Macroscale assessment of low-velocity impact on hybrid composite laminates

Composites are usually brittle materials and have low impact properties. Structural dimensions, stacking sequence, ply materials, ply thicknesses and ply angles are standard variables that influence composite‘s performance against impact loads. Stacking sequence in hybrid laminates affects the failure and impact resistance. Failure mechanisms at the low-velocity impact of a rigid object in hybrid laminates are complex, and the subsurface damage in a composite laminate cannot be detected directly. However, various simulation platforms make it easy to see the impact damage between the plies of laminate. This paper numerically investigated the effect of stack sequence and hybridization of two fiber types against low-velocity impact. The current study adopted four-layer composite laminates of carbon and glass fiber layers with a stacking plan [C/C/C/C], [C/G/C/G] and [G/C/G/C], having lay-up angles as [0°/45°/−45°/90°]. Keeping the impactor mass and the incident velocity constant, the laminates were subjected to low-velocity impact. The damage contours for a failure mode were recorded and compared at the ply level. The numerical study resulted in impact imitations showing comparisons of the damage contours using Hashin failure criteria. Hybrid laminates display better performance in absorbing impact energies; however, hybrid laminates experienced more subsurface damage due to more impact energy absorption.     

Fatigue life prediction of carbon fibre reinforced laminates by using cycle-dependent classical laminate theory

In this work a study about the adaption of the classical laminate theory for fatigue loads is presented. Cycle dependent stiffnesses of single UD 0°, UD 45° and UD 90° plies are implemented in order to calculate the fatigue-induced stiffness decrease of a multidirectional lay-up with the stacking sequence [0°/+45°/−45°/90°/90°/−45°/+45°/0°]. As second input alternative, UD 0°, UD 90° and ±45° plies are used. The calculated cycle-dependent stiffness parameters are compared to experimentally measured fatigue data of the multidirectional lay-up. The experimental test procedure used for the measurement of cycle-dependent stiffness parameters has been published previously. Results show that the experimentally measured stiffness decreases of the multidirectional lay-up can be estimated accurately based on the cyclic unidirectional input parameters.     

Modelling low-speed drop-weight impact on composite laminates

The impact responses of typical laminates are investigated numerically in this research. Delamination responses among plies and fibre and/or matrix damage responses within plies are simulated to understand the behaviours of laminates under different impaction conditions. Damage resistance of a laminate is highly dependent upon several factors including geometry, thickness, stiffness, mass, and impact energies (impact velocities), which are here considered by the finite element (FE) method. Three groups of composite laminates are simulated and the numerical results in general are in good agreement with corresponding experiments. Models containing different stacking sequences and impact energies are built to study their influence on impact responses and demonstrate that clustered (or nearly clustered) plies in the laminate can effectively reduce the degree of interface damage. Models containing different indenters and plate shapes are also built to systematically study their influence on the low-speed drop-weight behaviour of composite laminates. Suggestions are proposed for designing impact tests for particular purposes.     

The effect of stacking sequence on impact damage in a carbon fibre/epoxy composite

The effect of stacking sequence on impact damage in a carbon fibre/toughened epoxy composite was studied. The major form of damage was delamination, which initiated at almost every interface through the panel. During the impact event the force-time response was monitored and the energy absorbed analysed in terms of an initiation and a propagation energy. The energy absorbed in delamination initiation was influenced by the stacking sequence, being increased by placing 45° fibres in the surface plies and by increasing the number of dissimilar interfaces. The residual energy absorbed in delamination propagation was found to increase linearly with increasing total delamination area. The compression-after-impact strength was related to the maximum delamination area.     

