Strength Of Composite Laminates Research Articles

Damage progression and strength prediction of open-hole CFRP laminates containing ply gaps

Although automated fiber placement manufacturing can improve productivity, gaps and overlaps of plies occur due to the low position accuracy of the robot arm. This study develops the ply-by-ply shell layer model to investigate the mechanical effects of a gap on the damage progression and strength of composite laminates with a hole. The gaps and a hole are modeled independently of the mesh using the extended finite element method (XFEM) to reduce the preprocessing and computational costs using a simple square mesh. The opening of the resin rich area of the gap is represented by applying the cohesive zone model to the crack surface. The open hole tensile analysis was performed on a laminate with gaps. The predicted strength of specimen with gaps agrees well with the experiment and is slightly higher than that of specimen without gaps because the gap opening reduces the stress concentration at the hole.

Parametric Study on Low-Velocity Impact (LVI) Damage and Compression after Impact (CAI) Strength of Composite Laminates

A full-scale model for predicting low-velocity impact (LVI) damage and compression after impact (CAI) strength was established based on a subroutine of the material constitutive relationship and the cohesive elements. The dynamic responses of the laminate under impact load and damage propagation under a compressive load were presented. The influences of impact energy and ply thickness on the impact damage and the CAI strength were predicted. The predicted results were compared with the experimental ones. It is shown that the predicted value of the CAI strength is in good agreement with the experimental result. As the impact energy reaches a certain value, the CAI strength no longer decreases with the increase in the impact energy. Decreasing the ply thickness can effectively improve the damage resistance and CAI strength.

Mechanical properties of Assam’s bamboo-epoxy composite laminates – An experimental investigation

The dominance of contemporary composite materials has reached heights as well worth weight-saving materials. The current policies for manufacturing cost-effective and sustainable composites lead toward utilizing natural materials in polymeric matrixes. Bamboo is one of the rapidly growing crops which is as stronger as timber and can be utilized in developing semi-replaceable industrial products. Lack of adequate standardization and codes restricts its direct utilization for commercial use. The current study aims to investigate the mechanical properties of composite laminates from high-performance bamboo strips collected from the North-Eastern (NE) region of India. In this study, the alkali-treated bamboo samples were infiltrated with epoxy resin using VARTM (vacuum-assisted resin transfer molding) process. The Fourier transform infrared spectroscopic (FTIR) and Field Emission Scanning Electron Microscopy (FESEM) analysis is carried out to have a better understanding of the chemical composition, and surface morphology. It has been confirmed from the obtained results of FTIR that lignin content is greatly reduced from the delignified samples. In addition, the FESEM images have depicted an outsize layer of epoxy along the fiber cross-section and some pores have been observed as unfilled. On the other hand, these composites are also subjected to tensile, compressive, flexural, and impact tests. The composites achieved a maximum tensile strength and stiffness of 259 MPa and 16.8 MPa which is 253 % and 384 % higher than neat epoxy. However, the strength of the alkali-treated samples is found to be decreased which may be due to the loose fiber bundles and holes formed during the chemical treatment. Improvements in dimensional stability against moisture are achieved for the composites with treated fibers which can be related to better fiber-matrix interlocking. The investigation of the mechanical strength of these highly-cellulosic fiber composites is conducted through various experiments and the results are validated with theoretical equations. It is found that the theoretical values are in good agreement with the experimental data with an error < 15 %. These composites demonstrate large-scale production of lightweight, environmentally friendly, and sustainable future materials with high strength and stiffness. • Laminates with three species of bamboo from the North-Eastern region are developed. • A significant increment of 253 % in strength of composite laminates is observed. • Both longitudinal and transverse material properties are evaluated. • Experimental results differ by < 15 % error from theoretical. • Simultaneously, an increase in bending and toughness is observed.

The effects of cutout size and fibre orientation to the failure behaviour of glass epoxy and boron epoxy composite laminates

The composite laminate was widely used in aerospace, marine and automobile structure. The application of glass epoxy composite widely uses because of the lightweight demand from aerospace industry. The implementation of boron epoxy on the helicopter blade structure is one of the examples the of boron epoxy composite material. This research is to study the effect of square cut-out to failure behaviour of Glass Epoxy and Boron Epoxy composite laminates with various fibre orientations. The research is being conducted in two stages, which is numerical validation of the software used and failure analysis. The method of determining the failure analysis is accomplish by using the finite element method (FEM) with Maximum stress theory as the failure criteria. As the results, a significant variance of strength character between without cut-out and cut-out. The results for all three types of cut-out are somehow different with no cut-out results. The gap in between no cut-out and A1 cut-out is around 96.8% of percentage difference at 0° fibre orientation but at 90° fibre orientation, the different is reduce to 38.3% of percentage difference. By varying the result, the main finding for the research is that material removal effect due to cutting has an impact on the strength of composite laminates. The cut-out size and the direction of fibre orientation also does affect the failure behaviour of the composite laminates research result. Results gaps for all three cut-out sizes on 0-degree also different by around 5% in between A1 and A2 and decrease to 25.32% in between A2 and A3. Consistency is found on the results at 90-degree with different by 38.3% and 20.45%. Although it is not much, all the failure curves for cut-out shows gradually decrease by the increase of degree in fiber orientation after 50°. As the conclusion, it is proven that the current study is important in understanding the failure behaviour of the composite plate in which can be useful in contributing the knowledge for further study.

