F.L. Matthews

A progressive Damage Model for Mechanically Fastened Joints in Composite Laminates

A three-dimensional finite element model is developed to predict damage progression and strength of mechanically fastened joints in carbon fibre-reinforced plastics that fail in the bearing, tension and shear-out modes. The model is based on a three-dimensional finite element model, on a three-dimensional failure criterion and on a constitutive equation that takes into account the effects of damage on the material elastic properties. This is accomplished using internal state variables that are functions of the type of damage. This formulation is used together with a global failure criterion to predict the ultimate strength of the joint. Experimental results concerning damage progression, joint stiffness and strength are obtained and compared with the predictions. A good agreement between experimental results and numerical predictions is obtained.

Stress analysis and strength prediction of mechanically fastened joints in FRP: a review

A review of the investigations that have been made on the stress and strength analysis of mechanically fastened joints in fibre-reinforced plastics (FRP) is presented. The experimental observations of the effects of joint geometry, ply-orientation, lay-up and through-thickness pressure on the joint behaviour are described briefly for both single and multi-fastener joints. The analytical and numerical methods of stress analysis required before trying to predict failure are discussed. The numerical approaches cover both two and three-dimensional models and the effects of clearance, friction and geometry are assessed. The several methods that have been used to predict failure in single or multi-fastener joints are described. It is concluded that there are some issues that require further investigation. There is no general agreement about the method that should be used to predict failure, but progressive damage models are quite promising since important aspects of the joints behaviour can be modelled using this approach. In order to take into consideration several factors related to joint strength the use of three-dimensional models is suggested. These models require a three-dimensional failure criterion and an appropriate property degradation law.

A study of transcrystallinity and its effect on the interface in flax fibre reinforced composite materials

Abstract Cellulose fibres have long been used in the plastics industry as cost-cutting materials. Nowadays they are recognised as a potential replacement for glass fibres for use as reinforcing agents in composite materials. They have a number of certain advantages over glass fibres, such as low cost, high strength-to-weight ratio, biodegradability and ease of processing. In this study crystallisation from the melt of two different isotactic polypropylene matrices (iPP) in the presence of flax ( Linum usitatissinum ) fibres of four different types (green flax, dew retted flax, Duralin ® treated flax and stearic acid sized flax) was examined. The effect of processing parameters such as crystallisation temperature and cooling rate was investigated using hot stage optical microscopy. Differential scanning calorimetry (DSC) was used to investigate the inner morphology of the transcrystalline (TC) layer. Scanning electron microscopy (SEM) and X-ray diffraction were used in an attempt to identify the origin of the TC layer in connection with the structural characteristics of the fibres. The effect of transcrystallinity upon the mechanical properties of the interface was assessed using the single fibre fragmentation test. It was found that the interfacial adhesion is improved by the presence of a TC layer.

The Effect of Stacking Sequence on the Pin-Bearing Strength in Glass Fibre Reinforced Plastic

The pin-bearing strength of glass fibre reinforced plastic plates was measured using specimens possessing mid-plane symmetry and composed of eight layers, two each at 0° , 90° , and ±45° . The results suggest that placing the 90° layer (normal to the applied load) at or next to the surface increases the bearing strength. The ultimate failure mode was found to depend on stacking sequence.

A Finite Element Analysis of Single and Two-Hole Bolted Joints in Fibre Reinforced Plastic

The results are presented of a finite element analysis of bolted joints in fibre reinforced plastic. Although the finite element model uses only two- dimensional elements, through-thickness stresses being ignored, some cor respondence is demonstrated between the calculated strains and experimental data from tests on glass fibre reinforced epoxy resin.

Experimental and Numerical Studies of a Laminated Composite Single-Lap Adhesive Joint

The problem of a single-lap bonded joint with laminated polymeric composite adherents and with a spew fillet, subjected to tensile loading, is investigated. Experimental and numerical analyses of this problem are presented to address the mechanics and deformation of such material and bonding configuration. Strain gages are employed to record the geometrically nonlinear deformation of the specimen. Full-field moiré interferometr is used to measure the surface deformation of the adherends and adhesive (including a spew fillet). A geometrically nonlinear, two-dimensional finite element analysis is performed to check the mechanics assumptions by comparing with the experimental measurement. A good correlation between experimental and numerical solutions is obtained. It is observed that the composite single-lap joint deforms nonlinearly when subjected to tensile loading. The resulting displacement and strain fields for the adherend and adhesive layer are also presented. The moire results show that the transverse normal and shear strains, but not the longitudinal stain, for composite adherents on the free surface suffer from the fire edge effect, anticlastic as well as bending-twisting coupling effects. For the adhesive strains, the transverse normal (peel) component on the free surface is compressive throughout all the bond line except for a small region near the tip of the spew fillet, wile this component in the interior becomes tensile near the end of the overlap. The adhesive longitudinal strain in the spew fillet is insensitive to the three-dimensional defamation effect, but not the adhesive shear strain in the spew fillet. The adhesive longitudinal strain is not small enough to be neglected in the stress analysis.

