Advanced Materials Research Vols. 33-37

Abstract: The fatigue destruction is one way of expiration of pressure vessels, In order to avoid the accident occurring, it is extremely important to carry on the fatigue analysis to the pressure vessels. First, this article introduces the definition of fatigue destruction, the primary factors of affecting the fatigue expiration, and the advantages the analysis principle when the ANSYS finite element is applied to fatigue analysis; Then, this article carries on the stress analysis based on ANSYS software to the CNG storage gas pressure vessel, produces the stress distributed cloud chart, and gets the node number where is the maximum stress; Finally, This article carries on the fatigue analysis based on stress analysis result, the fatigue analysis demonstrates the CNG pressure vessel is effective in the establishment service life, Its fatigue accumulative damage coefficient is smaller than 1,Which explain it can satisfy the fatigue strength request.

Abstract: Uniaxial ratchetting-fatigue interaction of tempered 42CrMo alloy steel was observed by various cyclic stressing tests at room temperature. The ratchetting deformation and low cycle fatigue (LCF) property of the material as well as their interaction occurred in cyclic stressing were discussed. It is shown that progressive ratchetting deformation causes the decrease of fatigue life, and the fatigue life of the material depends greatly upon the applied mean stress, stress amplitude, maximum stress and stress ratio. Since tempered 42CrMo steel presents significant cyclic softening feature, a tertiary ratchetting is observed. Based on the experimental results, a simple and reasonable failure model convenient to engineering application was constructed to predict the fatigue life of the material in uniaxial cyclic stressing. The basic variables of the model are maximum stress and stress ratio, and the effect of cyclic softening feature on the ratcheting-fatigue interaction is also included in the model by introducing a new variable. It is shown that the predicted lives are in fairly good agreement with the experimental ones.

Abstract: Analytical studies were made on effect of size and location of a weld defect on fatigue life for argon-arc welded titanium alloy joint. In the analyses, a weld defect was assumed as an initial crack, and the crack growth life was taken as total fatigue life. By using the Isida and Noguchi’s stress intensity factor solution for a plate containing an embedded elliptical subsurface crack under tension, the life prediction code FASTRAN3.9 was revised. A small crack methodology based on the plasticity-induced crack-closure concept and the effective stress intensity factor range,
Keff , was used to predict the total fatigue life of welded joint, and to study the effect of the size and location of weld defect on fatigue life by means of the revised FASTRAN3.9 code. Limited amounts of experimental data were used to make comparison with the predictions. The predicted fatigue lives are in reasonable agreement with experiments, and the effect of both the size and location of the weld defect on fatigue life was found to be significant.

Abstract: As the implementations of the western development in China, more and more tunnels will get through the western mountains in China. In order to economize the construction costs, a new type of underground structural form called branching-out tunnel must be applied. The failure process of the branching-out tunnel under lateral overload action is also greatly complicated, which is related with the depth of the mountains, the branching-out angle, the in-situ stress field, the thickness of the middle wall and so on. This paper uses the 3D-physical model of geo-mechanical model tests to study the stability and failure process of this complicated structure, especially the part of the middle wall. The physical model is built up in a new kind of analogy material. In the process of the whole experiments, different lateral pressures imposed on both of the lateral planes of the physical model. According to different lateral pressures, we have attained the change of the stress and displacement field and looked into the failure process of the pivotal positions in the branching-out tunnel. We also use finite element method analysis software RFPA (Realistic Failure Process Analysis) to simulate the whole failure process of the branching-out tunnel. Finally, we have got the load-bearing safety reliability of this complicated structure through comparative analysis of physical modeling and numerical simulation.

Abstract: Fatigue crack growth under mixed mode loading conditions is simulated using S-FEM. By using S-FEM technique, only local mesh should be re-meshed and it becomes easy to simulate crack growth. By combining with auto-meshing technique, local mesh is re-meshed automatically, and curved crack path is modeled easily. Plural fatigue crack problem is solved by this technique. For two parallel crack problem, criteria of crack coalescence are proposed. By simulating this problem by S-FEM, it is verified these criteria are conservative ones.

