不同加载速度下脆性颗粒的破坏特性

对直径为8mm的K9玻璃球进行了加载速度为2×10-7和2×10-6m/s的准静态单轴压缩实验以及加载速度为3.4、7.1和10.6m/s的动态单轴压缩实验,研究了K9玻璃单颗粒破碎强度的Weibull分布特性,结合破碎产物的形貌特征,分析了不同加载速度下脆性材料拉伸破坏机制和剪切破坏机制的转变过程,提出了一种拉剪耦合的时序破坏模型,由此揭示了加载速度与3个破坏区的关系。考虑拉伸和剪切失效准则,应用ABAQUS软件进行数值模拟,并初步验证了该破坏模型的冲击过程。研究结果对于认识脆性颗粒材料的动态破坏具有很好的参考意义。     

酚醛层压材料的冲击力学行为及本构模型

为了研究酚醛层压材料的冲击力学行为并获得本构模型,利用万能试验机和整形修正的分离式霍普金森压杆(SHPB)装置,对材料试样进行了应变率范围为10-3~103 s-1的单轴压缩实验,得到了不同加载应变率下的应力应变曲线,对其在准静态、动态载荷下的压缩破坏机理进行了初步探讨。结果表明,酚醛层压材料具有较强的应变率效应,与准静态(1.67×10-3 s-1)时相比,在动态载荷(7×102 s-1)下,峰值应力增加了约10倍;破坏应变减少了约一半;在准静态和动态加载条件下试样力学性能的差异是由于纤维基体界面特性以及不同应变率下破坏模式的不同;采用朱-王-唐本构方程描述了酚醛层压材料力学行为,拟合得到了本构方程的系数,在加载过程中,理论计算值与实验结果吻合较好。     

基于数字图像相关性方法的脆性材料拉氏反分析实验技术

利用分离式Hopkinson压杆作为加载系统,借助超高速数字图像相关性分析方法,发展了长杆试件拉氏反分析实验技术,并用于研究脆性材料在小变形条件下的动态本构特性。通过超高速相机实时拍摄冲击加载下长杆试件变形的散斑图像,再对散斑图像进行数字图像相关性（digital image correlation,DIC）分析,获得长杆试件表面速度场和应变场。随后,以脆性材料PMMA（polymethyl methacrylate）为例,从DIC分析得到的速度场中提取出不同拉格朗日位置上质点速度时程曲线,构建路径线连接整个速度场,再结合零初始条件,数值求解得到了试件中的应力时程曲线,消去时间参数后,获得了脆性材料PMMA的动态应力-应变曲线,并与Hopkinson压杆实验和准静态压缩实验的结果进行了对比,揭示了PMMA材料在小应变条件下的黏弹性本构响应特征。     

高锁螺栓连接件动态拉伸响应与失效机理

飞机坠撞过程中结构的变形模式和吸能对乘员保护具有重要意义，而连接结构的载荷传递和失效形式是影响飞机结构变形的重要因素之一。为了获取航空高锁螺栓连接件在坠撞载荷下的动态响应和失效机理，基于抗剪型平头高锁螺栓设计了2种材料（2024-T3和7050-T7451）的单钉单搭接连接件，利用高速液压伺服材料试验机进行4种速度（0.01、0.10、1.00和3.00 m/s）下的拉伸测试，得到连接件的动态响应、极限载荷、能量吸收和失效模式随速度的变化规律，并分析了连接件的失效机理。结果表明，连接件的失效模式受母材和高锁螺栓/螺母材料强度影响较大，而受加载速度影响较小；当速度从0.01 m/s增加到3.00 m/s时，2024-T3连接件的极限载荷和能量吸收分别增加了2.17%和34.43%，7050-T7451连接件的极限载荷和能量吸收分别增加了5.53%和6.58%。     

水陆两栖飞机尾翼抗鸟撞设计与验证方法研究

为确保水陆两栖飞机尾翼结构的抗鸟撞性能,针对其不同结构部位提出不同的抗鸟撞设计思路。耦合SPH方法建立了尾翼结构的鸟撞数值模型,采用实验方法获得了结构铝合金材料的准静态和中低应变率拉伸实验数据以及不同冲击速度下带母材铆钉的极限拉伸载荷和极限剪切载荷数据。进一步开展了尾翼结构抗鸟撞分析,并通过鸟撞实验对数值分析结果进行验证。结果表明,针对水陆两栖飞机尾翼前缘结构提出的两种抗鸟撞设计思路合理,且具有较好的抗鸟撞性能;结构采用的3种铝合金存在较为明显的应变硬化效应,但应变率敏感性较弱;随着加载速度的增大,结构采用的4种铆钉拉伸载荷呈下降趋势,但总体幅度并不大,而剪切载荷变动较小;建立的尾翼结构鸟撞数值分析模型准确,较好预测了结构的破坏模式和鸟体冲击分散过程。     

