带中心孔形状记忆合金板在双轴加载下的响应研究

为了研究复杂应力下多孔SMA的超弹性特性,用一热力学有限元模型对带中心孔宽板在双轴拉应力作用下孔临界区域的应力分布以及相变情况进行了有限元模拟计算.初始时,平板处于奥氏体状态,在载荷作用下,在圆孔边界处首先发生相变,并随着载荷的增加,相变区域逐渐向内部扩展.在计算中,两个加载方向同时加载,并采用了不同的加载比例R.随着加载比例的增大,圆孔周围完全发生马氏体相变的区域逐渐增大,等效应力分布也发生变化.随着2-方向加载水平的增加,1-方向的应力-应变滞后环位置逐渐逐渐降低,马氏体相变临界应力也逐渐减小.同时确立了应力空间(σ11,σ12)上的相变初始面.     

双轴拉伸下高聚物的时间-应力等效原理试验研究

时间-应力等效原理(TSSP)能有效简化高聚物的黏弹性本构模型和力学性能测试。以往研究主要集中在单向应力状态,但工程构件通常处于双向或三向复杂应力状态。为此,首先利用有限元方法对双轴十字型试样进行合理设计与优化,使试样中心测试区的应力和应变基本满足均匀分布,并依此加工出硅橡胶试样。然后,对硅橡胶试样进行不同应力水平及应力比的双轴蠕变试验,获得一系列短期蠕变试验曲线。基于柯西应力和左柯西-格林变形张量定义合适的应力度量和变形度量以探讨双轴拉伸下的TSSP。按照主曲线移位方法,将不同应力水平下的短期性能曲线沿对数时间轴移位构建出宽广时域的主曲线。利用应力移位因子方程对移位结果进行非线性回归分析,获得相应的方程参数。研究结果可为复杂应力状态下黏弹性高聚物的长期力学性能加速表征和长期寿命预估提供理论基础和重要参考。     

双向玻纤织物复合材料双轴拉伸载荷下的力学行为

为研究双向玻纤织物复合材料在复杂应力状态下的力学行为,设计双轴加载十字型试样,对其进行不同载荷比的双轴拉伸实验,对比分析了材料在双轴拉伸载荷下的拉伸模量、拉伸强度及失效模式。结果表明:双向玻纤织物复合材料单轴拉伸行为表现为后期非线性、脆性断裂,双轴拉伸载荷下非线性现象更为显著;双轴拉伸模量随载荷的增大而增加,双轴拉伸载荷对材料的拉伸模量具有一定的强化作用;材料的双轴拉伸强度存在双向弱化效应,等比例双轴拉伸时,双轴拉伸强度最低,仅为单轴强度的60.5%;试样破坏发生于中心实验区域,材料不同载荷比的破坏形式有所不同,分别主要表现为纤维断裂、基体失效和玻纤布分层。     

轴类零件应力集中系数的理论研究

 应用Algor公司的SAPgl软件计算了受拉阶梯轴倒角处的应力集中系数，理论计算结果与前人的光弹实验结论比较吻台．研究表明：轴的倒角半径对应力集中效应有显著影响；当其他参数不变时，改变轴端力的作用方式或轴段的长度，均会产生不尽相同的应力集中系数．     

试样参数对划痕试验临界载荷的影响

本文有摩擦力监测的划痕法研究了试样参数（基体硬度、膜厚、基体表面粗糙度等）对硬质摩膜（或软膜）的临界载荷Ｌｃ的影响．样品基材是Ｗ６Ｍｏ５Ｃｒ４Ｖ２、５ＣｒＭｎＭｏ、４５钢、Ａ３钢和不锈钢．膜层为磁控溅射离子镀ＴｉＮ膜和Ｔｉ膜、化学镀ＮｉＰＣＵ膜，以及在Ｓｉ３Ｎ４上溅射镀Ａｌ膜．结果发现离子度ＴｉＮ膜和Ｔｉ膜与ＮｉＰＣｕ膜规律不同，ＴｉＮ膜和Ｔｉ膜的临界载荷Ｌｃ随基体硬度的提高而提高，但当基体硬度接近和超过膜层硬度时，Ｌｃ变化不大；基体粗糙度增加时Ｌｃ下降；随膜厚的增加Ｌｃ提高．而化学镀ＮｉＰＣｕ膜的临界载荷Ｌｃ随基体硬度的提高而下降，随基体表面粗糙度的提高而增加．以Ｗ６Ｍｏ５Ｃｒ４Ｖ２为基体的ＮｉＰＣｕ膜，随膜厚的增加Ｌｃ变化不大．本文对上述规律的实质进行了初步探讨，并对目前生产中正在推广的工具钢磁控溅射离子镀ＴｉＮ膜的划痕法标准提出建议．     

