John W. H. Price

Experimental apparatus for thermal shock fatigue investigations

Abstract An experimental rig developed for investigating crack growth in pressure vessels and piping equipment is described. The rig allows full scale modeling of cyclic thermal shock conditions that occur in operating thermal power station pressure equipment. It has the ability to apply a primary steady state mechanical load and to control the quenching environment allowing the study of the effect of welds and stress concentrators under conditions that simulate operational loadings. These results can be used for lifetime assessment. Preliminary results from the test rig analysing the effect of primary load on crack initiation and growth are presented and discussed briefly. A comparison with prediction methods from the ASME Boiler and Pressure Vessel Code is made. It is concluded that the primary load has little or no effect on crack initiation times, however it significantly affects the crack growth rate.

A generic asset management framework for optimising maintenance investment decision

This paper proposes a generic asset management framework that integrates enterprise decisions for optimising maintenance investment decisions. Optimisation involves finding the best results for an objective with given restraints and relationships. Three business decision dimensions are used–the annualised costs to the enterprise related to maintenance–namely loss of output cost, resources cost and risk cost. The objective is to reduce costs including those due to risks to the minimum. This paper addresses the relationship between these business decision dimensions within the proposed framework and implements the value per unit time approach for optimisation to occur in an integrated fashion.

A new fuzzy-c-means and assignment technique-based cell formation algorithm to perform part- type clusters and machine-type clusters separately

The incorporation of fuzziness in the cell formation problem by Chu and Hayya (1991 International Journal of Production Research, 29, 1475-1487) is a notable contribution, in which non-binary classification logic is used. However, despite this development, numerical illustrations performed in this research demonstrate that the Chu and Hayya approach can result in solutions with empty party-type cluster(s) and/or empty machine-type cluster(s). Further, it is noted that solutions based on the Chu and Hayya approach can contain non-empty part-type cluster(s) being assigned to empty machine-type cluster(s) and vice versa. Three strategies are offered in this research to overcome these inadequacies: the separate formation of part-type and machine-type clusters; modification of the stopping criterion; and the adoption of the assignment technique in the formation of the final manufacturing cell solutions. A new algorithm has resulted from these modifications and has been rigorously compared to the performance of...

Role of Welding Parameters Using the Flux Cored Arc Welding Process of Low Alloy Steels on Bead Geometry and Mechanical Properties

Welding parameters have direct effects on the bead geometry, microstructure, and mechanical properties of low alloy steels. A series of experiments have been carried out to examine some of these parameters using the flux cored arc welding process (FCAW). In this article, an experimental study was conducted to investigate the influence of welding parameters in FCAW process particularly welding voltage and travel speed on weld bead dimensions. The study also includes the effects of bead overlap and deposition sequence on the parent material and the heat-affected zone (HAZ) properties. It was found that an increase in the welding voltage leads to an increase in the weld bead width, and the increase in the welding traverse speed leads to a decrease in the weld bead width. When studying the bead overlap percentages, it was found that the 50% bead overlap can be considered to be practically a better option than the higher percentages of bead overlap (i.e., 70-90%). The experimental investigation of studying the deposition sequence showed that there were no significant differences in the microstructure, hardness, and the size of the refined HAZ between the two proposed deposition sequences. However, a significant improvement in the microstructure and the size of the refined HAZ, and a reduction in the hardness were achieved after depositing the second welding bead, irrespective of the depositing sequence.

A two-stage model for predicting crack growth due to repeated thermal shock

Abstract The growth of cracks in equipment that is exposed to repeated thermal down shocks presents a complex problem of analysis. The transient, highly non-linear nature of the stress profiles that are developed during the shock in addition to localized plasticity and environmental interactions makes difficult any accurate analytical predictions. The use of current analysis techniques based on linear stress approximations can result in overly conservative results that may lead to unnecessary and costly component replacements. This paper outlines results from an experimental investigation into crack growth in notched, flat plate specimens exposed to repeated one-dimensional thermal shocks. Analysis of the results shows that a simple two-stage growth model may be applicable for describing the crack growth. The model is comprised of a high strain fatigue region where crack growth is in the plastic range and a region where growth is described by linear elastic fracture mechanics. Allowances for the effects of mean loads and environment on the crack growth are also included in the model. The model is currently limited to the consideration of carbon steel components, operating at temperatures below the creep range.

