材料的双轴拉伸的规律的研究

汽车车身覆盖件是冲压成型的,其覆盖件的高精度回弹量计算非常困难,并少见有研究和报道。由于冲压成型过程是一个大应变和塑性屈服,常规的拉伸试验机无法测出其应力应变曲线,而目前汽车工业领域还没有建立用于修正回弹量的数据库,所以必须通过数值模拟和实验的方法对汽车车身覆盖件进行弹性模量、屈服强度和硬化规律进行研究。本实验研究的目的是通过对规定材料的双轴拉伸试验,应用数字图像处理的光测方法,确定材料屈服强化阶段复杂本构模型的材料特性常数,以达到正确合理描述复杂材料的硬化行为。试验中采用机械式拉伸试验机实现材料的双轴拉伸行为,利用数字相关光测方法获得拉伸材料屈服面屈服过程中大应变全场的数据,通过数字图像相关处理,得到材料在屈服强化阶段复杂本构模型的特性参数。实验研究的设备和仪器包括(1)双轴拉伸试验机;(2)计算机,高分辨率CCD和摄像头,图像卡,图像处理软件;(3)其它辅助设备等。实验研究设备的规范标准根据国家现行实验规范标准对本实验研究设备进行标定和计量审核。     

谈力的功的定义

已刊登的几篇关于讨论力的功的定义的文章中,关于功的定义是准确的,严格的,完整的,并不需要作补充或修正.但如果把功的定义改为     

谈力的功的定义

<正> 已刊登的几篇关于讨论力的功的定义的文章中,关于功的定义是准确的,严格的,完整的,并不需要作补充或修正.但如果把功的定义改为     

“关于力的功的定义”的讨论

<正> 本刊1987年第3期上刊登了上官飞的“关于力的功的定义”一文后,除了严宗达、王光远发表《对“关于力的功的定义”一文的答复》(见本期)以外,还有一些读者来文参加讨论,现将其主要观点摘要如下:绝大多数力学教材在讨论力的功时,都是从作用于质点上力的元功 δW=F·dr 出发,其中 dr 是力的     

随机激励的耗散的哈密尔顿系统的平稳解

本文首先为一般的随机激励的耗散的哈密尔顿系统得到精确的平稳解,然后在此基础上为类似而更为一般的系统发展了等效非线性系统法。     

随机激励的耗散的Hamilton系统理论的研究进展

近几年中,利用Ｈａｍｉｌｔｏｎ系统的可积性与共振性概念及Ｐｏｉｓｓｏｎ括号性质等,提出了高斯白噪声激励下多自由度非线性随机系统的精确平稳解的泛函构造与求解方法,并在此基础上提出了等效非线性系统法,提出了拟Ｈａｍｉｌｔｏｎ系统的随机平均法,并在该法基础上研究了拟Ｈａｍｉｌｔｏｎ系统随机稳定性、随机分岔、可靠性及最优非线性随机控制,从而基本上形成了一个非线性随机动力学与控制的Ｈａｍｉｌｔｏｎ理论框架．本文简要介绍了这方面的进展．     

随机激励的耗散的哈密尔顿系统的平稳解

本文首先为一般的随机激励的耗散的哈密尔顿系统得到精确的平稳解,然后在此基础上为类似而更为一般的系统发展了等效非线性系统法     

关于力的功的定义

在许多有关的力学教科书中,力的功的定义可以表述为:力F的大小与力的作用点沿力方向无限小的位移ds·cosθ的乘积的积分,即(参照图1) ...     

岩石断裂尺寸效应测试装置研制及实验研究

面向岩石断裂尺寸效应研究的实验装置需求,针对现有技术中三点弯曲装置对多组尺寸岩石试件适应性差、最小跨距的测试量程不足等问题,研制了一种可灵活用于岩石断裂尺寸效应测试的三点弯曲装里.装置采用"两体分离式"的设计,三种不同型号的滚子与压头体、支座体配合使用,有效避免了不同尺寸试件采用同一直径滚子测试带来的实验精度问题;同时...     

Crack path kinking under generalized stress state

Results of experimental investigations of three-point asymmetric bending applied to beams made of materials with substantially different structural, strength, and strain characteristics are presented. The effect of different types of fracture on the formation of crack kinks under a generalized stress state is revealed. Strength curves of the Coulomb-Mohr type are obtained for Plexiglas and fine-grained concrete, which are characterized by the quasi-brittle and brittle types of fracture, respectively. Dependences of the kink angles of the crack paths on the stress state in the case of three-point bending are described for these materials. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 50, No. 3, pp. 205–213, May–June, 2009.     

