不同加载状态下TA2钛合金绝热剪切破坏响应特性

一般认为绝热剪切现象在宏观上表现为材料动态本构失稳,即热软化大于应变硬化.本文采用帽型受迫剪切试样研究TA2钛合金的动态力学特性和本构失稳过程.首先对剪切区加载应力状态进行理论和数值分析,通过合理设计帽型试样,剪切区变形可近似按剪切状态处理;结合二维数字图像相关法(two-dimensional digital image correlation,DIC-2D)直接测试试样剪切区应变演化,给出帽型受迫剪切实验的等效应力-应变响应曲线.进一步,利用Hopkinson压杆对TA2钛合金开展动态压缩及帽型剪切对比试验研究,比较压缩、剪切试验得到的等效应力-应变曲线,采用"冻结"试样方法分析试样中绝热剪切局域化演化过程,探讨不同加载状态下TA2钛合金的绝热剪切破坏现象及其动态力学响应特性.实验结果表明,在塑性变形初始阶段,动态压缩及剪切加载下的等效应力-应变曲线符合较好,但随塑性损伤发展及绝热剪切带形成,两者出现分离,表明损伤及绝热剪切演化过程与应力状态相关.剪切试样实验得到的本构"软化"特性能够反映绝热剪切带起始、破坏演化过程的力学响应特性,而在动态压缩实验中,即使试样中已出现双锥形的绝热剪切带及局部裂纹分布,其表观等效应力-应变曲线并不出现软化特征,动态压缩实验无法得到关于绝热剪切起始、发展以及破坏的本构软化响应特性.     

不同应变率下冰破坏特性的试验研究

为研究寒区流冰对水工/船舶结构的影响,需要获得不同流速冰载荷的特征,准静态下不同应变率条件下冰材料变形至破坏的特征就成为一项重要的基础性问题。为此,开展了低温环境应变率10-2s-1至10-4s-1下淡水冰的单轴压缩试验。试验发现:随着应变率减小,极限应力由18.51MPa降低至8.44MPa,达到极限应力后,试件承载能力由瞬间消失变为逐渐降低至稳态,试件破坏形貌由劈裂转变为周向膨胀,即由脆性转变为韧性破坏,转变应变率约为10-3s-1;在双对数坐标系中,单轴压缩强度随应变率的增加近似呈线性增大。通过对应力-应变曲线进行积分,给出了不同破坏形式下冰变形至破坏的临界应变能密度,发现脆/韧转变状态下加载至破坏所需能量最大,该能量特征主要由冰中微裂纹的萌生、断面摩擦、再结晶引起。     

压剪复合加载下聚丙烯屈服行为的实验研究

为研究聚丙烯在压剪复合加载下的变形行为和屈服规律,设计了一种压剪复合加载的试件(Shear-Compression Specimen,SCS),分析其构型参数对变形行为的影响,并分别对倾斜角度为5°,15°,30°,45°,60°的SCS进行了准静态加载实验,对SCS压缩变形过程中测量区的压缩应力和剪切应力的演化规律进行了分析。结果发现:聚丙烯在压剪复合加载下屈服规律受静水压影响明显,偏离von-Mises屈服准则;而Hu-Pae屈服准则可较好地描述聚丙烯在不同压剪复合加载条件下的屈服规律。此外,对加载速率影响屈服的规律进行研究,发现聚丙烯在压剪复合加载下的屈服行为具有应变率敏感性。     

α-钛合金TA6的动态力学性能和剪切现象分析

诸多文献已经报道了β和(α β)钛合金冲击和爆炸加载下的绝热剪切现象,本文利用分离式的Hopkinson压杆和反射式Hopkinson拉杆对α-钛合金TA6的动态力学性能进行研究,得到不同应变率下材料的应力-应变曲线,发现钛合金TA6和其他钛合金一样也是一种应变率敏感材料。冲击拉伸加载时破坏形状45度剪切断口,然而其在冲击压缩应力作用下体现为典型的韧性,并未出现其它类型钛合金通常会出现的绝热剪切破坏。进一步分析表明:α-钛合金TA6不同于β和(α β)的钛合金,α-钛合金TA6相对于β和(α β)的钛合金是一种绝热剪切不敏感的材料。同时也利用应力状态的柔韧系数的概念对拉伸时45度的剪切断口解释。     

