Compression Fracture of CFRP Laminates Containing Stress Intensifications

Abstract

Abstract:For brittle fracture behaviour of carbon fibre reinforced plastics (CFRP) under compression, several approaches exist, which describe different mechanisms during failure, especially at stress intensifications. The failure process is not only initiated by the buckling fibres, but a shear driven fibre compressive failure beneficiaries or initiates the formation of fibres into a kink-band. Starting from this kink-band further damage can be detected, which leads to the final failure. The subject of this work is an experimental investigation on the influence of ply thickness and stacking sequence in quasi-isotropic CFRP laminates containing stress intensifications under compression loading. Different effects that influence the compression failure and the role the stacking sequence has on damage development and the resulting compressive strength are identified and discussed. The influence of stress intensifications is investigated in detail at a hole in open hole compression (OHC) tests. A proposed interrupted test approach allows identifying the mechanisms of damage initiation and propagation from the free edge of the hole by causing a distinct damage state and examine it at a precise instant of time during fracture process. Compression after impact (CAI) tests are executed in order to compare the OHC results to a different type of stress intensifications. Unnotched compression tests are carried out for comparison as a reference. With this approach, a more detailed description of the failure mechanisms during the sudden compression failure of CFRP is achieved. By microscopic examination of single plies from various specimens, the different effects that influence the compression failure are identified. First damage of fibres occurs always in 0°-ply. Fibre shear failure leads to local microbuckling and the formation and growth of a kink-band as final failure mechanisms. The formation of a kink-band and finally steady state kinking is shifted to higher compressive strains with decreasing ply thickness. Final failure mode in laminates with stress intensification depends on ply thickness. In thick or inner plies, damage initiates as shear failure and fibre buckling into the drilled hole. The kink-band orientation angle is changing with increasing strain. In outer or thin plies shear failure of single fibres is observed as first damage and the kink-band orientation angle is constant until final failure. Decreasing ply thickness increases the unnotched compressive strength. When stress intensifications are present, the position of the 0°-layer is critical for stability under compression and is thus more important than the ply thickness. Central 0°-layers show best results for OHC and CAI strength due to higher bending stiffness and better supporting effect of the adjacent layers.

The effects of loading direction on the compression after impact strength of quasi-isotropic face sheet honeycomb core sandwich structure

“…During the course of this experimental research activity, a more thorough literature review was taking place that specifically targeted studies on lay-up sequence and compression strength of quasi-isotropic laminates. A paper by Leopold et al 11 addressed the same issue found in this study. In that study, 24-ply quasi-isotropic carbon/epoxy laminates with varying lay-up sequence were tested for undamaged compression, OHC and CAI strengths.…”

“…The most notable increase in OHC and CAI strengths in Ref. 11 came when all of the 0° plies of the 24-ply laminate were clustered at the center of the specimen. This gave 45% higher OHC and CAI strengths.…”

A review of stress concentration studies on fibre composite panels with holes/cutouts

“…19 Open hole-compression experiments conducted by Leopold et al. 29 established that when ply thickness could influence stress concentration than the stacking sequence, failure initiation depended mainly on the stacking sequence, due to influence of 0° ply.…”

Numerical analysis of interlaminar stresses in open-hole laminates under compression

“…Many works can be found in the scientific literature on modelling of in-plane stresses and damage evolution around a hole in a laminate subjected to tensile [20][21][22][23] and compressive loads [9,10,16,24]. In spite of these works, there is a necessity to continuous working in the knowledge of the problem of open-hole laminates subjected to compressive loads.…”