Tests of Sheet Materials for Structural Elements in Transverse Shear

An experimental procedure to evaluate the energy spent for fracture of a circular-clamped thin plate in transverse shear, induced by impact loading with a flat-face body, is briefly outlined. Test results for the specimens of the three materials: mild steel, thickness = 1.0 mm, a D16T aluminum alloy, = 0.75 mm, and a PA6 shock-resistant composite, = 2.0 mm, are given. The analysis of experimental and calculated stress-strain data (within the circular area between the clamping contour and the loading surface) demonstrates the qualitative relation between the energy spent for deformation to fracture and dynamic strength and plasticity. Test results allow one to compare different materials by the energy spent for their fracture under impact loading. The specific work of deformation in shear characterizes this value more comprehensively than that determined in flexure, induced by impact of a spherical body, and is applicable over an expanded range of loading rates.     

A numerical investigation of ductile fracture initiation in a high-strength low-alloy steel

In this work, static and drop-weight impact experiments, which have been conducted using three-point bend fracture specimens of a high-strength low-alloy steel, are analysed by performing finite-element simulations. The Gurson constitutive model that accounts for the ductile failure mechanisms of microvoid nucleation, growth and coalescence is employed within the framework of a finite deformation plasticity theory. Two populations of second-phase particles are considered, including large inclusions which initiate voids at an early stage and small particles which require large strains to nucleate voids. The most important objective of the work is to assess quantitatively the effects of material inertia, strain rate sensitivity and local adiabatic temperature rise (due to conversion of plastic work into heat) on dynamic ductile crack initiation. This is accomplished by comparing the evolution histories of void volume fraction near the notch tip in the static analysis with the dynamic analyses. The results indicate that increased strain hardening caused by strain rate sensitivity, which becomes important under dynamic loading, plays a benign role in considerably slowing down the void growth rate near the notch tip. This is partially opposed by thermal softening caused by adiabatic heating near the notch tip.     

Computer Simulation of Normal and Oblique Impacts of Yawed Projectiles on Ceramic Targets

The investigation of a three-dimensional problem of normal and oblique interaction of yawed projectiles with ceramic plates in the velocity range up to 4000 m/s was carried out by the finite-element method. The paper presents an advanced constitutive model of AD995 Alumina. The model of a damaged medium is used; it is characterized by a possibility of crack initiation and propagation under impact loading. A kinetic fracture model of active type developed earlier for the simulation of fracture in various materials is used for numerical modeling of failure of ceramics at high velocity impact. Temperature effects are taken into account in the constitutive model.     

Impact Deformation and Fracture Mechanisms of Plates of Brittle Materials

We have studied the deformation and fracture mechanisms of plates of brittle materials under impact loading. The paper presents the results of numerical simulation of impact interaction between a projectile and a multilayer plate with a brittle layer. Methods of compression and perforation testing of brittle materials have been elaborated. The results of numerical simulation are compared with the experimental data.     

钢纤维增强水泥砂浆压剪联合冲击下动态剪切性能

基于唐志平等提出的剪切波跟踪技术(SWT),对钢纤维增强水泥砂浆进行了冲击速度40m/s~270m/s倾斜角为0~20°的斜撞击试验.研究结果表明,在此冲击速度下,材料处于损伤状态和由损伤状态向孔洞崩塌过渡阶段.此结果与聚丙烯纤维增强水泥砂浆的实验结果进行对比,发现钢纤维体积含量为0.5%时,对压缩性能的增强效果并不明显,但相应的剪切波速明显增加;当钢纤维体积含量为1%时对压缩性能和剪切波速均有较大的提高.由于钢纤维与水泥砂浆基体的强度相差太大,剪切强度有一定的改善,但数据较离散.同时通过对剪切结果分析表明,采用剪切波,尤其是卸载剪切波来探测脆性材料内部动态损伤非常有效.     

Theoretical-Experimental Analysis of Structural Components Separation upon Local Impulse Loading

Separation of structural components is considered as a high-speed directed local destruction of an object in the area of a specially applied concentrator. Elements in the form of plates and shells of arbitrary curvature have been taken as destruction objects. Calculations have been made by explicit finite-element methods. Results of the investigation performed have found practical applications.     

Damage and fracture mechanism of aluminium alloy thick-walled cylinder under external explosive loading

The damage and failure mechanism of aluminium alloy were investigated by means of the radial collapse of a thick-walled cylinder under high-strain-rate deformation. Adiabatic shear bands (ASBs) initiated at the inner surface of the aluminium alloy cylinder, and most of them were in spiral form along the cross-section of the cylinder towards counterclockwise direction. Tip of a shear band propagated along the direction of the maximum shear stress. The propagating trajectory of small shear band rather than developed shear band could be affected by grain boundary. The morphologies of ASBs such as bifurcation, crossing and annihilation were observed, and the evolution of ASBs was affected to a great extent by stress state. The nucleation of microcrack was mostly observed at the weak microstructure in ASBs. The coalescence of microcracks formed the crack within ASBs, and when the critical crack length is reached catastrophic fracture occurs.     

