高导无氧铜杆的冲击拉伸断裂系列实验以及基于样本体积单元的分析

采用Hopkinson装置和一种基于一级气体炮的高速冲击拉伸断裂装置,研究了无刻槽高导无氧铜 (OFHC)杆在一系列冲击拉伸速度下的断裂。当冲击拉伸速度大于40m/s时,断裂位置总在冲击拉伸端附 近,此速度被确定为OFHC的实验临界冲击拉伸速度。一种受单轴冲击拉伸荷载的、中心含椭球空穴的样本 体积单元被用于数值模拟所含空穴的增长与失稳的过程。OFHC的J-C与Z-A 本构关系用于描述基体材料 的动态响应。讨论了空穴失稳条件并提出以空穴形状演化为判据,比较了空穴失稳时的样本体积单元平均径 向应变与无刻槽杆的冲击断裂应变。也用这种样本体积单元模型分析了OFHC的实验临界冲击拉伸速度。     

高导无氧铜的临界冲击拉伸速度

基于气炮系统,提出了一种新的临界冲击拉伸速度实验方法。实验装置主要由2部分组成:气体炮系统驱动弹丸和弹丸击靶牵引多根拉伸杆试件。采用该实验方法对高导无氧铜进行了临界冲击拉伸速度实验。对于一维应力的临界冲击拉伸速度实验,探讨了一种完全热耦合的在颈缩区域计及塑性约束系数以及空穴增长与聚集效应的数值模拟方法。高导无氧铜的临界冲击拉伸速度实验的数值模拟结果表明,采用Zerilli-Armstrong本构关系预估的实验临界冲击拉伸速度比采用Johnson-Cook本构关系预估的更符合实验。     

Cr15Mn9Cu2Ni1N不锈钢的高温拉伸变形特性分析

利用热/力模拟试验机,对Cr15Mn9Cu2Ni1N不锈钢进行了950℃~1200℃高温范围内的拉伸试验;采用有限元方法对试样的均匀变形过程进行了分析。高温拉伸过程中,试样在达到最大应力后并不立即颈缩,而是还要经历一段宏观均匀变形后才颈缩。分析结果表明:最大应力之后,试样端部区域等效应力降低,横截面积收缩量减小,而中心区域横截面积收缩量增大,形成了潜在颈缩区;在应变速率敏感性的作用下,潜在颈缩区的变形抗力随应变速率的增大而增加,使变形不能在该区域集中,而转向其它位置,保持了试样的宏观均匀变形,且颈缩未在最大应力后立即发生;在高温拉伸条件下,材料应变速率敏感性的增大是颈缩延迟发生的主要原因,随着变形温度升高,应变速率敏感性增大,也使得试样颈缩前的均匀变形量增大。     

套管式冲击拉伸实验装置的研制

介绍了几种常见的冲击拉伸实验装置,并做了简要的评述.提出了一种改进后的套管式冲击拉伸实验装置,该装置直接产生拉伸脉冲,消除了反射式冲击拉伸实验装置中两种干扰信号的影响,得到了远比反射式冲击拉伸实验更为理想的透射波形.运用改进后的套管式冲击拉伸试验装置对几种板材材料进行了动态拉伸实验,得到了这几种板材在600/s,1200/s,1800/s应变率下的应力应变曲线,与静态应力应变曲线相比较,反映了这几种板材具有明显的应变率效应.根据实验结果中动态与静态应力应变曲线的一致性,分析了这种套管式冲击拉伸实验装置的优越性与有效性,它能更好地反映材料的动态力学性能.     

