Experimental determination of residual stresses in composite laminates [02/θ2]s

The present work aims to determine the residual stresses in carbon fiber/epoxy composite laminates, by means of the incremental hole-drilling method. Based on mechanical theories of composite laminates and an elastic plate with a circular hole, the relationship between the relaxed strains on the surface of laminates and the residual stresses in laminates was established. This newly deduced theoretical formula was adapted into the incremental hole-drilling method and allowed us to further study the residual stresses in the through-thickness direction for various composite laminates. Related numerical modeling of composite laminate with a hole was built to calibrate the coefficients within the formula. Experiments were conducted and the residual stresses in composite laminates [0₂/θ₂]_s are presented. The proposed approach was validated with the consistence between our results for cross-ply laminates and those in literature.     

Fatigue behaviour of carbon fibre-reinforced aluminium laminates

A new hybrid composite (CARALL), consisting of thin layers of carbon fibre/ epoxy prepreg sandwiched between aluminium sheets, has been developed. It is shown that this class of materials offers higher modulus, higher tensile strength and lower density than 2024-T3 alloy in the longitudinal direction. Under tension-tension fatigue loading, the hybrid laminates showed superior fatigue crack propagation resistance in the longitudinal direction, which may be attributed to the bridging effect imposed by the intact fibres in the crack wake. It has also been shown that the effectiveness of fatigue crack growth reduction increases with the thickness of the carbon fibre/epoxy layer. The resistance to fatigue crack propagation can be further improved by introducing compressive residual stresses in the aluminium layer by postcure stretching the laminate in the plastic region of the aluminium alloy.     

无限大多层层合板隔声量分析

多层层合板结构被广泛应用于隔声结构设计中.为探究多层层合板的隔声性能,在无限大单层薄板隔声量计算公式的基础上,通过引入由经典层合板理论所得出的弯曲刚度,提出一种用于计算无限大多层层合板结构隔声量的理论方法——等效薄板法,并分别计算三层、四层和五层层合板的隔声量,通过与统计能量法仿真结果进行比较,表明等效薄板法在质量控制...     

CALL混杂复合材料的弯曲试验研究

本文用高灵敏度云纹干涉法对CALL混杂复合材料在纤维方向和垂直于纤维方向的弯曲及破坏特性进行了实验研究,得到了弯曲试件横截面上的剪应变分布规律及破坏形式。实验结果表明:碳纤维/环氧树脂层的剪应变明显大于铝层的剪应变,但各自沿截面呈抛物线分布。纤维方向弯曲试件的破坏形式是分层或碳纤维/环氧树脂层剪切破坏;垂直于纤维方向弯曲试件的破坏由受拉面碳纤维/环氧树脂层的拉伸破坏所致。本文工作为进一步深入研究CALL材料的力学性能提供了重要的实验依据。     

Ti/CFRP超混杂复合材料残余应力研究

由于组成Ｔｉ／ＣＦＲＰ超混杂复合材料层板的炭纤维、钛合金薄板及树脂的热膨胀系数的差异,以及树脂固化过程的收缩,在层间有残余应力形成,残余应力的存在会对材料的力学性能及加工性能产生影响。因此采用应变片包埋法和非对称层板法对该Ｔｉ／ＣＦＲＰ的残余应力进行了研究,推导出计算层板残余应力的计算公式,经修正后的计算结果与测试结果基本吻合。     

Measurement of residual stresses in CFRP laminates by X-ray diffraction method

The fabrication of CFRP laminates from prepregs involves curing at elevated temperatures. Residual stresses are set up due to the difference in thermal expansion coefficient between the matrix and the fibre. In this investigation, the X-ray diffraction method is used to measure the curing stresses in CFRP laminates by incorporating a very fine layer of aluminium particles during the lay up of the laminate. A calibration procedure is followed to correlate the strain in the crystalline particles, as measured by X-rays, with the composite strain and stress. Curing stresses measured by this technique are quite close to the value calculated from the differential coefficient of thermal expansion.     

