Drilling of fiber reinforced plastics: A review

An extensive experimental program has been performed to investigate the effect of scaling on the tensile strength of notched composites. Hole diameter, ply and laminate thickness, were investigated as the independent variables, whilst keeping constant ratios of hole diameter to width and length, over a scaling range of 8 from the baseline size. In most cases strength decreased as specimen size increased, with a maximum reduction of 64%. However the reverse trend of strength increasing with in-plane dimensions was found for specimens with plies blocked together. As well as the variation in strength, three distinct failure mechanisms were observed: fibre failure with and without extensive matrix damage, and complete gauge section delamination. Despite these differences, similar sub-critical damage mechanisms were seen in all specimens, with the extent of the damage determining the failure stress and mechanism. Damage propagated across the gauge section via delamination at the hole, which was controlled by the ply thickness to hole diameter ratio. This same mechanism can explain both the increasing and decreasing strengths observed. Simple analytical criteria for determining notched strength were found to be accurate for fibre failure in the absence of extensive sub-critical damage, but could not account for those conditions where delamination propagated across the width prior to failure.

An experimental investigation into the tensile strength scaling of notched composites

The effects of hole machining defects on strength and fatigue life of carbon/epoxy laminates subjected to static and fatigue loading are presented. Dry specimens were subjected to pin loading and uniaxial compressive loading at room temperature. The KTH method, a new method which gives defect-free holes, was used to machine holes in the specimens. For comparison, holes were also machined using two traditional drilling techniques causing varying extents of damage. X-ray techniques were used to detect the hole machining defects. The permanent deflection of compressively loaded specimens was monitored during cyclic loading. Hole machining defects significantly reduced the static and fatigue strengths of pin-loaded laminates; the effects on the strengths of compressively loaded laminates were less pronounced.

Effects of hole machining defects on strength and fatigue life of composite laminates

Recommendations for designers and researchers resulting from the world-wide failure exercise

A parametric finite element analysis was conducted to investigate the effect of failure criteria and material property degradation rules on the tensile behaviour and strength of bolted joints in graphite/epoxy composite laminates. The analysis was based on a three-dimensional progressive damage model (PDM) developed earlier by the authors. The PDM comprises the components of stress analysis, failure analysis and material property degradation. The predicted load–displacement curves and failure loads of a single-lap single-bolt joint were compared with experimental data for different joint geometries and laminate stacking sequences. The stiffness of the joint was predicted with satisfactory accuracy for all configurations. The predicted failure load was significantly influenced by the combination of failure criteria and degradation rules used. A combination of failure criteria and material property degradation rules that leads to accurate strength prediction is proposed. For all the analyses performed, the macroscopic failure mechanism of the joint and the damage progression were also predicted.

Strength prediction of bolted joints in graphite/epoxy composite laminates

Most methods currently used for predicting tensile strength of composite laminates containing holes and cracks adopt a characteristic distance approach such as the Point Stress Criterion (PSC) or the Average Stress Criterion (ASC). These and similar ap proaches are attractive to designers since they are simple to apply. The limitation of such approaches, however, is that the characteristic distance is not a physical parameter but is an empirically determined constant, dependent on the geometry of the specimen. The Damage Zone Criterion (DZC) presented in this paper is based on fundamental physical principles. A damage zone is assumed to be present in the maximum stress region of the laminate when the tensile stress reaches the tensile strength of the unnotched laminate. Taking into account the stress redistribution followed by a damage development, a closed form analytical expression of the strength is derived from the equilibrium conditions of the specimen. The strength of laminates with holes and cracks...

Strength of Tensile Loaded Graphite/Epoxy Laminates Containing Cracks, Open and Filled Holes

On damage development in mechanically fastened composite laminates

One of the basic failure modes of bolted composite laminates is bearing fail ure. This mode of failure occurs in the material immediately adjacent to the contact points of bolt and laminate, and is caused primarily by compressive stresses acting on the hole boundary. The results of an experimental program which measured the bearing strengths of two different types of graphite/epoxy specimens are presented. The influence on the bearing strength of several important parameters including lateral constraint and ply orien tation are shown. Strains were measured in the vicinity of the loaded hole to quantify the extent of bearing failure. A new approach to predicting bearing failure is outlined. The Delamination Buckling Model (DBM) assumes that ply buckling following delaminations is a major feature of bearing failure. Quantitatively good agreement was found between ex periment and model results.

On the bearing strength of bolted graphite/epoxy laminates

The propagation of hole machining defects in pin-loaded carbon/epoxy laminates subjected to uniaxial tension fatigue loading was investigated using two non-destructive examination (NDE) techniques....

Propagation of Hole Machining Defects in Pin-Loaded Composite Laminates

Stress and failure analyses of composite laminates containing holes are presented. Both the finite element method (FEM) and an analytical method, based on complex stress functions, are used to determine the stress distributions around the holes. The Point Stress Criterion (PSC) and the Damage Zone Criterion (DZC) are used to predict the tensile strength of test specimens subjected to complex loading conditions. A comprehensive test program was carried out to establish criteria parameters and to generate data to validate the stress and failure analyses. For this purpose a special test fixture was designed. Good agreement between predicted and measured results was found.

Ingvar Eriksson

Papers

Strength of Tensile Loaded Graphite/Epoxy Laminates Containing Cracks, Open and Filled Holes

On damage development in mechanically fastened composite laminates

On the bearing strength of bolted graphite/epoxy laminates

Propagation of Hole Machining Defects in Pin-Loaded Composite Laminates

Strength of Composite Laminates Containing Holes and Subjected to Complex Loading Conditions