铁电陶瓷PZT53多轴力电耦合下的屈服行为

研究了不同拉-扭与压-扭比例载荷和电场耦合作用下铁电陶瓷PZT53的屈服行为。通过自己设计的力电加载装置,结合材料试验机和电压放大器对PZT53在多轴力电耦合载荷作用下应力-应变行为进行测量,结合陈和卢的理论,得出了极化和未极化的PZT53陶瓷在多轴力载荷下的初始屈服面和破坏面。结果表明初始屈服面类似于Drucker-Prager屈服面,本文提出了一个屈服准则,较好的符合实验结果。实验发现PZT53陶瓷破坏行为符合最大拉应力破坏准则。对于极化的PZT53陶瓷,还研究了偏置电场对应力-应变行为的影响,结果表明偏置电场大于0．6kV／mm时,PZT53陶瓷的屈服现象消失。本文工作为发展铁电陶瓷PZT53的多轴力电耦合唯象本构模型提供了实验基础。     

方形钢筋混凝土板的近场抗爆性能

为了探讨钢筋混凝土板在爆炸载荷作用下的抗爆性能,对方形钢筋混凝土板在单向支撑条件下进行了近场爆炸加载实验,实验中采取TNT装药对钢筋混凝土板进行加载.并利用AUTODYN软件采用流固耦合算法,建立了混凝土和钢筋三维分离式实体模型,对钢筋混凝土板的动态响应过程进行数值模拟,且考虑应变率对钢筋和混凝土材料的动态本构特性的影...     

一种新的高应变率复合压剪实验技术

提出了一种新的基于Hopkinson杆实验技术的在102~103s-1高应变率下实现压剪复合加载的实验装置,并给出了相应的理论分析和数值模拟。为了获取材料在复杂应力下的本构关系,借助斜飞片冲击实验的思想,对Hopkinson杆进行改造,将入射杆的末端改进为截锥形,以便在试样中同时产生压缩和剪切应力。利用有限元分析软件LS-DYNA对试样中的应力波传播进行模拟计算,并利用改进装置进行了初步实验。计算和分析结果表明,利用所设计的装置可以实现对试样的动态压剪复合加载,获得材料在高应变率复杂应力加载下的本构响应,进而建立材料在复杂应力状态下本构行为的描述。     

压剪复合平板冲击加载技术进展及其应用

自20世纪70年代末发明压剪炮以来, 压剪复合冲击加载实验技术和诊断技术有了长足进展, 应用也日益广泛.由于压剪联合加载波直接反映了材料的动态剪切特性, 对于认识材料的屈服、损伤演化、失效、相变、界面滑移等动态行为和机理, 构筑更全面的本构模型能够提供必要的附加信息.本文主要讨论气炮实验中压剪复合应力波的产生方式, 诊断技术, 以及在压剪复合塑性波和动高压本构模型、聚合物压剪冲击行为、剪切波跟踪法(SWT)和水泥基复合材料的损伤和失效、界面动摩擦行为、冲击相变、动态损伤和断裂等方面的研究与应用进展.      

金属材料在复杂应力状态下的塑性流动特性及本构模型

工程应用中,金属材料和结构往往处于复杂应力状态。材料的塑性行为会受到应力状态的影响,要精确描述材料在复杂应力状态下的塑性流动行为,必须在本构模型中考虑应力状态效应的影响。然而,由于在动态加载下材料的应变率效应和应力状态效应相互耦合、难以分离,给应力状态效应的研究和模型的建立造成很大困难。通过对Ti-6Al-4V钛合金材料开展不同加载条件下的力学性能测试,提出了一个包含应力三轴度和罗德角参数影响的新型本构模型,并通过VUMAT用户子程序嵌入ABAQUS/Explicit软件。分别采用新提出的塑性模型和Johnson-Cook模型对压剪复合试样的动态实验进行了数值模拟。结果表明,新模型不仅在对材料本构曲线的拟合方面具有较强的优势,而且由该模型所得到的透射脉冲和载荷-位移曲线均更加准确。因此,该模型能够更精确地描述和预测金属材料在复杂应力状态下的塑性流变行为。     