三维机织碳/碳复合材料双轴压缩载荷下的力学行为

基于三维机织碳/碳复合材料的细观结构特征, 设计平板十字形试样, 在材料双轴力学性能试验机上开展了复合材料单轴、 双轴加载压缩试验, 对比分析了三维机织碳/碳复合材料在双轴压缩载荷下的力学行为。研究表明: 三维机织碳/碳复合材料的压缩行为表现为非线性、 脆性断裂; 双轴载荷作用下非线性特征更为显著, 压缩模量随应力的增加而增大, 强度与模量相较于单轴有较大幅度增加, 双轴压缩载荷作用下材料的强化效应显著; 试样破坏位置并未出现在试样中心区, 而是发生在试样的加载端部或十字形试样的加载分枝根部, 主要表现为基体开裂、 纤维断裂和层间脱粘, 碳布及其层间界面剪切强度的强弱直接影响材料的压缩强度。     

矩形拉伸试样夹持位置对金属材料强度测试值的影响

通过X70管线钢板和S690QL高强钢板的室温拉伸试验,研究了矩形拉伸试样夹持位置对钢材强度测试值的影响。结果表明:在试验人员、试验机及引伸计、试样、试验方案、试验环境均相同的前提下,试样夹持在夹头的边缘与夹持在夹头的中间相比,测得的下屈服强度ReL、上屈服强度ReH和抗拉强度Rm几乎没有变化,但会造成试样的拉伸应力-应变曲线弹性变形阶段异常,从而影响对规定塑性延伸强度Rp0.2及规定总延伸强度Rt0.5的测试。     

拉伸试样弯曲度对屈服强度测试值的影响

分析了输送管管体横向拉伸试样弯曲度对屈服强度测试值的影响,从理论上初步阐明了管体横向拉伸试样的弯曲会引起屈服强度测试值的降低,并提出在相关的标准中应对管体横向试样的展平程度作出规定.     

搭接区端部细观结构对受拉复合材料单搭接头力学响应的影响

在复合材料单搭接头的加工过程中,在搭接区端部会形成一些细观结构,从而在这些区域常存在比较严重的应力集中。应用实验和有限元方法研究了胶瘤和复合材料端部毛刺这2 种搭接区端部细观结构对受拉复合材料层合板单搭接头力学响应的影响。应用数字图像相关方法测量了搭接区端部的应变场分布情况,同时利用基于子模型技术的非线性有限元方法分析了搭接区端部细观结构的作用。实验结果与有限元分析结果吻合较好。实验和有限元结果都表明胶瘤分担了部分载荷,可以降低搭接区端部的应力集中。复合材料端部毛刺的作用与毛刺的具体结构关系密切,不同结构的毛刺对搭接区端部应力应变分布的影响是不同的。     

一种含目视几乎不可见损伤(BVID)复合材料层压板面内失效准则的双轴加载验证

在复合材料飞机结构设计中,由于应力的铺层相关性,工程上通常采用应变进行结构强度预测.另外,复合材料飞机需考虑损伤容限设计,必须对含损伤,尤其是目视几乎不可见损伤(Barely visible impact damages,BVID)结构的适航符合性进行评估.因此,民用飞机结构设计上更需要基于应变的并考虑含损伤的复合材料...     

An analysis of fracture under biaxial loading using the non-singular T-stress

Finite element analysis using a two-dimensional modified-boundary-layer approach was used to model the effects of biaxial loading on crack tip stress fields. Loadings were applied corresponding to an elastic KI field, non-singular T-stress and a biaxial stress. For through-thickness cracks the T-stress inherent in the specimen geometry is augmented by the external biaxial stress. For surface-notched specimens the biaxial stress acts out of the crack plane. This effect was modelled with generalized plane strain elements. Results were analysed using the Anderson-Dodds approach for cleavage and the Beremin model in the ductile regime. Biaxial loading is predicted to have a large effect on the toughness of a through-thickness crack but little effect on a surface crack. Experimental results from a previous series of large-scale biaxial fracture tests are generally consistent with these predictions.     