水力劈裂问题的态型近场动力学建模

将态型近场动力学理论引入水力劈裂问题的模拟。构建了能反映岩土类材料准脆性断裂特征的态型近场动力学本构模型,并在物质点间相互作用力模型中加入等效水压力项,以实现在新生裂纹面上跟踪施加水压力。同时,考虑裂纹面间的接触,引入物质点间的短程排斥力作用,并设计了相应的接触算法。通过自编程序将模型和算法应用于含初始裂纹、不含初始裂纹以及含坝基软弱结构面的混凝土重力坝在高水头作用下的水力劈裂过程模拟,并与扩展有限元等模拟结果对比,验证了本文模型和算法的可行性和准确性。     

I-II复合型裂缝断裂角的分层剪滞模型与分析

采用修正的剪滞理论建立了岩石、混凝土等准脆性材料的I-II复合型裂缝在单向拉伸荷载作用下的计算模型,得到了与实验相吻合且优于传统S判据的断裂角。通过对远场应力、斜裂缝区应力以及子层位移的合理简化,得到了求解剪滞分析模型的边界条件,进而得到了含斜裂缝的各子层位移分布函数。引入最大应力集中因子,对I-II复合型裂缝前缘应力场进行简化;基于斜裂缝沿最大应力集中因子方向扩展,得到裂缝的断裂角。根据斜裂缝的应力分布,设置不同的子层分区,得到了更为细化的位移分布模式。通过对计算数据的分析,针对单向拉伸荷载作用下的I-II复合型裂缝,建立了按应力场分区设置子层的分层剪滞模型,得到更为精确的斜裂缝断裂角。     

岩石动态断裂韧度的尺寸效应

采用两种圆孔裂缝平台巴西圆盘试件(一种为直径分别为42、80、122、155 mm的几何相似试件,另一种为直径80 mm、仅裂缝长度不同的单一尺寸试件)对岩石动态断裂韧度的尺寸效应进行了研究。给出了在霍普金森压杆系统上对试件进行径向撞击产生的应变波形和断裂模式。实验结果表明,对于几何相似试件,动态断裂韧度的测试值随着尺寸的增大而增大,而对于单一尺寸试件,其测试值随着中心裂缝长度的增加呈现先增大后减小的趋势。裂缝前端的断裂过程区长度和孕育时间是岩石动态断裂韧度测试值表现为尺寸效应的主要原因,为了减小尺寸效应,建立了考虑这两个参数在空间-时间域对动态应力强度因子的分布进行积分后再平均来确定岩石动态断裂韧度的方法。     

Critical defect size distributions in concrete structures detected by the acoustic emission technique

Extensive research and studies on concrete fracture and failure have shown that concrete should be viewed as a quasi-brittle material having a size-dependent behavior. Numerous experimental techniques have been employed to evaluate fracture processes, and a number of modeling approaches have been developed to predict fracture behavior. A non-destructive method based on the Acoustic Emission (AE) technique has proved to be highly effective, especially to assess and measure the damage phenomena taking place inside a structure subjected to mechanical loading. In this paper, comparing AE frequency-magnitude statistics in solids subjected to damage processes with defect size distributions for disordered materials, critical parameters defining instability conditions for monitored structures are found. In addition, an experimental investigation conducted on concrete and RC structures by means of the AE technique is described. Experimental results confirm the described theories.     

Microcracking and Fracture Process in Cement Mortar and Concrete: A Comparative Study Using Acoustic Emission Technique

This article reports the acoustic emission (AE) study of precursory micro-cracking activity and fracture behaviour of quasi-brittle materials such as concrete and cement mortar. In the present study, notched three-point bend specimens (TPB) were tested under crack mouth opening displacement (CMOD) control at a rate of 0.0004 mm/sec and the accompanying AE were recorded using a 8 channel AE monitoring system. The various AE statistical parameters including AE event rate $ \left( {\frac{dn }{dt }} \right) $ , AE energy release rate $ \left( {\frac{dE }{dt }} \right) $ , amplitude distribution for computing the AE based b-value, cumulative energy (ΣE) and ring down count (RDC) were used for the analysis. The results show that the micro-cracks initiated and grew at an early stage in mortar in the pre peak regime. While in the case of concrete, the micro-crack growth occurred during the peak load regime. However, both concrete and mortar showed three distinct stages of micro-cracking activity, namely initiation, stable growth and nucleation prior to the final failure. The AE statistical behavior of each individual stage is dependent on the number and size distribution of micro-cracks. The results obtained in the laboratory are useful to understand the various stages of micro-cracking activity during the fracture process in quasi-brittle materials such as concrete & mortar and extend them for field applications.     