激光金属沉积GH4169在不同应变率下的剪切特性及破坏机理研究

为了能在传统的分离式Hopkinson压杆上准确可靠地测试激光金属沉积GH4169的动态剪切特性，基于数值模拟方法对比分析了三种不同动态剪切试样形式及尺寸对剪切区应力分布的影响，结果表明:经过尺寸优化后的双剪切试样的剪切区剪应力占主导地位，可实现近似纯剪切的动态剪切实验。利用此试样形式，系统测试了不同取向（扫描方向、沉积方向）的LMD GH4169试样在不同应变率下的剪切应力应变曲线，并对破坏后试样进行了SEM分析观察。结果表明:(1) 本文中选用的试样形式剪切纯度高，应力沿剪切区宽度厚度分布均匀，可以更好地得到材料的动态剪切特性；(2) 对实验所得剪应力-剪应变曲线进行分析，发现本材料在扫描路径方向和沉积方向并没有表现出明显的各向异性，但随着应变率的增加，具有明显的应变率强化效应；将单轴压缩和动态剪切应力应变曲线同时转换为等效应力应变曲线，对比证实了试样形式能很好反应材料的剪切特性；(3) 通过对LMD GH4169剪切变形破坏试样的微观分析发现，随着应变率升高，断口韧窝尺寸和深度减小，韧性降低，在更小的变形量下容易剪切失效。初始微观缺陷容易导致材料的动态剪切破坏。     

单轴压缩荷载下“十字形”交叉裂隙类岩石试件的破坏模式与力学特性试验研究

通过在类岩石材料中预制特定裂隙的方法,来模拟含"十字形"交叉裂隙的岩体,并对其在单轴压缩情况下的力学特性与破坏模式进行了试验研究。研究表明:(1)交叉裂隙试件中,次裂隙倾角对主裂隙倾角为45°时试件的抗压强度与起裂应力的影响最大,而对主裂隙倾角为90°时的影响最小。(2)单裂隙试件的抗压强度与起裂应力小于完整试件。交叉裂隙试件相对于单裂隙试件而言,当主裂隙倾角为0°和45°时,其抗压强度与起裂应力减小;当主裂隙倾角为90°时,其抗压强度与起裂应力增大。(3)在单裂隙试件中,当裂隙倾角为0°、45°和90°时,试件分别发生拉剪复合型破坏、剪切破坏和张拉破坏;相对于单裂隙试件而言,交叉裂隙试件破坏时产生的裂纹相对较多,破坏模式大多为拉剪复合型破坏,同时,在破坏过程中交叉裂隙试件发生复合其他破坏模式或直接导致试件破坏的滑移。上述结果对岩体工程的开挖和建设具有一定的参考价值。     

平纹编织陶瓷基复合材料面内剪切细观损伤行为研究

采用约西佩斯库(Iosipescu)纯剪切试件,研究了平纹编织SiC/SiC和C/SiC复合材料的面内剪切应力-应变行为和细观损伤特性.通过试验获得了材料不同方向上的单调和迟滞应力-应变行为,对比分析了两种材料的剪切损伤特性,结果表明材料的剪切损伤演化规律受热残余应力水平影响严重.由试件断口电镜扫描结果发现剪切加载状态下桥连纤维承受显著的弯曲载荷和变形,据此提出了纤维弯曲承载机制,并结合裂纹闭合效应分阶段阐释了材料的剪切迟滞环形状.基于材料的剪切细观损伤机制,通过两个损伤变量表征了材料的剪切损伤演化进程,得到了材料的面内剪切细观损伤演化模型.对比发现2D-C/SiC复合材料45°方向基体裂纹的起裂应力明显小于2D-SiC/SiC复合材料,而两者0°/90°方向裂纹的起裂应力基本相同.      

含瓦斯煤的破坏和流动法则

讨论了含瓦斯煤破坏形式及相应的破坏条件,以应变分量表示应力分量建立了关联于Drucker-Prager准则和Lagrange准则的正则流动法则,考虑了剪切屈服和拉伸破坏两种状态之间的转换。剪切屈服后的流动是双向的,而拉伸破坏后流动是单向的;对于剪切屈服和拉伸破坏后的流动,应变变化路径不同,应变增量与应力增量之间的关系也不同;应力增量与应变增量的关系,不能由几个方程简捷地表示,需要用流程才能表达。     

复杂应力状态下GCr15轴承钢的动态剪切及熔融特性研究

本文探究了GCr15轴承钢压剪试样在冲击载荷下的绝热剪切和熔融破坏特性．首先,实验研究发现GCr15轴承钢的力学性能具有拉-压不对称性、应力状态及应变率敏感性．因此,通过引入应力三轴度、洛德角、应变率和绝热剪切温升机制扩展了经典J-C本构模型,并对GCr15轴承钢的动态单轴压缩和压剪试验结果进行了数值实现．研究结果表明,应力三轴度和洛德角是影响动态冲击下压剪试样的绝热剪切产生和熔融特性的重要因素．     

