胶补加筋板应力强度因子及胶层剪应力计算

在铆钉力法的基础上，提出计算胶接修补加筋板结构应力强度因子及胶层剪应力的解析法，对分析胶接修补的加筋板强度具有现实意义．利用影响系数，可以得到筋条、补片及蒙皮位移的表达式，根据位移协调条件，建立联立方程，从而得到胶层剪应力及铆钉力．通过解析式，计算裂纹尖端的应力强度因子，分析补片对结构修补的效果，综合考虑了筋条及补片对应力强度因子的影响．     

胶补加筋板应力强度因子及胶层剪应力计算

 在铆钉力法的基础上，提出计算胶接修补加筋板结构应力强度因子及胶层剪应力的解析法, 对分析胶接修补的加筋板强度具有现实意义. 利用影响系数，可以得到筋条、补片及蒙皮位 移的表达式，根据位移协调条件，建立联立方程，从而得到胶层剪应力及铆钉力. 通过解析 式，计算裂纹尖端的应力强度因子，分析补片对结构修补的效果，综合考虑了筋条及补片对 应力强度因子的影响.     

复合材料补片加固钢板的粘接应力与失效模拟

在ANSYS有限元软件中建立三维有限元模型,对复合材料补片单面加固钢板进行数值模拟,使用内聚力单元模拟胶层脱粘和扩展过程,有限元模型计算结果与试验结果吻合较好,弹性极限载荷和失效载荷的相对误差分别为9.6%和4.2%。计算得到了模型在拉伸载荷作用下的载荷-位移曲线、胶层剪应力和剥离应力分布情况以及补片x轴向应力分布情况。结果表明,胶层端部最先达到极限强度后出现开裂,脱粘从补片两端开始逐渐向中心扩展,且扩展过程是非对称的;当补片发生部分脱粘后,补片应力集中在未脱粘处,承载长度逐渐减小、承载能力逐渐降低。     

复合材料层合圆柱壳的有限元分析

本文根据Reissner-Mindlin型的全局位移场(一阶和三阶)，应用有限元预测一修正法，数值计算和分析了机械载荷作用下复合材料层合圆柱壳的挠度和横向剪应力。首先按照一般的有限元分析过程(没有引入剪切修正系数)计算出层合圆柱壳的挠度预测值；然后利用Lagrange插值构造横向剪应力的一般形式，使得满足层间连续和表面上为零的条件，通过最小二乘法拟合三维应力平衡方程获得横向剪应力；最后在单元上计算和引入剪切修正系数，再经过有限元分析计算出层合圆柱壳的挠度修正值。数值计算结果与三维线弹性解的比较表明，挠度修正值和横向剪应力的精度是十分满意的。     

钢纤维混凝土层裂强度的实验研究

钢纤维能显著提高混凝土抵抗层裂破坏的能力,但纤维含量、长细比和形状对高应变率下钢纤维混凝土层裂强度的影响仍缺少系统研究.本文利用大直径Hopkinson杆对混凝土细长杆件进行冲击加载,通过放置在试件后方的空心铝合金杆上应变波形确定试件的层裂强度,系统研究了钢纤维含量、长细比和形状对钢纤维混凝土层裂强度的影响.实验结果表明钢纤维混凝土层裂强度具有应变率效应,即应变率越高层裂强度越高.钢纤维对混凝土层裂强度的增加与纤维影响系数α、纤维长细比和纤维含量三者之乘积具有线性关系.动态加载条件下的系数α高于静态,其原因主要是动态加载时纤维快速从基体拔出时动态剪应力增加.波浪形纤维混凝土的系数α比扁头型纤维高.根据实验结果建立了钢纤维混凝土层裂强度经验公式,公式预测结果能与实验结果较好吻合.     