一种基于SHPB的冲击膨胀环实验技术

设计了一种基于分离式Hopkinson压杆(SHPB)的冲击膨胀环实验装置,实验装置包括一个液压腔,一侧为驱动活塞,另一侧为圆环试件封闭。对活塞施加轴向冲击,利用液体体积近似不可压缩的特性,通过液压腔截面积的大比例缩小,将较低速度的对活塞冲击转化为圆环试件沿径向的高速膨胀,驱动试件发生拉伸变形直至断(碎)裂。使用这种冲击膨胀装置,获得了LY12铝环在不同撞击速度下碎裂过程的初步结果。实验结果显示,随着撞击速度增大,圆环试件碎裂产生的碎片的尺度减小,试件的表观断裂应变增加。这为研究材料的动态拉伸碎裂问题提供了一种加载方式。     

激光选区熔化增材制造GP1不锈钢动态拉伸力学响应与层裂破坏

采用选择性激光熔化增材制造技术,制备了GP1不锈钢单轴拉伸板条试样和层裂圆片试样,并对材料微观结构进行了表征。借助Zwick-HTM5020高速拉伸试验机,并结合数字图像相关性全场应变测量技术,开展了增材制造GP1不锈钢材料的轴向拉伸力学性能实验研究,得到了不同应变率下材料的拉伸应力-应变曲线,结果显示:(1)GP1不锈钢流动应力具有比较显著的应变强化效应;(2)通过回收试样的电子背散射衍射表征,发现GP1不锈钢在拉伸变形过程中会发生奥氏体与马氏体之间的相变;(3)GP1不锈钢的屈服应力随着应变率呈幂指数增大,断裂应变在中低应变率下保持不变,但在高应变率下则显著减小。采用一级轻气炮实验装置和激光干涉粒子速度测量技术,开展了增材制造GP1不锈钢的层裂实验,发现GP1不锈钢的层裂强度随着飞片撞击速度增大而减小。单轴拉伸试样断口和层裂试样断口的显微分析结果表明:随着应变率增大,单轴拉伸断裂模式和断裂机理都发生了转变;层裂损伤易成核于激光熔池边界线的交汇处,断口韧窝形貌明显区别于单向拉伸断口。     

超高强度钢单轴拉伸下形变与断裂过程的研究

1.引言单轴拉伸试验是材料基础力学试验之一,研究它的形变与断裂过程,历来受到人们的重视。关于超高强度钢单轴拉伸的形变规律,一般来说,当外加负荷较小时,材料处在弹性阶段,这时试样形状为圆柱体,应力与应变服从虎克定律。随外加负荷增加,材料进入塑性状态。从屈服至最大负荷这一段,试样仍保持为圆柱体,应力与应变关系服从二段幂乘硬化律。从最大负荷到断裂这一段,试样发生了局部变形,即所谓颈缩现象,     

粘塑性损伤模型模拟准超塑性单轴拉伸行为

发展了Chaboche粘塑性本构模型的大变形隐式算法，用损伤(DM)和无损伤(NDM)模型模拟准超塑性单轴拉伸。发现变形过程可分为三个阶段：均匀变形、颈缩发展、断裂破坏阶段。DM可准确模拟前两个阶段变形，NDM只能较好地模拟均匀变形阶段，表明DM可以较精确地描述稳定发展的动态过程。由于有限元方法只能描述连续介质，因此对于断裂破坏阶段，NDM模拟载荷大于试验结果，DM的载荷小于试验结果，这是由高应变速率敏感性造成。DM能够描述试验中出现地多处颈缩现象，局部应变速率分布随时间演化反映了颈缩发展程度。严重颈缩部位的距离代表着超塑性变形能力，距离越大，抗颈缩能力越好。     

