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

Abstract

a b s t r a c tCreep models for unidirectional ceramic matrix composites reinforced by long creeping fibers with weak interfaces are presented. These models extend the work of Du and McMeeking (1995) [Du, Z., McMeeking, R. 1995. Creep models for metal matrix composites with long brittle fibers. J. Mech. Phys. Solids 43, 701-726] to include the effect of fiber primary creep present in the required operational temperatures for ceramic matrix composites (CMCs). The effects of fiber breaks and the consequential stress relaxation around the breaks are incorporated in the models under the assumption of global load sharing and time-independent stochastics for fiber failure. From the set of problems analyzed, it is found that the high-temperature deformation of CMCs is sensitive to the creep-compliance of the fibers. High fiber creep-compliance drives the composite to creep faster, leading however to greater lifetimes and greater overall strains at rupture. This behavior is attributed to the fact that the greater the creep-compliance of the fibers, the higher the creep rate but the slower the matrix stress relaxation -since the matrix must deform with a rate compatible with the more creep-resistant fibers -and therefore the less the load carried by the main load-bearing phase, the fibers. As a result, fewer fibers fail and less damage is accumulated in the system. Moreover, the greater the creep-compliance of the fibers, the slower the matrix shear stress relaxation -and thus the lower the levels of applied stress for which this effect becomes important. The slower the shear stress relaxes, the slower the ''slip" length increases. Due to the Weibull nature of the fibers, the fiber strengths at the smaller gauge length of the slip length are stronger; therefore fewer fibers undergo damage. Hence, high fiber creep-compliance is desirable (in the absence of any explicit creep-damage mechanism) in terms of composite lifetime but not in terms of overall strain. These results are considered of importance for composite design and optimization.

Review: creep of fibre-reinforced ceramic matrix composites

“…It was assumed the transverse cracks initiated in the 90°plies gradually extend in the 0°plies because of creep of the bridging fibres in the 0°plies that relaxes the bridging traction (Figure 7). The models concentrated on matrix crack growth considering that this is the major component of lifetime [see for instance 62,113,[126][127][128][129][130]. Models for steady-state, constant velocity crack growth were developed for a composite with brittle elastic matrix containing creeping fibres [127].…”