Abstract: In the ASME Code Section III ‘design by analysis’ approach, stresses are determined by numerical method and compared with corresponding stress limits. This approach provides several stress criteria for fatigue life assessment and procedures for categorizing the representative stress components. Since the stress criteria were derived from two-dimensional basis, however, it may inappropriate to delineate structural components with complex geometry. In this paper, detailed transient analyses are performed for modular pressurizer with an asymmetric geometry, which includes perforated parts to mount various piping and equipments. Also, the applicability of an effective elastic modulus to consider the perforation and the appropriateness of stress linearization method using stress classification line are assessed. Then, the cumulative usage factor as well as stress intensities at critical locations of the pressurizer are calculated and compared with corresponding allowable design stress limits. The key findings of this work can be used to make regulatory guides for evaluation and confirmation of structural intensity of components with asymmetric perforated parts.

Abstract: In-situ observation of the fatigue crack growth and 90o domain switching was carried out for BaTiO3 ferroelectric single crystals under alternating electric field. It is shown that during the electric cycling, the crack propagates continuously. Parallel lines of 90o domain boundaries can be seen and they flip at each reversal of the alternating electric field. The width of the 90o domain switching zone grows with the number of cycles and its frontal always lies ahead of the crack tip. It is suggested that the cyclic stress field induced by the repeated 90o domain switching at the crack tip, as well as the stress field caused by the electrically activated material between the electrode and the material under the electrodes contribute to the observed fatigue crack growth.

Abstract: This paper shows how the number of lap of spot welded joints in automotive steel sheets changes the life -cycle transform of itself. There has been significant developments in the life extension of automotive steel sheets, as well as in passenger safety. We verified the validity of the S-N curve of materials by QSTS test. Moreover, we used EZNCEN and HS40R, which are commonly used as automotive steel sheets because of their excellence in increasing fuel efficiency. The purpose of research was to compare the fatigue life of multi - lap of spot welded joints to there. In addition s, we used the two different kinds of sheets together in the same welding under the same condition. Through this whole process, the hypothesis on the life cycle of lethargy coefficients was found to be in good agreement with the result of the experiment. That is, when using sheets of the same materials in one welding, the fatigue life of two laps of a spot welded joint was found to be much superior by EZNCEN 5~14%, HS40R 17~30%,and when using each sheet of different materials, we found the decrease in fatigue life by 6~16%. Finally, the best value from the data was chosen based on the experiment for the analysis of the fatigue life of each layer.

Abstract: HS40R and EZNCEN are extensively used as components of vehicle bodies because they satisfy environmental standards and improve fuel ratio. Because vehicle body sheets are becoming thinner and stronger, it is difficult to satisfy the design standards of spot welding joints. This research presents the optimization of the welding condition for various welding variables of a spot welding specimen by genetic algorithm (GA). After obtaining the optimum welding condition by use of the genetic algorithm, the fatigue properties of HS40R and EZNCEN spot welded joints are evaluated by using the finite element method (FEM) to determine the durability and stability of a structure.

Abstract: The adhesive bonding between the steel beam and carbon fibre reinforced polymer (CFRP) plate is the weakest link and fatigue performance is a major consideration. This paper gives details of a fatigue test programme of a series of small-scale steel beams bonded with a CFRP plate. Two phases of the fatigue life, including crack initiation life and crack propagation life, are considered. Backface-strain technique was applied to monitor crack initiation. An S-N curve was developed from the test results. The curve correlates the maximum principal interfacial stress at the plate end to the crack initiation life. The fatigue limit of the S-N curve was found to be about 30% of the ultimate static failure stress. In accordance with Paris Law, moreover, an equation was developed to predict the number of cycles during the crack propagation. The empirical coefficients of the equation were obtained from the fatigue test results. This equation can correctly predict the crack propagation life. The fatigue load range affects the fatigue life, but its significance is much less than the magnitude of the maximum load in the load range.