45钢柱壳爆炸膨胀断裂的SPH模拟分析

金属柱壳爆炸膨胀断裂存在拉伸、剪切及拉剪混合等多种断裂模式,目前其物理机制及影响因素还不清晰。本文中采用光滑粒子流体动力学方法（smoothed particle hydrodynamics, SPH）对45钢柱壳在JOB-9003及RHT-901不同装药条件下的外爆实验进行了数值模拟,探讨柱壳在不同装药条件下发生的剪切断裂、拉剪混合断裂模式及其演化过程,模拟结果与实验结果一致。SPH数值模拟结果表明:在爆炸加载阶段,随着冲击波在柱壳内、外壁间来回反射形成二次塑性区,沿柱壳壁厚等效塑性应变演化呈凸形分布,壁厚中部区域等效塑性应变较内、外壁大;在较高爆炸压力（JOB-9003）作用下,柱壳断裂发生在爆轰波加载阶段,损伤裂纹从塑性应变积累较大的壁厚中部开始沿剪切方向向内、外壁扩展,形成剪切型断裂模式;而在RHT-901空心炸药加载下,虽然裂纹仍从壁厚中部开始沿剪切方向扩展,但随后柱壳进入自由膨胀阶段,未断区域处于拉伸应力状态,柱壳局部发生结构失稳,形成类似“颈缩”现象,裂纹从剪切方向转向沿颈缩区向外扩展,呈现拉剪混合断裂模式。拉伸裂纹占截面的比例与柱壳结构失稳时刻相关。可见,柱壳断裂演化是一个爆炸冲击波与柱壳结构相互作用的过程,不能简单将其作为一系列膨胀拉伸环处理。     

PMMA膨胀环动态拉伸碎裂实验研究

在强动载作用下,脆性材料的碎裂问题是一个重要的研究课题,而脆性材料在冲击拉伸载荷下的力学行为的实验研究相对较匮乏.提出了一种动态拉伸断(碎)裂的液压膨胀环实验技术,可用于准脆性/脆性材料的动态拉伸.利用该技术对有机玻璃(PMMA)圆环试件进行了不同膨胀速度下的动态碎裂实验研究.从回收碎片的断口形貌和碎片内部残余裂纹观察可知试件的破碎由环向拉伸应力造成,碎片断口处发出的稀疏波会将周围的拉伸应力卸载,从而抑制其他裂纹的进一步发展.利用超高速相机记录了试件的膨胀碎裂过程,利用DISAR激光速度干涉仪获得了试件外表面粒子的径向膨胀速度历史,通过试件上的应变片获得了试件的应变历史和断裂应变.实验结果表明:在拉伸应变率150～500 s~(-1)范围,材料的动态断裂应变低于准静态加载下的断裂应变,体现出"动脆"现象;随着加载应变率的提高,PMMA材料的碎片尺寸减小;无量纲化的PMMA圆环的平均碎片尺寸介于韧性碎裂模型和脆性碎裂模型的预测数值之间,反映出材料的准脆性特性.     

Effects of superimposed hydrostatic pressure on fracture in round bars under tension

The effect of superimposed hydrostatic pressure on fracture in round bars under tension is studied numerically using the finite element method based on the Gurson damage model. It is demonstrated that while the superimposed hydrostatic pressure has no noticeable effect on necking, it increases the fracture strain due to the fact that a superimposed pressure delays or completely eliminates the nucleation, growth and coalescence of microvoids or microcracks. The experimentally observed transition of the fracture surface, from the cup-cone mode under atmospheric pressure to a slant structure under high pressure, is numerically reproduced. It is numerically proved that the superimposed hydrostatic pressure has no effect on necking for a damage-free round bar under tension.     

45钢的J-C损伤失效参量研究

为了在结构碰撞效应的有限元分析中描述材料行为,通过开展45钢在不同应力状态和温度下的准静态材料力学性能实验及拉伸SHB实验,考察了应力状态三轴度、温度和应变率对材料失效应变的影响。由实验数据得到了Johnson-Cook失效模型参量,并通过出现失效的Taylor撞击实验和数值模拟进行了一定的验证,表明模型描述与实验结果的趋势一致。     

A combined necking and shear localization analysis for aluminum sheets under biaxial stretching conditions

