Influence of the laminate lay-up on the fatigue behaviour of SiC-fibre/BMAS-matrix composites

The influence of the laminate lay-up on the mechanical response and the damage development during fatigue loading of SiC fibre toughened BMAS glass-ceramic matrix composites was studied at room temperature. Uni-directional, cross-plied, angle-plied, and quasi-isotropic laminates were investigated. The different materials survived 10⁶ fatigue cycles for maximum fatigue stresses below a stress level dependent on the laminate lay-up. The thus defined fatigue limit decreased from 375 MPa for uni-directional to 70 MPa for angle-plied SiC/BMAS. Finally, the fatigue damage mechanisms were identified and the mechanical response related to the fatigue damage.     

Thermodynamic analysis of fatigue failure in a composite laminate

We put forward a thermodynamic approach for analyzing fatigue failure in a composite laminate. We show that fatigue is an irreversible progression of increasing entropy that accumulates until it reaches a critical value called the fracture fatigue entropy (FFE) at the onset of failure. Extensive series of fatigue tests are carried out that involve load-controlled tension-tension, and displacement controlled fully-reversed bending fatigue with three different stress ratios as well as constant- and variable-loading. The role of hysteresis energy in the entropy generation is investigated. FFE values are calculated based the experimental data obtained for temperature and hysteresis energy of a woven Glass/Epoxy (G10/FR4) laminate. The concept of tallying entropy accumulation and the use of FFE are useful for determining the fatigue life of composite laminates undergoing cyclic loading.     

A mechanically fastened composite laminate joint and progressive failure analysis

A comprehensive procedure for a mechanically fastened composite laminate joint (ASTM D5961 Proc. A, B) is demonstrated from fixture design to analysis of test results. The ASTM tests are applied to evaluate the standard laminate properties and the composite joints. Composite laminate mechanical joints were analyzed using the finite element method (FEM), and the results were compared to test results. A progressive failure analysis (PFA) was applied to the FEM to predict the overall failure behavior of the test specimens. Three laminate failure theories – maximum stress, maximum strain, and Tsai–Wu – were applied to the PFA to predict the test failure load, displacement and strength. The PFA method was suitable to predict the initial test range of test and maximum test load except for the excessive failure area.     

Prediction of static tensile failure of double-bolt joint for composite laminate

Based on the micro-mechanics of failure (MMF) theory, the accelerated testing methodology (ATM) and the VUMAT subroutine in ABAQUS, this study put forward a novel microscopic analysis approach for the large deformation failure analysis of composite. The predicted strength accuracy of the MMF approach for the bolted structure was then compared with those calculated based on the Tsai–Wu and Hashin theories, and with the experimental results. Results showed that the MMF method enjoyed the highest strength accuracy with an error of 7.2%, and the component damage initial occurred in matrix of the 90-degree layer and the initial failure of fiber occurred in the 0-degree layer. The failure mode predicted by the MMF approach behaved as shearing mode, which was in agreement with the experimental results. The long-term static strength of the bolted structure was predicted based on the MMF/ATM approach, and a comparison with the test results was conducted.

     

Effects of composite lay-up configuration and thickness on the damage self-sensing behavior of carbon fiber polymer-matrix composite

The lay-up configuration (unidirectional, crossply and quasi-isotropic) and thickness (8–24 laminae) affect the damage self-sensing characteristics of continuous carbon fiber epoxy-matrix composites. The damage is by drop impact directed at the top surface of the laminate. The oblique resistance (i.e., resistance at an angle between the longitudinal and through-thickness directions) is an effective damage indicator for all lay-up configurations and thicknesses. The surface resistance of the bottom surface is an effective damage indicator for thin (8-lamina) composites, though it is less sensitive to minor damage than the oblique resistance. The surface resistance of the top surface is less effective than that of the bottom surface for 8-lamina multidirectional composites. The through-thickness resistance is an effective damage indicator for 16- and 24-lamina quasi-isotropic composites, but is ineffective for 8-lamina composites of any lay-up configuration. In general, effectiveness means a monotonic and significant increase of the resistance with damage extent.     

On the influence of laminate stacking on buckling of composite cylindrical shells subjected to axial compression

The buckling loads of laminated cylinders can strongly depend on the position of the differently oriented layers within the shell. This paper deals with two different laminated orthotropic cylinders with opposite stacking sequence of the laminate layers. Cylinders of this construction had been thoroughly tested within a BRITE EURAM project. Analytical and semi-analytical methods have been used to predict the buckling loads, and the results are reported in this paper as well as test results for comparison. An explanation of the striking influence of stacking sequence is given. With some more examples the findings are verified. It is suggested that the presented results can be used for benchmarking purpose.     

The influence of microstructural anisotropy on the mode of plate failure during projectile impact

Changes in the failure mode of rolled steel plate during projectile impact were examined in terms of the angle between the planes of microstructural inhomogenieties in the steel and the plane of the target. Two failure modes were identified: (1) discing failure involving multiple shear band formation in the rolling plane with separation along the shear bands to form a disc segment, and (2) tensile star cracking with the propagation of a tensile tear through the rear surface of the plate. Both the ballistic resistance and the predominant mode of failure were critically dependent on the orientation of the microstructural banding in the plate.     

