Israel Herszberg

Effect of weaving damage on the tensile properties of three-dimensional woven composites

This research paper examines the damage mechanisms and reductions to the tensile properties of E-glass yarns during weaving of three-dimensional (3D) fabrics for polymer-based composites. The paper also assesses the influence of weaving damage to load-bearing glass yarns on the tensile properties of 3D orthogonal woven composites. It is found that damage occurs to yarns at most stages of the 3D weaving process due to abrasion and breakage caused when sliding against the loom machinery. The abrasion damage causes a large reduction (∼30%) to the tensile strength of the dry woven yarns, although the tensile stiffness remains unaffected. The damage and reduction to the tensile properties of the dry yarns at different weaving stages are described. Tensile studies performed on single yarn/resin composites and larger coupons of 3D orthogonal woven composites reveal that weaving damage is responsible for a significant reduction to the tensile strength.

Mechanical performance of 2-D braided carbon/epoxy composites

Tension, shear, and compression tests were conducted to evaluate the mechanical performance of two-dimensional braided carbon/epoxy composites. Several different braid architectures were considered and their performance compared with those of carbon/epoxy composites made from unidirectional fabrics and prepreg tape. From this comparison, it was found that braided composites have comparable tension and compression stiffness but considerably reduced tension and compression strength which was attributed to fibre damage and fibre tow waviness. Tension tests on fibre tows revealed a 20% reduction in tension strength resulting from fibre damage during the braiding process. Results from shear tests remain inconclusive due to the unsuitability of current test methods.

The shear properties of woven carbon fabric

This paper presents experimental and theoretical studies of the shearing properties of carbon plain weave fabrics and prepregs. The shearing characteristics of these materials are determined by the use of a picture frame shear rig which is loaded by a mechanical test machine. The shear force/angle curves are presented for the experiments conducted with the various test materials. A proposed shear model based on previous research which idealizes the fabric yarns as beam elements is presented. Using fabric geometric and material parameters, the model predicts the initial slip region of the fabric, as well as the more dominant elastic deformation range. Comparisons of the experimental and theoretical results were conducted to validate the model. A discussion of the findings from the analysis is also given, with particular focus relating to the accuracy, limitations and advantages offered by such a model. Results indicated that the slip model gives modestly accurate predictions, whilst the elastic modulus model showed very good correlation with experimental data.

Failure of transversely stitched RTM lap joints

Abstract An experimental evaluation has been undertaken to investigate the effect of transverse stitching on the strength of composite single-lap joints. Balanced single-lap joints were considered, and the lay-up for the adherends was (0/ ±45/90)s. Specimens were stitched with Kevlar® thread in a zigzag pattern and were manufactured by using the resin-transfer moulding (RTM) technique. Experimental results indicated that stitched joints were over 20% stronger than their unstitched counterparts. The scanning electron microscopy study of the fracture surfaces indicated that the failure is mainly dominated by peel and the stitch threads break inside the adherends and then pull out.

The effect of binder path on the tensile properties and failure of multilayer woven CFRP composites

An investigation has been carried out on the in-plane tensile properties of two orthogonally woven structures with different binder paths. Measurements were made along both the longitudinal (warp) and transverse (weft) axes on samples manufactured from two commercially available carbon fibres which are known to have different resistance to fibre damage induced through weaving. In general, this research revealed that the more damage-resistant fibre produces composites with higher strength and stiffness. It was also found that, independent of the fibres used, the in-plane fibre yarns in the structure with the longer binder path are less crimped and this, by and large, translates to better or unchanged tensile modulus, strength and strain-to-failure. In addition to fibre fractures, the superior strength of this composite was also observed to promote an extensive amount of longitudinal splitting, thus resulting in a relatively large failure zone. In the structure with the shorter path length where the in-plane fibres are more crimped, failure is due predominantly to fibre fractures. Groups of fibres are pulled out during failure of both structures, despite good fibre wetting and fibre/matrix adhesion having been achieved with either fibre.

Longitudinal and transverse damage taxonomy in woven composite components

This work addresses the issue of micro-structural damage in the longitudinal direction of the woven during a deformation process, primarily in the tensile mode. This paper extends the insight into self-inflicted damage in dimensional multilayer woven composites subjected to uniaxial tensile and shear loading. Two composites made of multilayer woven architectures, hereby named: orthogonal and normal layer interlock forms the basis of this study. It identifies the physical characteristics which initiate the various damage modes, what these modes are and when they occur. This work complements the work already reported on the transverse direction in woven composites.