Interplay of size effects in concrete specimens under tension studied via computational stochastic fracture mechanics

We attempt the identification, study and modeling of possible sources of size effects in concrete structures acting both separately and together. We are particularly motivated by the interplay of several identified scaling lengths stemming from the material, boundary conditions and geometry. Methods of stochastic nonlinear fracture mechanics are used to model the well published results of direct tensile tests of dog-bone specimens with rotating boundary conditions. Firstly, the specimens are modeled using microplane material law to show that a large portion of the dependence of nominal strength on structural size can be explained deterministically. However, it is clear that more sources of size effect play a part, and we consider two of them. Namely, we model local material strength using an autocorrelated random field attempting to capture a statistical part of the complex size effect, scatter inclusive. In addition, the strength drop noticeable with small specimens which was obtained in the experiments is explained by the presence of a weak surface layer of constant thickness (caused e.g., by drying, surface damage, aggregate size limitation at the boundary, or other irregularities). All three named sources (deterministic-energetic, statistical size effects, and the weak layer effect) are believed to be the sources most contributing to the observed strength size effect; the model combining all of them is capable of reproducing the measured data. The computational approach represents a marriage of advanced computational nonlinear fracture mechanics with simulation techniques for random fields representing spatially varying material properties. Using a numerical example, we document how different sources of size effects detrimental to strength can interact and result in relatively complex quasibrittle failure processes. The presented study documents the well known fact that the experimental determination of material parameters (needed for the rational and safe design of structures) is very difficult for quasibrittle materials such as concrete.     

Asymptotic expansion homogenization of discrete fine-scale models with rotational degrees of freedom for the simulation of quasi-brittle materials

Discrete fine-scale models, in the form of either particle or lattice models, have been formulated successfully to simulate the behavior of quasi-brittle materials whose mechanical behavior is inherently connected to fracture processes occurring in the internal heterogeneous structure. These models tend to be intensive from the computational point of view as they adopt an “a priori” discretization anchored to the major material heterogeneities (e.g. grains in particulate materials and aggregate pieces in cementitious composites) and this hampers their use in the numerical simulations of large systems. In this work, this problem is addressed by formulating a general multiple scale computational framework based on classical asymptotic analysis and that (1) is applicable to any discrete model with rotational degrees of freedom; and (2) gives rise to an equivalent Cosserat continuum. The developed theory is applied to the upscaling of the Lattice Discrete Particle Model (LDPM), a recently formulated discrete model for concrete and other quasi-brittle materials, and the properties of the homogenized model are analyzed thoroughly in both the elastic and the inelastic regime. The analysis shows that the homogenized micropolar elastic properties are size-dependent, and they are functions of the RVE size and the size of the material heterogeneity. Furthermore, the analysis of the homogenized inelastic behavior highlights issues associated with the homogenization of fine-scale models featuring strain-softening and the related damage localization. Finally, nonlinear simulations of the RVE behavior subject to curvature components causing bending and torsional effects demonstrate, contrarily to typical Cosserat formulations, a significant coupling between the homogenized stress–strain and couple-curvature constitutive equations.     

Tensile fracture behavior of heterogeneous materials based on fractal geometry

Many of heterogeneous structural materials, like concrete, have different behavior under tensile stresses in comparison to their behavior under compressive stresses. The aim of this paper is to interpret behavior of such materials subjected to tensile stresses, by using newly introduced concept of fractal geometry. In the first part of this paper, tensile behavior of granular composites has been studied by using fractal geometry. It is shown that the fractality of the cross section in this kind of composites can be used to interpret the size effect on tensile strength. In fact, this work is a modification with innovations on the previous studies on fractal based size effect.This hypothesis that the fracture surfaces of quasi-brittle materials are fractals has been verified by several investigations. Accordingly, in the other part of this paper, softening process in heterogeneous materials is studied. Resulting from presented approach, a new softening curve for quasi-brittle materials is proposed. This new softening curve is denominated “Quasi-fractal softening curve” and is consisted of two parts, a linear portion in beginning part and an exponential portion in rest of the curve. This makes it very compatible to the pre-existing softening curves.