基于时域间断有限元方法的生物组织非傅立叶热行为数值分析

在热传导分析中,当热流与温度梯度存在时间延迟时,需采用非傅立叶热传导模型进行分析。生物组织具有较强的热松弛时间系数,承受激光、微波及烧烫等作用时,其呈现出较强的非傅立叶行为。本文对脉冲热源作用下生物组织的非傅立叶热传导进行研究,针对强脉冲引起的温度场在空间域的高梯度变化、波阵面的间断行为以及通用传统时域数值方法会带来虚假数值振荡的特点,提出采用所发展的时域间断Galerkin有限元法（DG-FEM ）进行求解计算。对多种脉冲热源作用下的非傅立叶热传导过程进行数值模拟,通过考量强脉冲作用下温度场分布和热致生物组织损伤行为的影响,表明了本文所发展的DGFEM 能够有效、准确地描述温度场空间分布和热传导过程以及非傅立叶行为下的生物热损伤更为明显,在生物组织热行为分析中应该受到重视。     

电磁加热条件下皮肤组织的生物热力学行为

研究目的是开发一种数学方法来计算传热过程、热引起的力学响应以及相应的疼痛等级, 从而对临床上应用的各种加热疗法之间的差别进行定量评估. 采用基于有限差分法的数值模拟方法, 基于无限大和均匀化假设, 分析了各种热疗法中皮肤组织的温度、烧伤和热应力分布. 研究发现: 充血对热损伤的影响很小, 但对皮肤的温度分布影响很大, 而这又反过来显著影响由此产生的热应力场; 对于激光加热, 光波越短则峰值温度越高, 但峰值更接近皮肤表面温度; 激光和微波加热所产生的热应力集中于表皮顶层, 因为发热量沿皮肤深度方向呈指数衰减; 薄角质层(常常被忽略)对皮肤组织的热力学响应起主导作用.      

金属铍的压缩变形行为

利用材料试验机及Hopkinson杆装置系统开展热等静压金属铍在不同温度下的静动态压缩力学行为研究,获得了温度、应变率对金属铍屈服强度和加工硬化行为的影响规律。结果表明:金属铍在压缩应力状态下呈现出良好的塑性,同时其力学性能具有显著的应变率敏感性与热软化效应,屈服强度和流动应力随应变率提高呈明显增大趋势,随着温度升高逐渐降低。同时,室温下其加工硬化行为随着应变增大表现为分段硬化特征,随温度升高则趋于理想塑性。最后,采用修正的Johnson-Cook本构模型对实验结果进行了拟合,模型计算结果与实验结果吻合较好。     

极端环境下连续碳纤维增韧的陶瓷基复合材料的力学行为

连续纤维增韧的碳化硅复合材料(以下简称C/SiC),作为超高速飞行器热结构使用时,有可能在高温环境下受到高速撞击的作用,因此,掌握其在极端环境(高温、高应变率)下的力学性能是进行结构安全设计的基础。本文采用具有高温实验能力的分离式Hopkinson杆,在293~1273K温度范围内进行了动态压缩力学性能测试,研究了环境温度和加载速率对材料力学性能的影响。结果表明:C/SiC复合材料的高温压缩力学性能主要受应力氧化损伤和残余应力的共同影响。实验温度低于873K时,应力氧化损伤的影响很小,而由于增强纤维和基体界面残余应力的释放使界面结合强度增大,复合材料的压缩强度随温度的升高而增大;当实验温度高于873K时,应力氧化损伤加剧,其对压缩强度的削弱超过残余应力释放对强度的贡献,材料的压缩强度随温度的升高逐渐降低。由于应力氧化损伤受应变率的影响很大,当温度由873K升高至1273K时,高应变率下压缩强度降低的程度要比应变率为0.0001/s时低得多。     

20#钢动态拉伸断裂行为及其临界损伤度研究

首先对一维应变平面冲击载荷下20#钢的动态拉伸断裂行为进行了实验研究,考察了加载应力和拉伸应变率影响;然后基于损伤度函数模型,对实验进行了数值模拟研究,结果表明：模型中定义的特征物理量一断裂临界损伤度与外载荷条件（加载应力和拉伸应变率）的无关性或不敏感性;最后,采用Ls-dyna动力有限元程序并嵌入损伤度函数模型,对滑移爆轰下20#钢柱壳内爆驱动层裂进行二维数值模拟研究,结果表明：一维应变平面层裂实验和模型计算确定的损伤模型参数,也适用于描述柱壳构形复杂应力环境下的动态拉伸断裂问题．从而初步证实了断裂临界损伤度可以看作是一个表征延性金属动态拉伸断裂的内禀物性参数．     

