充内压圆柱壳在激光辐照下的初始裂纹萌生机制研究

通过高速摄影在实验中观测到充压柱壳上初始裂纹萌生于激光辐照区中心．通过数值模拟、断口分析和激光辐照区材料的金相分析等手段揭示了裂纹初始萌生的物理力学机制，并进一步证实了裂纹初始萌生位置的问题．根据研究结果，确定了影响裂纹起始萌生的力学量．本项目工作的完成有助于激光辐照下充压柱壳断裂问题的深入研究．     

充内压圆柱壳在激光辐照下的实始裂纹萌生机制研究

通过高速摄影在实验中观测到充压柱壳上初始裂纹萌生于激光辐照区中心，通过数值模拟、断口分析和激光辐我材料的金盯分析等手段揭示了裂纺初始萌生的物理力学机制，并进一步证实了裂纹初始萌生位置的问题，根据研究影响裂纹起始萌生的力学量。本项目工作的完成有且副上充压柱壳断裂问题的深入研究。     

激光辐照下充压柱壳失效的三维离散元模拟

应用三维离散元方法模拟了充压柱壳在激光辐照下的破坏过程。结果表明 ,在较低功率密度和较大光斑情况下 ,光斑中央产生裂纹并沿轴向扩展引起破坏。在高功率密度和小光斑情况下 ,则产生中央穿孔破坏。对于铁合金 ,当材料的延展性较大时 ,可能产生第三种破坏形式 ,即在数十微秒量级产生较大面积的整体粉碎性爆裂。模拟结果与实验对比吻合较好。     

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。     

热-力联合作用下柱壳结构变形的计算分析

本文研究了激光连续加热和恒内压作用下柱壳结构的变形规律。以4340钢材料作为研究对象，其本构方程选用热粘塑性本构模型（Johnson-Cook模型）；选取某一应变率为临界值（本文中临界应变率取为1s^-1)。考察了激光功率密度，预载荷大小，激光作用时间等对结构变形规律的影响。主要结论有：结构失稳是导致激光辐照下充内压柱壳破坏的重要原因；激光功率密度和内压越高，结构破坏时间越短；根据辐照时间的长短，预测到了结构急速破坏，延迟破坏和不破坏三种模式。本工作对于深入认识激光作用下预载结构的热－力联合破坏有一定意义。     

激光辐照下圆柱壳结构的稳定性分析

针对航天工程中常用的承受轴压作用的薄壁圆柱壳,分别采用解析方法与特征值屈曲有限元方法分析了圆柱壳结构在均匀轴压作用下的稳定性能,得到了屈曲承载力.并进行了对比,验证了有限元模型的合理性.采用线性屈曲特征值有限元方法分析了强激光辐照作用下圆柱壳的稳定性能,分析表明激光辐照导致壳体局部温度上升并由此带来材料参数的改变,是因为激光辐照在壳体中引起了热应力与热应变,使轴压圆柱壳的屈曲承载力明显降低.论文还着重对加筋圆柱壳结构的稳定性进行了研究,数值分析结果表明,加筋能有效提高圆柱壳结构的抗压承载力,激光辐照作用下,加筋对轴压作用下圆柱壳的屈曲承载力的提高作用更为明显.     

表面粗糙度对6061铝合金薄壁管冲击膨胀断裂性能的影响

采用分离式霍普金森杆实验技术,对表面加工后不同粗糙度的6061铝合金薄壁圆柱管进行动态膨胀断裂冻结回收实验,并对薄壁金属圆柱管动态膨胀断裂过程中裂纹萌生、扩展情况以及最终断裂模式等进行了研究。结果表明:相同冲击压力条件下,薄壁金属圆柱管表面粗糙度越大,材料越容易发生膨胀破裂;裂纹萌生于外壁面,由外向内扩展,并且裂纹的扩展主要受裂纹处应力状态的影响;薄壁金属圆柱管的断裂模式由拉伸和剪切断裂机制起主导作用,其断口为拉剪混合型断口。     

激光辐照下充内压柱壳动态爆裂的数值模拟

采用动力有限元显式积分程序,对承受内压、在侧面受到激光辐照的圆柱壳的热 力学响应进行了数值模拟,展现了圆柱壳热软化、鼓包和爆裂的动态失效过程。获得的柱壳失效方式与实验现象基本吻合;还给出了不同内压作用下柱壳的破坏时间,获得一些定性的认识。     