A combined necking and shear localization analysis is adopted to model the failures of two aluminum sheets, AA5754 and AA6111, under biaxial stretching conditions. The approach is based on the assumption that the reduction of thickness or the necking mode is modeled by a plane stress formulation and the final failure mode of shear localization is modeled by a generalized plane strain formulation. The sheet material is modeled by an elastic-viscoplastic constitutive relation that accounts for the potential surface curvature, material plastic anisotropy, material rate sensitivity, and the softening due to the nucleation, growth, and coalescence of microvoids. Specifically, the necking/shear failure of the aluminum sheets is modeled under uniaxial tension, plane strain tension and equal biaxial tension. The results based on the mechanics model presented in this paper are in agreement with those based on the forming limit diagrams (FLDs) and tensile tests. When the necking mode is suppressed, the failure strains are also determined under plane strain conditions. These failure strains can be used as guidances for estimation of the surface failure strains on the stretching sides of the aluminum sheets under plane strain bending conditions. The estimated surface failure strains are higher than the failure strains of the forming limit diagrams under plane strain stretching conditions. The results are consistent with experimental observations where the surface failure strains of the aluminum sheets increase significantly on the stretching sides of the sheets under bending conditions. The results also indicate that when a considerable amount of necking is observed for a sheet metal under stretching conditions, the surface failure strains on the stretching sides of the sheet metal under bending conditions can be significantly higher.     

Determination of tensile flow stress beyond necking at very high strain rate

The objective of this effort was to extend the Bridgman analysis of tensile necking to obtain stress-strain data beyond the point of onset of necking from a split Hopkinson bar. For this purpose, combined analytical and experimental techniques were considered. The analytical efforts were focused on validating the use of Bridgman solutions for high rate of deformation through a finite-element analysis of a tapered tensile specimen. The experimental technique involved the development of a photographic system using a light-emitting diode and a 35-mm rotating drum camera for the observation of necking during dynamic tensile tests conducted with a split Hopkinson tension bar. The developed new technique was successfully used to measure neck profiles of 6061-T6 aluminum, HY100 and 1020 steel tensile specimens. The measured profiles were used with the Bridgman analysis and stress-strain data were obtained to over 70-percent strain.     

A numerical study on the deformation and fracture modes of steel projectiles during Taylor bar impact tests

A heterogeneous material model based on macro-mechanical observations is proposed for simulation of fracture in steel projectiles during impact. A previous experimental study on the deformation and fracture of steel projectiles during Taylor bar impact tests resulted in a variety of failure modes. The accompanying material investigation showed that the materials used in the impact tests were heterogeneous on scales ranging from microstructure as investigated with SEM to variation in fracture strains from tensile tests. A normal distribution is employed to achieve a heterogeneous numerical model with respect to the fracture properties. The proposed material model is calibrated based on the tensile tests, and then used to independently simulate the Taylor bar impact tests. A preliminary investigation showed that the simulations are sensitive to assumptions regarding the anvil behaviour and friction properties. A flexible anvil and a yield-limited friction law are shown to be necessary to correctly reproduce the experimental behaviour. The proposed model is then shown to be capable of correctly reproducing all fracture modes but one, and also predict critical impact velocities for projectile fracture with reasonable accuracy. Fragmentation at velocities above the critical velocity is not well reproduced due to excessive element erosion. Measures to make the element erosion process more physical are proposed and discussed with their respective drawbacks. The use of a simple fracture criterion in combination with an element erosion technique accentuates the effect of distributing the fracture parameter.     

Necking evolution in dynamically stretched bars: New experimental and computational insights

This paper presents new results on dynamic neck evolution in steel bars of varying diameters. Dynamic tensile tests were carried out in a Kolsky apparatus using cylindrical steel specimens with various cross-section diameters ranging from 1.5 mm to 4 mm. A high speed digital camera was used to record the deformation of the specimen during the loading process. Video recording of the tests enabled accurate experimental measurements of the necking evolution, specifically its growth rate as a function of the diameter. The experiments show that increasing the specimen cross-section slows down the neck development. This behavior has been further investigated using two different kinds of numerical calculations: (1) axisymmetric finite element simulations and (2) one-dimensional finite difference computations. While the finite difference model only considers the normal stress along the longitudinal direction of the bar, the finite element model does not entail any simplification on the stress state of the specimen during the loading process. In agreement with the experiments, the finite element calculations show a decrease of the necking growth rate with the increase in the cross-section of the sample. On the contrary, the damping effect of the specimen cross-section on the necking evolution is not captured by the finite difference computations. We postulate that this result comes from the one-dimensional nature of the finite difference model. This work uncovers, by means of combined experiments and modelling, the key role played by stress multiaxiality in the growth rate of dynamic necks.     

