热冲击下 A3 钢力学性能的研究

对一定拉伸预载下的Ａ３钢试样进行了快速加热实验,研究了温升率对Ａ３钢力学性能的影响。结果表明:温升率对自由热膨胀影响不大;温升率越大,产生相同塑性变形需要的温度越高;温升率明显影响Ａ３钢内部变化机制;温升率将决定试样热冲击后的晶粒大小。     

预压下锆基块体非晶合金的热冲击变形与破坏

实验研究了不同预压载荷与加热速率下Zr51Ti5Ni10Cu25Al9块体非晶合金的失效温度和破坏规律, 发现预压力和温升率较低时, 随着温度的升高, 材料强度减小, 样品最后发生塑性变形; 预压力和温升率较高时样品则发生剪切断裂, 且发生剪切破坏时样品的温度高于其玻璃化转变温度.基于变温条件下的结构弛豫模型, 分析了块体非晶合金在快速加热条件下的变形过程,给出了材料发生屈服时的温度与温升率、预压力与屈服温度之间的相互关系, 并得出了实验结果的拟合关系式. 对回收样品断裂面进行分析, 发现了与恒温压缩断裂明显不同的断裂特征. 最后分析了预压载荷下快速加热过程中上述材料发生剪切破坏的临界条件.      

混凝土高温动态劈拉行为细观数值分析

为研究高温作用下混凝土的动态劈裂拉伸破坏行为,考虑了力学性能的高温退化与应变率增强效应的联合作用,结合混凝土材料内部非均质性,建立了细观尺度数值分析模型与方法。将该数值方法分为两个步骤:首先对混凝土进行热传导行为模拟,进而将输出结果作为初始条件对混凝土动态劈裂拉伸行为进行细观模拟。在模拟结果与已有试验现象良好吻合的基础上,分析了高温下混凝土动态劈裂拉伸行为及其细观破坏机制,对比了不同应变率及加热温度下混凝土的劈裂拉伸应力-应变关系,揭示了混凝土应变率效应与温度退化效应的相互影响规律。研究结果表明:(1) 高温作用后,试件损伤区域较常温下更集中;(2) 名义应变率较大时,破坏过程急促,常温下骨料发生破坏,而经历高温后骨料基本没有破坏;(3) 由于混凝土试件细观结构的非均质性,其内部应力呈枣核状不连续分布;(4) 相比于应变率效应,混凝土劈裂拉伸强度受温度退化作用的影响更显著。     

高温环境下T300/BMP350拉伸力学行为研究

对不同温度下的耐高温树脂基复合材料T300/BMP350的0°和90°单向层合板进行静载拉伸实验,得到材料在不同温度下的应力-应变响应,分析了温度对材料的力学性能影响。通过高温应变片采集到材料在高温环境下的热输出和破坏时的极限应变,分析了材料的热行为。通过分析材料的应力-应变曲线和失效模式,研究了材料的损伤和失效机理。研究结果表明:T300/BMP350树脂基复合材料具有很好的耐高温性能。高温下0°方向的拉伸强度和模量保持率达到80%以上,90°方向的拉伸强度和模量保持率可以达到50%以上。高温环境对材料的极限应变影响不大,材料破坏模式均为脆性破坏。基于实验结果,对材料的强度随温度的变化进行拟合,预测了该材料在350℃时0°和90°的拉伸强度。     

一种改进的金属材料的高温动态拉伸实验技术

在旋转盘冲击拉伸实验装置上,利用金属材料自身的导电特性,对试样施加电流.使其在电流作用下发热,实现自加热,形成了试件快速加热而波导杆温升很小的金属材料的动态高温高应变率拉伸实验技术.应用该实验技术获取了45 #钢从室温到1000℃温度范围和应变率650s-1时的材料动态拉伸应力—应变曲线.实验结果表明,45 #钢具有明显的热软化效应,其流动应力和屈服应力随温度的升高而降低.     

