Advances in Fracture and Failure Prevention

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Authors: Xi Qiao Feng, Xi Shu Wang
Abstract: It is of interest to understand damage and failure mechanisms of microcracks and their evolution as a function of loading history, especially in the case of complex loading. Owing to their superior mechanical and physical properties, carbon nanotubes (CNTs) seem to hold a great promise as an ideal reinforcing material for composites of high-strength and low-density. HOWEVER, In most of the experimental results, only modest improvements in the strength and stiffness have been achieved by incorporating carbon nanotubes in polymers. There are many factors that influence the overall mechanical property of CNT-reinforced composites, e.g. the weak bonding between CNTs and matrix, the waviness and agglomeration of CNTs. In the present paper, we use the Mori-Tanaka method to evaluate the effect of these factors on the moduli of CNTs-CNT-reinforced composites. It is established that the waviness and agglomeration may significantly reduce the stiffening effect of CNTs, while the interface between the matrix and CNTs influence the moduli of CNTs-reinforced composites little.In this paper, the frictional sliding of microcracks under complex triaxial loading is analyzed, and the obtained results are incorporated into the constitutive relation of microcrack-weakened brittle materials.
Authors: D.A. Liu, Li Yun Li, Q.F. Yuan, D.X. Cheng
Abstract: A virtual crack vector model is proposed in this paper to simulate the branched crack propagation and simplify its calculation of energy release rate. An energy gradient fracture criterion is constructed based on this model. It can take account of the effects of biaxial loads or T-stress on brittle mixed mode fracture and agrees well with the test data of biaxial tensile fracture and the specially conducted experiment of the influences of ox on the mixed mode fracture of plexiglas. Considering its advantage of predicting initial propagation angle and critical fracture stress independently without the determination of the core region radius, it can be used as a practical fracture criterion for brittle mixed mode fracture under biaxial tension.
Authors: Jin Xi Liu, Ai Ping Liu, Z.Q. Jiang, Ai Kah Soh
Abstract: A screw dislocation interacting with a semi-infinite interfacial crack in two dissimilar piezoelectric layers is studied. The complex variable method and the conformal mapping technique are employed to obtain the solution of the problem. The stress and electric displacement intensity factors are given explicitly. We find that the stress and electric displacement intensity factors depend on the effective electro-elastic material constants. Numerical example shows that the influence of piezoelectric effect on the crack tip shielding is significant.
Authors: Han Ki Yoon, D.H. Kim, Yi Hyun Park, Yu Sik Kong, Akira Kohyama
Abstract: Reduced activation ferritic (JLF-1) steel is one of the leading candidates for blanket/first-wall structures in D-T fusion reactors. In fusion applications, structural materials will suffer the effects of repeated temperature changes. Therefore, a database of tensile strength and fracture toughness are very important as the temperature is 400 ° C and it's TIG welded joint. In this paper, fracture toughness JIC and KIC) and tensile tests were carried out at room temperature (RT) and at elevated temperature (400 ° C). The tensile properties of the TIG welding joint of JLF-1 steel was also investigated. It was shown that the tensile strength and fracture toughness values of the JLF-1 steel are slightly decreased with as increase in temperature. The fracture toughness values of JLF-1 steel at room temperature and at 400°C show excellent JIC values of about 530 kJ/m2 and 340 kJ/m2, respectively.
Authors: Fang Juan Qi, Li Xing Huo, You Feng Zhang, Hong Yang Jing
Abstract: Butt-fusion welding is the main technology to join high-density polyethylene (HDPE) plastic pipes, which are widely used in transport the water, gas and corrosive liquid. Investigation shows that one of the failure modes of HDPE pipe is the crack slowly grows across the thick direction and leads to failure at last, so that it is very important to study the resistance to crack initiation of HDPE pipe and its butt-fusion welded joint. In this study, the elastic-plastic fracture mechanics parameter, crack opening displacement (COD) is used to describe the fracture initiation behaviors for the HDPE materials and its butt-fusion welded joints. The resistance to initiation fracture of HDPE pipe materials and butt-fusion welded joints were investigated at different temperature by using multiple specimen resistance curve method and silicon-rubber replica method. The results show that saturation initial crack COD- δis of HDPE pipe materials and butt-fusion welded joints decreases with the decreasing temperature. The δis of butt-fusion welded joints is lower than that of HDPE pipe materials. Investigation also proved that the silicon-rubber replica method is more suitable for HDPE engineering material than the multiple specimen method. At the same time the statistic distribution of the δis of HDPE butt-fusion welded joint was conducted. The results show that the value of the δis has the statistic variance inherently. The optimum fitting distribution of COD is Weibull distribution with three parameters.