岩石材料单轴压缩损伤变量与纯剪切损伤变量间的关系

本文以岩石材料损伤应变能释放率为基础，利用不同的岩石屈服准则，探讨了岩石材料单轴压缩损伤变量与纯剪切损伤变量间的关系。借助于岩石材料单轴压缩或纯剪切中任一试验，即可求得两种受力状态下的损伤变量。     

花岗岩动静态压剪复合加载实验研究

岩石为天然含有微裂纹微孔洞等缺陷的复杂介质,其在复杂应力状态,尤其是在压剪应力状态下的性能研究,对工程应用有着十分重要的意义。本文利用双斜面垫块改进现有装置对花岗岩进行了动静态下的压剪复合加载实验,其中静态实验基于MTS材料试验机,动态实验基于分离式Hopkinson压杆（以下简称SHPB）,由此得到不同压剪载荷联合作用下花岗岩的静态和动态力学性能。通过改变倾斜端面的角度就能够得到岩石在不同压剪加载路径下的力学性能,由此可以得到岩石的静动态破坏面。结果表明:花岗岩的破坏强度具有明显的加载速率效应。本研究为探讨岩石在复杂应力状态下的力学性能提供了新的方法。     

压应力对压剪裂纹扩展的影响研究

本文针对压剪裂纹的启裂及扩展问题,研究了脆性材料中裂纹面压应力变化对其扩展的影响规律.借助双轴加载试验机可自由调节两个轴位移和力的优势,设计了一种单边对称双裂纹压剪试样.试验中,施加裂纹面压应力至不同的预设值后,使剪应力单调增大直至裂纹启裂及扩展,得到不同预设压应力下压剪裂纹启裂及扩展规律. 随着预设压应力增大,启裂角增大,裂纹扩展路径与初始裂纹的偏角也增大.但随着压应力增大,启裂角增大至一定值后趋于稳定,实验发现,依据裂纹是否闭合,压应力对压剪裂纹扩展的作用大致可分为两个阶段:闭合前,压应力对裂纹启裂载荷及启裂角、扩展路径均有影响,预设压应力增大,裂纹启裂载荷增大、启裂角增大,扩展路径愈来愈偏离初始裂纹方向;闭合后,压应力虽然增大,启裂角和临界压剪应力比始终恒定,压应力对临界剪力和扩展路径存在一定影响.研究表明,裂纹启裂角与启裂时的压剪应力比存在一定的对应关系.启裂时的压剪应力比增大,启裂角增大,启裂时的压剪应力比恒定,启裂角不变.      

FAILURE MODE AND CONSTITUTIVE MODEL OF PLAIN HIGH-STRENGTH HIGH-PERFORMANCE CONCRETE UNDER BIAXIAL COMPRESSION AFTER EXPOSURE TO HIGH TEMPERATURES

An orthotropic constitutive relationship with temperature parameters for plain highstrength high-performance concrete （HSHPC） under biaxial compression is developed. It is based on the experiments performed for characterizing the strength and deformation behavior at two strength levels of HSHPC at 7 different stress ratios including a=σs ： σ3=0.00：-1,-0.20：-1,-0.30 ： -1,-0.40：-1,-0.50：-1,-0.75：-1,-1.00：-1, after the exposure to normal and high temperatures of 20, 200, 300, 400, 500 and 600℃, and using a large static-dynamic true triaxial machine. The biaxial tests were performed on 100 mm×100 mm×100 mm cubic specimens, and friction-reducing pads were used consisting of three layers of plastic membrane with glycerine in-between for the compressive loading plane. Based on the experimental results, failure modes of HSHPC specimens were described. The principal static compressive strengths, strains at the peak stress and stress-strain curves were measured; and the influence of the temperature and stress ratios on them was also analyzed. The experimental results showed that the uniaxial compressive strength of plain HSHPC after exposure to high temperatures does not decrease dramatically with the increase of temperature. The ratio of the biaxial to its uniaxial compressive strength depends on the stress ratios and brittleness-stiffness of HSHPC after exposure to different temperature levels. Comparison of the stress-strain results obtained from the theoretical model and the experimental data indicates good agreement.     

Simulation and interpretation of diffuse mode bifurcation of elastoplastic solids

The diffuse mode bifurcation of elastoplastic solids at finite strain is investigated. The multiplicative decomposition of deformation gradient and the hyperelasto-plastic constitutive relationship are adapted to the numerical bifurcation analysis of the elastoplastic solids. First, bifurcation analyses of rectangular plane strain specimens subjected to uniaxial compression are conducted. The onset of the diffuse mode bifurcations from a homogeneous state is detected; moreover, the post-bifurcation states for these modes are traced to arrive at localization to narrow band zones, which look like shear bands. The occurrence of diffuse mode bifurcation, followed by localization, is advanced as a possible mechanism to create complex deformation and localization patterns, such as shear bands. These computational diffuse modes and localization zones are shown to be in good agreement with the associated experimental ones observed for sand specimens to ensure the validity of this mechanism. Next, the degradation of horizontal sway stiffness of a rectangular specimen due to plane strain uniaxial compression is pointed out as a cause of the bifurcation of the first antisymmetric diffuse mode, which triggers the tilting of the specimen. Last, circular and punching failures of a footing on a foundation are simulated.     

