Impact and post-impact properties of a carbon fibre non-crimp fabric and a twill weave composite

The impact and post-impact static and fatigue tensile properties of a carbon fibre/epoxy NCF composite were determined and compared to those of a carbon fibre/epoxy woven fabric composite, for two impact energies (3.5 and 7 J). The projected damage area after impact was larger for the NCF composite than that for the woven fabric composite for both impact energies. Impacted samples were subjected to static tensile tests and tensile–tensile fatigue tests. It was found that even a relatively low energy impact has already a significant negative influence on the residual properties in both static and fatigue tests, in the fibre direction as well as in the matrix dominated direction. In the matrix dominated directions the post-impact behaviour of the two materials is very similar. In the fibre direction, however, the properties of the non-crimp fabric composite are degraded more by an impact than those of the woven fabric composite.     

Relation Between Elastic Properties and Stress Intensity Factors for Composites with Rigid-Line Reinforcements

Stiffness and failure resistance are two properties of a composite material that are known to be difficult (if not impossible) to improve simultaneously. They are of a competing nature: while stiffness is an effective property of a homogenized microstructure, failure resistance is highly sensitive to local microstructural details. In the present work we establish a direct link between these two properties on the example of a composite material reinforced with rigid lines. More specifically, we relate rigid lines’ contribution into effective compliance to stress intensity factors at their tips. Such a link provides useful insights into the source of competition between stiffness and failure resistance and suggests a way to overcome this persistent trend.     

Compression behaviour of a fibre bundle with grafted carbon nanotubes

When carbon nanotubes (CNTs) are grown on carbon fibres, with the goal to increase toughness of a carbon fibre reinforced composite, the compressibility of the carbon fibre bundle or a fabric decreases significantly. The pressure, necessary to achieve the desired fibre volume fraction in a composite, should be increased by several bars. The paper proposes modelling approaches for calculation of the change of compression resistance of the CNT-grafted fibre bundle and fabric. The models use a previously developed algorithm for calculation of the compression resistance of a random assembly of CNTs. Two possible scenarios for the CNT-growth positioning are considered: a CNT assembly that homogeneously fills the free space between the fibres and a CNT assembly that is localised on the surface of the fibres. The model is validated against measurements of the compression resistance of carbon fibre bundles and fabrics with CNTs grown using the CVD method.     

Mechanical properties of epoxy composites reinforced with a low volume fraction of nanosilica fillers

In this paper we focus on the preparation and mechanical properties of the nanosilica-reinforced, epoxy resin Epikote 828LVEL. Epoxy composites containing two sizes of spherical silica nanoparticles, 130 nm and 30 nm, were prepared at a fixed volume fraction (V_P = 0.5%). To prevent agglomeration, the silica fillers were initially pre-treated with diglycidyl ether of bisphenol A (BADGE). Due to the low content of silica fillers, their inclusion in the matrix was confirmed by the increased roughness of a fracture surface compared to the smooth surface of the neat epoxy. Raman spectroscopy was employed to obtain additional information about the crack-propagation path. The mechanical properties, characterized by a three-point bending test, revealed a 10–20% increase in the composite's modulus of elasticity with 30-nm and 130-nm silica-filler inclusions. Elongation at break, on the other hand, decreased for 5–10% in both composites compared to neat epoxy, suggesting brittle fracture behavior in silica/epoxy composites. The fracture toughness results showed a 25–30% improved toughening for both composites compared to the pure epoxy. The composite's resistance to failure in terms of the impact energy was, however, strongly dependent on the size of the silica: we observed a 30% increase for the 130-nm, and a 60% increase for the 30-nm, silica/epoxy composites, compared to the pure epoxy.     

The effect of carbon nanotubes on the damage development in carbon fiber/epoxy composites

The aim of this study is to investigate the effect of carbon nanotubes (CNTs) on the initiation and development of damage in a woven carbon fiber/epoxy composite under quasi-static tensile loading. The composite is produced using resin transfer moulding and contains 0.25 wt.% of CNTs in the matrix. The results in the fiber direction report no improvement of the Young’s modulus and a slight improvement of the strength and strain-to-failure. The most important result of the study is a notion that CNTs have a hindering effect on the formation of transverse cracks. The conclusion is drawn from a combined analysis of the acoustic emission measurements (reporting a pronounced shift of all damage development thresholds towards higher strains by more than 30%) and X-ray/SEM observations (revealing a lower crack density in the CNT modified composite). The same analysis also indicates that the mechanism of energy dissipation through transverse microcracking is partially replaced by another mechanism that promotes (distributed) damage through fiber debonding.     

