Gross plastic deformation of axisymmetric pressure vessel heads

The gross plastic deformation and associated plastic loads of four axisymmetric torispherical pressure vessels are determined by two criteria of plastic collapse: the ASME twice elastic slope (TES) criterion and the recently proposed plastic work curvature (PWC) criterion. Finite element analysis was performed assuming small and large deformation theory and elastic–perfectly plastic and bilinear kinematic hardening material models. Two plastic collapse modes are identified: bending-dominated plastic collapse of the knuckle region in small deformation models and membrane-dominated plastic collapse of the cylinder or domed end in large deformation models. In both circumstances, the PWC criterion indicates that a plastic hinge bending mechanism leads to gross plastic deformation and is used as a parameter to identify the respective plastic loads. The results of the analyses also show that the PWC criterion leads to higher design loads for strain hardening structures than the TES criterion, as it takes account of the effect of strain hardening on the evolution of the gross plastic deformation mechanism

Design by analysis of ductile failure and buckling in torispherical pressure vessel heads

Thin shell torispherical pressure vessel heads are known to exhibit complex elastic-plastic deformation and buckling behaviour under static pressure. In pressure vessel Design by Analysis, the designer is required to assess both of these behaviour modes when specifying the allowable static load. The EN and ASME boiler and pressure vessel codes permit the use of inelastic analysis in design by analysis, known as the direct route in the EN Code. In this paper, plastic collapse or gross plastic deformation loads are evaluated for two sample torispherical heads by 2D and 3D FEA based on an elastic-perfectly plastic material model. Small and large deformation effects are considered in the 2D analyses and the effect of geometry and load perturbation are considered in the 3D analysis. The plastic load is determined by applying the ASME twice elastic slope criterion of plastic collapse and an alternative plastic criterion, the Plastic Work Curvature criterion. The formation of the gross plastic deformation mechanism in the models is considered in relation to the elastic-plastic buckling response of the vessels. It is concluded that in both cases, design is limited by formation of an axisymmetric gross plastic deformation in the knuckle of the vessels prior to formation of non-axisymmetric buckling modes

Education and social cohesion for economic growth

Relevant theoretical underpinnings suggest that higher education, continuous professional development and training provide numerous opportunities for societal advancement. This contribution posits that interventions in the realms of education can play a significant role in shaping key performance indicators for laudable social outcomes. It suggests that education leadership may contribute to create a fair, just and equitable society for all. This article discusses how education fosters social cohesion. This paper sheds light on Malta’s National Reform Programme in order to meet the European Union’s (EU’s) 2020 strategy. It presents an assessment of the economic, social and environmental situation in Malta. The smallest EU state is pursuing its policy efforts to reduce early school leaving. At the same time, it is striving to address skills gaps (and mismatches) in its domestic labour market. This case study indicates that with better education leadership, there may be implications for economic growth, job creation and competitiveness. It shows that family-friendly measures including better access to childcare, more flexible working schemes and employer incentives can help individuals to return to work. In conclusion, this contribution maintains that the pursuit towards continuous improvements in education leadership and social progress can create a virtuous cycle of productivity outcomes and economic growth.peer-reviewe

Advances in friction stir welding of steel : Project HILDA

A microstructure and property evaluation of friction stir welded DH36 6mm plate has been undertaken. The study examined a wide range of process parameters and, from this, a process parameter envelope has been developed and an initial process parameter set established that gives good welding properties. Thermo-mechanical deformation studies were developed to generate flow stress regimes over a range of stain rates and temperatures and these data will support the on-going local numerical modelling development. A preliminary thermo-fluid model has been developed to predict temperature and material flow during the FSW of steel grade DH36. In this model, materials are considered as highly viscous incompressible fluid. The welded material is flowing around the rotating tool thanks to the modelling of the friction at tool/workpiece interface. In parallel, a global numerical model is being developed to predict the inherent residual stresses and distortion of FSW butt welded assemblies often in excess of 6m long plate

Recent developments in steel friction stir welding : Project HILDA

Friction stir welding of steel presents an array of advantages across many industrial sectors compared to conventional fusion welding techniques. Preliminary studies have identified many positive effects on the properties of welded steel components. However, the fundamental knowledge of the process in relation to structural steel remains relatively limited, hence industrial uptake has been essentially non-existent to this date. The European-funded project HILDA, the first of its kind in terms of breadth and depth, is concerned with enhancing the understanding of the process on low alloy steel, establishing its limits in terms of the two more significant parameters which can be directly controlled, tool traverse and rotational speed, thus improving its techno-economic competitiveness to fusion welding. A detailed study investigated the effect of process parameters on the evolved microstructure. In parallel, a full programme of mechanical testing was undertaken to generate data on hardness, impact toughness and fatigue. From this, it has been established that friction stir welding of steel produces high integrity joints that exhibit excellent fatigue properties. From a simulation perspective, a local microstructural numerical model has been developed to predict the microstructural evolution within the weld zone during friction stir welding of low alloy steel. This model concentrates on predicting grain size evolution due to dynamic recrystallization with respect to tool traverse and rotational speed. Furthermore, a computational efficient local-global numerical model capable of predicting the thermal transients, stir and heat affected zone, residual stresses and distortion produced by friction stir welding of DH36 plates is presented

