An application of the boundary integral equation method to dynamic fracture mechanics

In this paper the direct boundary integral equation method is applied to dynamic fracture mechanics, and the computational results are compared with experimental values. The comparison shows that the authors' computation is successful.     

On the role of compression overloads in influencing crack closure and the threshold condition for fatigue crack growth in 7150 aluminum alloy

A study has been made of the effect of single compression cycles on near-threshold fatigue crack propagation in an I/M 7150 aluminum alloy. Based on experiments at a load ratio of R= 0.10 on cracks arrested at the fatigue threshold (δk_th) in under-, peak and overaged microstructures, large compression overload cycles, of magnitude five times the peak tensile load, were found to cause immediate reinitiation of crack growth, even though the applied stress intensity range did not exceed ΔK_th. Following an initial acceleration, subsequent crack advance was observed to take place at progressively decreasing growth rates until rearrest occurred. Such behavior is attributed to measured changes in crack closure which vary the effective near-tip driving force for crack extension (ΔK_eff). Specifically, roughness-induced closure primarily is reduced by the application of compressive cycles via a mechanism involving crack surface abrasion which causes flattening and cracking of fracture surface asperities. Closure, however, is regenerated on subsequent propagation resulting in the rearrest. Such observations provide further confirmation that the existence of a fatigue threshold is controlled principally by the development of crack closure and are discussed in terms of the mechanisms of closure in precipitation hardened alloys.     

有机硅改性超支化聚氨酯的研究

首先制备了端基为羟基的超支化聚氨酯-脲（HPU）和异氰酸根封端的线性低聚物（A2）。以二者为原料制备了异氰酸根封端的超支化聚氨酯,并用适量的硅烷偶联剂（KH550）与异氰酸根反应进行改性。最后未反应的异氰酸根在大气环境下湿固化成膜得到了杂化聚氨酯。用红外光谱（FTIR）表征了产物的化学结构,讨论了产物中可能存在的氢键形式。研究了硅烷偶联剂（KH550）含量对聚合物热性能和力学性能的影响。研究结果表明,聚合物的耐热性能随着KH550含量的增加而增加,但其拉伸强度逐渐下降。     

Microstructure and grain growth in rapidly solidified aluminum

Grain structures in splat-quenched samples of pure aluminum and a dilute aluminum-manganese alloy have been studied, and the effect of heat treatment on grain size has been investigated. The quenched samples were examined by a novel technique allowing surface structures and grain structures to be observed simultaneously, and interesting discrepancies between these were revealed. The grain structures showed considerable resistance to grain growth on heat treatment. This has been explained and the surface and grain structures reconciled on the basis that the material self-anneals to a considerable extent immediately following solidification. The scope of this study is extended to characterize the relationship between grain structures in consolidated and extruded Al-1.3% Mn and the structure of the starting material.     

Tracing planar surface motion from a projection without knowing the correspondence

The 3-dimensional motion of a planar surface is detected only from the motion of its projected 2-dimensional contour image on the plane of vision. There is no need to know the correspondence of points. The motion is explicitly given by measuring “diameters” of the image contour on the plane of vision. No iterative or matching processes are involved. Numerical examples are also given.     

二硅化钼改性硅酸镱环境障涂层的高温抗氧化行为及机理

新一代高推重比航空发动机的发展对环境障涂层（EBCs）的服役寿命和温度提出了苛刻的要求。稀土硅酸盐作为EBCs材料应用于面层,在服役过程中易产生纵向裂纹,为腐蚀性介质的侵蚀提供通道,严重影响EBCs的寿命。二硅化钼（MoSi2）具有优异的高温性能,有望改善稀土硅酸盐 EBCs体系的高温性能。本工作中采用MoSi2改性Yb2SiO5面层,通过真空等离子喷涂技术（VPS）分别制备了Yb2SiO5-XMoSi2（X=0,5和10vol.%）为面层,Yb2Si2O7为中间层,Si为粘结层的三种环境障涂层体系。研究了涂层在1350 ℃长时间氧化前后显微结构演化过程。结果发现,掺杂MoSi2涂层体系在高温氧化500 h后仍能保持结构完整且面层裂纹减少。原因在于,MoSi2可消耗氧化剂,减缓涂层内部TGO的增长速率,反应产物Yb2Si2O7与中间层成分一致,具有较好的化学相容性,有效改善了涂层体系的高温性能。     

On cell structure and subgrain formation in type 304 stainless steel

The formation of subgrains in Type 304 stainless steel, deformed by warm working in tension, was investigated using transmission electron microscopy. The subgrains evolved from an arrangement of dislocation cells formed during the early stages of plastic deformation. Subgrain formation is considered to be a consequence of the recovery mechanisms taking place during Stage III plastic deformation, which is the main part of the stress-strain curves for such steels above 550°C.     

Electron microscopic determination of structural defects in liquid-quenched Ag-Sn alloys and their dependence on internal residual stresses

Here we report a transmission electron microscopy (TEM) investigation of rapidly quenched Ag-6 at.% Sn and Ag-8.2 at.% Sn alloys. Alloys of the same Sn concentrations have also been previously examined by means of x-ray diffraction techniques. By TEM investigations, intrinsic and extrinsic stacking faults, multiple ribbons, extended dislocation nodes, as well as stacking fault tetrahedra were observed. Deformation twins were also regularly present. Nonuniformly distributed dislocations were mostly found to be dissociated. The splitting of dislocations into partials and consequent appearance of stacking faults we attributed to residual stresses. The state of stress, which was found to be three times the stress necessary to dissociate a perfect dislocation in Ag-Sn alloys, we also established from the displacements of x-ray line profiles.