Notch‐Sensitivity of Fiber‐Reinforced Ceramic‐Matrix Composites: Effects of Inelastic Straining and Volume‐Dependent Strength

Abstract

The effects of circular holes and sharp notches on the tensile strength of two Nicalon‐reinforced ceramic composites have been investigated. The influence of inelastic straining on the redistribution of stress has been elucidated through measurements of the local strains in the regions of high stress concentration, coupled with finite element simulations of the test geometries, using a nonlinear constitutive law appropriate to ceramic composites. The scale dependence of strength has been inferred from tests performed on specimens of varying size. The utility of two failure models that incorporate both the inelastic straining and the scale dependence has been assessed: one based on the point stress failure criterion and the other on weakest‐link fracture statistics. Both approaches provide a reasonably consistent description of the experimental measurements. Some of the implications and limitations associated with the failure models are discussed.

An elastic–plastic constitutive model for ceramic composite laminates

“…Interestingly, these results are broadly consistent with the 'point stress' fracture criterion [40], which is based on the attainment of a critical stress over a characteristic length. It has previously been used to rationalize the notch sensitivity of PMCs [41] and CMCs [7]. In those studies, it had been found that, regardless of the matrix material, the characteristic length is on the order of 1 mm.…”

“…The inelasticity can play an important role in the distributions of stresses and strains in coupons or components containing holes or notches (much like plasticity in metals). One consequence is a reduced degree of notch-sensitivity of tensile strength (relative to truly brittle materials) [1][2][3][4][5][6][7][8][9]. A robust capability for modeling inelasticity in CMCs would enable the design of complex engineering components and could lead to designs that are less conservative than those that would emerge from purely elastic analyses.…”

Remediation of a constitutive model for ceramic composite laminates

“…Since the tangent modulus in the inelastic regime is usually very small (roughly 1-5 GPa) -that is, the response is nearly perfectlyplastic [1,2] -the factor 1 þ D 45 will have only a small influence on the shear flow stress. The predictions of the modified model are therefore expected to be almost indistinguishable from the experimental results for the loading scenarios considered in Genin and Hutchinson [1]: 0°tension, 45°tension, and pure shear.…”

“…The second is to remedy the model formulation to ensure stability for strain-hardening materials. The modifications to the original model are necessarily minimal, in order to retain the positive features of the original formulation: notably, the excellent agreement between experimental and predicted stress-strain curves for 45°tension and pure shear [1], as well as between experimental and predicted strains in notched tension tests [2].…”

Notch Sensitivity of Ceramic Composites with Rising Fracture Resistance

“…15 Alternatively, in systems with strong (dense) matrixes, it can occur in a tensile mode through multiple matrix cracking. 15,16 When the spatial extent of the inelasticity becomes comparable to the structural dimensions, the inelasticity must to be taken into account explicitly 13,16 and not simply embodied in the initiation toughness.…”