A Mirzaee-Sisan

PREDICTING HOW CRACK TIP RESIDUAL STRESSES INFLUENCE BRITTLE FRACTURE

A probability distribution model, based on the local approach to fracture, has been developed and used for estimating cleavage fracture following prior loading (or warm pre-stressing) in two ferritic steels. Although there are many experimental studies it is not clear from these studies whether the generation of local residual stress and/or crack tip blunting as a result of prior loading contribute to the enhancement in toughness. We first identify the Weibull parameters required to match the experimental scatter in lower shelf toughness of the candidate steels. Second we use these parameters in finite element simulations of prior loading on the upper shelf followed by unloading and cooling to lower shelf temperatures to determine the probability of failure. The predictions are consistent with experimental scatter in toughness following WPS and provide a means of determining the relative importance of the crack tip residual stresses and crack tip blunting. We demonstrate that for our steels the crack tip residual stress is the pivotal feature in improving the fracture toughness following WPS. The paper finally discusses these results in the context of the non-uniqueness and the sensitivity of the Weibull parameters.Copyright

Application of the Local Approach to Predict Load History Effects in Ferritic Steels

The effect of residual stresses on fracture behaviour of ferritic steels at low temperature has been investigated using a local approach based on the Beremin model [1]. The study aims to enhance the use of local approach in failure prediction when residual stress is present. A tensile residual stress field has been introduced in the laboratory specimens and their subsequent behaviour was investigated at low temperature. Local compression methods, including side-punching and in-plane loading, were employed to introduce residual stress fields. These methods are discussed and comprehensive range of experimental presented. The transferability of the Weibull parameters between the cracked specimens with different constraint, test temperatures and also from unstrained specimens to specimens with residual stresses are illustrated. The general scheme in failure prediction using the local approach is that the Weibull parameters in the Beremin type model calibrated to the as-received data should predict the failure following complex interaction of residual and applied stresses. The paper compares the predictions and the experimental results.Copyright

Generating Well Defined Residual Stresses in Laboratory Specimens

Residual stresses play an important role in increasing and decreasing the possibility of failure. The magnitude and direction of the residual stresses is an important factor in the integrity of engineering structures, including those containing defects. Ideally, we would like to gain insight into the integrity of a structure through testing laboratory samples. The purpose of this paper is to review methods of introducing residual stresses into laboratory specimens that are either subsequently loaded to fracture or used to assess the influence of residual stress on material damage mechanisms. Three methods, mechanical, thermal and welding, are scrutinized and illustrative examples provided. The advantages and disadvantages are explored. We conclude that new methods that do not introduce microstructural changes during the generation of residual stress should be sought if an improved understanding of the effects residual stress on fracture is required.Copyright

The Role of Constraint and Warm Pre-Stress on Brittle Fracture in Ferritic Steels

This paper presents the results of an experimental and numerical study carried out to investigate the effect of warm pre-stressing on cleavage fracture in ferritic steels using cracked and notched specimens. It is shown that the local approach based on Weibull theory predicts the increase in toughness following warm pre-stressing in highly constrained geometries. The observed effect of pre-loading in low constraint specimens such as round notched bars is less. The local approach could not predict the differences and it is suggested that the variation of triaxiality factor, the ratio of hydrostatic stress to Von Mises, in the plastic zone, is a contributing factor.

Comparison of Methods for Predicting the Influence of Residual Stresses on Brittle Fracture

This paper compares different methods for predicting the influence of residual stresses on brittle fracture. Comparison is made between fracture predictions of a local statistical approach based on the Beremin type model, with those of a well-known fracture assessment procedures, BS7910 and R6 that uses a failure assessment diagram (FAD). The results from both methods are discussed and compared. The study focuses on the behaviour of a ferritic steel pipe containing a part-through circumferential thumb-nail crack on the outer surface of the vessel.Copyright

Comparison of Global and Local Approaches to Predicting Warm Pre-Stress Effect on Cleavage Fracture of Ferritic Steels

Potentially both global and local approaches may be used to predicting the effect of loading history on cleavage fracture toughness distribution of ferritic steels. In this paper the dramatic increase in the apparent lower shelf fracture toughness of A533B steel following warm pre-stressing (WPS) has been predicted using these approaches. Extensive experimental evidence suggesting significant enhancement in fracture toughness of ferritic steels within the lower shelf temperatures following WPS are used to verify and compare the applicability and the extent of validity of the models. The global approach is based on the distribution of toughness data described by Wallin statistical model in conjunction with the Chell model for WPS effect. The local approach on the other hand is a Beremin type model that uses the Weibull stress to predict the WPS effect. Weibull stresses would essentially reflect the WPS effect on redistribution of stress-state around the crack tip. Predictions for apparent toughness using the two approaches are discussed in the light of the suggestion that residual stresses are the main cause of the enhancement, at least for the material and geometry used in this study.

Predicting the influence of residual stresses on brittle fracture using local approach

This paper explores the application of the local approach to brittle fracture to predict the influence of residual stresses in a relatively thick pipe. Three different surface flaw sizes were assumed on the outside surface of the pipe and failure predictions were made using the local approach. Then the results of local approach prediction were compared with the well-known fracture assessment procedure, BS7910 which uses a failure assessment diagram (FAD). It has found that the local approach has an advantage of taking into account the details of stress re-distribution of residual stress around the crack tip compared to the conventional assessment procedure.Copyright

Characterising Residual Stresses in Rectangular Beam Specimens Following Thermo-Mechanical Loading

The neutron diffraction (ND) technique was used to characterise residual stress fields in thin rectangular beam specimens containing residual stresses induced thermo-mechanically by partial quenching. Two types of material were considered, type 316H stainless steel and A533B ferritic steel. The work was motivated by a need to investigate the influence of residual stress on the fracture behaviour of steels. During quenching, specimens experienced a severe temperature gradient which induced thermal stresses resulting in plastic strains and a subsequent residual stress field. An extensive finite element (FE) analysis was undertaken to predict the residual stress following thermo-mechanical loading. It was shown that partial quenching generated a considerable residual stress field in 316H stainless steel. However, the level of residual stresses in the A533B steel specimens was lower than that 316H stainless steel specimens. There was acceptable agreement between the finite element simulations and measurements with simulations generally predicting higher tensile residual stresses following partial quenching than those measured in the 316H stainless steel, and lower tensile residual stresses than those measured in the A533B ferritic steel.Copyright