Papers by Keyword: Microvoid Coalescence

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Authors: K. Qin, Li Ming Yang
Abstract: Experiments show that the failure of ductile materials can be characterized by a rate-independent parameter, relative spacing d defined as the ratio of the distance between two voids and the radius of voids. In this study, this experimental phenomenon is analyzed via numerical simulations using 3-D finite element model. Considering that hydrostatic stress is a dominant factor in the evolution of microvoid nucleation, growth and coalescence in ductile materials, numerical simulations are performed to obtain the relationship between relative spacing d and hydrostatic stress in the ligament between voids. Numerical results show that hydrostatic stress along matrix ligament is sensitive to the change of the relative spacing. Further analysis shows that the failure of ductile materials can modeled by using a criterion of the threshold of local hydrostatic stress in the ligament. Based on such a criterion, a curve displaying the relationship between the strength of ductile material and strain rate is obtained numerically. It is concluded that the failure criterion of ductile materials can be described by using local hydrostatic stress and relative spacing between two voids, which is not sensitive to strain rates.
Authors: Indranil Roy, Manuel Marya, David Susnitzky, Hong Qi Deng
Abstract: Three Ni-Cr-Mo alloys with 140ksi minimum yield strength, namely 625Plus, 718, and 945X were tested in selected sour environments to evaluate and rank their environmentally-assisted cracking susceptibility. The testing revealed that none of the alloys, stressed to 90% of their actual yield point, cracked as a result of an agressive 30-day NACE level VII-modified environment (401 ± 9 oF, 500 psia H2S, 500 psia CO2 and 151,700 mg/liter chlorides). In a complementary attempt to rank the alloy cracking susceptiblity, quasi-static strain rate (SSR) tests per NACE TM0198-2004 were conducted in a modified H2S-saturated NACE TM0177 Solution A. Following a critical analysis of parameters such as time-to-failure (TTF), percentage reduction in ductility (%ε), percentage reduction of cross-sectional area (% RA) and fractomicrographs, 625Plus was confirmed to outperform its counterparts while some evidence of hydrogen embrittlement was found on 945X.
Authors: Teresa L.M. Morgado, Armando Sousa Brito, Carlos M. Branco
Abstract: This paper presents the results and main conclusions of a study made to analyze the cause of failure occurred with an austenitic 304 class stainless steel wire rope of a helicopter rescue hoist. The cable is made up of 19 strands, 12 outside and 7 inside. As each strand contains 7 wires, the whole cable is made up of 133 wires. The study includes the chemical and microstructural characterization of the material, as well as the determination of its hardness, mechanical properties and the fractographic analysis by scanning electron microscopy (SEM). Tensile tests were performed for three velocities simulating different work conditions: 250mm/min, 50mm/min and 5mm/min. The fractographic analysis shows that the cable suffered lateral loss of material due to friction and leading to the failure of the remaining material by ductile mode.
Authors: Xin Ming Zhang, Yang Xiao
Abstract: The fractographs of Mg-9Gd-4Y-0.6Zr alloy specimens which were tensioned at different temperatures were investigated by optical and scanning electron microscopy, respectively. The results showed that different slip systems were activated at different temperatures, which were responsible for varied deformation mechanisms and fracture mechanisms. At 25 °C, the enabled slip systems were few and only the slip systems on basal plane were able to be activated, so cleavage fracture was observed. At −196°Cthe number of enabled slip systems increased, prismatic or pyramidal slip maybe occurred, so that an obvious low-temperature plasticity was observed, while fracture mechanisms were mainly microvoid coalescence fracture with cleavage fracture in local areas. At 250 °C, 300 °C and 350 °C, the multisystem slips on the basal planes, the prismatic planes and the pyramidal planes were able to be activated, while fracture mechanisms were also microviod coalescence fracture. At 400 °C, recrystallization happened, grain-boundary sliding in new fine recrystallized grains made the plastic deformation easy, showing coarse-grain superplasticity phenomenon, and an intergranular shear fracture took place.
Authors: Jian Guo Ning, Zhu Ping Huang, Jiufeng Hao
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