氢对体心立方金属裂纹尖端变形过程的影响

利用扫描电镜(SEM)原位拉伸,观察了 Fe-3％Si 单晶体裂尖的变形和断裂过程。用网格法加计算机处理系统测出了裂纹尖端的应变场及氢对应变分布的影响。发现在Ⅰ型载荷下,主裂纹尖端应变呈不对称分布,形变带内应变梯度很大。形变带内应变分布为: ε_(yy)=f(x)exp(-Ar).氢不改变裂纹尖端塑性变形特征,但能明显促进裂纹尖端局部塑性变形的非均匀增加。     

力电耦合载荷作用下铁电陶瓷破坏行为的云纹干涉方法研究

本文介绍了如何用云纹干涉法实时地观察铁电陶瓷在力载荷和电载荷共同作用下裂尖的破坏行为．测量了三点弯试验中由电场和应力集中导致的裂尖的变形场．对变形的云纹图分析表明：当极化方向与裂纹扩展方向一致，且都与电场方向垂直，裂尖附近的正应变随电场的增加而增加，应变集中现象比较突出，电场促进和加速了裂纹的扩展．     

应用云纹干涉法测量力电耦合作用下铁电陶瓷的破坏行为

本文采用云纹干涉系统对的电陶瓷在力电耦合载荷作用下裂纹尖端的力学行为进行全场实时非接触动态细观测量,采用三点弯实验获取裂纹尖端区域在力电耦合作用下与电场集中有关的电致伸缩位移场,应变场,通过分析实验取得的云纹图得到了裂尖区域的位移场,应变场,发现裂尖区域就变随着与裂尖距离的增加衰减的速率比没有电场作用下的理论计算结果要快。     

利用焦耳效应提高含裂纹金属构件抗裂性能问题的研究

设一无限大金属薄板中含有一个线裂纹,对金属板施加恒定的电流场,在两个裂尖处产生的热量远远大于其余地方产生的热量,可简化成两个点热源．经求解得到了问题的解析解,包括裂纹尖端附近区域温度、应力、应变、应变能密度因子的解析表达式．计算结果表明,裂纹尖端处的材料发生熔化而形成一个焊点,裂纹尖端明显纯化,可抑制裂纹的进一步扩展,提高含裂纹金属构件的抗裂性能．     

界面裂纹尖端有限变形弹性场的渐近分析

采用完全非线性弹性理论，研究了一类新的可压缩超弹性材料形成的界面裂纹问题，给出了平面应变条件下裂尖场的渐近解．揭示了界面裂纹尖端场的变形特征．     

相变滞后对裂纹屏蔽效应的数值分析

本文针对奥氏体-马氏体双相材料,研究裂纹尖端区弥散分布的奥氏体颗粒在应变诱发时发生的相变对裂纹的屏蔽效应。鉴于实验中已发现的不同相变滞后对裂纹屏蔽效应的不同影响,本研究通过在裂纹尖端区不同位置嵌入相变颗粒,考虑到裂纹尖端区应力应变场的奇异分布及其诱发的相变,将裂纹尖端区相变滞后问题转化为相变颗粒在裂纹尖端区的位置问题。计及奥氏体-马氏体相变的体积膨胀效应进行了平面应力裂纹问题数值模拟,得到了单个相变夹杂对裂纹屏蔽效应的影响规律。结果表明：裂纹尖端区相变夹杂的位置对裂纹的屏蔽效应在距裂尖2倍夹杂直径以内影响极大,且以裂尖86度方向为界。其影响规律与McMeeking 和 Evans理论预言的60度方向不同。     

界面裂纹尖端有限变形弹性场的渐近分析

采用完全非线性弹性理论，研究了一类新的可压缩超弹性材料形成的界面裂纹问题，给出了平面应变条件下裂尖场的渐近解．揭示了界面裂纹尖端场的变形特征．     

可压缩粘弹性材料Ⅱ型动态扩展裂纹尖端场

为了研究粘性效应作用下的动态扩展裂纹尖端渐近场,建立了可压缩粘弹性材料II型动态扩展裂纹的力学模型,推导了可压缩材料Ⅱ型动态扩展裂纹的本构方程.在稳态蠕变阶段,弹性变形和粘性变形同时在裂纹尖端场中占主导地位,应力和应变具有相同的奇异量级r-1/(n-1).通过渐近分析求得了裂纹尖端应力、应变和位移分离变量形式的渐近解,并采用打靶法求得了裂纹尖端应力、应变和位移的数值结果,给出了应力、应变和位移随各种参数的变化曲线.数值计算表明,弹性变形部分的可压缩性对Ⅱ型裂尖应力场影响甚微,而对应变场和位移场影响较大.裂尖场主要受材料的蠕变指数n和马赫数M的控制.当泊松比ν =0.5时,可以退化为不可压缩粘弹性材料Ⅱ型动态扩展裂纹.     