Predicting the compressive engineering performance of carbon fibre-reinforced plastics

Abstract This paper examines the compressive strength data of a recent experimental study [Smith FC. The effect of constituents’ properties on the mechanical performance of fibre-reinforced plastics. PhD thesis. Centre for Composite Materials, Imperial College, April 1998] concerned with the evaluation of a range of engineering properties of continuous carbon fibre/epoxy composites subjected to static tensile and compressive loading. A plastic fibre kinking analysis [Budiansky B. Micromechanics. Comput Struct 1983;16(1):3–12] and a linear softening cohesive zone model (CZM) [Soutis C. Compressive failure of notched carbon fibre–epoxy panels. PhD thesis. Cambridge University Engineering Department, UK, 1989; Soutis C, Fleck NA, Smith PA. Failure prediction technique for compression loaded carbon fibre–epoxy laminates with an open hole. J Comp Mat 1991;25(5):1476–1498] are used for the prediction of the unnotched and open hole compressive strength (OHC) of unidirectional and multidirectional laminates made of six different commercially available CFRP prepregs. Damage introduced by drop-weight (low-velocity) impact is modelled as an equivalent open hole and the cohesive zone model [Soutis C. Compressive failure of notched carbon fibre–epoxy panels. PhD thesis. Cambridge University Engineering Department, UK, 1989; Soutis C, Fleck NA, Smith PA. Failure prediction technique for compression loaded carbon fibre–epoxy laminates with an open hole. J Comp Mat 1991;25(5):1476–1498] is applied to estimate compression-after-impact (CAI) strength. The unnotched strength is accurately predicted from the knowledge of initial fibre misalignment and the shear yield stress of the composite, while the difference between the theoretical and experimental OHC and CAI strength results in most cases is less than 10%.

Failure mechanisms in bolted CFRP

An experimental investigation on the damage mechanisms of mechanically fastened joints in composite laminates is presented. This information concerning damage mechanisms is required before developing damage progression models. For doublelap joints with fingertight washers, specimens that fail in the bearing, tension and shearout modes are investigated. For fully failed specimens and for specimens loaded to several percentages of failure load, the damage mechanisms are investigated using Xradiography and sectioning. It is concluded that failure occurs by a process of damage accumulation, where the failure mechanisms present are fiber fracture, delamination at the laminate loaded hole, matrix cracks and related fiber microbuckling, and internal delamination. Modelling this interaction between failure mechanisms requires further analytical efforts. Due to the importance of the through-thickness stresses present at the hole boundary, delamination has a significant effect on the joint strength, so the use of a threedimensional failure criterion is suggested.

Adhesively-bonded repairs to fibre-composite materials I. Experimental

The performance of carbon fibre/epoxy repair joints, bonded using an epoxy film adhesive, under static and fatigue loading has been investigated. The repair joints were immersed in distilled water at 50°C for periods of up to 16 months and the effect of the hot/wet environment on the static and fatigue strengths was evaluated. Residual strength tests, where repairs were subjected to fatigue followed by static loading, were also performed. The mechanical properties of the substrate and the adhesive forming the joint were determined. All tests were undertaken at room temperature. It was found that there was no major effect of the conditioning on the above properties and that the repair joints had a similar static strength to that of the parent material. In some cases, a video camera fitted with a macro-lens was used to record the repair during static loading; cracks of an average length of 2.6 mm were visible in the composite just before catastrophic failure took place. In contrast to the static properties, the fatigue behaviour of the repair joints was significantly inferior to that of the parent material. Finally, fatigue tests were also performed on relatively large repair carbon fibre/epoxy panels with centrally-placed repairs. The fatigue results obtained from the repair panels were in close agreement with the fatigue results obtained from the repair joints.

Adhesively-bonded repairs to fibre-composite materials II. Finite element modelling

Abstract Part I described the static performance (i.e. the performance under a monotonic rate of loading) of carbon-fibre reinforced-plastic (CFRP) composites which had been repaired by adhesively bonding and co-curing, a second section of CFRP prepreg to the original parent material. The mechanical behaviour of these repair joints, as well as of the adhesive and CFRP forming the joint, were determined both in the unaged condition and after ageing. The hot/wet ageing of the repair joints and materials was simulated by immersing the joints and materials in water at 50°C. In Part II, the mechanical properties of the adhesive and the CFRP have been used in conjunction with a finite element analysis (FEA) to determine failure criteria which would predict the experimentally observed failure paths and strength of the adhesively-bonded repair joints. Two material models were used for the adhesive: a linear elastic and linear elastic–plastic. Two models were also used for the composite. In the first model, the composite was assumed to be a homogeneous orthotropic material with smeared properties. In the second, it was modelled as a combination of individual plies of various orthotropic/anisotropic properties, depending upon the fibre orientation angle. Three possible types of failure for the repair joints were analysed in order to predict the expected failure paths and failure loads. The general agreement between the experimental observations, and predictions of the failure path and loads was found to be good.