Research on damping performance and strength of the composite laminate

In this paper, the influence of E-glass fiber volume fraction and laying angle on the damping and strength of composite laminates was comprehensively analyzed. By increasing the fiber laying angle and reducing the glass fiber volume fraction, the damping capacity of the composite laminate was increased, but the tensile strength of the laminate was reduced; By reducing the fiber laying angle and increasing the glass fiber volume fraction, the tensile strength of the composite laminate was increased, but the damping characteristics of the laminate was reduced. In addition, in the damping experiment of composite laminates, in order to avoid the influence of external damping sources, the vacuum non-contact damping test method was adopted in this paper, and the influence of air damping on the damping experiment results of composite laminates was comparatively analyzed. The results of comparative experiments showed that air damping had a very obvious influence on the damping of composite laminates, especially when the damping of composite laminates was small, the influence of air damping would be greater.

Coupled Analysis of Low-Velocity Impact Damage and Compression after Impact Strength of Composite Laminates

AbstractA coupled analysis of impact damage and compression-after-impact (CAI) strength of laminated composite plates was conducted using efficient analytical and semianalytical models with the goa...

Effect of Delamination in Carbon Fibre Reinforced Polymer during Abrasive Water Jet Machining

Delamination is a type of defect produced while machining composites or layered materials. Due to matrix crack, shear crack and bending crack delamination is caused. Delamination is usually a separation along a plane parallel to the surface as in the separation of a coating from a substrate or layers of coating from each other. The aim of this project is to explore the study of delamination in carbon Fibre/epoxy composites under abrasive water jet machining. An experimental investigation was held to study the effect of delamination due to abrasive water jet machining on carbon fibre reinforced polymer. Effect of various parameters such as transverse speed, standoff distance, abrasive mass flow rate and water pressure was analysed. Taguchi method was used for overall analysis of parameters. Effect of Kerf width in CFRP material and on fibre cut was analysed step by step. Further observation was done on scanning electron microscopes. It can affect the compression strength of composite laminate and it will slowly cause the composite to experience failure through the buckling. Here the composite of Carbon Fibre Reinforced Polymer is made using carbon fibers and epoxy resin. Further cutting of CFRP is done using Abrasive water jet machining and analysis of delamination at various phases of the material is done. Analysis is done at which parameters delamination is reduced to minimum.

A fast and efficient numerical prediction of compression after impact (CAI) strength of composite laminates and structures

Abstract In this paper, an equivalent damage model is proposed to quickly predict the compression after impact (CAI) strength of composite laminates and stiffened panels. Low-velocity impact (LVI) and CAI tests at various energy levels are carried out on laminates. Based on the measured impact damage sizes, the model simplifies the damage area into concentric circles with different stiffness and strength properties by adopting the soft inclusion method. Progressive failure analysis and virtual crack closure technique (VCCT) are used to simulate the intra-laminar and inter-laminar failure of impacted laminates under axial compression in commercial finite element software ABAQUS. The predicted failure modes and CAI strength of laminates are in good agreement with the experiment results from this paper and the literature at various impact energies, which proves the validity of the model. Whilst the numerical model achieves high computational efficiency, the effect of mass scaling on computational efficiency and accuracy is evaluated, which provides a reference for the parameter choice in engineering application. The model is further applied to predict the compressive failure load of the stiffened composite panel with skin bay impact damage as a practical application, showing this modelling approach is also suitable to composite structures.

An Estimating Method of Compressive Strength of Composite Laminates After Low-Velocity Impact

An Estimating Method of Compressive Strength of Composite Laminates After Low-Velocity Impact

Optimising ply orientation in structural laminated bamboo

Currently, only two forms of laminated bamboo are commercially available as structural materials: unidirectional beams and boards, and cross-laminated boards. As a natural quasi-unidirectional composite, the lamination of bamboo into plies with specific orientations would allow the design and manufacture of a family of multi-axial composite laminates with unique properties. In this study, we test the tensile mechanical properties of single- and two-ply laminated bamboo at various off-axis loading angles and laminate configurations. The data is then compared to micro-mechanical models for predicting modulus and strength of composite laminates. On the basis of our analyses, we believe there is significant scope to extend the current range of laminated bamboo products to include angle-ply laminates. Moreover, we demonstrate that composite laminate theory is applicable to this natural composite and may be used for design of products and structures.

