FINITE ELEMENT ANALYSIS OF DELAMINATION CRACKS IN BENDING OF CROSS-PLY LAMINATES

SUMMARY

A study of delamination crack growth due to bending in cross-ply laminates is presented. For the understanding of interlaminar fracture behaviour of laminated composites the modelling of delamination crack growth induced by bending and shear cracks in three point bending specimens is carried out. A plane strain two-dimensional (2-D) finite element analysis is used to determine the strain energy release rates during delamination of the laminated beam. Contact elements were used to prevent the material interpenetration on the crack surfaces. The solution of the contact problem taking into account friction along crack surfaces is obtained. Energy release rates G_I and G_II for Mode I and Mode II fracture are calculated by virtual crack closure integral (VCCI) methods. Comparison of total energy release rates, obtained by local energy methods, with an analytical solution based on the beam theory and a global energy method have been carried out. Good agreement of the results obtained by various methods have been observed. Comparison of the results obtained by the solution of the contact problem and without contact elements have been performed. Significant differences between the values of energy release rates obtained with and without using contact elements have been observed. The influence of the coefficient of friction on the energy release rates is insignificant.     

纤维增强塑料齿轮轮齿弯曲强度分析

本文由纤维增强塑料齿轮轮齿的细观结构建立了各向异性粘弹性力学的计算模型,采用Laplace变换和有限单元法相结合求解轮齿的粘弹性应力场。通过引入时间折算因子,改进了Schapery的数值反演过程,给出了一个可行的复合材料齿轮弯曲强度实用分析方法。     

纤维增强塑料齿轮轮齿弯曲强度分析

本文由纤维增强塑料齿轮轮齿的细观结构建立了各向异性粘弹性力学的计算模型,采用Laplace变换和有限单元法相结合求解轮齿的粘弹性应力场。通过引入时间折算因子,改进了Schapery的数值反演过程,给出了一个可行的复合材料齿轮弯曲强度实用分析方法。      

FINITE ELEMENT ANALYSIS OF CLOSURE BEHAVIOUR OF FATIGUE CRACKS IN RESIDUAL STRESS FIELDS

An elastic-plastic finite element analysis with high order elements is performed to examine closure behaviour of fatigue. cracks in residua1 stress fieids and the numerical results are then compared with experimental results. The finite element analysis, performed under plane stress using 8-node isoparametric elements, can predict fatigue crack closure behaviour through residual stress fields very well. The crack opening and closing behaviour through a compressive residual stress field is found to be complicated and influenced by the applied load magnitude and the location of the crack tip. Three different types of crack opening behaviour, namely, normal, unsymmetric partial and symmetric partial crack opening behaviour are observed through a compressive residual stress field. The partial crack opening stress intensity factor including the partial crack opening effect is recommended for the prediction of fatigue crack growth through a compressive residual stress field.     

DAMAGE MECHANICS APPROACH TO REMOVE THE CONSTRAINT DEPENDENCE OF ELASTIC–PLASTIC FRACTURE TOUGHNESS

It is now generally agreed that the applicability of a one-parameter J-based ductile fracture approach is limited to so-called high constraint crack geometries, and that the elastic-plastic fracture toughness J_1c, is not a material constant but strongly specimen geometry constraint-dependent. In this paper, the constraint effect on elastic-plastic fracture toughness is investigated by use of a continuum damage mechanics approach. Based on a new local damage theory for ductile fracture(proposed by the author) which has a clear physical meaning and can describe both deformation and constraint effects on ductile fracture, a relationship is described between the conventional elastic-plastic fracture toughness, J_1c, and crack tip constraint, characterized by crack tip stress triaxiality T. Then, a new parameter J_dc (and associated criterion, J_d=J_dc) for ductile fracture is proposed. Experiments show that toughness variation with specimen geometry constraint changes can effectively be removed by use of the constraint correction procedure proposed in this paper, and that the new parameter J_dc is a material constant independent of specimen geometry (constraint). This parameter can serve as a new parameter to differentiate the elastic-plastic fracture toughness of engineering materials, which provides a new approach for fracture assessments of structures. It is not necessary to determine which laboratory specimen matches the structural constraint; rather, any specimen geometry can be tested to measure the size-independent fracture toughness J_dc. The potential advantage is clear and the results are very encouraging.     

ELASTIC-PLASTIC FINITE ELEMENT ANALYSES OF CRACKED NOTCHES IN PLATES UNDER MIXED-MODE LOADING

Plates containing inclined elliptical notches with and without cracks are analysed for five different major axis inclinations, namely, θ equal to 0, 11.25, 22.5, 33.75 and 45?. Short cracks emanating from the roots of such inclined circular and elliptical notches are analysed. Under various mixed-mode loading conditions, when cracks are present, a simple method is described for the evaluation of the Mode I and Mode II stress intensity factors, for the elastic state. Values of the J integral along three different contours are also evaluated in the elastic and elastic-plastic states, as well as the Mode I and Mode II components of the crack tip opening displacenient (CTOD), the maximum principal stresses, the maximum average equivalent plastic strains and the location of the elements in which they occur. Comparisons of the different loading conditions are presented and the implications of the effects of mixed-mode loading on fracture and fatigue crack propagation are discussed.     

FRACTURE MECHANICS OF PHYSICALLY SHORT CRACKS

Mechanical Engineering Research Institute, Academy of Sciences of the U.S.S.R., Griboedova Str., 4 Moscow Centre, Moscow 101830, U.S.S.R.     