含半椭圆表面裂纹圆柱壳体的三维热弹性动态断裂

研究了含轴向半椭圆表面裂纹的圆柱壳体在热应力与冲击载荷作用下的动态断裂情况,并应用所研制的三维动态断裂有限元程序进行了大规模的数值计算,确定了圆柱壳体的三维温度分布及热-力耦合下的动态应力强度因子,所得结果在一定程度上揭示了热-力作用下圆柱壳体的边界表面、裂纹面、物质惯性和弹性波的相互作用在结构动态断裂中的重要性。     

论切缝药包爆破的剪应力作用

通过岩石试件的冲击损伤实验,分析了在剪应力作用下,岩石具有的断裂破坏特性.进行的切缝药包爆破数值模拟结果表明,切缝药包爆破的切缝套管切缝处具有明显的剪应力作用,该剪应力将使爆破裂纹首先从切缝处形成,从而具有定向断裂成缝效果.     

Energy Consumption for Deformation to Fracture of a Circular-Clamped Thin Plate under Impact Loading

An experimental procedure for evaluating energy spent for fracture of a circular-clamped sheet structural element upon transverse flexure, induced by impact loading of a spherical head-face body, is briefly outlined. Flexure test results for two sheet metals (a 20 mild steel and a D16T aluminum alloy 1.0 and 0.75 mm thick, respectively) and a 2.0-mm PA6 shock-resistant composite are cited. Experimental data analyses and stress-strain state calculations for a plate material within the circular boundary upon flexure made it possible to establish the relation between the work of deformation and the dynamic strength and plasticity. Sheet structural element materials are comparatively evaluated by their specific energy spent for deformation under transverse static and impact loading.     

Stress concentration factors in two-dimensional composites: effects of material and geometrical parameters

In the study of fracture processes in composite materials, the interactions between broken and intact fibers are of critical importance. Indeed, the redistribution of stress from a failed fiber to its unfailed adjacent neighbors, and the stress concentration induced in these, determine the extent to which a break in one fiber will cause more breaks in neighboring fibers. The overall failure pattern is a direct function of the stress concentration factors (SCFs). In this paper, we propose a new model for the SCFs in two-dimensional unidirectional composites containing broken fibers. A closed-form expression is derived for the SCF profiles as a function of material and geometrical parameters. The model differs significantly from earlier schemes, as the local effect of a fiber break on nearest neighbors is much milder than previously calculated, both as a function of the inter-fiber distance and of the number of adjacent broken fibers. Comparison with experimental results for silicon-carbide/epoxy composites demonstrates the validity of the proposed scheme. Since the overall fracture pattern in fiber composites is a direct function of the SCFs, the model may help shed light on fracture nucleation and growth in composites.     

仪器化冲击在评测断口剪切面积比例中的应用

根据示波曲线定量给出的特征载荷点分布关系,结合ESIS给出的评测夏比V型冲击断口剪切面积比例(PSF)经验公式,介绍了仪器化冲击试验在估算PSF数值中的具体应用。估算值与实际测量值所进行的误差离散性与置信区间的分析结果表明,选用合适的经验算法可以在试验过程中实时获得高度可靠的PSF结果。与常规光学手工评测技术相比,该方法不依赖于试验者的主观评测经验、不受特定客观试验条件的限制,具有广阔的应用前景,同时也为材料PSF评测以及动态断裂机理的研究提供了一套方便而有效的试验手段。     

壁厚对HR2钢柱壳爆轰加载下膨胀断裂行为的影响

通过对6、12 mm两种不同壁厚的HR2钢柱壳进行爆轰加载实验,对其断裂碎片的宏观形貌、断口的微观形貌以及横截面的变形微观结构进行系统表征,研究了金属柱壳在爆轰加载下的膨胀断裂机理。结果表明,在膨胀断裂过程中壳壁厚度的增大导致HR2钢柱壳由纯剪切断裂变为拉剪混合的断裂模式。断裂碎片的微观结构分析结果表明,柱壳的断裂实际上是剪切裂纹从样品内部剪切带形核并扩展、和拉伸裂纹沿柱壳外表面的形核扩展的共同作用及竞争的结果。薄壁柱壳断裂由样品内裂纹沿剪切带的形核和扩展主导发生剪切断裂,而厚壁柱壳中内侧的裂纹沿剪切带的形核和扩展,但是最外侧则为环向拉应力主导发生拉伸断裂,因此表现出拉剪结合的断裂模式。     

Studies on Localized Strain in Sheet Metals under Impact Tension and Shear

Results of studies on the microstructure of plane sheet specimens after their impact loading at different rates are discussed. Near the notch tips, in the area of localized strain, several layers of different microstructure were revealed. The formation of fine grains is assumed to be determined by the process of dynamic recrystallization at increased local temperatures due to intensive plastic deformation. The boundaries between the layers of different microstructure as well as increased pore concentrations in the area of localized strain near the outer surface point to the realization of the three-dimensional stress-strain state in the material.     

Dynamic response of bimaterial and graded interface cracks under impact loading

The interfacial fracture in bimaterial and functionally graded material (FGM) under impact loading conditions is investigated using experimental and numerical techniques that are valid for both type of interfaces. Experiments are conducted on epoxy based specimens in three point bend configuration and the complex SIF is measured using an electrical strain gage mounted close to the crack-tip. A complementary two-dimensional finite element simulation is performed using tup force and support reactions as input tractions, and the SIF-time history is determined using a displacement extrapolation technique. The experimentally determined SIF-histories match closely with numerical simulation up to the time of fracture initiation. The test results show that the mode-mixity remains nearly constant through out the test in both the materials, and the mixity values correspond to their respective static counterparts. The general dynamic response of the bimaterial and FGM specimens in terms of impact load, support reaction and the magnitude of complex SIF are comparable, and the mode-mixity is the parameter that distinguishes the graded interface from the bimaterial case.