高温冲击拉伸试验中的快速接触加温技术

本文阐述了高温冲击拉伸试验技术中基于大热容量热惯性温升极大值原理的快速接触加温技术;研制了稳定性好、加热效率高、总体热惯性大、断电温升至极大值的稳定时间长的一对新的加温炉。测试结果表明,可以在试件上获得了最高可达1073K的近似稳定和均匀的温度场。通过实验研究建立了试件温度、稳定炉温和加温炉加热自动断电设定温度之间的标定关系。利用此标定关系,可以根据试验所需要的试件温度方便地确定加温炉自动断电设定温度,并通过监控稳定炉温来实施的所需试件温度下的冲击拉伸试验。本文还对高温冲击拉伸试验中的相关问题进行了分析讨论。     

镁合金MB2在高应力三轴度下的拉伸破坏行为研究

通过光滑试件及不同曲率半径缺口圆柱试件的拉伸试验,实现对镁合金MB2的单向及多向应力状态加载。结合数值模拟分析,研究了不同试件在拉伸加载过程中应力状态的变化。以应力三轴度为参数,给出了镁合金MB2等效破坏应变的变化规律,在应力三轴度-等效破坏应变空间建立了镁合金MB2的失效破坏准则。利用扫描电镜对试件断口形貌进行观察,分析了导致材料宏观延性变化的微观损伤机理,对不同应力状态下镁合金MB2的失效破坏行为做出了合理解释。     

基于J-C模型的镁合金MB2动静态拉伸破坏行为

为了研究不同应力状态和应变率条件下镁合金MB2的拉伸破坏行为,利用材料试验机和分离式Hopkinson拉杆(SHTB),对镁合金MB2的光滑及缺口圆柱试件进行了动静态拉伸加载;拟合得到了镁合金MB2的动静态拉伸本构关系,建立了其修正的Johnson-Cook失效破坏准则,并对不同试件的拉伸破坏行为进行了数值模拟;利用SEM对宏观破坏模式对应的微观损伤机理进行了分析。结果表明,随着应力三轴度的增加,镁合金MB2的等效破坏应变先增大后减小,宏观破坏模式由剪切转为正拉断,微观损伤机制由混合断裂转变为韧窝断裂;而随着应变率的增加,等效破坏应变不断减小,破坏模式不发生改变。Johnson-Cook本构关系和修正后的Johnson-Cook失效破坏准则能较好地拟合动态静态拉伸实验结果并预测不同试件的杯锥形破坏特征。     

水泥砂浆的一个热粘弹性率型损伤本构模型

利用SHPB实验系统及自行研制的混凝土类材料快速高温加热设备,对水泥砂浆试件进行了不同 温度(20~600℃)和3种冲击速度下的实验,得到了不同温度和冲击速度下水泥砂浆试件的应力应变关系曲 线。基于ZWT粘弹性本构模型,并且考虑高温下水泥砂浆损伤演化规律都服从Weibull分布,提出了一个水 泥砂浆的热粘弹性率型损伤本构模型。通过数据拟合,获得了本构模型的相关参数,结果表明:理论预测和实 验结果吻合良好。     

扫描显微环境下电流加热试件时的温度控制

如何实现扫描电镜(SEM)腔体内电加热试件的恒温控制,对于金属及合金材料在超高温环境下的微观力学性能的原位测量具有重要意义。本文在MATLAB GUI以及串口通信的基础上,建立了SEM环境中高温电加热系统,提出了一种SEM环境下串口通讯控制样品温度的电流加热控制方法,以及高效、灵敏的反馈函数,设计了友好的软件控制和测量界面,在SEM环境中对不同材料进行了测试,实现了单向拉伸加载条件下电流加热钨丝试件1700℃时与镍基单晶合金试件1000℃时,试件的温度波动分别保持在±3℃与±1℃之内。随后,利用该系统完成了1400℃静态和1200℃单轴拉伸与疲劳实验,系统在此温度时依然可获得高质量的原位扫描显微图像。该实验验证了所提方法对电流加热试件温度控制的有效性,为焦耳热加热实现材料高温力学性能研究,以及研究电加热对材料热性能的影响等关键问题提供了可靠的实验平台。     

热应力作用下结构声-振耦合响应数值分析

考察飞行器结构热应力对结构及其内声腔声-振耦合特性的影响,建立考虑热应力因素的声-振耦合动力学有限元方程,对一个典型飞行器结构考虑热应力时的声-振耦合动力学响应进行分析。计算结果表明,热应力的存在对耦合模型的固有频率影响较小,受热应力影响较大的区域主要集中在机头及机身等部位,其固有振动特性有较明显的变化。通过对比结构加速度与内声腔声压级的响应结果发现,热应力的影响主要表现为系统响应幅值及峰值位置的改变。     

热对流条件下考虑球形夹杂分布的材料热弹接触摩擦热影响分析

机械传动关键活动零部件接触副往往受到力载荷和摩擦热载荷的耦合作用,使得接触界面间的接触力学行为的分析变得极其复杂. 利用基于等效夹杂方法建立的考虑热对流非均质材料热弹接触力学分析模型研究不同摩擦系数、夹杂位置和材料属性等参数对材料表面及内部温升及热应力分布影响规律. 此外,进一步分析了接触副材料中含分布球形夹杂时摩擦热造成的影响. 结果表明:接触副表面温升梯度受热对流系数的影响较大;下表面温升和热应力随摩擦系数增大而增大;分布夹杂则将接触副材料下表面温升及热应力分布变得更为复杂.      