The effect of edge stresses on the failure of (0°, 45°, 90°) CFRP laminates

Four (0°, 45°, 90°) CF RP laminates with different stacking sequences of plies were tested in tension, and the development of damage and failure processes was monitored by visual observation and acoustic emission. All four developed cracks across the 90° layers before final failure, the thicker 90° layers cracking at lower applied loads. Cracks also formed parallel to the plies at the edges of three of the four laminates and grew, with increasing load, towards the middle. A theoretical model was developed to calculate the normal stresses perpendicular to the plane of a laminate at its edges. Effects were included for the residual stresses caused by cooling from the moulding temperature and by moisture absorption by the epoxy resin. The theoretical predictions agreed well with the observed differences in longitudinal edge cracking and delamination tendency of the four laminates.     

Post-impact compressive strength of curved GFRP laminates

Low velocity impacts to fibre reinforced plastic composites cause a pattern of damage consisting in general of delamination, fibre breakage and matrix cracking. Such damage is accidental and may go unnoticed; therefore composite structures must be designed assuming impact damage exists. Previous work on flat composite laminates has resulted in a reasonable understanding of the mechanisms of compressive strength reduction. There are, however, many instances where curved laminates are used in structures where impact is likely. Furthermore, due to the mechanisms of strength reduction, it may be expected that curvature would have a significant effect on the behaviour of the laminates.The work described here consists of experimental measurement of the post-impact compressive strength of curved GFRP laminates. The laminates were of 8 plies of 0.3 mm thick pre-impregnated glass fibre/epoxy tape in a (0, ±45, 0°)_s lay-up. Each laminate was 200 mm in length by 50 mm wide with the plane of curvature normal to the length. Laminates were impacted on the convex surface of the laminate by dropping a steel mass from 1 m vertically above it.Impacted laminates were loaded in compression and the out-of-plane displacements of the top and bottom surfaces were recorded. Final failure was typically due to fibre breakage occurring through the centre of the impacted area of the laminate. Possible differences in the impact response, and measurable differences in the sizes of the impact damage area, were found to arise from these curvatures, and differences were observed in their post-impact buckling behaviour. However, perhaps unexpectedly, the post-impact compressive strength for a curved laminate was found to be similar to that for a flat laminate. The failure loads for the impact damage laminates are shown to be comparable with those for laminates containing artificial delaminations.     

The mechanical properties of non-crimped fabric-based composites

The mechanical properties of composites prepared from two types of non-crimped fabric (NCF), namely biaxial, ±45° and quadriaxial with a 0°, ±45°, 90°, −45° ply sequence, are examined as a function of fabric weight and compared with those of alternative composite forms. In general, the properties of NCF laminates decrease slightly as the areal weight of the fabric increases. Laminates produced from biaxial fabrics exhibit superior properties for a given volume fraction of reinforcement than do laminates produced using woven rovings or continuous fibre prepregs, while quadriaxial NCF laminates have equivalent properties to woven roving laminates at certain orientations but, unlike woven roving laminates, retain their properties when rotated through 45°. Biaxial NCFS loaded at 45° to the fibre and quadriaxial fabrics produce composites with superior properties to those predicted using finite element and laminate analysis for idealized laminates based on the same materials.     

Damage Assessment of CFRP [90/±45/0] Composite Laminates over Fatigue Cycles

The present paper develops a stiffness-based model to characterize the progressive fatigue damage in quasi-isotropic carbon fiber reinforced polymer (CFRP) [90/±45/0] composite laminates with various stacking sequences. The damage model is constructed based on (i) cracking mechanism and damage progress in matrix (Region I), matrix-fiber interface (Region II) and fiber (Region III) and (ii) corresponding stiffness reduction of unidirectional plies of 90°, 0° and angle-ply laminates of ±45° as the number of cycles progresses. The proposed model accumulates damages of constituent plies constructing [90/±45/0] laminates by means of weighting factor η ₉₀, η ₀ and η ₄₅. These weighting factors were defined based on the damage progress over fatigue cycles within the plies 90°, 0° and ±45° of the composite laminates. Damage model has been verified using CFRP [90/±45/0] laminates samples made of graphite/epoxy 3501-6/AS4. Experimental fatigue damage data of [90/±45/0] composite laminates have fell between the predicted damage curves of 0°, 90° plies and ±45°, 0/±45° laminates over life cycles at various stress levels. Predicted damage results for CFRP [90/±45/0] laminates showed good agreement with experimental data. Effect of stacking sequence on the model of stiffness reduction has been assessed and it showed that proposed fatigue damage model successfully recognizes the changes in mechanism of fatigue damage development in quasi-isotropic composite laminates.     