Elasto-plastic stress analysis and burst strength evaluation of Al-carbon fiber/epoxy composite cylindrical laminates

This paper concentrates on the elastic–plastic stress analysis and damage evolution of the Al-carbon fiber/epoxy composite cylindrical laminates under internal pressure and thermal residual stress. Firstly, the elastic stress analysis of the composite laminates is performed by using the classical laminate theory. Secondly, the elasto-plastic stress analysis of the liner layer is further conducted by employing the power hardening theory and the Hencky equation in the plastic theory. Finally, an universal solution algorithm based on the last-ply failure criterion is proposed to explore the damage evolution and the burst strength of the composite laminates. Effects of the winding angle and number of the composite layers as well as the thermal residual stress are addressed. The calculated burst strengths are also compared with the experimental results.     

Relationship between transverse residual stress and viscoelasticity of resin in aramid-aluminium laminate

In this paper, the fibre/resin layers in unidirectional aramid aluminium laminate was considered as viscoelastic layers and the modulus was expressed by the function of temperature and time. In order to determine the constant parameters in the function, dynamic mechanical property of the fiber/resin layers was tested under dynamic load with different frequency. The test temperature was range from room temperature to 200°C. After the parameters in the function was determined, the transvers residual stress in the laminate was calculated. In order to examine the calculated results, strain gages were embedded in the unidirectional aramid aluminium laminates to measure the transverse residual stresses. The calculated results are in good agreement with the experimental values.     

Investigation of fatigue crack growth rate in CARALL,ARALL and GLARE

Fibre metal laminates (FMLs) are being used to manufacture many structural components in aerospace industry because of their very high strength to weight ratios, yet the exact model for estimating fatigue crack propagation in FMLs cannot be developed because of many variable parameters affecting it. In this research, tensile strength, fatigue life and fracture toughness values of 2/1 configuration carbon reinforced aluminium laminate (CARALL), aramid reinforced aluminium laminate and glass laminate aluminium reinforced epoxy specimens have been investigated. Mechanical, chemical and electrochemical surface treatments were applied to AA 1050 face sheets to improve the adhesive properties of the laminates. The specimens were prepared using vacuum assisted resin transfer moulding technique and were cut to desired shapes. Fatigue tests were conducted on centre notched specimens according to ASTM Standard E399. Real time material data and properties of adhesive were used in definition of numerical simulation model to obtain the values of stress intensity factor at different crack lengths. It was observed that CARALL shows very superior tensile and fatigue strength because of stress distribution during failure. Numerical simulation model developed in this research accurately predicts fracture toughness of aramid reinforced aluminium laminate, CARALL and glass laminate aluminium reinforced epoxy with less than 2% error. An empirical analytical model using experimental data obtained during research was developed which accurately predicts the trend of FMLs fatigue life.     

An investigation on the bearing test procedure for fibre-reinforced aluminium laminates

Excellent fatigue, static strength and damage tolerance characteristics together with low density make fibre-reinforced aluminium laminates a prime candidate sheet material for application in fatigue- and fracture-critical aircraft structures. Their use requires that mechanical property design allowables be established for incorporation in design handbooks (e.g. MIL-HDBK-5). An experimental programme based on statistical design was conducted to establish a meaningful test procedure for determination of fibre-metal laminate bearing strength design allowables. The test procedures investigated are the pin-type bearing test method (ASTM E-238) and the bolt-type bearing test method, a modified method based on the procedure for bearing strength determinations in plastics (ASTM D-953). Results are presented from an experimental programme which measured the bearing strengths of two grades of S-2 glass-based and one grade of aramid-based aluminium laminates. The influences of lateral constraint and ply orientation on bearing strength and failure mode are shown. The bolt-type bearing test method, which combines the attributes of the two aforementioned methods, is recommended. The study also showed that the bearing properties for edge distance ratio e/D = 2 can be predicted by correlation with the aluminium volume fraction in fibre-reinforced aluminium laminates. In addition, diagrams of joint structural efficiency, shown to be comparable to those of aluminium alloy sheets, have been established.     

