Fatigue crack-growth from a circular notch under high-levels of biaxial stress

A series of experiments have been conducted on cruciform specimens to investigate fatigue crack growth from circular notches under high levels of biaxial stress. Two stress levels (Δσ₁= 380 and 560 MPa) and five stress biaxialities (λ=+1.0, +0.5, 0, −0.5 and −1.0; where λ=σ₂/σ₁ were adopted in the fatigue tests in type 316 stainless steel having a monotonic yield strength of 243 MPa. The results reveal that fatigue crack growth rates are markedly influenced by both the stress amplitude and the stress biaxiality. A modified model has been developed to describe fatigue crack growth under high levels of biaxial stress.

Crack tip behavior and crack-propagation in ductile materials

Dilatational plastic equations, which can include the effects of ductile damage, are derived based on the equivalency in expressions for dissipated plastic work. Void damage developed internally at the large-strain stage is represented by an effective continuum being strain-softened and plastically dilated. Accumulation of this local damage leads to progressive failure in materials. With regard to this microstructural background, the constitutive parameters included for characterizing material behaviour have the sense of internal variables. They are not able to be determined explicitly by macroscopic testing but rather through computer simulation of experimental curves and data. Application of this constitutive model to mode-I cracking examples demonstrates that a huge strain concentration accompanied by a substantial drop of stress does occur near the crack tip. Eventually, crack propagation is simulated by using finite elements in computations. Two numerical examples show good accordance with experimental data. The whole procedure of study serves as a justification of the constitutive formulation proposed in the text.

Mixed-mode fracture mechanisms near the fatigue threshold of aisi-316 stainless-steel

Near threshold, mixed mode (I and II), fatigue crack growth occurs mainly by two mechanisms, coplanar (or shear) mode and branch (or tensile) mode. For a constant ratio of ΔK_I/ΔK_II the shear mode growth shows a self-arrest character and it would only start again when ΔK_I and ΔK_II are increased. Both shear crack growth and the early stages of tensile crack growth, are of a crystallographic nature; the fatigue crack proceeds along slip planes or grain boundaries. The appearance of the fracture surfaces suggest that the mechanism of crack extension is by developing slip band microcracks which join up to form a macrocrack. This process is thought to be assisted by the nature of the plastic deformation within the reversed plastic zone where high back stresses are set up by dislocation pile-ups against grain boundaries. The interaction of the crack tip stress field with that of the dislocation pile-ups leads to the formation of slip band microcracks and subsequent crack extension. The change from shear mode to tensile mode growth probably occurs when the maximum tensile stress and the microcrack density in the maximum tensile plane direction attain critical values.

三维离散元可变形、断裂模型及其在边坡工程中的应用

该文主要是在三维面-面接触离散刚体单元模型基础上，提出考虑可变形、断裂三维离散单元模型，并推导其相应的物理方程及运动方程.在已有划分块体单元计算算法基础上，对单元的自动划分算法作了相应改进.针对具有倾斜表面边坡坡体，可以利用堆积法自动划分块体单元，更加贴近工程实际应用，使三维离散单元法程序应用更广.

涂层-梯度层-基底结构中的热残余应力和应力强度因子研究

热喷涂构件中的热残余应力分布对其使用寿命的影响很大，在涂层和基底之间加入功能梯度材料可以降低构件中的热残余应力。本文用数值方法研究了热喷涂涂层-基底结构中孔隙率的随机分布对结构中热残余应力的影响，以及结构中热残余应力和界面裂纹的应力强度因子与功能梯度层层数之间的关系，并与实验结果相验证。论文由四部分组成。首先，介绍了热喷涂技术的发展历史和目前的主要应用、热喷涂产品中存在的问题以及研究和解决这些问题的方法。其次，研究了涂层中随机分布的孔隙对构件中热残余应力的影响，第三，分析了功能梯度层分层的层数对构件中热残余应力的影响，最后，分析了功能梯度层分层的层数对构件界面裂纹应力强度因子的影响。主要结论为：在期望和方差相同的情况下，涂层中的孔隙率呈高斯分布还是呈均匀分布，对涂层-功能梯度层-基底中的热残余应力的分布影响不大；在基底、涂层和功能梯度层的相对厚度不变的情况下，功能梯度层的层数愈多（即材料性质的过渡愈平缓），涂层-（功能梯度层）-基底结构中的热残余应力愈小；在基底、涂层和功能梯度层的相对厚度不变的情况下，功能梯度层分层的层数愈多，涂层-（功能梯度层）-基底结构的界面裂纹的应力强度因子愈小。

液氦温区VM型脉冲管制冷机的研究

本文的研究成果是世界上第一台热压缩机驱动的液氮温区脉冲管制冷机．它的主要特点是采用热压缩机来驱动脉冲管制冷机，主要研究目标有以下两个：无阀压缩机驱动的高效率液氦温区制冷机和使用最小容积的氦3得到低于2K的最低温度．热压缩机的设计与VM制冷机类似，利用室温和液氮之间的温差产生压力波，但一个重要的发明是功传递管的引入使得本系统中的热压缩机没有低温下的运动部件．使用这种设计也是一个全新的研究，它的重要性可以与脉冲管的引入取消了制冷机低温下的运动部件比拟．笔者进行了最初原型的调试，提出并完成了两次重要改进；最后在压比小于1.3的情况下成功地获得了3.5K的最低温度；为以后的发展打下了好的基础．