Effect of thickness on the compressive performance of ballistically impacted carbon fibre reinforced plastic (CFRP) laminates

The response of structural elements under impact conditions is a particularly important consideration in the design of components made from composite materials. The understanding of this response includes both the impact behaviour and the influence of some design parameters and material properties. Thus, the dependence of the residual compressive strength of ballistically impacted carbon fibre reinforced plastic (CFRP) laminates on their thickness has been examined. A previously verified model developed by the authors, has been applied resulting in rather interesting findings about the effect of the thickness on the sensitivity of a laminate to impact. The model takes into account the number of plies, the impact energy and the stacking sequence. Experimental results derived from the literature have been used for the verification of the model and a close agreement between theoretical predictions and experimental results was found. Also, it can be concluded that the present work helps to optimize laminate impact behaviour by varying the laminate thickness. This revised version was published online in November 2006 with corrections to the Cover Date.     

Numerical analysis of bending and transverse shear properties of plain-weave fabric composite laminates considering intralaminar inhomogeneity

In this study, the bending and transverse shear properties of plain-weave fabric composite laminates are investigated by considering the intralaminar inhomogeneity through finite element analysis. Using homogenization procedures, the effective Reissner–Mindlin plate bending and transverse shear properties of composite laminates are calculated. Assuming the number of plies to be infinite, the effective three-dimensional (3D) continuum properties (elastic properties of the equivalent 3D bulk material) of the composite are also calculated. Then, the effect of the number of plies on the bending stiffness and transverse shear stiffness of the laminate, in other words, the effect of the intralaminar inhomogeneity on the laminate stiffness, is investigated. Through the numerical investigation, it is found that if the number of plies of the laminate is very small, the bending stiffness and transverse shear stiffness of the laminate are significantly lower than those evaluated using effective 3D continuum properties.     

Stacking layer sequence effects for glass fibre/epoxy resin cross-ply laminates

We describe the effect of the stacking sequence on the distribution of the deformation and stress fields through the thickness of the laminate. The laminates are made of epoxy resin and glass fibres, having the ratio of 0 to 90° plies equal to 1. This stacking sequence is applied to obtain composite laminates with different thicknesses. The study presented in this paper is focused on the linear static behavior of the cross-ply laminates by means of finite-element models developed using the commercial codes ANSYS 6.1 and LUSAS 15.3. The numerical models represent three-point bending loading cases. A viscoelastic analysis of the laminates based on the correspondence principle is also performed. The results from the finite-element models show good agreement with those obtained using the classical laminate theory, while the viscoelastic storage modulus and loss factors’ distribution indicate the optimal stacking sequences for structural dynamics applications, such as spring leaves for novel car suspension systems. __________ Translated from Problemy Prochnosti, No. 3, pp. 134–146, May–June, 2007.     

Impact and subsequent fatigue damage growth in carbon fibre laminates

The influence of the ply stacking sequence on the impact resistance and subsequent O-tension fatigue performance of carbon fibre laminates has been investigated. Drop-weight impact tests were conducted on a range of 16 ply carbon fibre laminates with either all non-woven plies or mixtures of woven and non-woven plies. Damaged coupons were tested in O-tension fatigue for up to 10⁶ cycles, scanned using an ultrasonic probe and then loaded in tension until failure.The impact resistance and subsequent fatigue performance have been found to be sensitive to the ply stacking sequence. The non-woven composites showed a marked sensitivity to impact loading, but increases in residual static strength were noted after cycling. The inclusion of a woven fabric served to improve the impact resistance of the laminates. Fatigue cycling resulted in considerably improved residual static strengths; by 10⁶ cycles any effect of the impact damage had been removed.     

