There are more scatter in experimental CREEP CRACK GROWTH data observed in the early stages of a test than later on. This scatter can be attributed to a combination of factors; such as stress redistribution from the initial elastic state to the steady state CREEP, primary CREEP effects and the buildup of damage local to the CRACK tip during the early CRACK GROWTH period. In this paper the redistribution of stress and the buildup of damage at a CRACK tip during CREEP in a CRACKed body is considered. For short times after application of load, CRACK GROWTH rate can be described by stress intensity factor K for long times, if the stress fields relax to their steady sate, then C* can describe the magnitude of the stress fields and so the CRACK GROWTH rate.A new method has been developed for the time needed for the stresses to relax from their elastic state to some fraction between the elastic and CREEP state. Also a new CREEP CRACK GROWTH model is proposed in terms of ductility exhaustion to account for buildup of damage at a CRACK tip. It is shown that this build-up of damage is the most likely cause of increased scatter.

There is an increasing need to assess the service life of components containing defect which operate at high temperature. This paper describes the current fracture mechanics concepts that are employed to predict CRACKing of engineering materials at high temperatures under static and cyclic loading. The relationship between these concepts and those of high temperature life assessment methods is also discussed.A model for predicting CREEP CRACK GROWTH initiation and GROWTH in terms of C* and the CREEP uniaxial ductility is presented and it is shown that this model gives good agreement with the experimental results. The effects of cyclic loading on CRACK GROWTH behavior are considered and fractography evidence is shown to back a simple cumulative damage concept when dealing with CREEP/fatigue interaction. Finally a discussion is presented which highlights the important aspect of life assessment methodology for high temperature plant.

In this paper the CRACK GROWTH process of the parts subjected to hot fluids has been studied. The fluid high temperature activates the CREEP process. In addition the effect of the fluid corrosion as an important factor in the CRACK GROWTH has been considered. The effective factors are divided into two groups of developing factors and resisting factors. For calculation of the developing forces, the inherited mechanical model has been used and after deriving the CREEP function, the amount of these forces has been determined in two stages of CRACK concealment and CRACK GROWTH. The resisting force has been calculated using Rabotnov method of accumulated small fractures in the CRACK region. The effect of corrosive environment has been considered in the calculations using penetration function and the fluid standard density variations. For solving the resulted equations, programming in the MATLAB software has been used. Then the effects of the CRACK initial length, the applied stress and the fluid corrosiveness have been investigated on the CRACK GROWTH curve.

To ensure the rail transportations safety, evaluation of fatigue behavior of the rail steel is necessary. High cycle fatigue behavior of a rail steel was the subject of investigation in this research using fracture mechanics. Finite element method (FEM) was used for analyzing the distribution of the stresses on the rail, exerted by the external load. FEM analysis showed that the maximum longitudinal stresses occurred on the railhead. To find out about the relation of CRACK GROWTH with its critical size, and to estimate its lifetime, the behavior of transverse CRACKs to rail direction was studied using damage tolerance concept. It revealed that transverse CRACK GROWTH initially occurred slowly, but it accelerated once the CRACK size became larger. Residual service life was calculated for defective segments of the rails. In addition, allowable CRACK size for different non-destructive testing intervals was determined; the allowable CRACK size decreased as the NDT intervals increased.

The extent of economic damage due to brittle fracture may depend significantly on the path of CRACK GROWTH. Therefore, it is important to develop appropriate techniques for predicting the path of CRACK GROWTH in CRACKed components and structures. Although several methods are available for this purpose, the applications of these methods have been studied mainly for mode I CRACKs. The aim in this paper is to evaluate two available methods for predicting the path of brittle fracture for a given specimen subjected to pure mode II: "the method of incremental CRACK GROWTH" and "the method of onset of fracture". The experimental path of fracture, already known from an earlier study, is used to investigate the accuracy of results obtained from the present methods. The maximum tangential stress criterion is employed to determine the fracture path in any of the above mentioned methods. The effect of a higher order term of stress on the path of CRACK GROWTH is also studied. The computational results show that any of these two methods can provide an acceptable prediction for path of CRACK GROWTH in the given specimen.    

In this study, fatigue GROWTH of external surface CRACKs on the autofrettaged cylinders under bending is investigated. Autofrettage is a process in which a thick-walled cylinder subjected to internal pressure with known amount, causing some portions on the inner zone of the cylinder deformed plastically. In this case, removing the pressure causes compressive residual stresses on the inner layers and tensile stresses on the outer wall. The goal is increasing the fatigue durability of the product by inducing residual compressive stresses into materials, but along with this, there are adverse tensile stresses which can decrease the life due to the outer defects. In this paper, the external CRACKs are in the forms of halfelliptical, semi-elliptical and semi-circle. Samples made by aluminum 2024 alloy. The cylinders were autofrettaged up to 40 and 60 percent. CRACKs were located in circumferential direction and normal to cylinder axis. The numerical simulations were performed by finite element method. Experimental data and numerical results were compared. Results show that the number of load cycles to fracture, in the 60% autofrettaged cylinders are smaller than those for 40% and also smaller than the state without autofrettage. Distribution of stress intensity factor along the CRACK front is symmetric and CRACK grows in its initial plane which indicating the dominance of the first mode of failure during the CRACK GROWTH.In all samples, after some steps of the GROWTH, CRACK front transforms to the semi-elliptical shape until complete fracture.

The CRACKing elements method (CEM) is a novel numerical approach for simulating fracture of quasibrittle materials. This method is built in the framework of conventional finite element method (FEM) based on standard Galerkin approximation, which models the CRACKs with disconnected CRACKing segments. The orientation of propagating CRACKs is determined by local criteria and no explicit or implicit representations of the CRACKs' topology are needed. CEM does not need remeshing technique, cover algorithm, nodal enrichment or specific CRACK tracking strategies. The CRACK opening is condensed in local element, greatly reducing the coding efforts and simplifying the numerical procedure. This paper presents numerical simulations with CEM regarding several benchmark tests, the results of which further indicate the capability of CEM in capturing complex CRACK GROWTHs referring propagations of existed CRACKs as well as initiations of new CRACKs.