Thermal softening induced plastic instability in rate-dependent materials

A linear perturbation analysis is performed for a class of rate-dependent materials, such as the Johnson-Cook model, in which the rate contribution to the stress can be separated from that of the plastic strain and temperature and in which the temperature rises adiabatically. The analysis is facilitated by perturbing both the rate of momentum equation and the momentum equation. An identical material stability/instability criterion is deduced from the characteristic spectral equations for one-dimensional deformation, one-dimensional shearing, and general three-dimensional field equations, and thus shows that the instability derived here is a material constitutive instability.The criteria indicate that the materials become unstable once the thermal softening overcomes the strain hardening, regardless of the strain rate. The strain rate enters the criteria through its effects on the accumulated temperature and the current stress. Based on the criterion, the three-dimensional instability surface is established in the space of plastic strain, plastic strain rate, and temperature. Instability surface is shown as a material property and independent of deformation histories or modes. Both necking and shear banding are simulated to validate the excellent predictive capability of the criterion.     

Investigations to the effect of heating-rate on the mechanical properties of aluminum alloy LY12

Two classes of experiments were conducted with a Gleeble 1500 thermal–mechanical testing system to investigate the effect of heating-rate and its history on the mechanical behavior of aluminum alloy LY12. In the first class of experiment, specimens were heated at different heating-rates to prescribed temperatures and then stretched until fracture. It was found that the specimen heated with higher heating-rate possesses lower rupture strength. In the second class of experiment, the specimens were preloaded and then heated at different rates until fracture. It was found that the higher the heating-rate was, the lower the failure temperature would be. Metallographical analysis showed that there are more defects in the specimens undergoing higher heating-rate. It was conjectured that higher heating-rate may cause stronger local thermal inconsistency due to the heterogeneous nature of the material. It may then cause local residual microstress fields, which, together with external thermal–mechanical load, may result in the changes in the microstructure of the material, such as recovery, recrystallization, nucleation and growth of microdefects, accounting for the changes in the macroscopic mechanical properties including hardening/softening, damage and failure, etc. A numerical simulation was performed, in which the mechanisms of local thermal inconsistency and the effect of the influencing factors were investigated.     

短脉冲激光加热分数阶导热及其热应力研究

短脉冲激光加热引起材料内部复杂的传热过程及热变形,现有的以Fourier定律或Cattaneo-Vernotte松弛方程结合弹性理论为框架建立起来热应力理论在刻画其热物理过程存在严重缺陷. 本文基于分数阶微积分理论, 以半空间为研究对象, 建立了分数阶Cattaneo热传导方程和相应的热应力方程, 给出了问题的初始条件和边界条件, 采用拉普拉斯变换方法, 给出了非高斯时间分布激光热源辐射下温度场和热应力场的解析解, 研究了短脉冲激光加热的温度场及热应力场的热物理行为. 数值计算中, 首先对理论解进行数值验证, 然后取分数阶变量$p=0.5$研究温度场和热应力场的变化特点及激光参数对温度和热应力的影响,最后数值计算分数阶参数对温度和热应力场的影响. 计算结果表明, 分数阶Cattaneo传热方程和热应力方程描述的温度和热应力任然具有波动特性,与经典的Fourier传热模型和标准的Cattaneo传热模型相比, 分数阶阶次越大, 热波波速越小, 热波波动性越明显; 反之, 则热波波速越大, 热扩散性越强.激光加热和冷却的速度越快, 温度上升和下降的速度越快, 压应力和拉应力交替变化越快, 温度变化幅值越小, 热应力幅值影响不明显.      