复合材料层合壳有限元分析的预测-修正法

对于复合材料层合壳的有限元分析，本文根据Reissner-Mindlin型的全局位移场给出了一个预测一修正法。首先按照一般的有限元分析过程(没有引入剪切修正系数)计算出全局响应(如挠度，频率和屈曲载荷等)的预测值；然后利用Lagrange插值构造横向剪应力的一般形式，使得满足层间连续和表面上为零的条件，通过最小二乘法拟合三维应力平衡方程获得横向剪应力；最后在单元上计算和引入剪切修正系数，再经过有限元分析计算出全局响应的修正值。     

膜-基复合材料界面剪应力的三维半解析计算

 采用影响系数法对膜-基复合材料的界面剪应力三维半解析进行 了分析研究.利用三维有限元方法对薄膜的影响系数进行了计算. 将 基体作为半无限大体，利用其平面边界作用单位力时的位移场解析 解，得到基体的影响系数. 结果表明，对膜-基复合材料界面剪应力 进行三维半解析计算，克服了完全用三维有限元对其进行计算的限 制，为该类问题的分析提供了新的途径.     

45#钢的损伤演化方程和层裂准则研究

通过唯象分析和细观物理统计相结合的方法建立了一种韧性材料的损伤演化方程。在试验结果和内变量理论的基础上得到了45#钢的含损伤热—粘塑性本构关系。用有限差分方法计算了45#钢平板撞击所产生的应力波传播规律、损伤演化规律及层裂进程。通过自由面速度历史的数值模拟,并基于计算结果与试验结果间的最佳一致性,得到了损伤演化方程中的材料参数和极限损伤数值,并以此为依据建立了材料的应力率层裂准则。     

压缩剪切测试中载荷传递行为的数字光弹性研究

结合光弹性四步彩色相移法,提出改进的六步相移法来计算全场剪应力,对径压缩圆盘实验证实了该方法的正确性,并进一步研究了同质材料连接结构在压缩剪切测试中的层间剪切行为。实验结果表明:全场等色线条纹级数和剪应力连续分布且随载荷而增加;中线附近剪应力趋于零,而条纹级数为峰谷。在连接界面上的条纹级数和剪应力出现峰值,特别是在连接界面端部为最高,在最大载荷下可分别达3.5级和2.4MPa,是导致连接界面脱粘开裂的危险区域。连接结构通过层间剪应力进行应力传递,在连接界面上剪应力与外载基本保持平衡。     

含氦泡辐照老化材料层裂损伤计算方法分析

辐照条件下,一些材料内部产生大量的氦泡等微缺陷,氦泡的大小和数密度随着辐照年限的增长而增长。氦泡分布特征的变化不仅影响材料本身的物理、力学性质,而且直接影响材料层裂损伤演化后期材料破坏颗粒度的分布特征。延性材料的层裂损伤演化过程一般包括孔洞的成核、增长和汇合,但因已有孔洞对新成核孔洞存在抑制作用,当初始孔洞数密度达到一定临界值时,材料内部没有新的孔洞成核,因此,层裂损伤的计算可以不考虑新孔洞成核的影响。本文中基于损伤早期演化的特征,给出了这一临界值的计算方法,并进一步探讨了含氦泡辐照老化钚材料层裂损伤的计算方法。同时,在完善孔洞增长(void growth, VG)层裂损伤模型中参数的确定方法的基础上,借助含氦泡常规铝材料的层裂实验结果,对此问题进行了定性的分析：在氦泡尺寸变化不大的情况下,当氦泡浓度低于临界氦泡浓度时,需要考虑初始氦泡以及新增孔洞的综合影响;反之,可以采用简单的层裂损伤模型,不需要计算孔洞成核,但由于增长孔洞之间的相互影响,损伤模型的初始损伤参数需要重新确定。     

Isoparametric shear spring element applied to crack patching and instability

In this work the isoparametric shear spring element is applied to the stress and energy analysis of a center-crack panel reinforced by a rectangular patch. In this model, only transverse shears are assumed to prevail in the adhesive layer. The stresses and crack-tip stress intensity factors are obtained for reinforcement on both sides and one side of the panel, and are found to be in agreement with those obtained by previous authors using the triangular shear spring element.Crack stability that tends to vary with patch thickness is determined from the local and global maximum of the minimum strain energy density function denoted, respectively, as [(dW/dV)_min^max]_L at point L and [(dW/dV)_min^max]_G at point G. The distance l between L and G gives the prospective path of subcritical crack growth and its magnitude provides a measure of the degree of crack stability. A patched panel with small l tends to be more stable than that with large l. By increasing the patch thickness beyond a certain value, l can be contained within the patch such that failure, if initiated, will be highly localized. Such a behavior is exhibited. Numerical results are provided to support the foregoing conclusion.     