不锈钢(00Cr18Ni9)动态累积损伤研究

用一级压缩气体炮加载技术和可变平板电容传感技术，对不锈钢（００Ｃｒ１８Ｎｉ９） 动态累积损伤进行了实验研究，给出了层裂累积损伤连续度量的经验关系式，并对损伤度与动载历史的相互关系进行分析、讨论。     

斜波压缩下锡的相变和层裂行为

获取不同热力学路径下锡的动态响应实验数据,是深入研究其相变和损伤物理过程的基础.利用小型磁驱装置CQ-4完成了金属锡的斜波加载实验,获取了锡含有相变和层裂损伤物理信息的实验数据.实验结果显示,在加载段锡依次经历了弹塑性转变和β-γ相变两种物理过程,屈服强度约0.194 GPa,相变压力随着锡厚度的增加从7.54 GPa减小到7.14 GPa.在卸载段出现了明显的层裂损伤,层裂强度约1.1 GPa,与相同加载压力下冲击实验结果有巨大差异,层裂片厚度约0.38 mm.结合由锡的多相Helmholtz自由能计算的多相状态方程、Hayes相变动力学方程和损伤度理论,对斜波压缩实验过程进行一维流体动力学数值模拟,计算结果可以很好描述锡的弹塑性转变、相变和层裂三个物理过程.     

脆性材料热-力耦合模型及热破裂数值分析方法

针对混凝土、岩石等脆性材料,利用热传导和热-力耦合的相关理论,并结合材料在细观尺度上的损伤演化规律,提出了一种考虑损伤的热-力耦合模型,并在原有材料破坏过程分析系统RFPA(Realistic Failure Process Analysis)模型的基础上建立了脆性材料热破裂过程分析的数值模拟方法.该方法考虑了脆性材料在细观层次上力学性质的非均匀性(包括强度、弹模、传导系数等),并通过统计分布函数建立了宏、细观力学性能之间的联系.对不同均匀程度材料的数值模拟结果表明:材料的非均匀性对热传导规律、热应力分布以及热破坏模式有较大的影响.材料热力学性质的非均匀性加剧了材料内部热应力分布的非均匀性,这是致使非均匀材料热破裂的一个重要因素.对稳态和瞬态热传导两种条件下的脆性介质破裂过程模拟分析表明,考虑瞬态热传导计算所得到的破裂区小于相同条件下稳态热传导所得到的结果,表明在热破裂过程分析中,应注重考虑瞬态热传导对破裂过程的影响.     

基于压电双脉冲剪切波加载和光学相干断层扫描的生物组织弹性测试技术

生物组织力学性能的定量测试对疾病发生和发展的研究及诊治非常重要。本文集成压电堆栈振动加载和光学相干断层扫描(OCT)提出对生物组织弹性进行定量测量的剪切波光学相干弹性成像技术。压电堆栈携带探针在双脉冲激励下对生物组织或仿体产生约2μm振幅的振动,生成剪切波。在OCT探测区域,生物组织的变形会引起OCT的相位信号变化,由此提出检测剪切波的相位差法。再根据剪切波的传播速度,计算获得组织的弹性模量。通过比较该方法与拉伸实验对生物组织仿体弹性模量的测量结果,验证了技术的可行性。进而对人体手指指腹皮肤的弹性进行了在体测试,表明该方法有用于在体皮肤力学性能测试的潜力。压电堆栈双脉冲剪切波OCE为离体生物组织弹性测试提供了新的简单有效的手段,并且有潜力进一步发展为生物组织复杂力学性能的测试技术,以及应用于临床上皮肤等疾病的诊断。     

传爆药静态压缩力学性能及损伤特性研究

为研究聚黑-14C（JH-14C）传爆药静态压缩力学性能及损伤特性,开展准静态压缩实验,获得了不同应变率下的应力-应变曲线,建立了描述不同应变率下JH-14C力学行为的非线性本构模型;利用扫描电镜（SEM）对回收试样进行细微形貌观测,获得了准静态压缩JH-14C损伤特性的表征。结果表明:JH-14C压缩强度随应变率的升高而提高;实验与计算结果对照验证了本构模型的有效性;准静态压缩实验中,JH-14C主要损伤模式为脱湿和穿晶断裂。     

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.     

NONLINEAR BUCKLING BEHAVIOR OF DAMAGED COMPOSITE SANDWICH PLATES CONSIDERING THE EFFECT OF TEMPERATURE-DEPENDENT THERMAL AND MECHANICAL PROPERTIES

On the basis of the first-order shear deformation plate theory and the zig-zag deformation assumption, an incremental finite element formulation for nonlinear buckling analysis of the composite sandwich plate is deduced and the temperature-dependent thermal and mechanical properties of composite is considered. A finite element method for thermal or thermo-mechanical coupling nonlinear buckling analysis of the composite sandwich plate with an interfacial crack damage between face and core is also developed. Numerical results and discussions concerning some typical examples show that the effects of the variation of the thermal and mechanical properties with temperature, external compressive loading, size of the damage zone and ply angle of the faces on the thermal buckling behavior are significant. Project supported by the National Natural Science Foundation of China (No. 59975013).     