金属柱壳在爆炸加载断裂中的单旋现象

系列实验结果表明,在较强的滑移爆轰加载下,许多金属材料的旋转柱壳会呈现一种奇特的单旋剪切失稳破坏模式受驱动壳壁结构上众多剪切滑移带及断口取向一致化,在45和135两族剪切带中,出现一族占优的现象。通过高速分幅和X光照相对剪切裂纹在壳壁内、外表的生成发展图像的记录和碎片样品回收,结合过程应力分析,对微剪切断裂发展过程中环向和纵向关联的可能机制进行了分析讨论,但疑点仍较多。在分析柱壳的高应变率失稳行为时,对于与加载条件及几何构形紧密耦合的材料动载剪切强度、各向异性性质所扮演的角色需要进一步认识。剪切断裂的单旋与聚能罩内层无破坏的射流整体旋转现象表现方式和程度虽有差异,但可能有着共同本质,值得深入探索。     

塑性动态断裂实验研究

本文利用自制的实验装置,对韧性材料在爆炸冲击载荷作用下的塑性动态断裂特性,进行了实验研究,其中包括高塑性应变速率下,塑性区裂纹扩展过程和扩展速度的测试;塑性动态断裂韧性CTOD,及其在不同裂纹扩展速度下变化规律的测试。同时,对不同裂纹扩展速度的试件断口进行微观分析。     

复合型韧性断裂实验和控制参数

通过对不同韧性材料在各种平面复合载荷形式下裂纹启裂阶段裂端变形、启裂位置和扩展方向的系统的宏微观实验验证及计算分析,考察了韧性断裂参数空穴扩张比的分布特征和裂纹启裂及扩展方向的关系．得到：对于不同韧性的材料,在裂端的钝化变形区域,空穴扩张比的极大值区对应干裂纹的启裂位置,裂纹启裂时钝化裂端前缘空穴扩张比的临界值不敏感于复合比的变化．而对于裂纹启裂后的扩展方向,则需根据具体材料在相应的特定区域中比较空穴扩张比参数极大值的分布特征,需经进一步的分析,从而确定裂纹的扩展方向．实验及计算结果表明,尽管复合型断裂时裂纹启裂及扩展的机理极其复杂,用于韧性材料复合型断裂的空穴扩张比参数仍能很好地预测裂端的启裂及扩展方向,可作为复合型韧性断裂过程的控制参数．     

Process zone in the Single Cantilever Beam under transverse loading: Part I: Theoretical analysis

Single Cantilever Beam (SCB) specimen loaded with a transverse force parallel to the crack front is proposed for the analysis of crack propagation phenomena under mixed mode conditions. The stress redistribution in the adhesive layer in the vicinity of the crack front so as the beam deformation are estimated using a Timoshenko beam on elastic foundation model. This model emphasizes the Mode II contribution due to flexural beam rotation but also the cleavage due to the beam torsion induced by the eccentricity of the adhesive layer with respect to the beam neutral axis. Finally, one dimensional representation is assessed by comparing analytical solution with finite elements calculations, proving that this analysis is suitable for the analysis of the SCB test under transverse loading.     

A numerical study of crack tip constraint in ductile single crystals

In this work, the effect of crack tip constraint on near-tip stress and deformation fields in a ductile FCC single crystal is studied under mode I, plane strain conditions. To this end, modified boundary layer simulations within crystal plasticity framework are performed, neglecting elastic anisotropy. The first and second terms of the isotropic elastic crack tip field, which are governed by the stress intensity factor K and T-stress, are prescribed as remote boundary conditions and solutions pertaining to different levels of T-stress are generated. It is found that the near-tip deformation field, especially, the development of kink or slip shear bands, is sensitive to the constraint level. The stress distribution and the size and shape of the plastic zone near the crack tip are also strongly influenced by the level of T-stress, with progressive loss of crack tip constraint occurring as T-stress becomes more negative. A family of near-tip fields is obtained which are characterized by two terms (such as K and T or J and a constraint parameter Q) as in isotropic plastic solids.     

