复合材料加筋层合板的极限强度分析

基于更新拉格朗日格式,应用非线性层合三维退化壳元,结合有效的复合材料失效准则、刚度退化模型以及提出的刚度矩阵奇异判断准则,对复合材料加筋层合板在轴向载荷作用下的压缩极限强度问题进行深入研究。讨论了铺层方式、板厚等对极限强度的影响。通过与试验结果进行比较,表明基于提出的刚度矩阵奇异判断准则,结合增量更新拉格朗日格式下非线性层合三维退化有限元的计算方法,能有效计算复合材料加筋层合板的轴向压缩极限强度,并具有很高的精度。     

铺层方式对CFRP-铝合金单搭接胶接接头低速冲击损伤及剩余拉伸性能的影响

采用热压罐成型方法制备铺层方式为[+45/-45]_4s、[0/+45/-45/90]_2s和[0/90]_4s的碳纤维复合材料层合板,通过胶黏剂与铝合金板进行粘接获得异质材料单搭接胶接接头。利用落锤冲击试验机和万能电子试验机分别对三种接头进行低速冲击与冲击后静态拉伸测试,获得接头接触力-时间曲线和拉伸强度。通过CT扫描技术和数字图像相关(DIC)方法表征接头冲击损伤模式及表面应变演化过程,研究了铺层方式对接头抗冲击性能、冲击损伤模式以及剩余拉伸性能的影响。结果表明,复合材料铺层方式为[+45/-45]_4s时,接头在冲击载荷作用下具有较高的抗冲击性能,但胶层界面脱粘损伤较为严重。与胶层界面脱粘相比,接头剩余强度对层合板分层损伤更为敏感。相较于[+45/-45]_4s,[0/+45/-45/90]_2s和[0/90]_4s铺层方式接头的胶层界面脱粘范围与冲击后失效载荷退化程度较小,同时接头冲击后拉伸载荷主要集中于临近胶层的0°铺层且失效模式较为复杂...     

复合材料层合板低速冲击逐渐累积损伤预测方法

针对复合材料层板在冲击载荷下,各种损伤的产生和扩展是一个随载荷、时间和空间而演变的过程,发展了复合材料层合板低速冲击逐渐累积损伤预测方法.采用刚度退化技术和改进的Chang-Chang失效准则、显式有限元法来模拟复合材料层合板受到低速冲击下逐渐损伤过程.使用所发展的方法分析了[0m/90n/0m]铺层的复合材料层合板在低速冲击过程中的逐渐损伤扩展,结果表明本文的方法能较好地模拟复合材料层板在低速冲击下的损伤扩展及变形过程,计算结果与实验结果吻合较好;对不同冲击能量下层合板损伤扩展研究表明,冲击能量与分层损伤面积成线性关系.     

不同粒子改性环氧树脂基碳纤维复合材料低速冲击及冲击后压缩性能

使用单层纳米氧化石墨烯(GO)、纳米SiO2、陶瓷粉对环氧树脂进行改性处理,采用真空辅助树脂传递模塑成型(VARTM)工艺分别制备了[±45/0/90]S、[908]T、[08]T三种铺层角度下的碳纤维增强复合材料(CFRP)层合板.通过落锤冲击实验、超声C扫描检测、冲击后压缩实验等对不同粒子改性CFRP进行实验研究.结果表明:纳米粒子改性可以显著提升CFRP的抗冲击性能及冲击后压缩性能,与其他铺层角度相比,[±45/0/90]S铺层CFRP有效抑制了冲击裂纹的扩展,且单层纳米GO改性下的[±45/0/90]S铺层层合板最大冲击载荷及冲击后压缩强度分别达到3470 N、124.8 MPa,冲击损伤面积仅有580 mm2.与无粒子改性同种铺层层合板相比,最大冲击载荷及冲击后压缩强度相应提高了30％、47.3％,冲击损伤面积减小了15.5％.     

三维复合材料层合板渐进损伤非线性分析模型

为有效反映复合材料层合板层间相互作用和材料损伤非线性,建立了中等尺度的三维复合材料层合板渐进损伤分析模型。非线性渐进损伤分析过程包括应力求解、材料损伤失效判据及材料性能退化方案3个方面。讨论了损伤材料性能退化方案,引入与材料损伤模式相对应的损伤变量表征材料点的损伤状态,材料的刚度矩阵按损伤变量退化。基于该模型可成功预测复合材料层合板损伤起始、扩展直至最终失效的整个过程和极限强度。经文献试验数据验证,12种不同铺层顺序层合板的计算强度与试验数据均吻合较好,表明该模型在复合材料层合板极限强度预测上的有效性。     

