Xucheng Wang

Finite element stress analysis for steady creep

Abstract Based on the virtual work principle, an energy expression on the steady creep problem for three-dimensional structures is deduced in this paper. By use of the finite element method, nonlinear algebraic equations with respect to the steady creep strain rates are obtained. These nonlinear algebraic equations are incorporated into a three-dimensional linear elastic finite element program. The strain rates and stresses of the steady creep are calculated directly by the modified Newton-Raphson iteration method with the Aitken method for accelerating convergence. This numerical scheme for analysis on the steady creep is an efficient one which can be applied to calculate accumulated creep strains and further evaluate the integrity of high temperature structures during sustained loading.

General steady creep analysis for various constitutive laws. Part 1: theory and formulation☆

Generalized formulations of the finite element method with penalty multiplier for steady creep problems are proposed as being suitable for various constitutive relations. A simple expression is derived to obtain mean stress by identifying the physical quantity of the penalty multiplier. To solve the nonlinear problem, the Newton-Raphson or modified Newton-Raphson process with steepest descent technique is adopted. The detailed expressions of secant and tangential constitutive matrixes are deduced for Norton, Prandtl, Dorn and Garofalo creep laws. Some advice on numerical implementation is given.

Simplified method based on the deformation theory for structural limit analysis—I. Theory and formulation

Abstract Based on the deformation theory of linear hardening materials, a simplified finite element analysis method is proposed to calculate the limit load of structures. Compared with the general elasto-plastic incremental finite element analysis, the proposed method can avoid the incremental iteration of nodal displacements and the constitutive equation integration at each Gauss integral point. Compared with the mathematical programming method combined with finite element analysis, the proposed method shows theoretical simplicity and avoids bringing in complex mathematical theories. It is also easy to put into code. Compared with some current limit analysis methods, such as the GLOSS R-Node method and the elastic compensation method, the present method gives a remarkably accurate estimation of limit loads instead of just a simple lower or upper bound. Numerical examples have demonstrated that it is effective and the computational results are accurate enough to meet the need of engineering practice.

Simplified method based on the deformation theory for structural limit analysis-II. Numerical application and investigation on mesh density

A simplified finite element method based on the deformation theory for structural limit analysis is applied to the estimation of the limit load of two widely used pressure vessel components. One is the nozzle-sphere intersection under internal pressure and radial load: the other is the torispherical pressure vessel head under internal pressure. The results are compared with existing solutions and the detailed elasto-plastic finite element analysis. It can be seen that this method gives remarkably accurate results for various configurations of the components analyzed. In the meantime, the effects of mesh density on the accuracy of the solution are also examined. The investigation shows that with quite coarse mesh this method can obtain adequately accurate results and further reduce the computation resource.

Simplified method for elasto-plastic analysis of structures under variable cyclic loading

Abstract A simplified method is proposed based on a cyclic elasto-plastic constitutive model of a metal for elasto-plastic analysis of mechanical structures under cyclic loading. The results from this method show a satisfactory consistency with those from detailed analyses by the elasto-plastic finite element method.

An improved simplified method for determining multiaxial relaxation - compared with the GLOSS method and the ASME N-47 method

Abstract Three simplified methods for determining multiaxial relaxation, the improved multiaxial relaxation simulation, the GLOSS time-scaling method and the ASME N-47 method, are compared in this paper. The improved simplified method proves to be better for simulating the evolution of relaxed stress during the whole stress relaxation.

Numerical schemes for defining reference stress in components containing defects

Abstract In R6 and R5, the standards of integrity assessments of components containing defects established by the former Central Electricity Generating Board (C EGB ) in the UK, the basic procedure in calculating the crack driving force parameters, J -integration and C *-integration is to define the reference stress, σ ref , on the defective section. Three methods of directly defining the reference stress, σ ref [multiple steady-state creep analysis (M SCA ) method, creep module-modified iteration (C MMI ) method and elastic module-modified iteration (E MMI ) method], are suggested in this paper, differing from the common methods in which the limit loads of the components are first calculated and then the σ ref is convertibly determined. The basic conceptions and calculational steps of these methods are discussed and compared in the context, and the validity is demonstrated by several examples.

Simplified analysis of effective strain range and accumulated strain in structures under cyclic mechanical loading

The Equivalent Tracing method is a simplified method used to predict cyclic effective stress/strain evolution in a structure subjected to cyclic mechanical loading. It adopts an approximate equation for calculating the effective strain amplitude in every loading branch and a simple and effective constitutive model characterizing the cyclic plastic properties of materials. Beside the required effective strain range, the maximum accumulated strain is also an important control variable in design/analysis. A further extension of the Equivalent Tracing method to the simulation of cyclic stress/strain components makes the accumulated strain components available.

Simplified analysis of cyclic elastoplastic creep responses in elevated temperature structures under cyclic loading

Abstract A simplified analysis of cyclic elastoplastic creep responses in structures subjected to cyclic mechanical loads with a holding time at high temperature is presented in this paper. The main feature of this method is to transform the complicated response analysis to another approximately equivalent problem—the elastoplastic analysis of a structure subjected to a cyclic loading with variant load amplitude, while the load amplitude variation in each loading branch is determined by the amount of stress relaxation during the holding time before the subsequent load reversal. Then two methods which are used in simulating cyclic elastoplastic responses and creep stress redistribution respectively as well as a load equivalence technique to deal with their interaction are combined to track the response evolution of cyclic elastoplastic creep at a critical location concerned in a structure.

Simplified analysis of creep stress redistribution for materials exhibiting primary creep

A simplified analysis of creep stress redistribution for structures under sustained loads and materials exhibiting primary creep properties is proposed in this paper. This simple method is put into practice by use of the simplified analysis of creep stress redistribution for materials with the secondary creep property proposed previously by the authors and a time scale transform considering the primary creep property. Numerical examples show that the simplified analysis is simple and effective compared with the detailed finite element analysis.