Modelling magnetohydrodynamics to investigate variation of shielding gases on arc characteristics in the GTAW process

Gas tungsten arc welding requires a gas shield to be present in order to protect the arc area from contamination by atmospheric gases. As a result of each gas having its own unique thermophysical properties, the shielding gas selected can have a major influence on the arc stability, welding speed, weld appearance and geometry, mechanical properties and fume generation. Alternating shielding gases is a relatively new method of discreetly supplying two different shielding gases to the welding region in order to take advantage of the beneficial properties of each gas, as well as the inherent pulsing effects generated. As part of an ongoing process to fully evaluate the effects of this novel supply method, a computational fluid dynamics model has been generated to include the gas dependent thermodynamic and transport properties in order to evaluate the effects that an alternating gas supply has on the arc plasma. Experimental trials have also been conducted to validate the model arc profile predictions

Numerical modelling techniques applicable for the prediction of residual stresses and distortion due to mild steel DH36 frictions stir welding

Friction stir welding involves a multi-physics phenomena, including visco-plasticity, material flow, metallurgical transformation, heat generation, thermal straining and structural interaction. Numerical modelling provides an efficient and cost effective tool capable to analysis and predict the different phenomena. This study integrates different numerical modelling strategies to ultimately develop a robust yet computationally efficient modelling technique capable of predicting residual stresses and distortion due to FSW. A computational efficient local-global numerical model capable of predicting the material visco-plastic flow, thermal transients, stir/heat affected zone, residual stresses and distortion developed due to friction stir welding of DH36 plates is described. Different thermo-elasto-plastic modelling strategies ranging from analytical to transient numerical models are explored and the most robust and computational efficient strategy is identified through cross-reference with the realistic experimental test results

Evaluating plastic loads in torispherical heads using a new criterion of collapse

In ASME Design by Analysis, the plastic load of pressure vessels is established using the Twice Elastic Slope criterion of plastic collapse. This is based on a characteristic load-deformation plot obtained by inelastic analysis. This study investigates an alternative plastic criteria based on plastic work dissipation where the ratio of plastic to total work is monitored. Two sample analyses of medium thickness torispherical pressure vessels are presented. Elastic-perfectly plastic and strain hardening material models are considered in both small and large deformation analyses. The calculated plastic loads are assessed in comparison with experimental results from the literature

Recent developments in steel friction stir welding : project HILDA

Friction stir welding of steel presents an array of advantages across many industrial sectors compared to conventional fusion welding techniques. Preliminary studies have identified many positive effects on the properties of welded steel components. However, the fundamental knowledge of the process in relation to structural steel remains relatively limited, hence industrial uptake has been essentially non-existent to this date. The European-funded project HILDA, the first of its kind in terms of breadth and depth, is concerned with enhancing the understanding of the process on low alloy steel, establishing its limits in terms of the two more significant parameters which can be directly controlled, tool traverse and rotational speed, thus improving its techno- economic competitiveness to fusion welding. A detailed study investigated the effect of process parameters on the evolved microstructure. In parallel, a full programme of mechanical testing was undertaken to generate data on hardness, impact toughness and fatigue. From this, it has been established that friction stir welding of steel produces high integrity joints that exhibit excellent fatigue properties. From a simulation perspective, a local microstructural numerical model has been developed to predict the microstructural evolution within the weld zone during friction stir welding of low alloy steel. This model concentrates on predicting grain size evolution due to dynamic recrystallization with respect to tool traverse and rotational speed. Furthermore, a computational efficient local-global numerical model capable of predicting the thermal transients, stir and heat affected zone, residual stresses and distortion produced by friction stir welding of DH36 plates is presented.peer-reviewe

Mitigation of welding distortion and residual stresses via cryogenic CO2 cooling-a numerical investigation

Fusion welding remains the most common and convenient fabrication method for large, thin- plate welded structures. However, the resulting tendency to out-of-plane distortion exacts severe design and fabrication penalties in terms of poorer buckling performance, lack of fairness in external appearance, poor fit-up and frequent requirements for expensive rework. There are several ways to mitigate welding distortion and this study concentrates on the use of cryogenic CO2 cooling to reduce distortion. A feasible combination of welding process and cooling parameters, was investigated computationally and the resulting effects on final deformation were predicted. Three different computational strategies were developed and applied to butt-welding and fillet-welding processes, with and without the inclusion of cryogenic cooling. In the first method, a fully transient, uncoupled thermo-elastoplastic model was investigated. This method is comprehensive but not readily applicable to predict welding distortions in complex, industrial-scale, welded structures, due to the large computational requirement. More computationally efficient models are needed therefore and two further models of this type are suggested in this study. The results show good agreement between the different models, despite substantial differences in computational budget. In butt-welded plates, a significant decrease in out-of-plane distortion is obtained when cryogenic cooling is applied. In fillet-welded plates, cooling had much less effect on welding distortion. This was largely due to the size and configuration of the test case assemblies and the fact that the attached stiffener greatly increased the overall stiffness and resistance to contraction forces.peer-reviewe