铺层方式对CFRP-Al单搭接接头胶接性能的影响

使用热压罐制备[45/-45]_4s、[0/90]_4s和[0/45/-45/90]_2s三种铺层方式的CFRP层合板,然后在室温下与Al胶接制备出单搭接试样。使用电子万能试验机、数字图像相关法(DIC)和扫描电子显微镜(SEM)等手段测量胶接接头的拉伸载荷-位移曲线和应变分布并观察断口形貌。基于试验数据分析不同铺层方式下CFRP-Al单搭接接头的拉伸性能,研究了铺层方式对CFRP-Al单搭接接头胶接性能的影响和铺层方式胶接接头的破坏机制。结果表明,在拉伸过程中[45/-45]_4s试样出现塑性变形阶段其拉伸位移最大,而[0/45/-45/90]_2s和[0/90]_4s试样的拉伸位移较小且发生了脆性断裂。铺层方式从[45/-45]_4s到[0/45/-45/90]_2s再到[0/90]_4s,试样的极限载荷和纤维束断裂数量增加、层间剪切力减小、应变集中程度和分层破坏程度降低。     

基于断裂韧性预报复合材料层合板的开孔拉伸强度

开孔层合板的强度预报往往取决于孔边的临界长度,它不仅与材料性能,而且与铺层、孔径都有关。本文基于线弹性断裂力学,提出了一种预报对称铺层层合板开孔拉伸强度的新方法,只需提供正交层合板的断裂韧性和无缺口层合板的拉伸强度,显著降低对实验数据的依赖性。首先,将临界长度表作为层合板断裂韧性和无缺口拉伸强度的函数,再通过正交层合板[90/0]_8s的紧凑拉伸试验和虚拟裂纹闭合技术,确定出0°层断裂韧性,进而计算得到任意对称铺层层合板的断裂韧性。本文测试了T300/7901层合板[0/±45/90]_2s和[0/±30/±60/90]_s的开孔拉伸强度,孔径分别为3 mm、6 mm和9 mm。理论预报结果与试验值吻合较好,最大误差为15.2%,满足工程应用需求。      

修理温度对复合材料层合板拉伸强度的影响研究

分别对[45/0/0/45]、[45/0/45/0/45]和[45/0/45/0/45/0/45]等3种铺层形式的复合材料层合板的表面划伤损伤进行中温和常温贴补修理,对贴补修理后的层合板拉伸强度进行拉伸测试和有限元模拟,评价中温和常温修理的修理效果。结果表明:中温和常温修理都能满足修理要求,中温的修理效果要好于常温的修理效果。有限元模拟结果表明:有限元计算和实验结果偏差在10%以内,建立的有限元模型可以准确预测层合板修理后的拉伸强度。     

表面带金属层的复合材料层合板低速冲击数值模拟

使用Abaqus/Explicit建立表面带金属层的复合材料层合板和复合材料裸板低速冲击有限元模型,与已有文献对比验证结果的可靠性,研究结果对复合抗弹结构有很好的借鉴和参考价值.采用Johnson-Cook本构关系模拟铝合金和钛合金层的力学行为,选用Hashin准则对复合材料层内损伤进行失效判断,用二次应力准则来模拟粘结层Cohesive单元的层间失效.结果表明,相同铺层与冲击能量下,表面带铝合金层合板对内部纤维的保护性能优于表面带钛合金层合板,表面带钛合金层合板的抗冲击性能优于复合材料裸板;[§/0°/90°/0°/90°/0°]s铺层层合板的抗冲击性能优于[§/-45°/90°/0°/45°/-45°]s铺层层合板的抗冲击性能;在子弹刚冲破层合板与子弹完全离开层合板阶段,表面带铝合金层合板对子弹动能吸收率最大.     

压缩载荷下碳/环氧复合材料的动力学响应行为

以铺层顺序为[45/0/-45/00-45/90/0]s的新型碳纤维增强改性环氧树脂复合材料为研究对象,采用分离式Hopkinson(SHPB)压杆装置为加载手段,在高速冲击载荷条件下,对复合材料层合板在厚度方向和平面内纵向的动态压缩性能进行实验研究,分别得到在四种不同应变率下的应力一应变关系;并借助SEM对复合材料断口损伤形貌进行表征.结果表明:在高应变率条件下,层合板厚度方向的动态压缩强度及失效应变明显大于平面内加载方向;基体开裂、分层及剪切断裂是复合材料在动态压缩条件下的主要损伤及断裂模式.     