Investigation on effect of geometric, material and load parameters on strength of composites with cutouts

Composite applications require presence of multiple holes for mechanical fasteners or cutouts in laminates. Unlike isotropic materials, composite materials experience change in stress values due to different parameters such as geometric, material and loading parameters. The present study is devoted to primarily determine whether geometric or material parameters have dominant influence on strength of composite laminates. Computational study using ABAQUS CAE software is employed for the analyses. Results reveal that geometric parameters have much significant influence on stress concentration factor and thereby the strength of composite laminates, when compared to material parameters. An elliptical cutout is seen to have comparatively more adverse effect on strength of laminate, when compared with other cutout shapes. Further, effect of load parameters - in-plane tension, compression and shear, is also studied. However, no significant effect was evidenced in stress concentration factor due to load parameters.

Investigation on effect of geometric, material and load parameters on strength of composites with cutouts

Composite applications require presence of multiple holes for mechanical fasteners or cutouts in laminates. Unlike isotropic materials, composite materials experience change in stress values due to different parameters such as geometric, material and loading parameters. The present study is devoted to primarily determine whether geometric or material parameters have dominant influence on strength of composite laminates. Computational study using ABAQUS CAE software is employed for the analyses. Results reveal that geometric parameters have much significant influence on stress concentration factor and thereby the strength of composite laminates, when compared to material parameters. An elliptical cutout is seen to have comparatively more adverse effect on strength of laminate, when compared with other cutout shapes. Further, effect of load parameters - in-plane tension, compression and shear, is also studied. However, no significant effect was evidenced in stress concentration factor due to load parameters.

Layup Configuration Effect on Notch Residual Strength in Composite Laminates.

The current trend shows an increasing demand for composites due to their high stiffness to weight ratio and the recent progress in manufacturing and cost reduction of composites. To combine high strength and stiffness in a cost-effective way, composites are often joined with steel or aluminum. However, joining of thermoset composite materials is challenging because circular holes are often used to join them with their metal counterparts. These design based circular holes induce high stress concentration around the hole. The purpose of this paper is to focus on layup configuration and its impact on notch stress distribution. To ensure high quality and uniformity, the holes were machined by a 5 kW continuous wave (cw) CO2 laser. The stress distribution was evaluated and compared by using finite element analysis and Lekhnitskii’s equations. For further understanding, the notch strength of the laminates was compared and strain distributions were analyzed using the digital image correlation technique.

Study on low-velocity impact and residual strength at high temperatures of composite laminates

In this paper, low-velocity impact characteristics and residual tensile/compressive strength of composite laminates at high temperatures are experimentally and analytically investigated. Low-velocity impact tests at room temperature were performed using a drop-weight apparatus, and residual strength tests at high temperatures were performed using a hydraulic MTS machine. The experimental results show that both residual tensile and compressive strength decrease monotonically with the increase of impact energy, while the variation trend of residual tensile/compressive strength of composite laminates keeps the same with longitudinal tensile/compressive strength with the increase of temperature. In addition, a new stress-based delamination failure criterion was established, in which the delamination is considered to be controlled by the difference between through-thickness stresses of adjacent layers. Once delamination occurs, only the elements below the interface are marked with delamination, whereas the material properties of the elements on both sides of the interface are reduced simultaneously. In this way, delamination can be defined more precisely without cohesive elements, and a considerable reduction in CPU time can be achieved. Combined with extended Hashin failure criteria, an integrated finite element model was established to simulate low-velocity impact damage and to predict residual tensile and compressive strength of composite laminates. The numerical results show good agreements with experimental data.

A method to determine composite material residual tensile strength in the fibre direction as a function of the matrix damage state after fatigue loading

Abstract The phenomenon of residual strength in composite laminates after fatigue loading has been studied for decades but is generally expressed as a function of the number of cycles applied to the specimens. Phenomenological laws deduced from these tests are therefore loading dependent and identifying a valid model for any loading condition of a given laminate requires long fatigue testing campaigns. To overcome this difficulty, a testing procedure is proposed to express the residual tensile strength of a unidirectional ply in the fibre direction, as a function of its matrix damage. The resulting behaviour is then independent of the imposed loading and can be determined by only a few fatigue tests. Experimental results obtained on glass-epoxy non-crimp fabric laminates are presented and analysed.

