Optimization of the shear strengths of ultrasonically consolidated Ti/Al 3003 dual-material structures

The interfacial bonding between layers in ultrasonically consolidated structures is poor for some material combinations, resulting in relatively low bond strength. This makes resultant parts unsuitable for structural applications. This work discusses a study of the effects of post process heat treatment of ultrasonically consolidated commercially pure titanium and aluminum alloy 3003 dual-material systems. The lap shear strengths of as-consolidated specimens as well as heat treated ones were tested. The results show that there is significant improvement of the strengths of post processed specimens over the as-consolidated ones. The improvement is as a result of stress relieving of the strain hardened interface between the two materials and some interactions of the base materials across the interfacial boundaries at elevated temperatures, leading to stronger bonds. The study highlights the role of post process heat treatments for improving the mechanical properties of ultrasonically consolidated structures.     

Evaluating the strengths of thick aluminum-to-aluminum joints with different adhesive lengths and thicknesses

Composite joints are often the weakest elements in composite structures. In this paper, we propose a modified version of the damage zone theory based on the yield strain ratio. We use this framework to predict failure loads for various adhesive joints. Thick aluminum-to-aluminum joint specimens with eight different adhesive lengths and four adhesive thicknesses were manufactured and tested. The strengths of different adhesive lengths could be predicted to within 15.4% using the damage zone ratio method. In addition, the strengths of joints that feature different adhesive thicknesses were predicted to within 16.3% using the damage zone ratio method.     

Iosipescu shear deformation and fracture in model thermoplastic polyolefins

Shear deformation and fracture behaviors in polypropylene (PP)‐based model thermoplastic polyolefins (TPOs) were investigated with the Iosipescu shear test. The shear deformation process was monitored in situ via video camera to obtain experimental shear stress–strain curves of model TPOs. Shear fracture mechanisms were studied with optical microscopy and scanning electron microscopy. Macroscopically, the cracks in neat PP propagated along the maximum shear plane, which indicated that mode‐II shear failure existed in neat PP. Microscopically, it was shown that shear fracture initiated in the form of partial, discontinuous inclined microcracks that later coalesced and formed the final continuous crack. The incorporation of rubber in PP could transform the shear fracture process into a stretching process in the shear damage zone. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 3201–3214, 2001     

Numerical simulation of viscous fingering of shear‐thinning fluids

The viscous fingering instability of miscible shear‐thinning fluids has been examined using a pseudo‐spectral numerical technique based on the Hartley transform. The instability was studied for a flow in a rectilinear Hele‐Shaw cell, and the shear‐thinning character of the fluids has been modelied using the Carreau equation. New mechanisms of viscous fingering not previously observed in the case of similar Newtonian flow displacements have been identified. These mechanisms, which are reminiscent of the fractal patterns observed in experimental studies, were interpreted in terms of the velocity‐dependent mobility of the flow.     

聚丙烯腈—硫氰酸钠水溶液的流变方程

测定了聚丙烯腈 -硫氰酸钠水溶液在不同浓度下的流变性能 ,分别建立了剪切速率、温度、浓度与粘度的关系 ,进一步得到以剪切速率、温度、浓度为自变量的粘度关联式 ,将关联式应用于浓度的计算 ,计算值与实验值误差小于 2 %。     

连续刚构主墩桩基础设计

对铁路连续刚构桩基础计算模式进行了描述，对主墩桩基础承台进行了抗压、抗拉、抗剪承载力验算，计算结果表明该设计是满足要求的，为以后的设计工作提供了依据。     

TC11钛合金应变率相关的拉伸行为

为了获取TC11钛合金拉伸性能随应变率的变化规律,对该材料开展了宽应变率范围下的单轴拉伸试验。结果表明,随着应变率从准静态增加到动态,TC11钛合金的屈服强度略有上升,而应变硬化模量下降。此外,在准静态和动态拉伸下,TC11钛合金均发生了剪切断裂,但动态断裂面上韧窝尺寸小于准静态断面上韧窝尺寸。进一步对材料在变形过程中的温升进行了分析,结果发现,高应变率下材料断裂面上更小尺寸的韧窝和材料更容易发生应变软化归因于动态加载情况下材料中产生了更高的温升。     

