Rachid Boukhili

Thermal expansion responses of pressure infiltrated SiC/Al metal-matrix composites

Aluminium-matrix composites containing thermally oxidized and unoxidized SiC particles featuring four average particle diameters ranging from 3 to 40 μm were produced by vacuum assisted high pressure infiltration. Their thermal expansion coefficient (CTE) was measured between 25 and 500°C. Oxidation of the SiC particles in air produces the formation at their surface of silicon oxide in quantities sufficient to bond the particles together, and confer strength to preforms. After infiltration with pure aluminium, the composites produced showed no sign of significant interfacial reaction. The CTE of the composite reinforced with unoxidized SiC particles featured an abrupt upward deviation upon heat-up near 200°C, and a second abrupt decrease near 400°C. The first transition is attributed to an inversion of stress across particle contact points. When composites are produced with oxidized SiC particles, these two transitions were removed, their CTE varying smoothly and gradually from the lower elastic bound to the upper elastic bound as temperature increases. With both composite types, the CTE decreased as the average particle size decreased. This work illustrates the benefits of three-dimensional reinforcement continuity for the production of low-CTE metal matrix composites, and shows a simple method for producing such composites.

Dynamic mechanical analysis of prestrained Al2O3/Al metal-matrix composite

The effect of prestraining on the elastic modulus,E, and damping capacity, tanφ, of 10 and 20 vol% Al2O3 particle-reinforced composites has been investigated as function of temperature using dynamic mechanical analysis. Both elastic modulus and damping capacity were found to increase with volume fraction. At 10 vol% the modulus and damping were relatively insensitive to prestrain. However, at 20 vol% it was observed that the modulus decreased with increasing prestrain while damping increased significantly. These results are discussed in terms of fraction of broken particles, particle size, and differential in thermal expansion between the matrix and Al2O3 particulate.

Descriptive relationships between bearing response and macroscopic damage in GRP bolted joints

Abstract The aim of this investigation was to examine the relationships between laminate architecture, macroscopic damage and important bearing response characteristics such as damage load, ultimate failure load and post-failure behaviour in general purpose glass-fibre reinforced polyesters. Six representative laminates with varying 0° roving, 90° roving and chopped strand mat contents were tested for their bearing response in a single-bolt double shear lap configuration. Three specimen configurations were tested. The first specimen geometry was the standard coupon geometry of the ASTM D5961 bearing response test with a pitch distance ratio ‘w/D’ of 6 and an end distance ratio ‘e/D’ of 3. The second and third geometries used a ‘w/D=6, e/D=6’ and ‘w/D=2, e/D=3’. The diversity in laminating sequences and geometries led to equally diverse failure patterns. However, interesting general trends were observed that went beyond this diversity, thereby creating a global picture of failure in GRP bolted joints.

Bearing stiffness of glass fibre-reinforced polyester: influence of coupon geometry and laminate properties

The bearing stress–strain response of glass fibre-reinforced polyester (GRP) laminates in a single-bolt double lap joint was investigated experimentally and numerically. Six GRP laminate types were tested according to the ASTM D5961 procedures for three coupon geometries. A 2-D finite element model that included material non-linearity, clearance and bolt–hole friction predicted the bearing stiffness very accurately. However, both numerical and experimental data disagreed with existing joint flexibility models developed for metals or carbon fibre-reinforced epoxy laminates with tight fitting bolts. Clearly, some adaptation of the joint flexibility formulae is needed when they are used for design of GRP structures with large bolt–hole clearance. On average, joints with a reduced width (w/D=2) were 26% more compliant than three times wider standard joints (w/D=6). Doubling the end distance had a smaller effect on the bearing stiffness. Laminates with more axial reinforcement had a clearly higher bearing stiffness for the small coupon width (w/D=2). For wider coupons (w/D=6), this trend became less apparent.

Impact response of laminated composite plates: prediction and verification

Abstract Methods and procedures for predicting the impact response of laminated composite plates using a commercial finite element system are described. Results of element evaluation, procedure verification and a correlation study are presented and discussed. The need for a hybrid experimental-numerical approach and combined geometric and material nonlinear finite element analysis is identified. A methodology for the prediction of delamination (onset and growth) is outlined.

