Advances in Fracture and Failure Prevention

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Authors: Chul Kim, Jong Heun Lee, J.H. Kim, Hoon Sang Choi
Abstract: The optimal stacking sequence and wall thickness of the composite strut tubes were determined to minimize thermal strains during orbital operation using generic algorithms and finite element analyses. From the results of previous thermal analyses of composite struts with various stacking sequences, the axial deformation is a matter of prime importance. For this reason, the optimization focuses to minimize the axial strains. The balanced and symmetric stacking sequences are used to minimize the radial and the twisting deformations. The genetic algorithm is known to be very effective for the discrete optimization such as stacking sequences of composite materials. As a result, the thermal deformations of the strut with an optimal stacking sequence are almost zero. The optimal strut tube consists of 6 plies and the weight of a composite strut is 22.4% that of aluminum strut. Finite element analyses showed that the optimal design of composite strut tubes withstood combined launch loads without buckling and failure. To validate the analyses, four composite struts were fabricated and their thermal strains were measured under the temperature increase of 100°C. The thermal and vibration experiments showed excellent correlations with analytical results.
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Authors: Ouk Sub Lee, Jang Sik Pyun, Si Won Hwang, Kyoo Sung Cho
Abstract: This paper presents the effect of boundary conditions of various failure pressure models published for the estimation of failure pressure. Furthermore, this approach is extended to the failure prediction with the help of a failure probability model. The first order Taylor series expansion of the limit state function is used in order to estimate the probability of failure associated with each corrosion defect in buried pipelines for long exposure periods with unit of years. The effects of random variables such as defect depth, pipe diameter, defect length, fluid pressure, corrosion rate, material yield stress, material ultimate tensile strength and pipe thickness on the failure probability of the buried pipelines are systematically investigated for the corrosion pipeline by using an adapted failure probability model and varying failure pressure model.
803
Authors: Tae Hee Lee, J.J. Jung
Abstract: Nonlinear analysis of anisotropic structures is described by using Hill's yield criterion that anisotropic yield contour is assumed to be retained its shape during the hardening process. Nonlinear constitutive equation of anisotropic material is derived using plastic potential concept. Linear strain hardening model is utilized and forward Euler method is employed as a stress integration method. Newton-Raphson method is implemented for numerical nonlinear analysis. Direct differentiation method differentiating directly the equilibrium equation with respect to design variables is applied to design sensitivity analysis of nonlinear anisotropic plate. The results of design sensitivity analysis are compared with those of finite difference method to verify the accuracy. Optimization is accomplished for a rectangular plate using evaluated sensitivity coefficients.
809
Authors: Jing Xi Chen, Guang Zhang, Bin Hu
Abstract: This present paper, taking a large-scale underwater power station in China as background engineering works, carries out sensitivity analyses of various parameters of jointed rockmass on the scope of damage zones in the rockmass surrounding the works. It gives major sensitivity parameters and discusses the trend and possibility of these parameters’ effect on the damage zone of surrounding rocks as well as makes some suggestions on the exploration and construction
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Authors: Sung Gyu Jung, Chang Soon Lee, In Gyu Park, Se Hwan Lee, Tae Eun Jin
Abstract: In-service inspections (ISI) of pipes in the nuclear power plants are currently performed based on mandated requirements in the ASME Section XI, which is based on deterministic approach of the critical welds. The 20 years of ISI experience in U.S.A. has revealed less correlation between the critical welds and actual failures, and much conservatism in current ISI requirements. To reduce those problems, risk-informed ISI technology has been developed and proved to be useful. This paper presented a method for predicting piping failure probabilities in an application of risk-informed ISI, and analyzed the effect of input parameters on piping failure probabilities. Results generated using this approach revealed that the calculated failure probabilities can be sensitive to the different types of stressors, crack size distribution, inspection interval, etc..
821
Authors: Seok Yoon Han
Abstract: Shape optimization a cantilevered beam in mixed mode for prolonging fatigue life was accomplished by the linear elastic fracture mechanics and the growth-strain method. Linear elastic fracture mechanics (LEFM) was used to evaluate the stress intensity factor and fatigue life. The growth-strain method was used to optimize a shape. From the results, it was found that the optimal shape of a cantilevered beam greatly prolongs their fatigue life, and the growth-strain method is an appropriate technique for shape optimization of a structure having a crack.
827
Authors: Seok Yoon Han, S.S. Bae, S.J. Jung
Abstract: The growth-strain method was applied to cutout optimization in laminated composite plates. Since the growth-strain method optimizes a shape by generating the bulk strain to make the distributed parameter uniform, the distributed parameter was chosen as Tsai-Hill value. In this study, of particular interest is to see whether the growth-strain method developed for shape optimization in isotropic media would work for laminated composite plates. In volume control of the growth-strain method, it makes Tsai-Hill value at each element uniform in laminated composite plates under the predetermined volume. The shapes optimized by Tsai-Hill fracture index were compared with those of the initial shapes for the various load conditions and predetermined volumes of laminated composite plates. As a result, it was verified that volume control of the growth-strain method worked very well for cutout optimization in laminated composite plates.
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Authors: Seok Yoon Han, J.Y. Park, Y.J. Ma
Abstract: Stress control of the growth-strain method was applied to shape optimization of multiple cutouts in laminated composite plates. Since the growth-strain method optimizes a shape by generating the bulk strain to make the distributed parameter uniform, the distributed parameter was chosen as Tsai-Hill value, as volume control of the growth-strain method. In this study, of particular interest is to see whether stress control of the growth-strain method developed for shape optimization in isotropic media would work for laminated composite plates. The shapes optimized by Tsai-Hill fracture index were compared with those of the initial shapes for the various load conditions. As a result, it was verified that stress control of the growth-strain method also worked very well for multiple cutouts optimization in laminated composite plates.
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Authors: Vladimír Číhal, Rudolf Štefec, Tetsuo Shoji, Yutaka Watanabe, Vivenakand Kain
Abstract: The EPR test, designed to examine of the susceptibility to nonuniform, primarily intergranular corrosion, ranks among the more successful testing technique developments relating to stainless steels and alloys. One of its numerous advantages is that it lends itself to non-destructive, on-site examination. EPR enjoyed wide expansion over the years since first conceived by Čihal in 1969. Recent EPR measurements tend to focus on (1) double and/or single loop EPR as a modern technique used to establish the resistance of stainless steels and alloys to intergranular corrosion; (2) detecting integranular corrosion (IGC) and intergranular stress corrosion cracking (IGSCC) susceptibility in alloy steels and nickel alloys for nuclear engineering applications; and (3) studies of grain boundary precipitation and other minute local changes to alloy composition and structure.
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