Structural behaviour of aluminium self-piercing riveted joints: An experimental and numerical investigation

The present paper deals with the structural behaviour of self-piercing riveted joints based on aluminium and steel rivets. Two T-components made of two open aluminium profiles in alloy AA6063 temper T4 joined by 6 and 12 rivets, respectively, were designed and tested under quasi-static loading conditions. A new test device was designed to perform the tests of the T-components under two different load cases. Experimental results of the T-components joined by using aluminium self-piercing rivets were then compared with the corresponding components joined by using steel rivets in terms of force-displacement curves, deformation modes of the components as well as rivet failure modes. Further, the experimental results of the T-components based on aluminium rivets were used to validate a resultant-based point-connector model for self-piercing rivets proposed by Hanssen et al. (2010) using shell elements.     

压缩-剪切复合应力下PMMA材料的静态力学特性研究

通过压缩具有一定倾斜角(0°,10°,15°,20°和25°)试件和双剪切模型试件,实现了单轴压缩、压缩-剪切复合应力以及纯剪切三种应力状态,得到PMMA(聚甲基丙烯酸甲酯)在相应应力状态下的应力-应变曲线,同时对不同应力状态下试件的破坏模式进行了分析。结果表明:在不同受力环境中材料的强度和破坏的机理不同;同单轴压缩状态下相比,材料在压缩-剪切复合应力状态下屈服极限、强度极限以及破坏应变均不同程度的增大,呈现明显的"剪切增强"现象。单轴压缩与压缩-剪切应力状态下试件的破坏模式均为在试件短对角面上出现明显的剪切屈服带,由应力分析得出试件剪应力在短对角面上达到最大,引起在此平面上分子链间滑动从而产生应变软化形成剪切屈服带;双剪切试件的破坏模式为与剪切面呈45°的斜面。     

砌体填充墙的抗爆性能

为了揭示砌体填充墙的抗爆破坏机理,在野外实验中,测得了爆炸条件下砌体填充墙上的爆炸荷载及位移,得到了墙的抗爆性能、破坏模式以及碎片的飞散和分布情况。实验结果表明,墙体的破坏模式与荷载的大小有关,其破坏主要由灰缝的破坏引起。结合实验现象,采用分离式建模的精细化数值模拟方法,得到了不同荷载条件下裂缝的发展过程、墙体的边界条件对墙体的破坏模式的影响,确定了墙体不同破坏等级时的药量,进一步说明本文中数值模拟方法的合理性。     

A viscoplastic constitutive model for nickel-base superalloy,part 2: modeling under anisothermal conditions

In Part 2 of this study, extensive deformation tests were carried out on the nickel-base polycrystalline superalloy IN738LC under isothermal and anisothermal conditions between 450 and 950 °C. Under the isothermal conditions, the material showed almost no rate/time-dependency below 700 °C, while it showed distinct rate/time-dependency above 800 °C. Regarding the cyclic deformation, slight cyclic hardening behavior was observed when the temperature was below 700 °C and the imposed strain rate was fast, whereas in the case of the temperature above 800 °C or under slower strain rate conditions, the cyclic hardening behavior was scarcely observed. Unique inelastic behavior was observed under in-phase and out-of-phase anisothermal conditions: with an increase in the number of cycles, the stress at higher temperatures became smaller and the stress at lower temperatures became larger in absolute value although the stress range was approximately constant during the cyclic loading. In other words, the mean stress continues to evolve cycle-by-cycle in the direction of the stress at lower temperatures. Based on the experimental results, it was assumed that evolution of the variable Y that had been incorporated into a kinematic hardening rule in Part 1 of this study is active under higher temperatures and is negligible under lower temperatures. The material constants used in the constitutive equations were determined with the isothermal data, and were expressed as functions of temperature empirically. The extended viscoplastic constitutive equations were applied to the anisothermal cyclic loading as well as the monotonic tension, stress relaxation, creep and cyclic loading under the isothermal conditions. It was demonstrated that the present viscoplastic constitutive model was successful in describing the inelastic behavior of the material adequately, including the anomalous inelastic behavior observed under the anisothermal conditions, owing to the consideration of the variable Y.     