Experimental evaluation of the Wyoming-modified two-rail shear test method for composite materials

In-plane unidirectional shear properties of carbon/epoxy and glass/epoxy materials were measured using unidirectional and cross-ply specimens utilizing the Wyomingmodified two-rail shear test fixture. Simple test specimens of trapezoidal and rectangular geometries, and specimens with tabs were tested. Various specimen sspect ratios were also considered. Untabbed unidirectional specimens exhibited premature shear failures but cross-ply specimens produced reasonably reliable results with acceptable failure modes. Both bonded tab and integral tab 0° specimens were found to give comparable results. Unidirectional and cross-ply PEEK specimens, and angle-ply and quasi-isotropic AS4/3501-6 specimens, were also tested.     

Mechanical behaviour of the upper layers of a sandy loam soil under shear loading

The mechanical behaviour of the upper layers of a sandy loam soil was studied under standard triaxial compression and direct shear box tests. Variations of soil material properties were investigated at four different initial dry bulk densities of 1410, 1520, 1610 and 1670 kg/m³. Soil deformation and volume change under the triaxial compression loading were also studied at these bulk densities. Results from the two tests showed increases in the soil mechanical properties with the initial dry bulk density. The internal friction angle values measured with the triaxial compression apparatus exceeded those measured with the direct shear box. In contrast, the soil cohesion values measured with the direct shear box exceeded those measured with the triaxial compression apparatus. Under the triaxial compression test, the loose soil samples underwent contraction and volume reduction, whereas the dense samples swelled and failure cracks appeared clearly at various planes. The soil contraction for the former case characterizes the occurrence of soil compaction, whereas the cracks propagation and volume increase in the latter case characterizes the breaking up and loosening of soil during tillage operations. For the loose and moderately compacted states, the engineering Poisson's ratio increased with the axial strain until loading was completed. It also increased at the compacted and very compacted states until reaching given loading stages, after which its value started to decrease. This shifting in the engineering Poisson's ratio during loading may provide another identification of the moment of soil failure occurrence, in addition to that of the maximum shear stress.     

Shear Testing of Polypropylene Materials Analysed by Digital Image Correlation and Numerical Simulations

Distributions of shear strains and strain states (triaxiality) were analysed for two in-plane shear test fixtures (Iosipescu and V-notched rail), using digital image correlation and numerical simulations. Three different polypropylene-based materials (two talc-filled compounds and one unfilled homopolymer) were tested. The three materials behaved differently in the shear tests. Most notably, cracks developed in tension near the notches for the particle-filled materials, while the unfilled homopolymer did not fracture. There were also differences between the materials regarding strain localisation between the notches, strain rates vs. strain level (for a given cross-head speed), thickness change in the sheared section, and triaxiality. The yield stresses in shear, uniaxial tension and uniaxial compression showed pressure sensitivity. At least for equivalent strain rates below 1?s^?1, the strain rate sensitivity of the yield stress was approximately the same in these three stress states. The stress?Cstrain curves obtained with the two methods were quite similar for these materials. There were some differences between the methods regarding the ease of mounting and aligning specimens, the complexity of specimen deformation patterns, and the uniformity of the shear strain distribution between the notches.     

Ultrasonic nondestructive material evaluation method and study on texture and cross slip effects under simple and pure shear states

Ultrasonic wave velocities propagating in a plastically deformed medium are known to depend upon its microstructural material properties. Therefore, the authors have proposed the theoretical modeling of an ultrasonic nondestructive method to evaluate plastically deformed states. In the present paper, we verify the proposed theoretical modeling of an ultrasonic nondestructive method and examine its accuracy by comparing the experimental results with the simulated subsequent yield surfaces, the longitudinal and transverse wave velocities under combined stress states of an aluminum alloy using internal state variables of an anisotropic distortional yield model which were determined to achieve a good fit for the experimental results of the longitudinal and transverse wave velocity changes under uniaxial tension test. As a special case, the velocity changes of longitudinal wave under pure shear state subjected to the combinations of tension and compression are also studied, it shows a different result compared with that of longitudinal wave velocity under torsional tests of thin thickness cylinders, i.e., simple shear state. The effects on ultrasonic wave velocity changes due to texture and cross slip under simple and pure shear states are studied via a finite element polycrystal model (FEPM).