Effect of torsional strain-rate and lay-up sequences on the performance of hybrid composite shafts

In this study, the torsional behavior of hybrid composite shafts was examined by a combined experimental and numerical approach. Glass and carbon fiber reinforced hybrid shafts with three lay-up sequences were manufactured using filament winding technique. All three shafts had same amount of glass and carbon fiber. Angular velocities of 0.1°/min and 5°/min were used as torsion test speeds. The effect of torsional strain-rate and lay-up sequences on the response of hybrid shafts was studied. Torque–twisting angle changes were recorded. Test results revealed that changing angular velocities did not affect the torsional behavior of composite shafts significantly. However, three different lay-up sequences resulted in remarkably different torsional behavior.Torsional behavior of composite shafts was simulated using Finite Element (FE) software, Abaqus. The elastic orthotropic composite model was used for simulations. FE models were validated using experimental test results. Numerically and experimentally obtained results exhibited quite similar torsional behavior.     

Effects of material properties and crush conditions on the crush energy absorption of fiber composite rods

Crushing along the fiber axis of undirectional E-glass fiber composites rods was examined to determine the effects of fiber volume fraction, fiber diameter, matrix compressive yield strength, crush rate, fiber surface treatment, and crush plate geometry. The volume specific energy absorption was found to increase with fiber content, fiber diameter, matrix yield strength, and crush rate. The crush load stability was found to be independent of fiber content and fiber diameter but not of matrix yield strengt. The crush load became less stable as the yield strength increased. The crush behavior of specimens containing clean fibers was about the same as with sized fibers, but specimens with a release agent on the fiber surface crushed with less energy absorption that decreased even as the fiber content increased, but the crush load was more stable than with sized or clean fibers. The volume specific energy absorption was greater when the rod specimens were crushed against concave surfaces than against a flat plate. A relatively simple model was able to account for the dependence of the energy absorption on fiber volume fraction and matrix yield strength.     

复合材料夹芯梁屈曲破坏模式及极限承载

为研究复合材料夹芯梁在轴压作用下的屈曲、后屈曲特性及承载能力,进行了试验研究与有限元仿真。首先,开展了系列复合材料夹芯梁屈曲特性试验,研究了铺层比例、梁长度、表层厚度及芯层厚度等因素对其屈曲、后屈曲破坏模式及极限承载的影响;然后,基于非线性屈曲理论,采用三维内聚力界面单元模拟面芯脱粘,并引入初始预变形及材料损伤准则对复合材料夹芯梁在轴压下的屈曲特性及极限承载进行仿真研究。结果显示:界面脱粘是屈曲破坏的重要模式;仿真计算的极限承载与试验结果相比,误差控制在10%以内。所得结论表明该方法可有效预报复合材料夹芯梁的后屈曲路径、破坏模式及极限承载。      

Simulation and experimental studies of the repaired composite laminate

Firstly the compress experiment is undertaken to investigate the efficiency of repaired panels in this paper, and then modeling of the mechanical behavior of the repaired composite panel under compressive static load is conducted by using of the finite element method. The effect of geometric non‐linearity on the stress‐strain response is considered in the numeric analysis. Fatherly, the user material subroutine (UMAT) is integrated with the ABAQUS package with the geometric non‐linearity effect for studying the damage initiation and its progression in the composite structure, and quadrilateral, linear, thick shell elements (S8R) are adopted. Finally, the predicted strain distribution, damage evolution and strength of the laminate are compared with the test results.     

The influence of fibre surface properties on the mode of failure in carbon-fibre/epoxy composites

A series of mechanical tests have been performed on composites consisting of high-strength carbon fibres in an amine-cured epoxy resin. A comparison has been made between composites containing untreated, commercially treated (electrochemically), and plasma treated fibres. While both treatments improve interfacial adhesion, the manner in which the composite fails is totally different. In composites that contain electrochemically treated fibres failure is, in most cases, matrix dominated, whereas interfacial failure always occurs in samples made from plasma-treated fibres. This behaviour can be explained in terms of the nature of the fibre surface after each type of treatment.     

Off-axis transformation of the composite laminate stiffness properties

Most composite structures are orthotropic with respect to the major structural loading direction, i.e. the 0 ° fibres are along the principal bending axes of an aircraft wing (the spar line). The laminate stiffness properties are given with the laminate orthotropic axes aligned with the structural axes. If the orthotropic axes are not aligned to the structural axes, then the laminate stiffness properties are generated from classical laminated plate theory with individual ply angles rotated through the appropriae angle of transformation. In this discussion, the transformation of a laminate's stiffness from the on-axis position to an off-axis position is accomplished in one step. Two typical examples are shown to illustrate where and why such a transformation is used.     