An Investigation of the Structure-Property Relationship of Knitted Composites

The tensile and compressive behaviours of knitted composites in the wale and course directions were studied for their dependence upon knit architecture and varying knit/structural parameters, such as loop length and stitch density. Strength, modulus and strain-to-failure were investigated. In addition, Poisson’s ratio of the composites under tensile loading was also studied. Post-failure examination was carried out on the test specimens using stereo-optical and scanning electron microscopy to analyse their fracture mechanisms. It has been found that any change in the mechanical properties of the knitted composites with respect to architecture and knit/structural parameters are broadly related to accompanying modifications to the state of the microstructural imperfections, viz. fibre bending and fibre crossover junctions, and also to the relative fibre distribution along the two principal loading axes, in the knit structure.

Structural integrity analysis of embedded optical fibres in composite structures

Abstract The virtues of having sensors in manufactured goods for increased functionality purposes have been well documented. Benefits include sophisticated structures requiring less maintenance and repair, increased safety and reliability, and avoidance of ‘over design’. Though many schemes of sensing are available, these so-called ‘smart’ products in the near future, will increasingly rely on the optical fibres (OF) principles because of numerous inherent advantages. Optical fibres are small, lightweight, possess geometrical flexibility, Electromagnetic interference (EMI) immunity, operate over a wide range of environmental conditions, and can be configured to respond to many physical parameters. This paper will report on the suitability of embedding OF in commonly used carbon-fibre composites. These panels will be designed, manufactured and tested for the effects of typical fibre-optic geometrical and physical parameters such as types of fibre coating polymers, fibre diameter and fibre distribution. Corroboration of these test results with finite element (FE) results will be shown. Based on tensile and compression tests on OF-embedded composites, it is shown that significant deterioration on strength is observed beyond a certain OF density level. This paper will focus on the macroscopic effect of having optical fibres in composites from a structural integrity point of view. To this end, an exposition on the theoretical considerations using continuum mechanics and energy principles is provided.

Impact resistance and tolerance of interleaved tape laminates

This paper presents and discusses the results of low-velocity impact and compression-after-impact (CAI) tests conducted on interleaved and non-interleaved carbon/epoxy tape laminates. Olefin film interleaves provided a strong interface bond, resulting in a reduction in projected damage area. These interleaves changed the stress distribution under impact and restricted delamination formation at the ply interface. An investigation into the compression behaviour of these laminates revealed a reduction in undamaged strength using olefin interleaves. This was attributed to the lack of lateral support for fibres at the fibre/interleaf interface, allowing fibre microbuckling to occur at a low load. Low modulus copolyamide web interleaves resulted in an increase in damage area and minor changes to CAI strength. Examination of laminate cross-sections revealed that this was due to both the open structure of the interleaf and poor resin/interleaf adhesion. High shear modulus polyethylene interleaves resulted in a significant decrease in damage area at various impact energies, with CAI strength improved compared to the non-interleaved laminates.

Effects of Compaction on the Stiffness and Strength of Plain Weave Fabric RTM Composites

Changes in fiber architecture which result from the compaction of fabric plies can be a major cause for scatter and inconsistencies in measured mechanical properties. The need to quantify and provide insight into the influence of compaction on both the stiffness and strength of plain weave fabric composites, has led to the development of a model. The proposed model extends an existing modelling technique, a point-wise lamination approach using Classical Laminate Theory, to analyse the compaction problem. The analysis, which is valid for crimp angles less than 20°, is applied in both the warp and weft directions, with ply nesting being ignored at this point in time. In this paper, the formulation of the proposed model is presented. Numerical results generated from the model are compared with experimental data and other analytical methods, in order to validate the model. A discussion on the findings from the analysis are also given, with particular focus on the initial failure mode and stress/strain distributions predicted by the model for a longitudinal tensile load case at various levels of compaction. The model was found to easily model fabrics of varying fiber architecture, making it a useful tool in providing insight and quantification on the effects on stiffness and strength caused by fiber architecture changes resulting from compaction.