On the investigation of the biaxial fatigue behaviour of unidirectional composites

The paper illustrates the preliminary activity of an extensive research program oriented to investigate the multiaxial fatigue behaviour of unidirectional composite laminates, with particular attention to the analysis of the damage mechanisms and their correlation with the local multiaxial stress state to be used then as the basis for the development of multiaxial fatigue criterion. The definition of an effective experimental procedure for multiaxial fatigue testing is carefully discussed in terms of specimen geometry, specimen manufacturing and local stress state. Once identified in the thin-walled tubular specimens under tension–torsion loading the best test configuration for the aims of the research, the results of comparative fatigue tests investigating the influence of the tubes geometry (wall thickness to diameter ratio) on the transverse fatigue response are presented. In the final part of the paper the effects of an increasing shear stress component (σ₆) on the transverse (σ₂) fatigue strength and damage evolution in UD glass–epoxy tubes are illustrated.     

An application of the implicit gradient method to welded structures under multiaxial fatigue loadings

This paper deals with the problem of fatigue strength assessment of welded joints subjected to multiaxial loading. Three-dimensional solid modelling and linear elastic stress analysis, by means of numerical methods, are used to investigate the local stress field at weld toes and roots, geometrically regarded as sharp notches. Starting from the stress field obtained from a linear elastic analysis and taking advantage of the so-called implicit gradient approximation, an effective stress index connected with the material strength is calculated. In particular, there will be an investigation into the possibility of applying the implicit gradient approach to welded structures, under both uniaxial and multiaxial loading conditions, by introducing a multiaxial criterion into the implicit gradient framework. The multiaxial criterion consists of an improvement of the well-established Crossland-like criteria. It will deal with multiaxiality caused by external loadings as well as multiaxial stress fields caused by severe stress raisers. In order to validate the devised approach, theoretical fatigue damage estimations are compared with experimental data. In particular, the proposed approach is applied to a series of applicative examples taken from scientific literature and related to welded joints subjected to uniaxial or in-phase multiaxial fatigue loading.     

Fatigue behavior of laminated composites with a circular hole under in-plane multiaxial loading

A modified fiber failure fatigue model is presented for characterizing the behavior of laminated composites with a central circular hole under in-plane multiaxial fatigue loading. The analytical model presented is based on minimum strength model and fiber failure criterion under static loading available in the literature. The analysis starts with the determination of location of a characteristic curve around the hole and the stress state along the characteristic curve under in-plane multiaxial fatigue loading. Number of cycles to failure and location of failure are determined under given fatigue loading condition. Based on ply-by-ply analysis, ultimate fatigue failure and the corresponding number of cycles are determined. Analytical predictions are compared with the experimental results for uniaxial and multiaxial fatigue loading cases. A good match is observed. Further, studies are carried out for different in-plane biaxial tension–tension and biaxial compression–compression loading cases.     

On the estimation of the material fatigue properties required to perform the multiaxial fatigue assessment

To accurately perform the fatigue assessment of engineering components subjected to in‐service multiaxial fatigue loading, the adopted design criterion must properly be calibrated, the used information usually being the fatigue strength under both pure uniaxial and pure torsional fatigue loading. Because of the complex fatigue response of metallic materials to multiaxial loading paths, the only reliable way to generate the necessary pieces of calibration information is by running appropriate experiments. Unfortunately, because of a lack of both time and resources, very often, structural engineers are requested to perform the multiaxial fatigue assessment by guessing the necessary fatigue properties. In this complex scenario, initially, the available empirical rules suitable for estimating fatigue strength under both pure axial and pure torsional fatigue loading are reviewed in detail. Subsequently, several experimental results taken from the literature and generated by testing metallic materials under a variety of proportional and non‐proportional multiaxial loading paths are used to investigate the way such empirical rules affect the accuracy in estimating fatigue strength, the damage extent being evaluated according to the modified Wöhler curve method. Such a systematic validation exercise allowed us to prove that under proportional loading (with both zero and non‐zero mean stresses), an adequate margin of safety can be reached even when the necessary calibration information is directly estimated from the material ultimate tensile strength. On the contrary, in the presence of non‐proportional loading, the use of the empirical rules reviewed in the present paper can result, under particular circumstances, in a non‐conservative fatigue design.     