Delamination analysis of woven fabrication laminates using cohesive zone model

A new test method was proposed to evaluate the cohesive strength of composite laminates. Cohesive strength and the critical strain energy for Mode-II interlamiar fracture of E-glass/epoxy woven fabrication were determined from the single lap joint (SLJ) and end notch flexure (ENF) test, respectively. In order to verify their adequacy, a cohesive zone model simulation based on interface finite elements was performed. A closed form solution for determination of the penalty stiffness parameter was proposed. Modified form of Park-Paulino-Roesler traction-separation law was provided and conducted altogether with trapezoidal and bilinear mixed-mode damage models to simulate damage using Abaqus cohesive elements. It was observed that accurate damage prediction and numerical convergence were obtained using the proposed penalty stiffness. Comparison between three damage models reveals that good simulation of fracture process zone and delamination prediction were obtained using the modified PPR model as damage model. Cohesive zone length as a material property was determined. To ensure the sufficient dissipation of energy, it was recommended that at least 4 elements should span cohesive zone length.

Residual strength estimation of CFRP laminates subjected to impact at different velocities and temperatures

This paper demonstrates the results of an experimental study on cross ply carbon/epoxy composite laminates fabricated from high temperature hardener HT972 subjected to impact loading at different velocities and temperatures. The carbon fiber reinforced plastic (CFRP) samples were impacted at velocities 1.5 m/s and 2.5 m/s, each at a temperature level of 30°C, 60°C, 90°C, and 120°C. The impact response of the material towards various velocities and temperatures was determined using impact parameters like peak force, absorbed energy, maximum deflection, and rebound velocity. Result reveals that the velocity and temperature play a significant role in the impact response of the material. The variation in the trend of Flexural After Impact (FAI) strength of composite laminates at different velocities and temperatures was determined using FAI test and these results were further correlated with impact results. The dominating failure modes affecting the residual strength of the samples were found using acoustic emission (AE) monitoring. POLYM. COMPOS., 38:2182–2191, 2017. © 2015 Society of Plastics Engineers

A three-dimensional micro-mechanical model for predicting the layer longitudinal compression strength of composite laminates

A three-dimensional micro-mechanical model for predicting the compressive strength of composite laminates is presented in this paper. The effect of initial misalignment of fiber angle on the compressive strength of fibrous composites is investigated. The compressive strength of fibrous composites is estimated using the actual initial misalignment of fibers in the rotated plane. The initial misalignment angles in both directions are defined as a curve in the form of a cosine function. Equilibrium equations are then derived for a Representative Volume Element (RVE) along the axis of the fibers using total potential energy principle. Minimization of the total potential energy with respect to the amplitudes defined by the initial waviness function of the fibers yields a formula used to define the maximum compressive strength of fibrous composite materials. The formula obtained in this paper is used to study continuous fiber/polymer matrix composites. The results obtained by the model presented in this paper are in a good agreement with experimental data obtained from compression tests on [±60°/0°] specimens and theoretical results available in the literature.

The Effects of Hydrothermal Ageing on the Crushing Behavior of Glass/Epoxy Composite Pipes

The paper discusses the crushing behavior of glass fibre reinforced epoxy (GRE) pipes under hydrothermal ageing condition. This study determines the behavior of the GRE pipes when subjected to different ageing periods and temperatures. Hydrothermal ageing has been found to cause degradation between resin and fibre interface thus causing the reduction in the strength of composite laminates. The pipes were subjected to hydrothermal condition to simulate and precipitate ageing by immersing the pipe samples in water at 80°C for 250, 500, and 1000 hours. Compression tests were carried out using Universal Testing Machine (UTM) for virgin condition and aged samples in accordance with ASTM D695 standard. The maximum force at the initial failure region is observed for each of the conditioned pipes. The results show that the strength of the matrix systems was considerably degraded due to the plasticization of the matrix system.

The effects of curvature and internal pressure on the compression-after-impact strength of composite laminates

This work presents an experimental characterization of the curvature effects on the compression-after-impact strength of laminated composite shells. Curved panels impacted on the outer (convex) face and with normal pressure on the inner (concave) face with three different curvatures at three different impact energy levels were tested. A compression-after-impact testing setup was designed and implemented to evaluate the impact-induced damage tolerance of the composite shells. An analytical modeling methodology for compression-after-impact strength predictions based on the Mar-Lin and Whitney-Nuismer failure criteria is also proposed. The approach proposed herein consists of replacing the damaged area of the impacted coupon by an equivalent hole. The analytical compression-after-impact predictions obtained using the Mar-Lin and Whitney-Nuismer failure criteria were compared with experimental results. A good agreement between analytical predictions and experimental results was found. The experimental results also indicate that the compressive residual strength of the composite shells is significantly affected by the shell curvature and internal pressure effects.