The fracture toughness and fatigue behavior of DRA

‘In this study, the fracture toughness and elevated-temperature tensile and fatigue behavior of discontinuously reinforced aluminum (DRA) were examined. The effects of heat treatment and specimen thickness on fracture toughness were studied in a 7093/SiC_p composite. The toughness of the DRA was lowest in the peak-aged condition, but increased considerably in the overaged condition. The highest toughness was obtained at a critical value of specimen thickness; this critical value was used to fabricate a laminated composite consisting of alternating layers of DRA and unreinforced alloy. Elevated-temperature tensile and fatigue behaviors were investigated in a 2080-T6/SiC_p composite at different volume fractions and particle sizes. Increasing reinforcement volume fractions resulted in increases in tensile and fatigue strength. Exposure tests for 300 h at 150°C produced no significant reduction in ultimate tensile strength or yield strength, indicating good thermal stability of the 2080 matrix. For more information, contact A.B. Pandey, Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson AFB, Ohio 45433; (937) 255-1320; fax (937) 255-3007; e-mail awadh.pandey@ml.afrl.af.mil.     

The aging response and creep of DRA composites

The hardening rates of 6061/SiC/20w and 6061/Al₂O₂/15p are significantly higher than that of wrought 6061 Al. The whisker composite retains its tensile strength over a broad range of aging times (up to 500 hours). Under identical aging conditions, the tensile strength of the whisker composite is about 65% higher than that of the particulate composite and wrought aluminum. Likewise, the stiffness of the whisker composite is approximately 60% higher than that of the other two materials (which have similar stiffnesses). At 350°C, the whisker composite has about 55% higher strength than the particulate composite and the wrought aluminum, in comparison with a 111% advantage at room temperature. The whisker composite shows better resistance to creep than the particulate composite and wrought aluminum; the improvement is almost two orders of magnitude in terms of the steady-state creep rate.     

The fracture resistance and crack-tip micromechanics of in-situ intermetallic composites

The Cr₂Nb intermetallic compound is attractive as a high-temperature material because of its high melting point, low creep rate, and good oxidation resistance. However, this material is also extremely brittle at ambient temperature. Consequently, substantial efforts have been made to develop Cr₂Nb-based in-situ composites containing a ductile phase with the aim of imparting fracture resistance while maintaining most of the high-temperature properties. This article presents an overview of the effects of matrix composition on the fracture resistance and crack-tip micromechanics for Cr₂Nb-containing in-situ composites based on the Nb-Cr-Ti system. Crack-tip strain measurements are presented to illustrate the important effects of titanium addition on the ductility of the solid-solution phase and the fracture process in the in-situ composites.     

Micro- and nanoscale tensile testing of materials

This article reviews concepts and techniques for performing instrumented tensile testing of materials at small dimensions. State-of-the-art methods to probe tensile behavior of micro- and nanoscaled materials span many orders of magnitudes of force and displacement, often requiring a custom solution for each new material discovery. We discuss the experimental opportunities, challenges, and pitfalls in concert with the scientific insights revealed from tensile investigations at length scales where conventional wisdom is challenged on how materials deform.     

微纳米生物玻璃的体外成骨性能研究

微纳米生物活性玻璃因其具有特殊的形态结构和理化性能目前引起众多研究者的关注,但是目前对微纳米生物活性玻璃的结构及形态对细胞性能的影响研究的较少。本文通过溶胶一凝胶法结合模板仿生技术合成了具有特殊结构和形态的微纳米生物活性玻璃,并通过体外细胞实验,研究了这种微纳米结构对人骨髓间充质干细胞成骨性能（ALP,Runx2,Coilal和OPN）的影响,结果证明具有规则形态的生物活性玻璃（SBG）相比不规则形态的生物活性玻璃（IBG）更能促进细胞的体外成骨性能,为将来设计具有特定结构的生物活性玻璃提供了参考依据。     

内高压成形开裂问题的影响因素研究

内高压成形技术最常见的问题是管材开裂。而对导致开裂的因素的研究成为内高压成形技术的主要课题。经实际生产经验积累,对内高压开裂问题做简要分析阐述。其影响因素主要有：材料属性、加载曲线、工艺造型、成形速率、型腔与管材的摩擦系数等。     

双相钢电阻点焊接头断裂形式研究

针对双相钢电阻点焊过程焊接工艺窗口窄、易出现熔核断裂的问题,文中通过焊点力学性能与微观金相试验,研究了不同焊接工艺参数变化时对双相钢焊点质量的影响规律,建立其焊接工艺窗口,分析了双相钢熔核断裂的成因及焊接参数对熔核断裂的影响规律.研究表明,对于一定板厚的试件,临界熔核直径将是决定双相钢点焊接头是否出现熔核断裂的重要参数之一,并给出避免熔核断裂的合理焊接工艺参数.     

Understanding the factors controlling the interfacial failure strength of advanced high-strength steel resistance spot welds: hardness vs. fracture toughness

The failure of advanced high-strength steels’ spot welds is a critical issue for automotive crashworthiness. This paper deals with understanding the underlying factors of the tensile-shear strength of automotive steels’ resistance spot welds during interfacial failure. It was found that the ratio of the fracture toughness to the hardness of the fusion zone is the critical factor governing the interfacial failure mechanism: ductile shear failure (controlled by the fusion zone hardness) vs. cleavage crack propagation (controlled by the fracture toughness). This clarification could pave the way for more accurate modelling of interfacial failure of advanced steel resistance spot welds and shed light on the design of proper post-weld heat treatment for improving the weld mechanical performance.     

Effect of Micro- and Nano-SiC Particulate Reinforcements in Magnesium-Based Metal Matrix Composites

The effects of the volume ratios of micro and nano-SiC particles (SiCp) on the grain refinement, distribution of SiCp particle, and tensile properties of the as-cast AZ31B (Mg-3Al-1Zn-0.3Mn) magnesium-based metal matrix composites have been investigated. As the volume fraction of micron SiCp decreases to 9 vol.% and the nano-SiCp increases to 1 vol.%, excellent grain refinement effect is achieved. This is due to both the uniform distribution and refining effects of micro- and nano-SiCp. Moreover, the micron SiCp distribute along the grain boundaries, while nano-SiCp is mainly distributed around the micron SiCp. The room-temperature tensile results show that the optimal room-temperature yield and tensile strengths are achieved with a 9/1 ratio of micro to nano-SiCp, while the 9.5/0.5 ratio yielded the highest elongation.     

外板圆角区域拉深破裂及影响因素研究

汽车覆盖件整体侧围外板由于型面复杂,拉深成形中极易产生起皱、拉薄和拉裂等缺陷.针对某车型右侧围外板现场生产中一圆角区域间断产生拉裂的问题,通过网格应变分析技术,分析了圆角区域各部位的变形特征及材料产生拉薄、拉裂的原因.基于有限元仿真平台,建立了与该区域具有相似变形特征的分析模型,系统研究了板料尺寸和拉深筋阻力对拉深破裂极限的影响规律,并分析了两种修模方案的效果.最后,通过现场生产,验证了分析结果的准确性和修模方案的有效性.