Local strain in a 5-harness satin weave composite under static tension: Part I - Experimental analysis

This paper presents an experimental method for determining the local strain distribution in the plies of a thermoplastic 5-harness satin weave composite under uni-axial static tensile load. In contrast to uni-directional composites, the yarn interlacing pattern in textile composites causes heterogeneous strain fields with large strain gradients around the yarn crimp regions. In addition, depending on the local constraints that are imposed by the surrounding plies, the deformation behavior of the laminate inner layers may vary from that of the surface layers, which are relatively more free to deform, compared to the inner layers. In order to validate the above hypothesis, the local strains on the composite surface were measured using digital image correlation technique (LIMESS). Internal strains in the composite laminate were measured using embedded fibre optic sensors (FOS).Based on the DIC results, the strain profiles at various locations on the composite surface were estimated. Using the FOS results, the maximum and minimum strain values in the laminate inner layers were evaluated. Comparison of the local strain values at different laminate positions provides an estimate of the influence of the adjacent layers on the local longitudinal strain behavior of a satin weave composite. Part II of this paper elucidates the local strain variation computed using the meso-FE simulations. In addition to the comparison of numerical and experimental strain profiles, Part II presents the maximum and minimum strain envelopes for the carbon-PolyPhenelyne Sulphide (PPS) thermoplastic 5-harness satin weave composite.     

Damage development in woven carbon fiber/epoxy composites modified with carbon nanotubes under tension in the bias direction

The study investigates the effect of carbon nanotubes (CNTs) on the damage development in a woven carbon fiber/epoxy composite under quasi-static tension in the bias direction. The composite is produced by the resin transfer molding and contains 0.25 wt.% of CNTs in the matrix. The tensile tests are carried out till different strain levels and are accompanied with acoustic emission (AE) registration. The nano-modified composite possesses a higher stiffness and strain-to-failure. It also exhibits a significantly increased AE activity, both in terms of the number of events and the energy level, but reveals a lower crack density. The combined analysis of the AE data and X-ray images indicates that in the nano-modified composite cracks progress through the material in smaller jumps than in the virgin composite. The crack faces in the composite with CNTs also display a fine web of secondary fractures, which is not detected in the virgin composite.     

Analysis of knitted fabric reinforced composites: Part II. Stiffness and strength

The influence of the knit structure on the stiffness and strength in tensile and in share loading of glass warp knitted fabric epoxy composites is studied. The average strength depends on the fibre content and on the linear density of the yarn. The anisotropy in tensile and shear properties is related to the orientation tensor components a₁₁₁₁ and a₁₁₂₂, respectively. By making use of these relationships, a knit structure can be evaluated with regard to the mechanical properties of its composite with only two measurements: (1) measurement of the achievable fibre content; and (2) measurement of the fibre orientations.     

Tensile behaviour of thermally bonded nonwoven structures: model description

Nonwovens are complex three-dimensional anisotropic structures and consisting of fibres orientated in certain directions, which are bonded by thermal, chemical, mechanical entanglement or a combination of these techniques. Thermally bonded are further classified in two categories, i.e. through-air and calendared nonwoven structures. In this study, a modified micromechanical model describing the tensile behaviour of thermally bonded nonwovens is proposed by incorporating the effect of fibre re-orientation during the deformation. The anisotropic behaviour of through-air bonded structures is demonstrated through theoretical stress–strain curves and the relationship between the fibre re-orientation and fabric strain is also analysed. Furthermore, the failure criterion of thermally bonded nonwovens is analysed using pull-out behaviour of fibres in the system. A parametric study revealing the dependencies of various structural and geometrical characteristics of fibres on pull-out behaviour of fibres in thermally bonded nonwovens is also discussed.     

Tensile behaviour of nonwoven structures: comparison with experimental results

Nonwoven structures have been recently explored for numerous novel applications ranging from composites to scaffolds. The tensile property of nonwovens is a pre-requisite and indeed, one of the main parameters to determine their performance for such applications. In the first part, a modified micromechanical model describing the tensile behaviour of thermally bonded nonwovens was proposed by incorporating the effect of fibre re-orientation during the deformation (Rawal et al., J Mater Sci 45:2274, 2010). In this study, an attempt has been made to compare the theoretical and experimental stress–strain curves of thermally bonded and spunbonded nonwoven structures. These theoretical findings have been obtained from the most popular analytical tensile models of nonwovens available in the literature in addition to our modified tensile model. Poisson’s ratio has also been determined experimentally in order to predict the stress–strain behaviour of nonwoven, and its relationship with longitudinal strain has clearly distinguished between the randomly and preferentially orientated types of structures. In thermally bonded nonwovens, the tensile strength in various test directions is computed through pull-out stress and a comparison is made with the experimental results.