evolving spatial stresses.

Mechanical hysteresis in ceramic matrix composites

“…After saturation of matrix multicracking, the stress applied on the composite is sustained only by unbroken fibres, and as far as this stress increases the fraction of broken fibres increases. When all fibres are broken the strength of the composite is reached [4,5].…”

“…For example in airplane engines, materials have to maintain their good long-term mechanical properties under high temperature and oxidizing atmospheres even when subjected to complex mechanical loads. Ceramic matrix composites (CMCs) are interesting candidates for such applications, because they have low density, are tolerant to damage and have better mechanical properties at high temperature than metallic alloys [1][2][3][4][5][6][7][8][9].…”

On the effect of fiber creep-compliance in the high-temperature deformation of continuous fiber-reinforced ceramic matrix composites

“…Curtin (1991) established, under global load sharing rule, fiber failure stochastics for time-independent fibers dependent upon the gauge length and a constant shear stress across the slipped portion of the interface. The time-dependent breakdown of the fibers was included in a number of studies based mainly on the Coleman's lifetime distribution (Coleman, 1958;Ibnabdeljalil and Phoenix, 1995;Newman and Phoenix, 2001;Mahesh and Phoenix, 2004) and an empirical crack growth power law (Iyengar and Curtin, 1997a;Halverson and Curtin, 2002). Moreover, the effects of fiber pullout and that of the matrix damage state were also included in Halverson and Curtin (2002).…”

“…The time-dependent breakdown of the fibers was included in a number of studies based mainly on the Coleman's lifetime distribution (Coleman, 1958;Ibnabdeljalil and Phoenix, 1995;Newman and Phoenix, 2001;Mahesh and Phoenix, 2004) and an empirical crack growth power law (Iyengar and Curtin, 1997a;Halverson and Curtin, 2002). Moreover, the effects of fiber pullout and that of the matrix damage state were also included in Halverson and Curtin (2002). Du and McMeeking (1995) have solved a single broken fiber model (cell model) to understand the evolution of interfacial shear and then developed an averaging approach for the entire composite (the effect of interface relaxation was accounted for by an average time-dependent sliding interfacial shear stress).…”

Mechanical Behavior of SiC Fiber-Reinforced Ceramic Matrix Composites

“…When all fibers are broken the strength of the composite is reached (Evans, 1997;Halverson & Curtin, 2002). After saturation of matrix multicracking, the stress applied on the composite is sustained only by unbroken fibers, and as far as this stress increases the fraction of broken fibers increases.…”