A Detailed Investigation of the Micromechanisms of Compressive Failure in Open Hole Composite Laminates

Abstract

Compression failures in composite laminates containing circular holes are often preceded by the development of a damage zone which grows with increasing com pressive load. This damage zone which appears similar to a flat, tension fatigue crack in metals, initiates at the edges of the hole and propagates across the width of the laminates leading to final failure. The damage zone is initiated by local fiber buckling and/or shear crippling into the edges of the hole. The length or size of the damage zone increases with increasing load, propagating stably until it reaches a critical size. Then, unstable growth begins, and the zone completely traverses the specimen. Subsequently, a more cata strophic failure occurs giving brooming and/or delamination. Several tests of 2.54 cm wide AS4/PEEK containing a 0.16 cm diameter hole were inter rupted prior to catastrophic damage growth to allow for a careful examination of the fiber shear crippling and/or microbuckling. These observations were accomplished using scan ning electron microscopy. Direct observation of the specimen revealed the surface size of the shear crippling zone. To determine the extent of this damage through the thickness of the laminate, systematic sectioning techniques were employed. From these sectioning studies, the extent and mode of failure were determined, and a three-dimensional schematic of the damage zone was developed.

Compression fracture of unidirectional carbon fibre-reinforced plastics

“…This is quite unusual, as in traditional materials the former may be used for tension but not for compression. To describe the effect of defects for a multilayered CFRP, Guynn and Braydley [31] composites under tensile load [32]. For a unidirectional composite, modification of the linear fracture mechanics is not necessary.…”

Effect of geometry on compressive failure of notched composites

“…1. Soutis et al [1] and Guynn et al [2] find that damage is initiated in multi-directional carbon fibre-epoxy and carbon fibre-PEEK laminates respectively at the edge of holes by fibre microbuckles in the 0 ° plies. These microbuckles nucleate and grow in a stable manner under an increasing remote load, so that the panel fails at a higher load than that at which damage initiates.…”

A Parametric Study of Variables That Affect Fiber Microbuckling Initiation in Composite Laminates: Part 2 — Experiments

“…Previous work [3][4][5][6] on tough matrix composite laminates containing center holes indicates that the compressive failure of these composites initiates with microbuckling of the 0° fibers toward an unsupported surface in 0° plies either in-plane for interior 0° plies or out-of-plane for surface 0° plies. Growth of this initial damage to a critical size at which catastrophic failure occurs requires little additional load.…”

“…This result suggests that the compression strength is controlled by initiation of fiber microbuckling. Additionally, the inand out-of-plane fiber microbuckling appears to precede shear crippling damage in the systems studied in References [3][4][5][6]. It should be noted that the term &dquo;fiber microbuckling&dquo; used throughout this paper, refers to a gradual increase with increasing load of the lateral deflection of initially wavy fibers, leading to fiber breakage, rather than a bifurcation instability.…”

“…The experimental observations [3][4][5][6] and the literature review [1] suggest at least six factors that affect the determination of the strain levels at which fiber microbuckling initiates and thus, partially controls the composite's compression strength. These factors are (1) the orientation of the supporting plies adjacent to the 0° plies through the thickness of the laminate, (2) the effects of the free surfaces, (3) the fiber/matrix interfacial bond strength, (4) the degree of initial fiber waviness, (5) the thickness of resin-rich regions between plies, and (6) the nonlinear resin shear constitutive behavior.…”