碳纤维复合材料层合板低速冲击损伤应力分析

目的 为掌握碳纤维复合材料板在低速冲击载荷作用下的损伤规律,延缓失效破坏,对其冲击损伤的应力状态进行研究。方法 基于ABAQUS平台,建立碳纤维复合材料层合板低速冲击有限元模型,采用Hashin失效准则和VUMAT用户子程序,对碳纤维复合材料层合板的冲击过程进行数值模拟,同时考虑层合板层内与层间失效,以此来研究低速冲击条件下复合材料的损伤机理,分析冲击损伤过程中的应力变化趋势,讨论应力的分布状态。重点研究铺层角度及铺层距离冲头远近对应力的影响。结果 不同角度铺层的应力传播轨迹均沿着纤维方向和垂直于纤维方向同时扩展,应力均先增加至极限值而后迅速下降;铺层角度越大,板料的承载能力越弱,0°铺层的极限应力为1 432 MPa,而90°铺层的极限应力降至1 206 MPa;离冲头越远的铺层应力越小,达到峰值的时间更早且率先下降,说明远离冲头的铺层更早发生失效。结论 揭示了碳纤维层合板在低速冲击载荷作用下的应力状态及其对损伤的影响规律,能够为复合材料层合板零件设计提供参考。     

铺层混杂对复合材料层压板侵彻性能的影响

本文利用MTS和冲击侵彻测试装置，研究了由芳香族聚酰胺纤维、高强聚乙烯醇纤维制成的织物通过不同铺层方式与酚醛/PVB树脂复合的层压板的准静态和冲击侵彻性能。结果表明，芳香族聚酰胺织物层的加入能显著提高高强维纶织物树脂复合材料层压板的准静态侵彻刚度。随着芳纶混杂体积分数的提高，铺层混杂复合材料层压板的准静态侵彻阻力、穿孔能量（或单位面密度穿孔能量）将随之增加。从防护装具性能/重量比和性能/价格比的角度考虑，在芳香族聚酰胺与高强聚乙烯醇织物铺层混杂复合材料层压板中，高强聚乙烯醇纤维混杂体积分数可以确定为20%左右。     

Querrißbildung in 0/90/0 CFK-Laminaten

Cross-ply Cracking in 0/90/0 CFRP Laminates Cross-ply cracking in carbon fibre reinforced plastics (CFRP) with thermoset as well as thermoplastic matrix systems was investigated on 0/90/0 laminates with varying 90°-ply thickness. Since in an angle ply laminate the 90°-ply fails first, the higher strength of the other plies cannot be taken advantage of. For this reason efforts have to be done to increase the transverse strength (strain) of fibre reinforced plastics. In the first place thus it is necessary to investigate the influence of the different parameters which contribute to the transverse strength. In this work the influence of matrix (fracture strain), fibre/matrix interface, voids and constraining effect of neighbouring plies is investigated. With the aid of two-parameter Weibull distributions of the 90°-ply fracture strain, which describe the phenomenon of multiple cracking in a specimen, it was found that the constraining effect due to neighbouring plies, improved fibre/matrix interface and matrix ductilty increase, whereas voids decrease the transverse fracture strain.     

Optimization of laminated composites subject to uncertain buckling loads

Optimal design of composite laminates under buckling load uncertainty is presented. The laminates are subjected to biaxial compressive loads and the buckling load is maximized under worst case in-plane loading which is computed using an anti-optimization approach. The magnitudes of the in-plane loads are not known a priori resulting in load uncertainty subject to the only constraint that the loads belong to a given uncertainty domain. Results are given for continuous and discrete fibre orientations which constitute the optimization problem coupled to load anti-optimization problem leading to a nested solution method. It is observed that the stacking sequence of a laminate designed for a deterministic load case only differs considerably from that of a robust laminate designed taking load uncertainties into account. Consequently the buckling load carried by a deterministic design is considerably less than the one carried by a robust design when both are subjected to uncertain loads.     