Shear localization behavior in hat-shaped specimen of near-α Ti−6Al−2Zr−1Mo−1V titanium alloy loaded at high strain rate

The microstructure characteristics in early stage shear localization of near-α Ti?6Al?2Zr?1Mo?1V titanium alloy were investigated by split Hopkinson pressure bar (SHPB) tests using hat-shaped specimens. The microstructural evolution and deformation mechanisms of hat-shaped specimens were revealed by electron backscattered diffraction (EBSD) method. It is found that the nucleation and expansion of adiabatic shear band (ASB) are affected by both geometric and structural factors. The increase of dislocation density, structure fragment and temperature rise in the deformation-affected regions provide basic microstructural conditions. In addition to the dislocation slips, the extension twins detected in shear region also play a critical role in microstructural fragmentation due to twin-boundaries effect. Interestingly, the sandwich structure imposes a crucial influence on ASB, which finally becomes a mature wide ASB in the dynamic deformation. However, due to much larger width, the sandwich structure in the middle of shear region is also possible to serve as favorable nucleation sites for crack initiation.     

热障涂层TGO界面应力分布及裂纹扩展行为的研究进展

热障涂层以其优异的抗氧化? 隔热? 耐腐蚀性而广泛应用于热端部件表面.在超过1000℃高温的服役环境下,外界的氧元素通过陶瓷层扩散到粘接层界面,与其中的金属元素发生氧化反应生成一层热生长高温氧化物(TGO)? 随着服役时间的增加,TGO不断生长,TGO界面产生较大的热应力,导致裂纹的萌生与扩展,使得涂层大面积剥落,因此TGO氧化失效的研究既是难点也是热点问题.总结了研究TGO界面建立的主要几种模型,例如同心圆界面模型? 曲线弦界面模型和真实界面模型,其中曲线几何结构通常只需要振幅和波长即可很好地描述界面,因此大多数模型建立采用了曲线模型.在此基础上,重点综述了界面形貌、界面粗糙度和TGO厚度对应力分布的影响,以及从应变能释放率和裂纹路径两个角度探讨了界面损伤行为.上述研究很好地阐明了TGO生长过程,但是仅考虑了形态特征对应力分布的影响,接下来的研究应考虑真实TGO界面并完善模拟的准确性,同时发展有限元技术实现在实际条件下裂纹扩展路径和使用寿命的预测.     

2A12铝合金动态腐蚀-疲劳耦合失效机理研究

目的 针对某飞机吊挂结构用2A12铝合金开展预腐蚀后的动态腐蚀-疲劳耦合试验,很快发生断裂行为,寻找失效原因并提出解决措施.方法 利用自制的卧式动态腐蚀-疲劳试验装置,开展2A12铝合金预腐蚀后的腐蚀-疲劳协同试验直至样品断裂.同时,对比开展2A12铝合金腐蚀与疲劳交替试验,分析断口腐蚀形貌、元素含量、价态变化,获得2A12铝合金在两种试验方法下的腐蚀疲劳机理.结果 2A12铝合金主要由铝基体以及弥散分布其中的多种合金强化相组成.当有预裂纹时,2A12铝合金在腐蚀与疲劳交替作用下,很快发生疲劳断裂,且裂纹几乎贯穿整个断面,在整个裂纹附近存在较多的腐蚀产物Al2O3.2A12铝合金在预腐蚀后,基体与其表面的氧化膜之间形成腐蚀电池,初期的点蚀孔快速发展成明显腐蚀坑并产生大量腐蚀产物Al2O3,腐蚀坑底部由于应力集中现象而成为裂纹源,在动态腐蚀-疲劳耦合作用下快速萌生裂纹并呈放射状扩展,很快发生疲劳断裂行为,同时裂纹扩展区无腐蚀产物.结论 2A12铝合金用作飞机吊挂结构件时,必须进行表面防腐处理,避免形成腐蚀坑,减缓吊挂结构发生腐蚀-疲劳断裂进程.