Load-Inversion Device for the High Strain Rate Tensile Testing of Sheet Materials with Hopkinson Pressure Bars

A high strain rate tensile testing technique for sheet materials is presented which makes use of a split Hopkinson pressure bar system in conjunction with a load inversion device. With compressive loads applied to its boundaries, the load inversion device introduces tension into a sheet specimen. Two output bars are used to minimize the effect of bending waves on the output force measurement. A Digital Image Correlation (DIC) algorithm is used to determine the strain history in the specimen gage section based on high speed video imaging. Detailed finite element analysis of the experimental set-up is performed to validate the design of the load inversion device. It is shown that under the assumption of perfect alignment and slip-free attachment of the specimen, the measured stress–strain curve is free from spurious oscillations at a strain rate of 1,000 s^?1. Validation experiments are carried out using tensile specimens extracted from 1.4 thick TRIP780 steel sheets. The experimental results for uniaxial tension at strain rates ranging from 200 s^?1 to 1,000 s^?1 confirm the oscillation-free numerical results in an approximate manner. Dynamic tension experiments are also performed on notched specimens to illustrate the validity of the proposed experimental technique for characterizing the effect of strain rate on the onset of ductile fracture in sheet materials.     

Dynamic fracture toughness of high strength metals under impact loading: increase or decrease

An elusive phenomenon is observed in previous investigations on dynamic fracture that the dynamic fracture toughness(DFT) of high strength metals always increases with the loading rate on the order of TPa.m 1 /2.s 1.For the purpose of verification,variation of DFT with the loading rate for two high strength steels commonly used in the aviation industry,30CrMnSiA and 40Cr,is studied in this work.Results of the experiments are compared,which were conducted on the modified split Hopkinson pressure bar(SHPB) apparatus,with striker velocities ranging from 9.2 to 24.1 m/s and a constant value of 16.3 m/s for 30CrMnSiA and 40Cr,respectively.It is observed that for 30CrMnSiA,the crack tip loading rate increases with the increase of the striker velocity,while the fracture initiation time and the DFT simultaneously decrease.However,in the tests of 40Cr,there is also an increasing tendency of DFT,similar to other reports.Through an in-depth investigation on the relationship between the dynamic stress intensity factor(DSIF) and the loading rate,it is concluded that the generally increasing tendency in previous studies could be false,which is induced from a limited striker velocity domain and the errors existing in the experimental and numerical processes.To disclose the real dependency of DFT on the loading rate,experiments need to be performed in a comparatively large striker velocity range.     

Size dependent response of large shell elements under in-plane tensile loading

Large-scale thin-walled structures with a low weight-to-stiffness ratio provide the means for cost and energy efficiency in structural design. However, the design of such structures for crash and impact resistance requires reliable FE simulations. Large shell elements are used in those simulations. Simulations require the knowledge of the true stress–strain response of the material until fracture initiation. Because of the size effects, local material relation determined with experiments is not applicable to large shell elements. Therefore, a numerical method is outlined to determine the effect of element size on the macroscopic response of large structural shell elements until fracture initiation. Macroscopic response is determined by introducing averaging unit into the numerical model over which volume averaged equivalent stress and plastic strain are evaluated. Three different stress states are considered in this investigation: uniaxial, plane strain and equi-biaxial tension. The results demonstrate that fracture strain is highly sensitive to size effects in uniaxial tension whereas in plane strain or equi-biaxial tension size effects are much weaker. In uniaxial and plane strain tension the fracture strain for large shell elements approaches the Swift diffuse necking condition.