复合材料分层断裂判据及扩展准则研究

针对现有层间开裂判据及扩展准则未考虑断裂模式耦合性的现状,本文提出了基于模式混合度的复合材料分层断裂判据及扩展准则。依此模型计算了蒙皮/凸缘复合材料加筋层合板结构的分层扩展行为。计算结果表明,所提出的开裂判据及扩展准则能够较好的预测蒙皮/凸缘复合材料加筋层合板结构的断裂力学行为,所得结果对确定含损伤复合材料加筋层合板结...     

Instabilities in multilayered soft dielectrics

Experimental observations clearly show that the performance of dielectric elastomeric-based devices can be considerably improved using composite materials. A critical issue in the development of composite dielectric materials toward applications is the prediction of their failure mechanisms due to the applied electromechanical loads. In this paper we investigate analytically the influence of electromechanical finite deformations on the stability of multilayered soft dielectrics under plane-strain conditions. Four different criteria are considered: (i) loss of positive definiteness of the tangent electroelastic constitutive operator, (ii) existence of diffuse modes of bifurcation (microscopic modes), (iii) loss of strong ellipticity of the homogenized continuum (localized or macroscopic modes), and (iv) electric breakdown. While the formulation is developed for generic isotropic hyperelastic dielectrics, results are presented for the special class of ideal dielectrics incorporating a neo-Hookean elastic response. The effect of material properties and loading conditions is investigated, providing a detailed picture of the different possible failure modes.     

The Effect of Grain Size on Deformation and Failure of Ti<Subscript>2</Subscript>AlC MAX Phase under Thermo-Mechanical Loading

An experimental investigation was performed to analyze the effects of grain size on the quasi-static and dynamic behavior of Ti₂AlC. High-density Ti₂AlC samples of three different grain sizes were densified using Spark Plasma Sintering and Pressureless sintering. A servo-hydraulic testing machine equipped with a vertical split furnace, and SiC pushrods, was used for the quasi-static experiments. Also, a Split Hopkinson Pressure Bar (SHPB) apparatus and an induction coil heating system were used for the dynamic experiments. A series of experiments were conducted at temperatures ranging from 25 °C to 1100 °C for strain rates of 10^?4 s^?1 and 400 s^?1. The results show that under quasi-static loading the specimens experience a brittle failure for temperatures below Brittle to Plastic Transition Temperature (BPTT) of 900–1000 °C and large deformation at temperatures above the BPTT. During dynamic experiments, the specimens exhibited brittle failure, with the failure transitioning from catastrophic failure at lower temperatures to graceful failure (softening while bearing load) at higher temperatures, and with the propensity for graceful failure increasing with increasing grain size. The compressive strengths of different grain sizes at a given temperature can be related to the grain length by a Hall-Petch type relation.     

CDM模型及在纤维增强层合材料侵彻数值模拟中的应用

基于纤维增强层合材料的一种三维连续损伤本构模型（continuum damage constitutive model, CDM）,采用动态有限元程序LS-DYNA对Kevlar纤维增强层合材料的弹道侵彻过程进行了数值模拟,预测了弹丸的剩余速度以及靶板的变形和破坏模式,给出了弹道冲击下不同损伤模式的分布及演化。通过与实验结果的比较验证了数值计算的有效性和合理性。     

Simulation of damage evolution in ductile metals undergoing dynamic loading conditions

A set of constitutive equations for large rate-dependent elastic-plastic-damage materials at elevated temperatures is presented to be able to analyze adiabatic high strain rate deformation processes for a wide range of stress triaxialities. The model is based on the concepts of continuum damage mechanics. Since the material macroscopic thermo-mechanical response under large strain and high strain rate deformation loading is governed by different physical mechanisms, a multi-dissipative approach is proposed. It incorporates thermo-mechanical coupling effects as well as internal dissipative mechanisms through rate-dependent constitutive relations with a set of internal variables. In addition, the effect of stress triaxiality on the onset and evolution of plastic flow, damage and failure is discussed.Furthermore, the algorithm for numerical integration of the coupled constitutive rate equations is presented. It relies on operator split methodology resulting in an inelastic predictor-elastic corrector technique. The explicit finite element program LS-DYNA augmented by an user-defined material subroutine is used to approximate boundary-value problems under dynamic loading conditions. Numerical simulations of dynamic experiments with different specimens are performed and good correlation of numerical results and published experimental data is achieved. Based on numerical studies modified specimens geometries are proposed to be able to detect complex damage and failure mechanisms in Hopkinson-Bar experiments.