Results 1 - 10 of 221
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[en] This paper proposes an engineering method to estimate the creep C-integral for realistic creep laws to assess defective components operating at elevated temperatures. The proposed estimation method is mainly for the steady-state C*-integral, but a suggestion is also given for estimating the transient C(t)-integral. The reference stress approach is the basis of the proposed equation, but an enhancement in terms of accuracy is made through the definition of the reference stress. The proposed estimation equations are compared with extensive elastic-creep FE results employing various creep-deformation constitutive laws for six different geometries, including two-dimensional, axi-symmetric and three-dimensional geometries. Overall good agreement between the proposed method and the FE results provides confidence in the use of the proposed method for defect assessment of components at elevated temperatures. Moreover, it is shown that for surface cracks the proposed method can be used to estimate C* at any location along the crack front
[en] The method is not new in the sense that the stress-modified fracture strain model is based on the well-known concept that the fracture strain for ductile fracture strongly depends on the stress state. Virtual testing using FE damage analysis should ultimately be used to simulate failure of large-scale components such as full-scale pipe tests (possibly with long, stable crack growth). To simulate long, stable ductile crack growth in full-scale cracked pipes, existing methods need to be modified to incorporate larger element sizes. In this paper, an element-size-dependent damage model based on the stress-modified fracture strain model is proposed to simulate failure of full-scale cracked plates including the influence of residual stresses. The proposed method is then compared with published experimental full-scaled cracked plates with and without electron-beam welds. This paper has presented the results from an element size dependent model to predict the influence of residual stress effects on ductile tearing behavior of AL2024 and AL5083 alloys. The proposed method is based on the stress-modified fracture strain model, proposed previously by the authors. Incremental damage is defined by the ratio of the plastic strain increment to the fracture strain. In our work, progressive cracking was assumed to occur when the accumulated damage becomes unity and provided a proper finite element size. It has been shown that this method worked very well to simulate the ductile crack growth in laboratory specimens, simulating long, stable crack growth in large-scale cracked plates containing residual stresses
[en] Although the pipe bend is the most commonly used component in pipeline system, useful assessment solutions for stress intensity factor of pipe bends are not exist. This paper shows the trend of the stress intensity factor for pipe bends with through-walled crack under internal pressure based on the FE analysis. The trend is different from existing solutions of straight pipe.
[en] In the present paper, approximate plastic limit load solutions for pipe bends under combined internal pressure and bending are obtained from detailed three-dimensional (3-D) FE limit analyses based on elastic-perfectly plastic materials with the small geometry change option. The present FE results show that existing limit load solutions for pipe bends are lower bounds but can be very different from the present FE results in some cases, particularly for bending. Accordingly closed-form approximations are proposed for pipe bends under combined pressure and in-plane bending based on the present FE results. The proposed limit load solutions would be a basis of defective pipe bends and be useful to estimate non-linear fracture mechanics parameters based on the reference stress approach
[en] Based on three-Dimensional (3-D) FE limit analyses, this paper estimates effect of internal pressure on plastic limit loads for elbows with circumferential through-wall crack under in-plane bending incorporating large geometry change effects. Circumferential through-wall crack in extrados is considered. The FE limit analyses using the large geometry change option provide plastic collapse loads (using the twice-elastic-slope method). For the bending mode, closing bending is considered. Other relevant variables affecting plastic limit loads are systematically varied, related to pipe bend geometry (the mean radius, thickness and bend curvature) and defect geometry (the length of circumferential through-wall crack)
[en] This paper briefly describes the new engineering method, called the enhanced reference stress method, to estimate J(or C*) for non-linear fracture mechanics analysis of defective components, recently proposed by authors. The proposed method offers significant advantages over existing methods in terms of its accuracy, simplicity and robustness. Examples of application of the proposed method to typical piping integrity problems such as through-wall cracked pipes under combined loading, and surface cracked pipes under internal pressure and bending are given. Excellent agreements between the FE J and C* results and those of the proposed method provide sufficient confidence in the use of the proposed method. One notable point is that the proposed method can be used to estimate J (or C*) along the crack front of surface cracks. Moreover simplicity of the proposed method makes it easy to extend to more complex problems. Thus the proposed method is attractive to assess the significance of defects under practical situations
[en] This paper proposes a method to determine the elastic follow up factors for the C(t) integral under secondary stress. The rate of creep crack growth for transient creep is correlated with the C(t) integral. Elastic follow up behavior, which occurs in structures under secondary loading, prevents a relaxation of stress during transient creep. Thus, both the values of C(t) and creep crack growth increase as increasing elastic follow up. An estimation solution for C(t) was proposed by Ainsworth and Dean based on the reference stress method. To predict the value of C(t) using the solution, an independent method to determine the elastic follow up factor for cracked bodies is needed. This paper proposed that the elastic follow up factors for C(t) can be determined by elastic plastic analyses using the plastic creep analogy. Finite element analyses were performed to verify this method
[en] Laser shock peening(LSP) is an innovative surface treatment technique, and it has been successfully used to improve the fatigue performance of metallic components. It is widely known, that cracks caused by metal fatigue occur only at the location where the metal is subject to tension, and not at the location where the metal is subjected to compression. Therefore, LSP can be employed to improve fatigue life because it generates a high-magnitude compressive residual stress on the surface and interior of metallic components. In this study, we analyzed the applicability of the LSP method in improving fatigue performance and evaluated the various parameters that influence the compressive residual stress. Further, we analyzed the change in the mechanical properties such as surface dynamic stress and the compressive residual stress on the surface and interior of metallic components
[en] In this paper, the estimation method of C(t)-integral for combined mechanical and thermal loads is proposed for elastic-plastic-creep material via 3-dimensional FE analyses. Plasticity induced by initial loading makes relaxation rate different from those produced elastically. Moreover, the interactions between mechanical and thermal loads make the relaxation rate different from those produced under mechanical load alone. To quantify C(t)-integral for combined mechanical and thermal loads, the simplified formula are developed by modifying redistribution time in existing work done by Ainsworth et al..
[en] This paper proposes engineering estimation equations of elastic-plastic J and crack opening displacement (COD) for axial through-wall cracked pipes under internal pressure. On the basis of detailed 3D finite element (FE) results using deformation plasticity, the plastic influence functions for fully plastic J and COD solutions are tabulated as a function of the mean radius-to-thickness ratio, the normalised crack length, and the strain hardening. On the basis of these results, the GE/EPRI-type J and COD estimation equations are proposed and validated against 3D FE results based on deformation plasticity. For more general application to general stress-strain laws or to complex loading, the developed GE/EPRI-type solutions are re-formulated based on the reference stress (RS) concept. Such a re-formulation provides simpler equations for J and COD, which are then further extended to combined internal pressure and bending. The proposed RS based J and COD estimation equations are compared with elastic-plastic 3D FE results using actual stress-strain data for Type 316 stainless steels. The FE results for both internal pressure cases and combined internal pressure and bending cases compare very well with the proposed J and COD estimates