Estimation of the fatigue strength distribution in high-cycle multiaxial fatigue taking into account the stress–strain gradient effect

Based on the weakest link concept, a probabilistic approach in high-cycle multiaxial fatigue is developed to predict, for a given number of cycles, the probability distribution of the fatigue strength for metallic structural components. A three-parameter Weibull distribution is combined with the energy-based and volumetric high-cycle multiaxial fatigue criterion proposed by Banvillet et al. in 2002 [Banvillet A, Palin-Luc T, Lasserre S. A volumetric energy based high cycle multiaxial fatigue criterion. Int J Fatigue 2003;26(8):755–69; Banvillet A, Palin-Luc T, Lasserre S, Vittori JF. Energy based high cycle multiaxial fatigue criterion depending on stress–strain distribution. In: Blom AF, editor. Fatigue 2002: eighth international fatigue congress, vol. 1, EMAS: Stockholm; 2002, p. 283–90]. Whatever the stress state and the loading type are, the corresponding fatigue strength probability distribution can be deduced from three usual experimental fatigue limits. The scale effect is also predicted. Experimental probability distributions and theoretical predictions of the fatigue strength of smooth specimens are in good agreement for the five materials investigated: the 30NiCrMo16 and 35CrMo4 quenched and tempered steels, the C20 annealed steel, the EN-GJS800-2 nodular cast iron and the Ti–6Al–4V titanium alloy.     

Selection of the critical plane orientation in two-parameter multiaxial fatigue failure criterion under combined bending and torsion

The present paper is focused on engineering application of the algorithm of fatigue life calculation under multiaxial fatigue loading. For that reason, simple two-parameter multiaxial fatigue failure criterion is proposed. The criterion is based on the normal and shear stresses on the critical plane. Experimental results obtained under multiaxial proportional, non-proportional cyclic loading and variable-amplitude bending and torsion were used to verify the proposed two-parameter criterion and other well-known multiaxial fatigue criteria. Elastic–plastic behaviour of the bulk material was taken into account in calculation of the stress/strain distribution across the specimen cross-section. It is shown that the proposed two-parameter multiaxial fatigue failure criterion gives the best correlation between the experimental and calculated fatigue lives.     

STRENGTH PREDICTION AND DAMAGE MECHANISMS OF GLASS/EPOXY WOVEN LAMINATES WITH CIRCULAR HOLES

Abstract— The notched strengths of four woven laminates (two orthotropic, one quasi-isotropic and one square symmetrical) under monotonic uniaxial loading, predicted with the point and average stress criteria, the two parameter criterion, the progressive degradation model, and the initial and improved point and minimal strength models, have been respectively compared with the experimental data. The damage mechanisms for the laminates were tentatively studied. The results show that, for minimal experimental information, the notched strengths predicted by the improved minimal strength models are the most precise and the progressive degradation model can numerically illustrate the damage mechanisms in detail. The damage mechanisms of the woven laminates were found to be very different from those of non-woven laminates.     

Optimum in situ strength design of laminates under combined mechanical and thermal loads

In this paper, optimum laminate configurations are sought for multidirectional fibre-reinforced composite laminates under combined in-plane mechanical and thermal loads. The design objective is to enhance the value of the loads over and above the first-ply-failure loads which are judged by a transverse failure criterion and the Tsai-Hill criterion, respectively. The in situ strength parameters previously obtained are incorporated in these criteria. It is found that the optimum designs under combined mechanical and thermal loads are not the same as those under pure mechanical loads for three of the four loading cases studied. For all cases the optimum loads are significantly larger than those for a quasi-isotropic design.     

Off-axis notched strength of fiber–metal laminates and a formula for predicting anisotropic size effect

Experimental and theoretical studies of the effects of notch size and fiber orientation on the off-axis notched strength of the fiber–metal laminate GLARE-3 have been conducted. The notched strength decreased with increasing notch size, regardless of the fiber orientation. The notch sensitivity in GLARE-3 was highest in the fiber direction, and it decreased with increasing fiber orientation angle to the lowest in the 45° direction. A new multiaxial criterion for the tensile failure of notched orthotropic fiber composites was developed from a phenomenological point of view. From the proposed multiaxial failure criterion, an anisotropic size effect law for predicting the off-axis notched strength of orthotropic composite laminates was derived. By comparison with the experimental results on GLARE-3, it was demonstrated that the anisotropic size effect law can accurately and efficiently predict the off-axis notched strength of GLARE-3, regardless of the notch size and fiber orientation.     

Multiaxial fatigue life estimation in welded joints using the critical plane approach

Many engineering structures experience multiaxial fatigue states of stress–strain in the vicinity of welded joints. Fatigue assessment of welded joints under proportional (in-phase) cyclic loading can be performed by using conventional hypotheses (e.g., see the von Mises criterion or the Tresca criterion) on the basis of local approaches. On the contrary, the fatigue life predictions of welded joints under non-proportional (out-of-phase) cyclic loading are generally poor if these conventional hypotheses are used. In the present paper, the critical plane-based multiaxial fatigue criterion proposed by Carpinteri and Spagnoli for smooth and notched structural components is extended to the fatigue assessment of welded joints under in- and out-of-phase loadings. The applicability of this criterion, expressed in terms of nominal stresses, to the fatigue life prediction of welded specimens is investigated by using experimental data available in the literature.