An Experimental-Numerical Hybrid Method for Elastic Contact Problems*

ABSTRACT

An experimental-numerical hybrid technique for determining the contact stress distribution between two elastic bodies having both frictionless as well as bonded contact is discussed in this paper. The hybrid method makes use of experimental data collected at a section far from the contact surface and the numerically generated influence coefficients, in terms of the applied unit normal and shear stresses. The experimental data, i.e., the differences in normal stresses and the shear stress, are obtained using photoelastic analysis for the examples illustrated in this paper. When substituted into equations corresponding to the unit normal and shear stress applied in the contact region, this results in a set of algebraic equations which, when solved, allow the contact stress distribution to be obtained. This method is illustrated with examples involving simple and complex geometries of the contacting bodies.     

复合材料加筋壁板的优化设计

本文用简化的方法,考虑了受压及压剪联合作用的复合材料加筋壁板的总体屈曲和局部屈曲、材料强度、刚度、轴向应变等因素以及其它工程要求的影响,用数学规划法提出壁板的优化设计问题,用乘子罚函数法求解。以石墨-环氧帽型加筋板为例,所得优化设计结果与NASA得出的最优曲线一致,证明了所用方法的可靠性,并可进一步推广应用于工程实践。     

Nonlinear analysis of beams of variable cross section,including shear deformation effect

In this paper the analog equation method (AEM), a BEM-based method, is employed for the nonlinear analysis of a Timoshenko beam with simply or multiply connected variable cross section undergoing large deflections under general boundary conditions. The beam is subjected in an arbitrarily concentrated or distributed variable axial loading, while the shear loading is applied at the shear center of the cross section, avoiding in this way the induction of a twisting moment. To account for shear deformations, the concept of shear deformation coefficients is used. Five boundary value problems are formulated with respect to the transverse displacements, the axial displacement and to two stress functions and solved using the AEM. Application of the boundary element technique yields a system of nonlinear equations from which the transverse and axial displacements are computed by an iterative process. The evaluation of the shear deformation coefficients is accomplished from the aforementioned stress functions using only boundary integration. Numerical examples with great practical interest are worked out to illustrate the efficiency, the accuracy and the range of applications of the developed method. The influence of the shear deformation effect is remarkable.     

Nonlinear analysis of a composite panel with a cutout repaired by a bonded tapered composite patch

This study presents a solution method to analyze the geometrically nonlinear response of a patch-repaired flat panel (skin) with a cutout under general loading conditions. The effect of induced stiffening due to tensile loading on the in-plane and, particularly, the out-of-plane behaviors of the patch-repaired skin are investigated. The damage to the skin is represented in the form of a cutout under the patch. The patch with tapered edges is free of external tractions. The skin is subjected to general boundary and loading conditions along its external edge. The solution method provides the transverse shear and normal stresses in the adhesive between the skin and the patch, and in-plane and bending stresses in the patch and skin. Both the patch and skin are made of linearly elastic composite laminates, and the adhesive between them is homogeneous and isotropic, exhibiting a bi-linear elastic behavior. Modified Green’s strain–displacement relations in conjunction with von Karman assumptions are employed in determining the in-plane strains in the skin and patch; however, the transverse shear strains in the adhesive are determined based on the shear-lag theory. The present solution method utilizes the principle of virtual work in conjunction with complex potential functions.     

Evaluation Using Digital Image Correlation of Stress Intensity Factors in an Aerospace Panel