On the identification of an effective cross section for a cruciform specimen

Biaxial tensile testing of sheet metals is becoming increasingly popular for sheet metal forming. Determining equivalent stresses in biaxial tensile specimens is more complicated than in conventional uniaxial tensile specimens. In the present study, we compare four different approaches to calculate effective stresses during biaxial tensile loading of a cruciform specimen: (a) partial unloading method, where stresses are determined based on force–strain curves; (b) identification with uniaxial tensile testing; (c) an analysis of equivalent biaxial tests; and (d) numerical simulations. Considering experimental results for an AA1050 aluminium alloy and for a low‐carbon steel DC06, we show that, for the cruciform sample studied here, two methods do not yield physically reasonable results: The uniaxial approach does not properly take into account the effect of transverse loading, and the equivalent biaxial approach exhibits uncertainties in strain measurement data. The most comprehensible approach is the numerical method, because it also yields detailed information about the local stress and strain states. The numerical results are in excellent agreement with the partial unloading method in terms of the initial flow stress and of effective stress–strain curves for strains up to 0.02, with both methods predicting a similar effective cross section of 18.0 mm² for the considered specimen.     

In‐plane and out‐of‐plane constraint for single edge notched bending specimen and cruciform specimen under uniaxial and biaxial loading

Considering fracture constraint is an efficient way to describe stress–strain field and fracture toughness more accurately, so it is necessary to realise the relationship with in‐plane and out‐of‐plane constraint for different standard specimens. In this paper, three‐dimensional finite element method is applied to study the in‐plane and out‐of‐plane constraint for both cruciform specimen and single edge notched bending specimen made from commercial pure titanium. Crack length and in‐plane loading as the factors affecting in‐plane constraint, and thickness as the factor affecting the out‐of‐plane constraint are used to study the effect on both in‐plane and out‐of‐plane constraint in this paper. From the results, in‐plane and out‐of‐plane constraint are both related to specimen geometries and loading styles. And there exist relationships with in‐plane and out‐of‐plane constraint because of factors for different specimens. Depending on crack length, out‐of‐plane constraint increases with in‐plane constraint. While depending on transverse loading, out‐of‐plane constraint decreases with in‐plane constraint. In addition, when the in‐plane constraint of a specimen is higher, in‐plane constraint increases with out‐of‐plane constraint (thickness). When the in‐plane constraint is lower, in‐plane constraint almost remains unchanged with out‐of‐plane constraint.     

含孔CFRP正交层合板的双轴拉伸力学行为

采用加载臂开槽的中心开孔等厚度十字形试样,实验研究了正交对称铺层碳纤维增强聚合物基复合材料（CFRP）层合板在双轴拉伸载荷作用下的力学行为,分析了3种双轴加载比对其拉伸强度和破坏行为的影响。研究表明:纤维被切断的铺层部分在拉伸作用下容易与其相邻铺层脱粘,导致层合板承载力下降;等双轴加载时,在孔边的被切断纤维与连续纤维间基体在横向拉伸和纵向剪切组合作用下首先开裂;非等双轴加载时,在垂直于快速拉伸方向的铺层中沿孔边应力集中处先出现基体裂纹;随着加载比的增大,快速拉伸方向的细观结构损伤随载荷的增大发展更快,刚度下降更快,破坏时主裂纹的扩展方向更趋于垂直于快速拉伸方向;强度包络线的分析表明快速拉伸方向的拉伸强度随加载比的增大呈缓慢增大的趋势。      

Failure prediction for a glass/epoxy cruciform specimen under static biaxial loading

Material models were developed to predict the mechanical behavior of glass/epoxy multidirectional laminates under complex stress states. An incremental plane stress analysis was performed, taking into account the anisotropic material non-linearity, separate damage onset conditions and distinct post-failure stiffness degradation rules. Theoretical formulations were implemented in a shell element of the 1st order shear deformation theory. Numerical results were validated via comparison with test data from cruciform specimens subjected to static biaxial tensile loading. Local strain gauge and full-field strain measurements, obtained using the Digital Image Correlation (DIC) technique, corroborated numerical predictions. Improved strength and failure mode results were derived when, in addition to stiffness reduction, compressive strength degradation in the fiber direction was also considered.     