Analysis of the bearing response test for polymer matrix composite laminates: bearing stiffness measurement and simulation

Abstract This paper is the first part of a project that aims to investigate the mechanical and fracture behaviour of bolted joints in general purpose glass fibre-reinforced polyesters (GRP). In the present study a procedure is set up to measure the bearing stiffness of a GRP laminate in a single-bolt double lap joint. With a three-dimensional finite element model it is shown that the bolt and fixture deformations affect the stiffness results. Hence the experimental displacement data were corrected before calculating the coupon bearing stiffness. The coupon bearing stiffness was also simulated by a two-dimensional finite element model. Provided that bolt–hole clearance, material non-linearity and bolt–hole friction are taken into account, good agreement is observed with experimental data. Bearing strain and bearing stiffness are based on the bearing deformation of the coupon, not on the hole elongation. This makes the stiffness data useful for design and allows an easy installation of the displacement measurement devices.

Evaluation of the Physical and Mechanical Properties of Braided Fabrics and their Composites

This study aims to evaluate the permeability and the basic mechanical properties of braided fabrics intended for hollow composites to be manufactured by resin infusion processes. A simple procedure has been devised to obtain flat specimens for the required permeability and mechanical tests. Permeability measurements have been performed on 2-D biaxially braided glass fabrics at three braiding angles. The change of the braiding angle affects the fiber volume fraction, which in turn affects the permeability. Tension, compression, and shear tests have also been conducted on unidirectional (UD) and braided glass-epoxy composites. From the experimental UD properties, the lamination theory (LT) has been used to predict the properties of angle ply () laminates equivalent to the braided composites tested. From the comparison of the predicted and experimental results, the effects of the fibers’ intertwining and undulation on the mechanical properties have been derived. The undulation of the fibers is responsible for the compression strength reduction, mainly at low braiding angles, but has no effect on the tensile strength. For all angles, the braided composites have a higher modulus than the predicted angle ply laminates. The Iosipescu shear method is not suitable for the characterization of composites containing braided fabrics because of their high shear stiffness and strength.

Fatigue Mechanisms Under Low Energy Repeated Impact of Composite Laminates

This investigation deals with the fatigue behaviour under low energy repeated impact (LERI) of quasi-isotropic graphite/epoxy composite plates. The damage growth during the test was monitored by ultrasonic C-scan imaging. The damage area plotted as function of the number of impacts displays a three stages pattern behaviour. Stage I corresponds to a matrix cracking of the region under the impactor, stage II corre sponds to a rapid delamination propagation and stage III to a slow delamination propaga tion. The relative extent and the damage growth rate in stage II increase with the impact energy E I. The number of impacts NIF corresponding to the transition between stage I and stage II is governed by an impact-fatigue curve in the form of EI = C log (NIF) + E 0 where E0 is the threshold impact energy that induces a delamination in one impact and C constant denoting the damage tolerance of the material.

An Inverse Approach Based on Laminate Theory to Calculate the Mechanical Properties of Braided Composites

A new approach based on plate laminate theory (PLT) is developed to calculate the mechanical properties of woven and braided composites. It supposes that, for each woven or braided composite oriented at ±θ, there is an equivalent ±θ angle-ply laminate made out of two unidirectional plies. An inverse algorithm based on PLT was developed to calculate, from the known mechanical characteristics of a composite reinforced by engineering fabrics, the properties of the mechanically equivalent angle-plies. The virtual angle-ply and cross-ply laminates obtained by inverse calculation include the effects of undulation and strand shear. The difference in properties between experience and theory is clear, especially for Poisson’s ratio. Comparisons performed between numerical predictions and experimental tests have shown a good correlation.

Finite element simulation of impact tests of laminated composite plates

Abstract Numerical simulation of static indentation and lateral impact tests of laminated composite plates, using a commercial finite element system are presented. Extensive results from a parametric study are also provided to supplement the experimental result. It is concluded that a nonlinear and transient response analysis was needed to predict the deformation and damage measured in the tests.