蠕变材料Ⅱ型动态扩展裂纹尖端场

为了研究粘性效应作用下的动态扩展裂纹尖端渐近场,建立了蠕变材料Ⅱ型动态扩展裂纹的力学模型,在稳态蠕变阶段,弹性变形和粘性变形同时在裂纹尖端场中占主导地位,应力和应变具有相同的奇异量级,即(σ,ε)∝r-1/(n-1)。通过渐近分析求得了裂纹尖端应力、应变和位移分离变量形式的渐近解,并采用打靶法求得了裂纹尖端应力、应变的数值结果,数值计算表明,裂尖场主要受材料的蠕变指数n和马赫数M的控制。通过对裂纹尖端场的渐近分析,从应变角度出发,提出了蠕变材料Ⅱ型动态扩展裂纹的断裂判据。     

裂尖曲率对裂纹前缘塑性区的影响

考虑尖端为圆弧形的钝头裂纹模型，在外围取线弹性无裂纹体的解，应用线场分析方法。形成一套估计钝头裂纹前缘塑性区尺寸的方法。对含径向裂纹和圆弧形裂尖的圆盘受均匀张力作用情况，给出了塑性区的裂纹前缘尺寸与裂纹尖端曲率的关系。得到的结论是，塑性区的裂纹前缘尺寸与裂纹尖端曲率有关；对于给定的塑性区的裂纹前缘尺寸，载荷反比于外缘尺寸的平方。前一结论说明了塑性区的裂前尺寸作为裂纹失稳扩展判断的局限性；后一结论说明了裂纹体强度失效的尺寸效应规律：抗断强度与总体线尺寸的平方成反比。     

Analysis of dynamic growth of a tensile crack in an elastic-plastic material

The influence of inertia on the stress and deformation fields near the tip of a crack growing in an elastic-plastic material is studied. The material is characterized by the von Mises yield criterion and J₂ flow theory of plasticity. The crack grows steadily under plane strain conditions in the tensile opening mode. Features of the stress and deformation state at points near the moving crack tip are described for elastic-perfectly plastic response and for several crack propagation speeds. It is found that inertia has a significant effect on the elastic-plastic response of material particles near the crack tip, and that elastic unloading may occur behind the crack tip for higher speeds. The relationship between the applied crack driving force, represented by a remote stress intensity factor, and the crack tip speed is examined on the basis of a critical crack tip opening angle growth criterion. The calculated result is compared with dynamic fracture toughness versus crack speed data for a 4340 steel.     

Strain and failure analysis of graphite/epoxy plates with cracks

The objective of this investigation was to study the deformation and failure of uniaxially loaded graphite/epoxy plates with cracks and to determine the influence of notch size on failure. The specimens were quasi-isotropic laminates with cracks of various lengths. They were instrumented with strain gages, photoelastic coatings and moiré grids. Strains near the crack tip show two distinct points of rate change at strain levels of 0.002 and 0.006, the latter corresponding to the ultimate strain of the 90-deg plies. Failure near the crack tip takes the form of a damage zone consisting of ply subcracking along fibers, local delamination and fiber breakage. Failure occurs when this damage zone reaches some critical value. Measured maximum strains at failure exceeded twice the ultimate strain of the unnotched laminate. The average stress over a characteristic distance (5 mm) from the crack tip was used as a criterion to describe the influence of crack length on failure. Comparison of results with those from similar specimens with circular holes showed that strength was nearly independent of notch geometry in this case, i.e., specimens with holes and cracks of the same size had nearly the same strength.     

三点弯曲试样动态应力强度因子计算研究

利用Hopkinson压杆对三点弯曲试样进行冲击加载,采集了垂直裂纹面距裂尖2mm和与裂纹面成60°距裂尖5mm处的应变信号。根据裂尖附近测试的应变信号计算试样的动态应力强度因子,并与有限元计算结果进行比较,结果表明由于裂尖有一段疲劳裂纹区,通过裂尖附近应变信号来计算动态应力强度因子时,如果裂尖位置确定不准及粘贴应变片位置不够准确对计算结果将带来很大影响。因此利用应变片法计算动态应力强度因子时,为了获得更准确的计算结果,在实验后应对试件裂纹面进行分析测量,重新确定裂尖位置,必要时需对应变片至裂尖距离进行修正后再计算动态应力强度因子值。     

A finite element study of the near crack tip deformation of a ductile material under mixed mode loading

In ductile fracture, voids near a crack tip play an important role. From this point of view, a large deformation finite element analysis has been made to study the deformation, stress and strain, and void ratio near the crack tip under mixed mode plane strain loading conditions, employing Gurson's constitutive equation which has taken into account the effects of void nucleation and growth. The results show that: (i) one corner of the crack tip sharpens while the other corner blunts, (ii) the stress and strain distributions except for the near crack tip region, can be superimposed by normalizing distance from the crack tip by a crack tip deformation length, i.e., a steady-state solution under a mixed mode condition has been obtained, (iii) the field near a crack tip can be divided into four characteristic fields (K field, HRR field, blunted crack tip field, and damaged region), and (iv) the strain and void volume fraction become concentrated in the sharpened part of a crack tip with increasing Mode II component.     

