Crack Growth and Fracture of Continuous Fiber Metal Matrix Composites: Analysis and Experiments

Abstract

It is reasonably well accepted that the standard procedures developed for isotropic homogeneous metals using linear elastic fracture mechanics models are not appropriate for either continuously or discontinuously reinforced metal matrix composites. For example, the ASTM plane strain fracture toughness test methods typically give widely different values of fracture toughness depending on the particular test specimen geometry as well as the fiber orientation. For unidirectional boron/aluminum composites one finds approximately a factor of two difference between the measured values of fracture toughness obtained from a center-notched test coupon and that given by a compact tension specimen. In particular for unidirectional composites, and to a slightly lesser degree for angle ply laminates, the dominant controlling mechanism for this behavior is matrix plasticity. A secondary toughening mechanism, resulting from the matrix plasticity, is stable transverse fiber failure. The present paper will focus on both the influence of the large plastic zone at the end of the notch and on the constraint that the fibers impose on the shape of this zone, as well as the transverse crack growth. First, a review of some particular experimental studies and methods of analysis for predicting crack growth and fracture of notched unidirectional metal matrix composites is given. Next, two mechanistic models for unidirectional composites with damage are presented. The first is an improved shear lag model that accounts for both of the above damage modes, and the second describes a recent extension of the shear lag concept in an attempt to include transverse stresses. A related finite width laminate model is then discussed, and it is indicated that an isotropic finite width correction factor is reasonably accurate for most center-notched test coupons.

Asymmetrical dynamic fracture model of bridging fiber pull-out of unidirectional composite materials

“…Applying the same technique as that of (26) and, (27), substitute x = α 1 t, x = −α 2 t into (45) while V 1 , V 2 and t are regarded as known constants, the fibrous fracture speeds α 1 and α 2 can be facilely attained.…”

“…If the fiber failure occurring at the crack plane is governed by maximum tensile stress, the fiber breaks and hence the crack extension should also appear in a selfsimilar fashion. The fiber breaks lie along a transverse line and therefore present a notch [3,25,26]. When a crack of composite materials moves at high speed, bridging fiber pull-out still exists in the dynamic circumstance, whose fiber pull-out is more significant than those in the static case.…”

A dynamic asymmetrical crack model of bridging fiber pull-out in unidirectional composite materials

“…Load is transferred between adjacent fibers through the matrix by a straightforward shear mechanism. The shear stresses are independent of transverse displacements and the equilibrium equation in the fiber direction decreases to an equation in the longitudinal displacements alone, as is a typical of shear-lag theory [17,22,23]. The approach and modeling procedure developed by Ref.…”

“…When relevant parameters are put into Eqs. (23), (24), (41), (42) to easily plot K 1 (1) (t) and K 1 (2) (t) as a function of time t, respectively, and the numerical solutions of them are readily obtained. The following constants [40,41,36] are presumed: (t) and K 1 (2) (t) decay gradually to slow and trend to constants finally and have obvious singularity with the increase of time, because only variable t lies in the denominator of the two expressions; moreover the rest quantities are regarded as real constants.…”

“…If the fiber failure is governed by maximum tensile stress, which occurs at the crack plane, the fiber breaks and hence the crack extension should also appear in a self-similar fashion. The fiber breaks lie along a transverse line and therefore present a notch [3,22,23]. When a crack moves at higher speed, bridging fiber pull-out of composite materials still exists in the dynamic circumstance, which is more significant than those in the static case.…”