Damage evolution of angle-ply SCS-6/Ti composites under static and fatigue loading

The effect of fibre orientation and laminate stacking sequence on the tensile and fatigue behaviour of SCS-6/Ti 15-3 composites were investigated. The laminates used in this study were: (90)₆, (0/ ± 45)_s, (0/90)_s, and (90/ +-45)_s. The initiation and progression of microstructural damage at various stress levels was thoroughly characterized. It was found that fatigue life at high applied stresses were controlled by fibre fracture; progressive damage involving fibre fracture, interfacial debonding and matrix cracking became dominant at low applied stresses. Observation of the damage mechanisms in the angle-ply laminates under cyclic loading suggests that increasing the fibre-matrix bonding strength may improve the load carrying capability and fatigue life of laminates containing off-axis plies.     

Tensile Response of Hoop Reinforced Multiaxially Braided Thin Wall Composite Tubes

This paper presents the tensile response of thin-walled composite tubes with multi-axial fibre architecture. A hybrid braid-wound layup has the potential to optimise the composite tube properties, however, stacking sequence plays a role in the failure mechanism. A braid-winding method has been used to produce stacked overwound braid layup [(±45°/0°)₅/90°₄]_T. Influence of stacking sequence on premature failure of hoop layers has been reported. Under tensile loading, a cross-ply composite tube with the alternate stacking of hoop and axial fibre show hoop plies splitting similar to the overwound braided composite tube. However, splitting has been restricted by the surrounding axial plies and contained between the adjacent axial fibre tows. This observation suggests hoop layers sandwiched between braid layers will improve structural integrity. A near net shape architecture with three fibre orientation in a triaxial braid will provide additional support to prevent extensive damage for plies loaded in off-axis. Several notable observations for relatively open braid structures such as tow scissoring, high Poisson’s ratio and influence of axial tow crimp on the strain to failure have been reported. Digital Image Correlation (DIC) in conjunction with surface strain gauging has been employed to capture the strain pattern.     

Development of novel cost effective hybrid ply carbon-fibre composites

A carbon-fibre/epoxy hybrid material has been developed which comprises ultra-high-performance fibres in the principal load-bearing direction and standard carbon fibres in the secondary orientations. The effect of stacking sequence on mechanical properties, including impact damage tolerance, has been studied and the results compared with non-hybrid data. By optimising the stacking sequence the hybrid material exhibited a mechanical performance similar to that of the ultra-high-performance material and a performance superior to the standard material. The energy absorbed during low velocity impact was analysed in terms of an initiation and propagation energy. The energy absorbed through delamination initiation was increased by placing 45 ° fibres in the surface plies and by placing ultra-high-performance fibres in the 0 ° plies. The energy absorbed during delamination growth was independent of the fibre type and determined solely by the matrix material. On the basis of current pre-preg prices the hybrid material corresponds to a significant cost saving of 12% through the use of lower cost standard fibres in the secondary stressed layers.     

Some aspects of crack growth and failure in fibre reinforced composites

A theoretical analysis, previously developed to deal with the machanics of matrix cracking in unidirectional composites and with transverse ply cracking in cross ply laminates, has been developed further to deal with the tensile failure of unidirectional fibrous composites in with the fibres have a known distribution of strengths. It is proposed that, under the application of a tensile load, stable transverse cracks are formed which originate from regions of initial damage and which become unstable at some critical strain value. The model takes account of various parameters including the interfacial fibre/matrix debonding energy, the residual frictional shear strength of the debonded interface and the elastic properties of fibres and matrix. Comparisons are made between the predictions of the model and the observed failing strains of the 0° plies in carbon fibre polymer matrix laminates. The relevance of the model to the study of delayed fracture in fibrous composites is discussed. The modification of this model, previously developed to describe crack growth in the transverse plies of 0°/90° laminates, is used to predict the initial cracking strains for a wide range of CFRP laminate geometries and initial crack sizes. Some aspects of the mechanics of crack extension across interply interfaces are discussed.