拉压加载下纤维增强铝合金层板疲劳裂纹扩展的压载荷效应与预测模型

基于增量塑性损伤理论与纤维增强金属层板疲劳裂纹扩展唯象方法, 推导出在拉-压循环加载下, 纤维增强金属层板疲劳裂纹扩展速率预测模型。并通过玻璃纤维增强铝合金层板在应力比R=-1,-2的疲劳裂纹扩展实验对预测模型进行验证。结果表明, 纤维增强铝合金层板疲劳裂纹扩展的压载荷效应分为两种情况: 在有效循环应力比R_C>0时, 表现为压载荷对铝合金层所承受残余拉应力的抵消作用; 当R_C<0时, 表现为压载荷抵消残余拉应力后, 对纤维增强铝合金层板金属层的塑性损伤, 对疲劳裂纹扩展存在促进作用。纤维铝合金层板疲劳裂纹扩展的压载荷效应不可忽略, 本文中得出的在拉-压循环加载下疲劳裂纹扩展速率预测模型与实验结果符合较好。     

Finite element calculation and measurement of thermal stresses in quenched plates of high–strength 7075 aluminium alloy

Abstract

Thick parts of high–strength aluminium alloys usually undergo a drastic quench which gives rise to thermal stresses and may cause distortion of products. The control of complex phenomena involved during quenching is achieved by determining the temperature distribution, thermal strains, and residual stresses using the MARC finite element program. In this approach, the thermo mechanical problem is assumed to be uncoupled, i.e. the thermal and mechanical calculations are solved consecutively. First a non–linear heat transfer analysis is performed taking the temperature dependence of the thermophysical properties and transient conditions of convection at the surface of the plate into account. This is followed by a thermo–elastoplastic stress analysis using the predicted temperature distributions, assuming an isotropic behaviour of the material and taking the temperature dependence of its mechanical properties into account. The calculation of thermal stresses occurring during the cold and hot water quenching of a 70 mm thick plate of 7075 alloy using this method shows a good agreement between theoretical predictions and experimental values of residual stresses, as measured by the layer removal method.

MST/2     

The role of carbon nanofibers on thermo-mechanical properties of polymer matrix composites and their effect on reduction of residual stresses

In this research, the effects of carbon nanofibers (CNFs) on thermo-elastic properties of carbon fiber (CF)/epoxy composite for the reduction of thermal residual stresses (TRS) using micromechanical relations were studied. In the first step, micromechanical models to calculate the coefficient of thermal expansion (CTE) and Young's modulus of CNF/epoxy and CNF/CF/epoxy nanocomposites were developed and compared with experimental results of the other researchers. The obtained results of the CTE and Young's modulus of modified Schapery and Halpin-Tsai theories have good agreement with the experimental results. In the second step, the classical lamination theory (CLT) was employed to determine the TRS for CNF/CF/epoxy laminated nanocomposites. Also, the theoretical results of the CLT were compared with experimental results. Finally, reduction of the TRS using the CLT for different lay-ups such as cross ply, angle ply, and quasi-isotropic laminates were obtained. The results demonstrated that the addition of 1% weight fraction of CNF can reduce the TRS that the most reduction occurred in the unsymmetric cross-ply laminate by up to 27%.     

Analysis of unsymmetric CFRP–metal hybrid laminates for use in adaptive structures

The potential of using multistable composite materials for adaptive structures is currently receiving interest from the aerospace community because they possess more than one single equilibrium configuration. Unsymmetric CFRP laminates are studied which have an inner isotropic metallic layer. These hybrid laminates are studied using analytical, finite element and experimental techniques. The thermal contraction of the isotropic layer upon cool down from cure induces large in-plane thermal loads which act remotely from the laminate’s neutral plane, increasing snap-through moments and out-of-plane displacements. The curvatures of the hybrid laminates can be doubled compared to pure unsymmetric CFRP laminates.     

