N.K. Naik

On the machining of fiber reinforced plastic (FRP) composite laminates

Abstract With the increasing use of fiber reinforced plastic (FRP) composites outside the defense, space and aerospace industries, namely, civilian industries, machining of these materials is assuming a significant role. Unit cost rather than solely performance at any cost will be the consideration for the implementation of FRP composites to consumer industries. The current knowledge of machining FRP composites, unfortunately, is inadequate for its optimum utilization in many applications. This paper presents some observations made on the orthogonal machining of unidirectional carbon fiber reinforced plastic (UD-CFRP) laminates with different fiber orientations. Iosipescu shear test was adopted to evaluate the inplane shear strength of varied fiber angle test specimens. A model for predicting the cutting forces and the dependence of cutting direction on machinability requirements is presented.

Elastic Behavior of Woven Fabric Composites: II—Laminate Analysis

Two-dimensional models are presented for the elastic analysis of the plain weave fabric laminates. A methodology to generate the laminate configuration from the lamina configuration is given. The effect of lamina configuration and their relative posi tions with respect to each other on the in-plane elastic constants of the woven fabric lami nates is studied. A laminate configuration with optimum shifts gives higher elastic proper ties compared to the other configurations. A comparison between in-plane elastic moduli of the unidirectionally laminated symmetric cross-ply laminates and plain weave fabric laminates is made. The in-plane elastic moduli of plain weave fabric laminates predicted by the models are found to be in good agreement with the experimental results.

Damage in woven-fabric composites subjected to low-velocity impact

Abstract The behaviour of woven-fabric laminated composite plates has been studied under transverse central low-velocity point impact by using a modified Hertz law and a 3D transient finite-element analysis code. The in-plane failure behaviour of the composites has been evaluated by means of a failure function based on the Tsai-Hill quadratic failure criterion. The effect of fabric geometry on the impact behaviour of woven-fabric composites has been studied. For comparison, the impact behaviour of balanced, symmetric, crossply laminates made of unidirectional layers and unidirectional composites has been included. The studies have been carried out with plate dimensions of 150 mm×150 mm×6 mm for a supported boundary condition. For these studies, incident impact velocities of 3 and 1 m/s and an impactor mass of 50 gm have been used. It is observed that the in-plane failure function is lower for woven-fabric laminates than for crossply laminates, indicating that woven-fabric laminates are more resistant to impact damage.

Prediction of on-axes elastic properties of plain weave fabric composites

Abstract Two fabric composite models are presented for the on-axes elastic analysis of two-dimensional orthogonal plain weave fabric lamina. These are two-dimensional models taking into account the actual strand cross-section geometry, possible gap between two adjacent strands and undulation and continuity of strands along both warp and fill directions. The shape functions considered to define the geometry of the woven fabric lamina compare well with the photomicrographs of actual woven fabric lamina cross-sections. There is a good correlation between the predicted results and the experimental values. Certain modifications are suggested to the simple models available in the literature so that these models can also be used to predict the elastic properties of woven fabric laminae under specific conditions. Some design studies have been carried out for graphite/epoxy woven fabric laminae. Effects of woven fabric geometrical parameters on the elastic properties of the laminae have been investigated.

Parametric studies on pulsed Nd:YAG laser cutting of carbon fibre reinforced plastic composites

Abstract Carbon fibre reinforced plastic (CFRP) composites are found to be cut satisfactorily by a pulsed Nd:YAG laser at the optimum process parameter ranges. Predictive models have been developed based on important process parameters, viz. cutting speed, pulse energy, pulse duration, pulse repetition rate and gas pressure. The responses considered are the heat-affected zone (HAZ) and the taper of the cut surface. The optimisation of process parameters was done using response surface methodology (RSM). The thermal properties of the constituent material and the volume fraction of the fibres are the principal factors that control the cutting performance. The effect of the process parameters on the output responses is also discussed.

A zigzag model for laminated composite beams

Abstract In the present work, a zigzag model for symmetric laminated beam is developed. This model uses a sine term to represent the non-linear displacement field across the thickness as compared to a third order polynomial term in conventional theories. Transverse shear stress and strain are represented by a cosine term as compared to parabolic term. This model satisfies displacement and transverse shear stress continuity at the interface. Zero transverse shear stress boundary condition at the top and bottom of the beam are also satisfied. The numerical results indicates that the present model predicts very accurate results for displacement and stresses for symmetric cross-ply laminated beam, even for small length to thickness ratio. The results are also compared with a simplified theory of same class.

An analytical method for plain weave fabric composites

Abstract A two-dimensional closed-form analytical method has been developed for the thermoelastic analysis of two-dimensional orthogonal plain weave fabric laminae. Strand undulation and continuity in both the warp and fill directions, actual strand cross-section and weave geometry, strand fibre volume fraction and possible gap between the two adjacent strands have been considered in the analysis. Good correlation is observed between the predicted thermoelastic properties and experimental results.

Failure Behavior of Plain Weave Fabric Laminates under On-Axis Uniaxial Tensile Loading: I—Analytical Predictions:

A two-dimensional woven fabric composite strength model is presented for the prediction of failure strength of two-dimensional orthogonal plain weave fabric laminates under on-axis uniaxial static tensile loading. Different stages of failure such as warp strand transverse failure, fill strand shear/transverse failure, pure matrix block failure and the failure of matrix and fiber in the fill strand in longitudinal tension are considered. Material and geometrical nonlinearities have been considered for predicting the stress-strain behavior. The studies were carried out for three idealized laminate configurations. The possible shift of layers with respect to each other along x-, Y- and -directions were considered for the laminates. A comprehensive experimental program was carried out on different material systems. A very good correlation is observed between the predicted and the experimental results.

Investigations into the effect of geometry of a trepanning tool on thrust and torque during drilling of GFRP composites

Abstract This paper presents the results of an experimental investigation into the effect of the geometry of a trepanning tool on thrust and torque during the drilling of uni-directional glass fibre-reinforced plastic (UD-GFRP) laminates. It is well-known that the most effective way of achieving good quality holes while drilling fibre-reinforced plastics (FRPs) is by reducing the thrust and torque. Therefore, this investigation was aimed at exploring the possibility of reducing the thrust and torque by using the concept of trepanning. The design considerations and development methodology of the trepanning tool are discussed. The appropriate tool geometry has been determined by using statistically planned experiments and analysis. Orthogonal arrays with analysis of means as well as analysis of variance have been used to assess individual factor and interaction effects and their significance levels. The investigations have revealed that the performance of the trepanning tool is superior to that of conventional twist drills in terms of thrust, torque and hole quality. Low production cost and ease of regrinding are its major additional advantages due to its simple geometry.

Elastic Behavior of Woven Fabric Composites: III — Laminate Design

Two-dimensional woven fabric composite models were developed for the elastic analysis of WF composites in Part I of this paper. The effect of undulation of yarn and fabric thickness on the elastic properties of woven fabric lamina was studied. The study was extended to woven fabric laminates and the effect of laminate configuration on the elastic properties was analyzed in Part II. In the present paper, the effect of gap be tween two adjacent yams on the elastic properties of woven fabric lamina and the effect of laminate configuration for different number of layers on the elastic properties of woven fabric laminates for different material systems are studied. An optimum gap between adja cent yarns gives higher Youngs moduli, and it depends upon the fabric structure and material system. A basic building block is found which gives higher elastic properties than with any other configuration with the same number of layers. Such a basic building block can then be used for making thicker laminates. The comparison of woven fabric laminates with UD cross-ply laminates indicated that, in general, the material system with lower de gree of orthotropy is desirable for woven fabric composites.