Effect of heating on the spalling failure of glass-cloth-base laminate

Results are presented for experimental determination of the nominal spalling failure of glass-cloth-base laminate at normal and elevated temperatures. It is established that on heating glass-cloth-base laminate to a temperature of 150°C there is an increase, and with further heating to 250°C there is a decease, in its resistance to spalling failure compared with the similar value under loading conditions at normal temperature.Translated from Problemy Prochnosti, No. 10, pp. 63–64, October, 1990.     

Variable-stiffness composite panels: As-manufactured modeling and its influence on the failure behavior

The introduction of tow-steered laminates in structural applications demands reliable design methodologies to predict and simulate their mechanical response. The full details about the mechanisms that lead to damage and failure of these novel panels are not yet known. In this work, a numerical analysis tool has been developed for structural simulations in a three-dimensional domain. This paper presents a comparison between different configurations and modeling approaches of these panels. Finite element analyses are carried out to simulate the first-ply failure of tow-steered panels under tensile load. The simulations show how matrix cracking is affected by discontinuities in the fiber angle between adjacent courses and fiber-free areas resultant from manufacturing effects; whereas fiber tensile failure is directly influenced by the orientation angles of the fiber, whose distribution depends on the chosen course cutting method: on one side, or on both sides.     

The influence of layer and bond strengths on the ductility of an all beryllium laminate

A limited study has been made of the effect of layer and bond strengths on the longitudinal tensile failure mode of an all beryllium laminate comprising a ductile (～ 5% elongation), high purity substrate and an adhering, brittle (<1%), layer. It is proposed that the overall ductility is related to the extent of delamination at the interface which, in turn, is determined by the ratio (R) of bond strength to layer strength. When R is 0.23 extensive delamination along the gauge length is obtained resulting in maximum ductility, but at R=0.41 little separation occurs and the elongation is reduced almost to the minimum, namely that of the brittle layer. A simple calculation indicates that for the material used in this work, the influence on ductility of this mode of failure in the brittle condition may begin to be reduced when the layer is <1/3 of the total composite thickness, when multipoint delamination may be anticipated.     

复合材料层合板静态压痕与损伤行为

发展了层合复合材料面内及分层失效模型,模型考虑了横向压剪残余剪切变形和分层残余相对位移。提出一种块体单元包含若干铺层的模型,以减少复合材料逐层分析时的计算量。对一种碳纤/环氧层合复合材料进行了横向压痕试验,测量了不同压力下凹坑的深度,用超声C扫测得损伤面积。用发展的模型在ABAQUS平台上通过用户材料子程序UMAT模拟了上述试验,分析了试验曲线特征点对应的损伤机制,通过与试验结果的对比,验证了本文模型的合理性。     

The mechanical behavior of optical fiber sensor embedded within the composite laminate

Mechanical behavior, such as tensile and fatigue strength, of the optical fiber sensor embedded within the composite laminate was investigated. Tensile and fatigue tests were performed to evaluate the static and fatigue characteristics of optical fibers embedded within three types of laminated composite specimens, [0₆/OF/0₆]_T, [0₂/90₄/OF/90₄/0₂]_T and [0₃/90₃/OF/90₃/0₃]_T. The initiation of damage and fracture of the optical fiber were detected by observation of the intensity drop-off of laser signal transmitted through the optical fiber during test. Experimental results showed that the fatigue strength of optical fiber embedded within the cross-ply laminate is much lower than the fatigue strength of optical fiber within the unidirectional ply laminate. It was also found that the optical fiber embedded within unidirectional ply laminate fractured due to the fatigue damage accumulation of internal defects of optical fiber itself. However the optical fiber embedded within the cross-ply laminate fractured due to the growth of transverse matrix crack of host composite laminate.     

The response of a multi-directional composite laminate to through-thickness loading

The through-thickness mechanical response of a carbon fibre/epoxy laminated composite of lay-up [0/45/−45]_ns is measured at low rates of strain. Uniaxial tension and compression experiments are carried out on dogbone specimens cut from a thick laminate along different directions, and failure mechanisms are observed via optical and electron microscopy. The effect of direct and shear stresses at the ply interfaces on the onset of failure is measured, and a failure envelope is constructed. The compressive response of specimens of different shape is investigated. Composite beams of different volume and aspect ratios are tested to failure in three-point bending and these tests reveal a strong dependence of the apparent out-of-plane tensile strength of the composite on the beam volume; this effect is modelled by Weibull theory.     

Delaminations at the free edge of a composite laminate

The concept of a fracture process zone where damage takes place is used to analyse the delaminations at the free edges of angle ply laminates under uniaxial tension. The use of a fracture process zone removes the singularity in the interlaminar stresses and enables the initiation and growth of delaminations to be modelled for a perfect laminate without any assumed prior defects. Two different models for the stress displacement relationship in the fracture process zone are examined: a constant stress up to critical displacement and a linear relationship. Finite element analysis shows that there is little difference in the predictions obtained from the two models. An approximate analysis is presented for the constant stress stress-displacement model which is shown to agree with a finite element solution and experimental data. Hence it is argued that the approximate method using a constant stress model for the fracture process zone is sufficient for accurate prediction of delaminations.