Papers by Keyword: Initial Crack

Abstract: Benefit from the gradient distribution of microstructure, gradient nanograined (GNG) metals have broad application prospect owing to their advantages of both high strength and good tensile ductility. Meanwhile, the fracture behavior of gradient nanograined metals is different from that of traditional homogeneous materials. Using molecular dynamics (MD) method, we simulated the propagation of a crack in a pre-cracked GNG Cu. Voronoi method was adopted to generate the polycrystalline topology with gradient grain size, and FCC copper atoms were filled into the topological structure. The crack was introduced by removing three layers of atoms. Then, the MD specimen was loaded to simulate the crack growth and/or blunting. The micro-defects were identified by the common neighbor analysis parameter. The effects of the grain size gradient and the crack tip initial position on the crack growth were also investigated.

Abstract: Damage or failure frequently occurs on gearbox gears. If this occurs in the gearbox of tractors, this can be severe. This study aimed to determine the cause of the spur gear fracture through empirical and simulation studies. The hardness test was undertaken employing the Rockwell method where the fracture surface was observed using a scanning electron microscope (SEM) in order to identify crack initiation and the type of fracture. The stress intensity factor was next analysed using the finite element method (FEM). The results of the chemical composition testing indicated that the material used was according to the AISI 8620 standard containing an element of Carbon (C) of about 0.142 %.The hardness value of the gear was 109 HRB. The observation of the fracture surface showed a brittle fracture surface, suggesting that an impact load had occurred. The simulation results using the FEM also showed that the maximum stress intensity factor and K_I value occurred at the centre of the tooth. The value of K_I was shown to be larger than the fracture toughness and (K_IC). Therefore, this result indicates that a crack will continue to propagate until final failure.

Abstract: An aluminum alloy is used in airplanes and aerospace to reduce the weight of structures. Recently, researchers have studied how to intensify the strength of aluminum alloys. Shot peening is one of method to reinforce strength by hitting the surface of materials to make residual stress. In this study, high cycle fatigue characteristics of aluminum alloy Al 7075-T6 and Al 2024-T4 were analyzed. Fatigue characteristics of before and after the shot peened materials were tested by cantilever-rotary bending fatigue test machine (YRB 200, Yamamoto). Also, fractographic analysis was performed by a Scanning Electron Microscopy (SEM). The results showed that the fatigue life of both shot peened materials increased more than others.

Abstract: Based on extended finite element method (XFEM) of the finite element software platform-ABAQUS, three-point bending numerical simulation of CFRP reinforced concrete beam with initial crack has been made to predict the 3D crack propagation path. The numerical tests present the crack propagation path under ultimate load and crack damage modes of non-CFRP and some different CFRP plies. The results show that the strength of CFRP reinforced concrete beam is obviously improved and the crack which previous penetrate through doesnt penetrate through concrete beam under CFRP reinforce based on multiple optometric numerical simulation results. The results of this paper can also provide some foundations for damage mode, the ultimate load and CFRP overlay design of reinforced concrete beams.

Abstract: nitial crack widely exits in the welded crane members. These cracks are induced by defect of design and manufacturing and effect of alternate fatigue stress. The fatigue performance and remaining life of welded components which have initial crack is significantly reduced. In the paper the typical box-type welded components of crane are discussed. Based on fatigue propagation experiment of box-type specimen with initially prefabricated crack, the fatigue propagation feature of Q235 steel is studied. Meanwhile finite element software Ansys is used to calculate the stress intensity factor. Combining experimental data and calculated result of stress intensity factor, two important propagation parameters of Q235 steel are acquired. The experiment result can be applied to evaluate the remaining fatigue life of welded crane members with initial crack.

Abstract: Three-dimensional stresses are analyzed in the tubing working based on the theory of thick-walled cylinder, and the stress intensity factor expression is derived on longitudinal and circumferential extending cracks in the tubing. Tubing fatigue life is calculated according to given condition through the preparation of the corresponding program. Influence of initial crack length, change of the pressure and the tubing wall thickness on the fatigue life are discussed.

Abstract: Abstract: In drilling process, drill pipes’ out surface always have lots of pits and scratches due to the corrosive and erosion of mud or wear of well wall in work environment. Through establishing drill pipe’s working model and constitutive equations with Finite Element Analysis Software, the paper have simulated stress distribution on drill pipe body under two different condition that without mechanical damage and with initial crack. Analysis results show that the maximum stress increases with the increasing of initial crack depth, and the maximum stress position turns to crack circumferential root from the crack radial root with the increasing of initial crack’s depth. The analysis result is completely consistent with the actual morphology of crack propagation, as a result, the simulation analysis provides a effective new methods and reference for remaining life assessment and failure analysis of drill pipe.

Abstract: It requires a damage tolerance assessment for all airframes and engines to set the inspection windows for safe operation. A certain rotorcraft main rotor yoke were analyzed with NASGRO model. The sensibility analysis of influence parameters in crack growth life has been done. In order to calculate the sensibility index of influence factors to crack growth life, the factors were divided into input parameters and crack growth model parameters. The results show that the input parameters have the following precedence ordering: fatigue crack growth threshold, fracture spectrum, initial crack, fracture toughness, the sensibility values are 11.25, 8.5417, 0.8333, 0.1125, respectively. The NASGRO model parameters have the following precedence ordering: n, p, C, q. the sensibility values are 6.0417, -3.9583, 1.25, 0.1812, respectively.

Abstract: By numerical simulation, the researches of effecting initial crack forming with slotted charge blasting are performed. The effecting factories conclude radial uncoupled parameter α and seam width W. Herein α adopts 1.43, 1.67 and 2, and 4, 6, 8 and 10 are adopted by W separately. The pressure peak is 15.5 GPa under α=1.67, and formed long cracks. With 6 and 8 mm seam widths, energy focus phenomenon is remarkable. The results show that initial damage of rock are decided by peak pressure of gas and function time. The seam width has significant effect to initial damage. With W adopting 6 and 8 mm, effective energy propagation is ensured and excessive damage area is avoided. The experiments validate the simulation results.

Abstract: The crack growth behavior and the fatigue life of welded members with initial crack in bridges under traffic loading were investigated. Based on existed fatigue experiment results of welded members with initial crack and the fatigue experiment result of welded bridge member under constant stress cycle, the crack keeps semi-elliptical shape with variable ratio of a/c during crack propagation. The calculated method of the stress intensity factor necessary for welded bridge member crack propagation was discussed. The crack remained semi-elliptical shape with variable ratio of a/c during crack propagation. The fatigue crack propagation law suitable for welded steel bridge member fatigue crack propagation analysis was deduced based on the continuum damage mechanics and fracture mechanics. The proposed fatigue crack growth model was then applied to calculate the crack growth and the fatigue life of existed welded member with fatigue experimental result. The calculated and measured fatigue life was generally in good agreement, at suitable initial conditions of cracking, for welded member widely used in steel bridges.