Stress intensity factors of a central slant crack with frictional surfaces in plates with biaxial loading

The stress intensity factor is a traditional topic in mechanics and there have been many solutions for many different cases. The closed frictional crack problem has been modeled in the rock mechanics field where fractures are mostly under compression. Further, the effect of finite plate dimensions under biaxial loading has not been considered in the literature. The key contribution of the present paper is to evaluate the effect of the crack length to plate width ratio on the mode I and II stress intensity factors (SIF) of a central slant crack with frictional surfaces in plates with biaxial loading of different patterns, i.e. tension-tension, tension-compression, compression-tension or compression-compression. A plane strain elastic two-dimensional finite element analysis was adopted. Crack length to plate width ratios equal to 0.1, 0.3 and 0.5 with biaxial ratios from –1 to 1, crack angles from 0° to 90° and friction coefficients from 0 to 1 were considered. Contact regimes and the effect of the crack length to plate width ratio were found dependent on biaxial ratio and pattern, friction coefficient and crack angle.     

Buckling analysis of orthotropic rectangular plate resting on Pasternak elastic foundation under biaxial in-plane loading

In this study, buckling of rectangular orthotropic plates resting on a Pasternak elastic foundation under biaxial in-plane loading by the power series method (the method of Frobenius) was analyzed. Similar to many studies, two opposite edges of loading are simply supported and two other edges are assumed clamped. In order to extract the characteristic equations of orthotropic rectangular plate under in-plane loading resting on a Pasternak elastic foundation, the classical plate theory, by considering the interaction between plate and foundation, is used. The results showed that in the aspect ratio of less than 2, the existing Pasternak foundation caused the buckling load to increase severely, but by increasing the aspect ratio, the effect of the foundation is negligible. Applying the in-plane load in the y-direction caused the buckling load to decrease, but by increasing the aspect ratios the effect of the load is negligible.     

3D meso-mechanical analysis of concrete specimens under biaxial loading

In recent years, the mechanics of materials group at ETSECCPB‐UPC has developed an approach for meso‐mechanical analysis of concrete using zero‐thickness interface elements in 2D and more recently in 3D. In this methodology, the meso‐structure is generated with in‐house developed computer programs based on Voronoï/Delaunay theory. In the analysis, continuum elements are assumed linear elastic. Non‐linearity and fracture phenomena are made possible by the systematic use of zero‐thickness interface elements inserted on a priori determined potential fracture planes. In this paper, the results obtained for a 3D specimen under biaxial loading are presented. The results turn out to be very satisfactory and, in particular, it is observed that even the specimens which contain a reduced number of aggregates (14 in the present calculations) lead to a realistic failure envelope under biaxial loading, and they also capture the tendencies of cracking and fracture orientations observed in experiments for different rates of biaxial loading. The special limit case of biaxial loading under restrained out‐of‐plane deformations is also analysed, leading to practically elastic behaviour as shown by available experimental evidence.     

Fatigue behaviour of a box-welded joint under biaxial cyclic loading: effects of biaxial load range ratio and cyclic compressive loads in the lateral direction

ABSTRACT The biaxial fatigue of a steel plate (JIS SM400B) having a box‐welded (wrap‐around) joint was experimentally studied. Special concerns were focused on the effects of the biaxial load range ratio and compressive cyclic loading in the lateral direction. The direction of fatigue crack propagation under biaxial cyclic tensile loading, which has a phase difference of π, changed according to the biaxial load range ratio, R_xy = ΔP_x/ΔP_y. When R_xy was less than 0.56, fatigue cracks propagated along the toe of the weld in the x‐direction because the principal tensile stress range Δσ_y at that location exceeded the orthogonal value Δσ_x at the box‐weld toe. The fatigue lives in biaxial tests related well to the data from uniaxial tests when invoking the Δσ₅ criterion. However, the location and direction of Δσ₅ should be chosen according to the R_xy value and the failure crack direction. An increase in Δσ₅, as induced by the Poisson's ratio effect from either the out‐of‐phase tensile loading or the in‐phase compressive loading in the y‐direction, leads to an increase in fatigue damage (decrease in fatigue resistance or specifically a faster crack propagation rate), and this effect can be successfully estimated from uniaxial fatigue test data.