Biothermomechanical behavior of skin tissue

Advances in laser, microwave and similar technologies have led to recent developments of thermal treatments involving skin tissue. The effectiveness of these treatments is governed by the coupled thermal, mechanical, biological and neural responses of the affected tissue： a favorable interaction results in a procedure with relatively little pain and no lasting side effects. Currently, even though each behavioral facet is to a certain extent established and understood, none exists to date in the interdisciplinary area. A highly interdisciplinary approach is required for studying the biothermomechanical behavior of skin, involving bioheat transfer, biomechanics and physiology. A comprehensive literature review pertinent to the subject is presented in this paper, covering four subject areas： （a） skin structure, （b） skin bioheat transfer and thermal damage, （c） skin biomechanics, and （d） skin biothermomechanics. The major problems, issues, and topics for further studies are also outlined. This review finds that significant advances in each of these aspects have been achieved in recent years. Although focus is placed upon the biothermomechanical behavior of skin tissue, the fundamental concepts and methodologies reviewed in this paper may also be applicable for studying other soft tissues.     

2.5维自愈合C/SiC复合材料的压缩力学行为

试验研究了2.5维自愈合C/SiC复合材料的压缩力学行为,根据材料的细观结构特点,建立了压缩载荷下的损伤力学模型,得到了经纬向压缩的非线性应力应变关系,预测结果与试验值吻合较好。结果表明,经向和纬向的力学行为不同,纬向的压应力逐渐增大时,切线模量逐渐增大,压缩强度为270.05MPa,而经向压应力逐渐增大时,层间损伤逐渐发生,经纱承受的弯矩越来越大,切线模量逐渐降低,破坏强度为128.66MPa。     

THE USE OF VISCO-ELASTOPLASTIC DAMAGE CONSTITUTIVE MODEL TO SIMULATE NONLINEAR BEHAVIOR OF CONCRETE

A visco-elastoplastic damage constitutive model is proposed for simulating nonlinear behavior of concrete. Based on traditional plastic theory, the irreversible deformation is simulated in effective stress space. In order to reflect different stiffness degradation mechanism of concrete under tensile and compressive loading conditions, both tensile and compressive damage variables are introduced, and then on the basis of energy release rate, the model is firmly derived within the concept of irreversible thermodynamics. The rate-dependent model is considered by introducing viscous regularization into the inelastic strain and damage variable, and combined with an additional elastic condition. Fully implicit backward-Euler algorithm is used to perform constitutive integration. Results of numerical examples using the proposed model agree well with test results for specimens under uniaxial tension and compression, biaxial loading and triaxial loading. Failure processes of single-edge-notched (SEN) beam and double-edge-notched (DEN) specimen are also simulated to further validate the proposed model.     

Analytical analysis of the dual-phase-lag model of bioheat transfer equation during transient heating of skin tissue

Dual-phase-lag model of bioheat transfer equation is utilized in treating the transient heat transfer problems in skin tissue considering prevalent heating conditions in thermal therapy applications, namely, pulse train and periodic heat flux. Comparisons between the presented analytical results for limiting cases and previous studies display an excellent agreement. The effects of temperature gradient relaxation time on the tissue temperature, damage, and also on the blood perfusion in skin tissue are studied.     

循环加载作用下皮肤应力软化的连续介质模型

皮肤组织作为富含纤维的非均匀材料,具有复杂的力学特性．皮肤组织在循环加载作用下,随着循环次数的增加,加载过程的应力响应逐渐降低,并最终达到不随循环次数增加而改变的稳定状态,这种现象被称为应力软化行为．本文对加载过程中纤维的延展机制对宏观力学响应的影响进行研究,认为在外界载荷较小时该机制主导了宏观层次上的应力软化行为,随着外界载荷的增大,拉伸过程中微观结构损伤的演化开始产生影响,而且此时内部微观结构的演化由两种机制共同影响,据此建立了连续介质模型,将宏观尺度上应力软化行为和微观结构的演化相关联．将所获得的应力响应理论结果与猪离体头部皮肤在循环加载作用下的实验结果进行对比分析,证明了该模型能够合理地描述皮肤组织在循环加载作用下的应力软化行为．     

考虑损伤累积的热弹塑性问题变分原理及其有限元方法

本文基于连续介质损伤力学中有效应力的概念，研究建筑结构抗火分析中遇到的热弹塑性问题与损伤的耦合，并运用参变量变分原理，建立起用于热弹塑性损伤问题结构分析的变分原理。本文给出了原理应用的有限元列式。具有明确的物理意义，表达形式规范，便于数值手段实现。