Ivica Skozrit

On the Calculation of Stress Intensity Factors and J-Integrals Using the Submodeling Technique

In the present paper, calculations of the stress intensity factor (SIF) in the linear elastic range and the J-integral in the elastoplastic domain of cracked structural components are performed by using the shell-to-solid submodeling technique to improve both the computational efficiency and accuracy. In order to validate the submodeling technique, several numerical examples are analyzed. The influence of the choice of the submodel size on the SIF and the J-integral results is investigated. Detailed finite element (FE) solutions for elastic and fully plastic J-integral values are obtained for an axially cracked thick-walled pipe under internal pressure. These values are then combined, using the General Electric/Electric Power Research Institute (GE/EPRI) method and the reference stress method (RSM), to obtain approximate values of the J-integral at all load levels up to the limit load. The newly developed analytical approximation of the reference pressure for thick-walled pipes with external axial surface cracks is applicable to a wide range of crack dimensions.

The Creep and Fracture Behaviour of the Polyethylene PE100

This paper presents an experimental and numerical study on the creep and fracture behaviour of the polyethylene PE100. The experimental procedure includes monotonic tests on the standard tensile specimen as well as creep-fracture tests on the axisymmetrically cracked specimens. Based on the experimental results, a new primary/secondary creep constitutive model is proposed to simulate the nonlinear and time dependent behaviour of considered material. The material parameters are computed from a leastsquare fit to experimental data obtained from tests at 80 °C. Within the framework of numerical investigations an algorithm for the integration of the constitutive law is derived. The derived algorithm in conjunction with the consistent tangent matrix is implemented in the finite element (FE) code ABAQUS by using the user subroutine CREEP. The accuracy of the proposed numerical algorithm is validated by comparing with experimental results.

Modelling of Nonlinear Creep and Recovery Behaviour of Cortical Bone

A new one-dimensional constitutive model for human cortical bone is proposed to simulate the viscoelastic–viscoplastic behaviour occurring during creep-recovery tests.The material parameters are determined by fitting experimental results of creep-recovery tests reported in the published literature. An efficient computational algorithm for the integration of the proposed constitutive model at the material point level is derived. The derived algorithm in conjunction with the Jacobian matrix is implemented in the finite element code ABAQUS. The model predictions are found to be in good agreement with the experimental data presented in literature.

Fracture Analysis of Medium Density Polyethylene

The paper deals with the application of the reference stress method (RSM) to estimate the J and C integrals of cracked thick-walled metal as well as medium density polyethylene (MDPE) pipes. Unlike the existing solutions, the newly developed analytical approximations of the plastic limit pressure and J-integral are applicable to a wide range of crack dimensions. Based on the experimental data from literature and analogy between plasticity and creep, the paper discusses a method used to develop the efficient computational strategy for modeling creep fracture mechanisms by slow crack growth in a MDPE pipes.

On Numerical Analysis of Creep Fracture Behaviour of Medium Density Polyethylene

An implicit numerical algorithm for the integration of the primary/secondary creep constitutive law at the material point level is derived. For the presented constitutive model, the experimentally obtained material parameters for the medium density polyethylene (MDPE) are taken from literature. The derived algorithm in conjunction with the Jacobian matrix is implemented in the finite element (FE) code ABAQUS by using the user subroutine CREEP. The accuracy of the proposed algorithm is demonstrated.

Numerical Modelling of Viscoelastic/Damage Behaviour of Cortical Bone

An efficient computational algorithm for the integration of the viscoelastic/damage constitutive model at the material point level is derived. For the presented constitutive model, material parameters, obtained experimentally for human cortical bone, are taken from literature. The derived algorithm in conjunction with the Jacobian matrix is implemented in the finite element (FE) code ABAQUS by using the user subroutine UMAT. The accuracy of the proposed algorithm is demonstrated.

Numerical Analysis of Surface Cracks in Steam Generator Tubes

Operating experience with steam generators has shown that axial surface cracks present one of the most common causes of loss of steam generator tube integrity. An accurate computation of the plastic limit pressure represents a key for the prediction of structural integrity and reliability of pressurized tubes. A greater number of existing solutions for limit pressure of a cracked tube have been developed either analytically, based on a simple equilibrium stress field, or empirically, based on test data [1]. These solutions are generally shown to be too conservative for components with part-through-thickness defects. Recently, a finite element based plastic limit pressure expression for cylinders with external axial semi-elliptical surface cracks has been developed in Ref. [2]. However, the proposed expression is applicable to a very limited range of crack dimensions, and therefore the new extended solutions are desirable.