Comparison of Neutron and Synchrotron Diffraction Measurements of Residual Stress in Bead-on-Plate Weldments

This paper explores the use of neutron and synchrotron diffractions for the evaluation of residual stresses in welded components. It has been shown that it is possible to achieve very good agreement between the two independent diffraction techniques. This study shows the significance of the weld start and end sites on the residual strain/stress distribution. Quantitative evaluation of the residual stress development process for multibead weldments has been presented. Some measurements were also taken before and after postweld stress relieving to establish the reduction and redistribution of the residual stress. The detailed measurements of residual stress around the weld achieved in this work significantly improve the knowledge and understanding of residual stress in welded components.

A maintenance prioritisation approach to maximise return on investment subject to time and budget constraints

Purpose – Asset maintenance activities need to be prioritised as budget and planned outage time is often limited for all maintenance work to be carried out. The purpose of this paper is to develop a technique for prioritising maintenance work that maximises the return on investment under the constraints of budget and time.Design/methodology/approach – Three indices are proposed to be used as indicators for prioritising maintenance. These indices are termed: maintenance investment index (MII), time index (TI) and budgetary index (BI). These indices permit prioritisation of asset maintenance based on the required emphasis on return on maintenance investment, time and budget respectively.Findings – It is found that approaches to prioritising maintenance which integrate the critical dimensions in the decision making process are lacking in the literature. There is a need for such an approach to assist decision makers to ensure enterprises objectives and targets are maximised with given budget and planned shut...

Hardness, Microstructure, and Residual Stresses in Low Carbon Steel Welding with Post-weld Heat Treatment and Temper Bead Welding

This paper investigates the effects of post-weld heat treatment (PWHT) and temper bead welding (TBW) on hardness, microstructure and residual stresses in multi-layer welding on low carbon steel specimens made with two different weld geometries, viz. (1) smooth-contoured and (2) U-shaped. It was found that the PWHT technique gave overall lower hardness than the TBW technique, but the hardness values in both techniques were acceptable. Microscopy analysis showed that the TBW technique was more effective in tempering the heat affected zone as the grain size decreased slightly at the fusion line in spite of the higher temperature at the fusion line. Residual stresses measured using the hole-drilling method showed that the residual stress is not reduced below yield stress near the last bead solidified in TBW. Only PWHT gives low residual stress results in this area. High tensile residual stresses may result in sensitivity to fatigue loading.

Residual Stresses Evaluation in Welds and Implications for Design for Pressure Vessel Applications

Welding residual stresses have important consequences on the performance of engineering components. High residual stresses may lead to loss of performance in corrosion, fatigue, and fracture but as yet these consequences are poorly quantified. The major cause of this is that residual stress often remains the single largest unknown in industrial damage situations since it is difficult to measure or estimate theoretically. One of the key issues in the study of residual stress is that the detail of the stress distribution on a small scale (in the order of millimeters) can be important. In this paper, the neutron diffraction technique is used which while it is a very expensive technique, is capable of nondestructively measuring residual stresses at this scale up to a depth of 35 mm. The investigation reported compares the residual stress characteristics due to various restraints for a single bead and in fully restrained samples with different numbers of beads. This paper considers the results of the neutron diffraction studies in relation to fitness for purpose guidance and implication for pressure vessel design.

Using the ASME and BSI codes to predict crack growth due to repeated thermal shock

Abstract This paper examines the use of the ASME and British Standard codes to estimate the growth of cracks driven mainly by thermal shocks. Thermal shock loading of operating pressure equipment is a common occurrence, particularly in thermal power stations. The tensile stresses that are produced at the surface of a heated component exposed to a rapid thermal down shock can be high, particularly in the presence of stress concentrations. Repeated application of the thermal shocks may lead to crack initiation and crack growth. The ability to use current codes and standards to describe this type of crack growth is desirable. Unfortunately, thermal shock is a complex transient situation with highly non-linear stress distributions and environmental effects that are not well described by some codes. This paper describes attempts to use the techniques described in the ASME Boiler and Pressure Vessel code Section XI and British Standard BS7910 to predict crack growth rates derived from Monash University experiments. Areas of large conservatism in the methods currently used in industry are identified and possible alternative, less conservative approaches are suggested. If the methods are fully applied, the possibility of crack growth slowing can be captured and the replacement or repair of equipment with thermal shock cracking might be avoided.