Novel Method for Analyzing Crack Growth in Polymeric Microtensile Specimens by <Emphasis Type="Italic">In Situ</Emphasis> Atomic Force Microscopy

In this paper a micro tensile test which allows the determination and observation of the crack growth behaviour in thin polymer layers is presented. The setup consists of micromanipulators and piezo actuators for straining the sample while an atomic force microscope (AFM) is used for scanning the crack tip area with high lateral resolution. The stress in the specimen is determined by an optical microscope for observation of the deflection of a force sensing beam. The material under investigation is an amorphous and strongly entangled thermoplastic polyimide which can be patterned photolithographically and is spin cast to form layers of 3 μm thickness. The results show the potential of the setup to measure crack length, crack tip opening and nominal stress. The stress-crack length-diagram then allows to determine different stages during crack growth.     

用压电材料进行损伤鉴别的理论与数值分析

对压电材料用于损伤监测的理论和数值分析做了一些研究。首先,设计了一种用压电材料进行损伤监测的模型。然后,对这个模型进行分析,找出简单有效的解答办法,将求解过程分解为断裂力学分析和压电分析两部分,并通过适当的假设,进行了详细的理论推导。通过正电有限元程序进行仿真计算,将数值计算结果与理论解进行比较以验证提出理论的正确性,并分析得到了裂纹参数与压电层表面电势变化之间的关系和普通弹性材料泊松比对波峰参数的影响。最后,用提出的方法验算了两个例题。从结果来看,理论结果和数值结果非常接近。     

采用显微网格法研究韧性材料平面应力状态下裂尖附近的应变场

用显微网格数字图象处理方法，测量了韧性材料平面应力条件下Ｉ型单边裂纹尖端附近的应变场。实验结果表明：在裂尖两侧存在与裂纹方向夹角约为５０°的大变形带。最大应变位于离裂尖一定距离的大变形带上，随着载荷增加，最大应变的位置逐渐靠近裂纹尖端。应变峰值轨迹线在裂纹顶端呈放射状，载荷增加时，应变峰值的轨迹线位置并不改变。     

Brittle versus ductile transition of nanocrystalline metals

The brittle versus ductile transition for conventional metals is dictated by the competition between dislocation emission and cleavage. For nanocrystalline metals with grain size below 25 nm, however, dislocation activities are suppressed and the classic theory fails to apply. In this paper, one of the competing mechanisms that control the brittle versus ductile transition of nanocrystalline metals is found to be the grain boundary dominated creep deformation versus the grain boundary decohesion. A model is proposed to quantify the crack propagation in nanocrystalline metals. The effects of material properties, initial configuration and applied loads on the property of crack propagation are addressed. It is concluded that either the increases in the initial crack length, the applied load and the grain boundary damage, or the deterrence in creep deformation, accelerate the crack propagation, and vice versa.     

Damage accumulation and fracture initiation in uncracked ductile solids subject to triaxial loading

A damage plasticity model for ductile fracture is proposed. This model is established on the cylindrical coordinate system of principal stress space. Experimental results show that fracture initiation in uncracked ductile solids is sensitive to the hydrostatic pressure and dependent on the Lode angle. The joint effects of pressure and Lode angle define a fracture envelope in principal stress space. Plastic deformation induced damage is calculated by an integral of the damage rate measured at current loading and deformation status with respect to the fracture envelope. A power law damage rule is proposed to characterize the nonlinearity in damage accumulation. A damage-related weakening factor is adopted to describe the material deterioration. The material parameters are calibrated from standard laboratory tests. The proposed model is numerically implemented. Four simulations with emphasis on crack path prediction are presented.     

粗糙面在梯度表面层上滑动接触的应力分布

对粗糙面在梯度表面层上的滑动过程进行应力分布研究,以模拟实际摩擦过程中,考虑塑性变形情况下,梯度覆层体中的应力分布规律,同时与均质体及单覆层体进行比较研究,分析了在表面载荷相同时滑动接触的应力分布。结果表明覆层体出现塑性变形后,在接触表面上的压力分布与弹性变形时有很大变化,在界面处梯度层的应力分布比单层膜更为理想,其应变梯度也较小;受相同表面载荷作用下产生塑性变形时,梯度层膜在基体产生塑性变形较小