Stress and energy density fields near a blunted crack front in power hardening material

Using Jaumann and Dienes rates of Euler stress in elastic-plastic constitutive equations of finite deformation, plane strain finite element analysis for a compact tension specimen with a blunted crack front is made. The Euler stress, Kirchhoff stress and volume strain energy density near a blunted crack tip are computed. Constitutive relations with different deformation rates affect the the near crack tip solution in a region within an order of magnitude of the crack opening displacement. The results differed from the corresponding solution of deformation plasticity (or nonlinear elasticity) with increasing deformation. They are smaller in a local region of about 2 to 10 times of the crack opening distance.The volume energy density near the crack tip is computed, the stationary values of which determine the locations of extensive yielding and possible sites of crack initiation. It remained nearly constant with increasing deformation. Such a character tends to support the volume energy density criterion as a means for quantifying the ductile fracture behavior of metals.     

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.     

A viscoplastic study of crack-tip deformation and crack growth in a nickel-based superalloy at elevated temperature

Viscoplastic crack-tip deformation behaviour in a nickel-based superalloy at elevated temperature has been studied for both stationary and growing cracks in a compact tension (CT) specimen using the finite element method. The material behaviour was described by a unified viscoplastic constitutive model with non-linear kinematic and isotropic hardening rules, and implemented in the finite element software ABAQUS via a user-defined material subroutine (UMAT). Finite element analyses for stationary cracks showed distinctive strain ratchetting behaviour near the crack tip at selected load ratios, leading to progressive accumulation of tensile strain normal to the crack-growth plane. Results also showed that low frequencies and superimposed hold periods at peak loads significantly enhanced strain accumulation at crack tip. Finite element simulation of crack growth was carried out under a constant ΔK-controlled loading condition, again ratchetting was observed ahead of the crack tip, similar to that for stationary cracks.A crack-growth criterion based on strain accumulation is proposed where a crack is assumed to grow when the accumulated strain ahead of the crack tip reaches a critical value over a characteristic distance. The criterion has been utilized in the prediction of crack-growth rates in a CT specimen at selected loading ranges, frequencies and dwell periods, and the predictions were compared with the experimental results.     

Interfacial mixed-mode fracture of adhesive bonds undergoing large deformation

Interfacial fracture of adhesive bonds undergoing large-scale yielding is studied using a combined experimental/finite-element approach. The full range of in-plane mode mixity is produced over bond thickness ranging from 30 to 500 μm using the scarf and the ENF joint geometries. Novel techniques for introducing pre-cracks and surface decoration, together with in situ observations, facilitate accurate determination of the bond-average and the local shear strains at the crack tip during the onset as well as the rest of the crack propagation event. The crack generally grew along one of the two interfaces of the bond, although the failure was always fully cohesive. The local shear strain at the crack tip is independent of the bond thickness, and, under quasi-static conditions, it remains constant throughout the growth, which make it a viable fracture parameter. This quantity strongly depends on the mode mixity, the sign of the phase angle (i.e., shearing direction) and the crack speed, however.A finite-element analysis is used to obtain the crack tip deformation field for an interface crack in adhesively bonded scarf and ENF joints. Large-strain and quasi-static conditions are assumed. A distinct material model in the fracture process zone that allows for volume change in the post-yield regime is incorporated into the analysis. The local deformation is characterized by a pair of bond-normal and tangential displacements corresponding to the nodal points adjacent to the crack tip. The critical values of these quantities are obtained when the FEM bond-average shear strain at the crack tip becomes equal to its experimental counterpart. The so defined critical local displacements, after an appropriate normalization, seem to conform to a single-valued, linear type interrelationship over the entire range of mode mixity. The fact that this relationship is independent of the bond thickness, and furthermore it encompasses both cases of positive and negative phase angles, makes it a viable candidate for characterizing mixed-mode interfacial fracture under large-deformation conditions.     

A new technique for viewing deformation zones at crack tips

A new technique has been developed to examine and view the deformation zone that occurs at a crack tip during loading. The technique, referred to as the “image-distortion technique”, is based on the optical distortion that results on a highly polished surface ahead of a crack when it is strained. The “image distortion” is a direct result of the thickness change occurring at the crack tip. While the technique was developed using monotonic loading, it is believed that it also will be applicable to cyclic-loading crack-tip studies. Additional studies are underway that will yield strain fields at the crack tip from the observed images of the deformation zones.     

Finite element analysis of void growth near a blunting crack tip

Large deformation finite element analysis has been used to study the near crack tip growth of long cylindrical holes aligned parallel to the plane of a mode I plane strain crack. The near crack tip stress and deformation fields are analyzed. The results show that the holes are pulled towards the crack tip and change their shape to approximately elliptical with the major axis radial to the crack. They also grow faster directly ahead of the crack than at an angle to the crack plane. Several crack-hole coalescence criteria are discussed and estimates for the conditions for fracture initiation are given and compared with experimental results. The range of estimates now available from finite element calculations coincides quite well with the range of experimental data for materials containing inclusions which are only loosely bonded to the matrix.