The fracture behavior of a crack propagating in a large (4.8 m × 1.4 m) aircraft panel was investigated quantitatively by experiment for the first time using digital image correlation. Mixed mode (I+II) stress intensity factors were evaluated using a methodology, which combined digital image correlation with the multi-point over-deterministic method to fit displacement field equations to the experimental data from around a crack tip. More than 800 images were taken during a 10-minute time period as the fracture of the panel occurred under monotonic loading. It was observed that the crack propagated through the skin of the panel at a relatively low speed, with an average crack tip velocity of 0.014 mm/s, and changed its propagation direction at particular points due to the reinforcement of the structure. In the later stages of the test, substantial shear lips were observed indicating a state of plane stress as would be expected in a thin, wide panel and the size of the plastic zone increased substantially. The value of the mode I stress intensity factor obtained from the measured displacement fields initially increased linearly to around 50 MPa√m (K_Ic = 37 MPa√m) and afterwards non-linearly reaching 300 to 400 MPa√m for crack extensions of the order of 100 mm. It is proposed that these high values of stress intensity factor do not represent an unrealistically high material fracture toughness but rather are indicative of the high resistance to crack growth of the structural assemblage of ribs, stringers and hole reinforcements in the panel which allow the skin to sustain a strain level that would otherwise cause unstable crack growth. Digital image correlation is demonstrated to be particularly powerful in elucidating this structural behavior.     

Fatigue crack growth analysis of adhesively repaired panel using perfectly and imperfectly composite patches

Fatigue crack behavior of cracked aluminum panel repaired with the imperfectly bonded composite patch is analyzed. The imperfection is in the form of debond which could result during the bonding of patch or the service life of the repaired structure. Debonds, of various sizes and at different locations with respect to the crack front, are investigated. An analytical procedure, involving two-dimensional finite element method having three layers to model cracked plate, adhesive and composite patch, is used to compute the stress intensity factors of test coupons. From the computed stress intensity factors, the crack growth rates are obtained analytically by assuming that the relationship between the stress intensity factor and the crack growth rate after repair is the same as the fatigue crack growth relationship for cracked panel material. The fatigue crack growth rates obtained experimentally and analytically are in good agreement with each other and they vary linearly with crack length inside the patch. The experimental results are bounded between its analytical counterparts at the mid-plane and free edge surfaces of the cracked panel. The present analytical procedure can, thus, be used to characterize the effects of imperfectly and perfectly bonded composite patch repairs on the durability and damage tolerance of the repaired structure.     

Shear deformation effect in non-linear analysis of composite beams of variable cross section

In this paper the non-linear analysis of a composite Timoshenko beam with arbitrary variable cross section undergoing moderate large deflections under general boundary conditions is presented employing the analog equation method (AEM), a BEM-based method. The composite beam consists of materials in contact, each of which can surround a finite number of inclusions. The materials have different elasticity and shear moduli with same Poisson's ratio and are firmly bonded together. The beam is subjected in an arbitrarily concentrated or distributed variable axial loading, while the shear loading is applied at the shear center of the cross section, avoiding in this way the induction of a twisting moment. To account for shear deformations, the concept of shear deformation coefficients is used. Five boundary value problems are formulated with respect to the transverse displacements, the axial displacement and to two stress functions and solved using the AEM. Application of the boundary element technique yields a system of non-linear equations from which the transverse and axial displacements are computed by an iterative process. The evaluation of the shear deformation coefficients is accomplished from the aforementioned stress functions using only boundary integration. Numerical examples are worked out to illustrate the efficiency, the accuracy, the range of applications of the developed method and the influence of the shear deformation effect.     

Nonlinear analysis of shear deformable beam-columns partially supported on tensionless three-parameter foundation

In this paper, a boundary element method is developed for the nonlinear analysis of shear deformable beam-columns of arbitrary doubly symmetric simply or multiply connected constant cross-section, partially supported on tensionless three-parameter foundation, undergoing moderate large deflections under general boundary conditions. The beam-column is subjected to the combined action of arbitrarily distributed or concentrated transverse loading and bending moments in both directions as well as to axial loading. To account for shear deformations, the concept of shear deformation coefficients is used. Five boundary value problems are formulated with respect to the transverse displacements, to the axial displacement and to two stress functions and solved using the Analog Equation Method, a BEM-based method. Application of the boundary element technique yields a system of nonlinear equations from which the transverse and axial displacements are computed by an iterative process. The evaluation of the shear deformation coefficients is accomplished from the aforementioned stress functions using only boundary integration. The proposed model takes into account the coupling effects of bending and shear deformations along the member as well as the shear forces along the span induced by the applied axial loading. Numerical examples are worked out to illustrate the efficiency, wherever possible, the accuracy and the range of applications of the developed method.