Authors: Jae Sil Park, Chang Sung Seok
Abstract: In order to perform elastic-plastic fracture mechanical analyses, fracture resistance curves for concerned materials are required. A standard CT specimen was used to obtain fracture resistance curves. However the fracture resistance curve by the standard CT specimen was very conservative to evaluate the integrity of the structure. Also the fracture resistance curve was affected by the specimen geometry, crack plane orientation, reverse cyclic loading and dynamic strain aging. The objective of this paper is to be certain the conservativeness of the fracture resistance curve by the standard CT specimen. For these purpose, fracture tests using the real-scale pipe specimen and standard CT specimen test were performed. A 4-point bending jig was manufactured for the pipe test and the direct current potential drop method was used to measure the crack extension and the length for the pipe test. From the result of the pipe and the standard CT specimen tests, it was observed that the fracture resistance curve of the standard CT specimen test was conservative compare to that of the pipe specimen test.
Authors: M.S. Han, S. Kim, Kyung Chun Ham, J.I. Song
Abstract: Partial penetration welding joint defines the groove welds that applies the one side welding which does not use steel backing and both side welding without back gouging, that is, the partial penetration welding joint leaves an unwelded portion at the root of the welding area. In this study, we analyzed the residual stress and fracture on the thick metal plates that introduced the partial penetration welding method. We performed the fracture behavior evaluation on the partial penetration multi pass welding with 25.4mm thick plate by using the Ĵ-integral, which finally led us the conclusion that the partial penetration multi-pass welding method is more applicable and effective in handling the root face with less than 6.35mm.
Authors: Guang Zhang, Jing Xi Chen, Bin Hu
Abstract: The brittle-ductile character is one of the important mechanical indexes of rocks and also one of the important affecting factors of rockburst. Both conventional and true triaxial tests have shown that the brittle-ductile character of rocks varies with the variation in rocks stress state and stress path, but these two kinds of tests have revealed totally different laws of brittle-ductile transition. This present paper analyses the results from two tests firstly and then summarizes the effect of rock’s brittle-ductile transition character on rockburst and finally points out the deficiency in present studies of rockburst.
Authors: A. Mirzaee-Sisan, Saeid Hadidi-Moud, Christopher E. Truman, David John Smith
Abstract: This paper presents the results of an experimental and numerical study carried out to investigate the effect of warm pre-stressing on cleavage fracture in ferritic steels using cracked and notched specimens. It is shown that the local approach based on Weibull theory predicts the increase in toughness following warm pre-stressing in highly constrained geometries. The observed effect of pre-loading in low constraint specimens such as round notched bars is less. The local approach could not predict the differences and it is suggested that the variation of triaxiality factor, the ratio of hydrostatic stress to Von Mises, in the plastic zone, is a contributing factor.
Authors: Majid R. Ayatollahi, David John Smith, M.J. Pavier
Abstract: Research studies for mode I cracks have shown that fracture toughness or the critical value of J for fracture initiation, Jcrit is not merely a material property but depends also on the geometry and loading configurations. The geometry dependency of fracture toughness can be attributed to the effect of the crack tip constraint. In this paper, the constraint effect is studies for the initiation stage in mode II ductile crack growth. Two major mechanisms of ductile fracture: 'void growth and coalescence' and 'shear band localization and de-cohesion' are considered. A boundary layer model is simulated using the finite element method and the effect of far-filed T-stress on the relevant stress parameters near the crack tip is studied. It is shown that the initiation of the ductile crack growth in mode II is influenced significantly by T for the mechanism of void growth and coalescence and is insensitive to T for the mechanism of shear localisation and de-cohesion.

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