Papers by Author: Saeid Hadidi-Moud

Abstract: Using simplified benchmark models, representative of the behavior of real structures, a unified framework for quantification of elastic follow-up (EFU) in structures has been provided. Closed form analytical solutions for evaluation of elastic follow-up are presented for model structures. The impact of elastic follow-up on the relaxation i.e. the redistribution of residual stresses has been explored and hence its significance in the integrity assessment of structures in general and in classification of residual stresses in particular has been highlighted.

Abstract: Characterisation of failure of components subjected to impact fatigue has received much interest in recent years. Critical stress intensity factor, i.e. fracture toughness, is a characteristic parameter for fracture conditions. Evaluation of this parameter is therefore of primary importance in the study of structures containing cracks. Due to its significance numerous research work have been carried out to provide dynamic stress intensity descriptions under cyclic, impulse and impact loading conditions. These methods are mainly based on numerical analyses and / or experimental techniques led to a range of approximate models. This paper firstly provides a review of fatigue failure due to impact loading and explains the principles of impact mechanics concepts including impact loading, stress wave equation and resulting stress distributions. Then, based on available experimental studies on developing and propagating cracks under impact loading, suggests a simple model leading to an approximate analytical solution for determination of dynamic stress intensity factor, kd under high strain rate loading. Calculated values based on the suggested solution compare well with the experimental data.

Abstract: Reliable prediction of fracture conditions is a major concern in the integrity assessment of structural components. This is specifically critical within the transition regime where there is a significant scatter in fracture test data. In recent years local stress based approaches that use a "Weibull distribution function" have been examined to predict probability of cleavage fracture at lower shelf temperature. Furthermore the role of constraint in toughness prediction has been noted. An extensive experimental programme known as "Euro fracture dataset" aimed at characterisation of the "Ductile-to-Brittle" transition (DBT) behaviour of ferritic steels. Recently this data set was used by authors to propose a set of "Global" equations for determination of temperature and thickness dependence of Weibull distribution parameters. In this paper finite element simulations of fracture tests are carried out firstly to verify the experimental findings and secondly to examine and validate the proposed "Global" equations. This objective has been achieved through the comparison between the experimental data, predictions of "Global" curves and the results of performed finite element simulations.

Abstract: Assessment of the integrity of structures such as reactor pressure vessels is a critical issue in relevant industries. In a full integrity assessment, the presence of initial residual stresses (RS) needs to be taken into account. An initial RS field is introduced into a type 316 stainless steel cylindrical vessel with no defects and to one with a partial circumferential crack on its outer surface. Relaxation of RS following several proof load cycles, in form of internal pressure, applied to the vessel is explored using finite element simulations. It is found that the proof loading process generally relaxes the RS and is proved to be beneficial to both cracked and un-cracked vessels with or without the presence of initial RS. Interaction of residual stresses with warm pre-stressing is further investigated using A533B steel at room and low temperature subjected to axial loading. The results are compared with similar analyses but with no introduction of an initial RS field to explore the interaction effects on fracture resistance, as well as the role of partial crack on the RS distribution / redistribution. The differences are discussed and illustrated.

Abstract: 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.

Abstract: 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.