Novel test method for accurate characterization of intralaminar fracture toughness in CFRP laminates

A novel initial crack insertion method, “intralaminar film insertion method”, was proposed to investigate the fracture toughness of unidirectional carbon fiber reinforced plastic (CFRP) laminates when the crack propagates inside the ply and not in the interlayer resin-rich area. Here, a release film was inserted inside a single lamina during the resin impregnation process of prepreg manufacturing. Mode I intralaminar fracture toughness tests were carried out for conventional CFRP laminates and interlayer toughened CFRP laminates. For comparison, two conventional methods were used to introduce initial cracks. One is the “interlaminar film method”, where a release film is inserted between two prepreg plies during the lay-up process. The other is the “machined slit method”, where a slit notch is machined in parallel to the layer of CFRP laminates. It was demonstrated that the proposed “intralaminar film method” can correctly evaluate the intralaminar fracture toughness of both conventional CFRP laminate and interlayer toughened CFRP laminate from the initial value to the propagation value. For this range, it was also found that the intralaminar fracture toughness of interlayer toughened CFRP laminate was the same as that of conventional CFRP laminate. Thus, the intralaminar fracture toughness was not influenced by interlayer toughening.     

Analysis of five-point bending of composite laminates

Composite laminates are subjected to five-point bending test conditions, resulting in cracks and delaminations in the laminate. A theoretical model is also developed to represent the five-point bending configuration, resulting in the calculation of the through-the-thickness distribution of stresses and strains. The results of the theoretical analysis and the five-point bending tests are presented. The stress and strain distributions arising from the theoretical model, together with knowledge of the laminate stacking sequence, can be used to predict the type and location of failures under five-point bending test conditions.     

基于有效三维损耗矩阵的复合材料层合板模态阻尼预测

为了在复合材料层合板阻尼分析中既考虑层合板厚度方向应力、应变对结构阻尼的贡献,又不增加厚度方向的单元数量,基于复合材料的有效三维阻尼矩阵预报理论,建立了新的高效复合材料结构模态阻尼的三维有限元预测方法。沿层合板厚度方向将原结构分成若干亚层,每一亚层包含若干单层。根据合成理论计算每一亚层的刚度矩阵、柔度矩阵和有效三维阻尼损耗矩阵。对亚层划分单元,进行结构的有限元模态分析。根据模态分析结果,利用有效三维阻尼矩阵求出各个模态对应的模态阻尼。利用该理论,分别计算了单向层合板、对称层合板以及厚板的结构模态阻尼。数值计算结果表明,该方法具有很好的适用性,其优点在于既考虑了板厚方向的阻尼贡献,又减少了厚度方向的单元数,提高了计算的准确性和效率。      

预应变法调整芳纶－铝层板的残余应力

采用预应变法能有效地调整芳纶-铝层板的层间残余应力[1]。本文分析了3/2铺层的芳纶-铝层板的残余应力与所施加的预应变的关系;并用腐蚀法测定了几种不同的预应变层板中铝层的残余应力;研究了施加预应变对层板的拉伸性能和疲劳性能的影响。      

纤维铝合金胶接层板残余应力分布

本文采用准三维有限元方法,计算对称铺设混杂复合层板在固化时产生的残余应力,给出了芳纶纤维/树脂-铝合金层板,在自由状态和施加预应力情况下的残余应力计算值,以及不同温度变化下残余应力计算值。本文还给出铝合金-玻璃纤维/树脂-芳纶纤维/树脂层板,在自由状态下固化时的残余应力值。某些计算结果与应变片包埋法实验测得的层板残余应力值十分吻合,表明了计算方法的正确性。用准三维有限元分析残余应力的方法,为设计和制造出满足最佳力学性能要求的层板提供了有价值的参考。     

粘弹性基体复合材料层合板的固化残余应力和曲率变化

粘弹性基体复合材料层合板因铺层取向差异而产生的固化残余应力会随层合板在室温下放置时间的推移而变化, 这一过程可通过非对称层合板的曲率变化得到反映。本研究通过非对称板的曲率变化实验来观察板内残余应力的变化过程, 并根据细观复合材料力学理论和适宜的变形分析方法建立一种与残余应力最终稳定分布相对应的铺层修正折减模量矩阵, 用于进行层合板固化残余应力和曲率变化的渐近值预测, 并依此确定固化后残余应力的变化范围。