铝-镁合金5A06在瞬态热冲击条件下的力学性能研究

通过热强度试验,测试并确定航空航天材料在复杂高速热冲击条件下的强度极限等关键参数,对于航空航天材料和结构的可靠性评定、寿命预测以及高速飞行器的安全设计具有重要的意义。针对强度设计手册中没有航空航天材料在高速热冲击环境下的强度极限等表征参数的现状,使用自行研制的高速飞行器瞬态气动热试验模拟系统,对铝-镁合金材料5A06在多种不同的瞬态热冲击条件下,进行气动加热模拟与热载联合试验研究,得到在瞬态热、力学环境的共同作用下铝-镁合金5A06材料的强度极限、承载时间等力学性能变化状况。为研究分析航空航天材料和结构在高速热冲击环境下的承载能力和结构减重提供了可靠依据。     

Stochastic analysis of a thermoelastic problem in functionally graded plates with uncertain material properties

The statistics (i.e., mean and variance) of temperature and thermal stress are analytically obtained in functionally graded material (FGM) plates with uncertainties in the thermal conductivity and coefficient of linear thermal expansion. These FGM plates are assumed to have arbitrary nonhomogeneous thermal and mechanical properties through the entire thickness of plate and are subjected to deterministic convective heating. The stochastic temperature and thermal stress fields are analysed by assuming the FGM plate is multilayered with distinct, random thermal conductivity and coefficient of linear thermal expansion in each layer. Vodicka’s method, which is a type of integral transform method, and a perturbation method are employed to obtain the analytical solutions for the statistics. The autocorrelation coefficients of each random property and cross-correlation coefficients between different random properties are expressed in exponential function forms as a non-homogeneous Markov random field of discrete space. Numerical calculations are performed for FGM plates composed of partially stabilised zirconia (PSZ) and austenitic stainless steel (SUS304), which have the largest dispersion of the random properties at the place where the volume fractions of the two constituent materials are both 0.5. The effects of the spatial change in material composition, thermal boundary condition and correlation coefficients on the standard deviations of the temperature and thermal stress are discussed.     

Effect of material non-homogeneity on the inhomogeneous shearing deformation of a Gent slab subjected to a temperature gradient

It is well known that most rubber-like materials are non-homogeneous due to either imperfect manufacturing conditions or the action of severe thermo-oxidative environments in many practical applications. In this study, within the context of finite thermoelasticity, we theoretically analyze the inhomogeneous shearing deformation of a non-homogeneous rubber-like slab subjected to a thermal gradient across its thickness. The major objective of this study is to investigate the effect of the material non-homogeneity, which is the material-coordinate dependence of the material response functions, on the stress-strain fields for a given temperature gradient. First, we show the existence of a simple shearing deformation from which the generalized shear modulus and the generalized thermal conductivity of the slab could be obtained. Based on this information, the Gent material model is generalized to take the material non-homogeneity and the temperature dependence of the stress into account. To analyze the inhomogeneous shearing deformation of the non-homogeneous slab, deformation and temperature fields are postulated; then the decoupled temperature field is obtained analytically by solving the local energy balance equation. Finally, the static equilibrium equations are solved considering the linear temperature field. Our results show that the spatial pattern and the degree of the material non-homogeneity have profound effects on the stress-strain fields. The shear strain becomes nearly homogeneous and the stresses are relatively small for a certain spatial variation of the material non-homogeneity. This result suggests the possibility of designing a novel class of materials: functionally graded rubber-elastic materials (FGREMs).     

Biothermomechanics of skin tissues

Biothermomechanics of skin is highly interdisciplinary involving bioheat transfer, burn damage, biomechanics and neurophysiology. During heating, thermally induced mechanical stress arises due to the thermal denaturation of collagen, resulting in macroscale shrinkage. Thus, the strain, stress, temperature and thermal pain/damage are highly correlated; in other words, the problem is fully coupled. The aim of this study is to develop a computational approach to examine the heat transfer process and the heat-induced mechanical response, so that the differences among the clinically applied heating modalities can be quantified. Exact solutions for temperature, thermal damage and thermal stress for a single-layer skin model were first derived for different boundary conditions. For multilayer models, numerical simulations using the finite difference method (FDM) and finite element method (FEM) were used to analyze the temperature, burn damage and thermal stress distributions in the skin tissue. The results showed that the thermomechanical behavior of skin tissue is very complex: blood perfusion has little effect on thermal damage but large influence on skin temperature distribution, which, in turn, influences significantly the resulting thermal stress field; the stratum corneum layer, although very thin, has a large effect on the thermomechanical behavior of skin, suggesting that it should be properly accounted for in the modeling of skin thermal stresses; the stress caused by non-uniform temperature distribution in the skin may also contribute to the thermal pain sensation.