ELASTIC–PLASTIC ANALYSIS OF THE FATIGUE LIMIT FOR A MATERIAL WITH SMALL FLAWS

Abstract Fatigue tests were carried out on mild steel with small cracks for which linear fracture mechanics is not effective. A fatigue limit criterion, based on the cyclic plastic zone size at a crack tip being a material constant at the fatigue limit, can effectively evaluate the effects of crack length and stress ratio. Regarding flaws as cracks, the theory gives fatigue limit values close to those obtained in experiments on specimens with natural defects, such as surface roughness, micro-shrinkage cavities, inclusions etc. The effect of water corrosion was also investigated.     

双相型不锈钢角焊缝连接承载性能有限元分析

为研究双相型S22053不锈钢角焊缝连接的承载性能,对不锈钢母材和焊缝金属圆棒试样进行材性试验,分别获得了母材和焊缝金属材料的本构关系;对不锈钢正面和侧面角焊缝连接试件进行拉伸试验,测得试件极限承载力和焊缝变形发展情况;利用ANSYS有限元软件对不锈钢角焊缝连接试件单向拉伸试验进行了非线性有限元模拟,并将试验结果与模拟结果进行了对比。对比结果表明,考虑不锈钢母材和焊缝金属本构关系差异并使用材料真实应力-应变曲线的有限元模型能有效模拟角焊缝连接承载性能拉伸试验。最后,利用经试验验证的有限元模型对角焊缝承载力影响因素进行了参数分析,并将有限元模拟值与按照美国规范SEI/ASCE 8-02、欧洲规范EN 1993-1-4,以及中国不锈钢结构技术规程CECS 410：2015计算求得的规范设计值进行了对比,得出了承载力模拟值与各国规范设计值的比值,可为不锈钢设计规范的完善和工程应用提供参考。     

轴向碰撞弹性直杆动力后屈曲有限元分析

利用显式动力学有限元方法对弹性直杆的动力后屈曲进行了分析;模拟了弹性直杆轴向碰撞动力屈曲的变形及发展过程。分析中将碰撞杆视为无初始缺陷的理想直杆,将弹性直杆动力屈曲双特征参数的解答作为非线性动力后屈曲求解的初始条件,实现了对无缺陷理想直杆的动力后屈曲分析。计算结果与文献中的实验数据获得了很好的一致。计算结果同时也揭示了直杆动力屈曲变形发展的机理,以及轴向应力波和屈曲变形的相互作用规律。     

五种本构模型在钢管混凝土有限元中的比较

采用Von Mises、Mohr Coulomb和Drucker Prager三种经典模型、以及两种适用于混凝土材料的Smeared Cracking模型和Damaged Plasticity模型,分别对带约束拉杆和无约束拉杆的方形、矩形钢管混凝土短柱轴压承载力进行有限元分析,简述每种本构模型的特点,并与已有的47个钢管混凝土短柱轴压试验结果对比。结果表明与静水压力无关且忽略混凝土塑性体积膨胀的VonMises模型无法考虑钢管对混凝土的约束作用以及约束拉杆的作用,采用相关流动法则而低估了塑性体积膨胀的Smeared Cracking模型分析结果略低于试验结果,其他三种考虑静水压力并采用不相关流动法则的模型更符合试验结果,而且三者计算结果很接近。     

混合结构房屋现浇楼板裂缝的有限元数值分析

现浇钢筋混凝土楼板开裂是混合结构房屋常遇到的问题之一。在对若干实际工程调查和模型试验的基础上,用有限元程序建立了两种结构形式的有限元模型,进行了非线性有限元分析。有限元分析结果包括混凝土主拉应力及应变、楼板裂缝和钢筋应力的分布和发展,其与试验结果符合程度较好。研究结论给出了楼板开裂的主要原因和影响因素、裂缝形态及范围、控制裂缝的关键措施和楼板钢筋合理的布置方式。     

短纤维复合材料刚度特性的有限元随机能量法预报

本文基于能量等效原理,采用有限元法和随机统计试验的Monte-Carlo方法相结合的随机分析数值方法,提出了可用于短纤维复合材料刚度预报的有限元随机能量法.文中还以碳纤维/AG3合金、碳纤维/GA321合金和钢纤维/尿烷复合材料为例,用本文方法对其刚度特性进行理论预报,所得理论预报与试验值吻合很好,说明本文提出的分析模型和方法是有效的.     

有限元线法的曲线单元

有限元线法是一种新近提出且正在开发中的一种新型结构分析方法。本文建立了有限元线法的曲线单元,并以弹性力学平面问题为例给出了相应的数值算例以表明曲线单元良好的适用性及有效性。     

FATIGUE CRACK PROPAGATION BEHAVIOUR OF SHORT CRACKS; THE EFFECT OF MICROSTRUCTURE

–Fatigue cracks shorter than some critical length tend to propagate anomalously quickly. This paper examines the concept of a ‘critical length’, identifying three regimes of behaviour for different crack lengths. Some published work is examined, covering a wide range of different materials. It is concluded that there is an approximate correlation between the critical length for short crack behaviour and the scale of the microstructure. LEFM is difficult, if not impossible, to apply to cracks shorter than this critical length because the material surrounding a crack cannot be assumed to approximate to a homogeneous continuum. Suggestions are made for a fatigue design philosophy which incorporates short crack behaviour.     

弹塑性有限厚条法及工程应用

本文采用大单元内划分小网格的方法判断塑性区范围。在弹性区及塑性区内选用统一的解析函数级数。以修正常刚度增量法为迭代方法，给出了塑性系数矩阵及塑性刚度矩阵的推导过程。假设小网格内的应力为常量，使求解过程方便化．编制了弹塑性有限厚条法在地下工程三维弹塑性围岩稳定性分析中的应用程序，并对圆形及椭圆形截面洞室进行了稳定分析