The aim of this study is to investigate the microscopic fracture process in titanium metal matrix composites (TiMMC) shafts based on finite element (FE) analysis. A unit cell approach is used as a tool to design a representative volume element to explain the complex failure phenomenon on a microscopic level. FE mesh convergence is examined and validated with the equivalent plastic strain and contact pressure. The accuracy of the numerical modelling is verified by comparison with experimental investigations on fractured TiMMC shafts. The results show that FE modelling can successfully analyse the complicated fracture process that occurs on a microscopic scale of TiMMC shafts.

Machining of slender (low rigidity) parts is associated with tool/workpiece deflections due to induced cutting forces resulting in machining error (dimensional inaccuracy of the machined surface). The development of smart fixtures is seen as an enabler for reduction of machining error. To reduce lead times, the smart fixtures need to be designed in an efficient way by using more virtual simulations and less physical iterations. This paper presents the development of a novel methodology for machining error prediction in milling of a fixture-workpiece system. The methodology integrates a cutting force model, a finite element based fixture-workpiece system and a multi-step error predictive approach. The methodology was first validated on a flexible thin-wall Ti6Al4V slender part where less than 6% difference was achieved between predicted and measured machining error. The difference between predicted and measured cutting forces was approximately 6%. After the gained confidence, the methodology was applied to the flexible thin-wall Ti6Al4V slender part encompassed by a fixture with three actuators acting as supports. The predicted machining error was reduced from the range of 0.2–0.33 mm (no actuators) to the range of 0.12–0.14 mm (with three actuators). This demonstrated the capability of the developed methodology to aid the design of future smart fixtures with the potential to reduce lead times during their development.

The process of fusion arc welding of steel pipes in power generation plants induces residual stresses which may be detrimental to the integrity and endurance of plant pipelines. P91 is high-grade steel used in the construction of pipelines carrying hot steam at high pressure, conditions which cause creep during service. Welded P91 pipes are usually subjected to post-weld heat treatment (PWHT) to mitigate the magnitude of residual stresses and temper the material, hence improving its resistance to creep. In this paper, the finite element (FE) method of modelling residual stresses due to PWHT in a circumferentially butt-welded P91 pipe is presented. The PWHT hold temperature is 760 degrees C. The paper describes the X-Ray Diffraction (XRD) and Deep-Hole Drilling (DHD) experimental techniques and how they are applied to measure residual stresses in the welded P91 pipe after PWHT. The material property data, necessary for the FE simulation of PWHT, has been obtained from stress-relaxation tests on P91 uniaxial tensile specimens at 760 degrees C. Good agreements have been achieved between the results of the FE method and the two sets of experimentally-measured residual stresses.

In this paper, the low cycle fatigue behaviour of MarBN steels at elevated temperatures is discussed. Four cyclic loading waveforms were applied to investigate both the mechanical and microstructural features. The mechanical results exhibit continuous cyclic softening and decelerated stress relaxation, recoverable viscoelasticity at both short and long time scales and irrecoverable viscoplastic mechanisms. Detailed microstructural characterisation was also conducted to investigate the rupture behaviour of the investigated steel. The microstructural findings indicate typical transgranular and ductile fracture behaviour, extensive sub-grain formation and structural refinement as well as significant decrease in hardness close to the fracture surface.

Improving the energy efficiency of power plants by increasing steam operating temperature up to 700 degrees C can be achieved using novel engineering design concepts such as coated steam pipe systems. This paper presents an optimised design for a novel coated dual pipe system to be used in advanced ultra-supercritical power plant. The approach developed in this study uses a combination of an optimisation algorithm and FE simulation, based on the reduction of the hoop stress at top coat/bond coat interface generated by the thermal and mechanical stresses. This allows determination of the optimum dimensions and material properties of the system. A unified viscoplastic model which combines a power flow rule with non-linear anisothermal evolution of isotropic and kinematic hardening has been used for the thermo-mechanical analysis of the coated dual pipe system under the cyclic loading. The results of the optimisation show that the value of the hoop stress at the top coat/bond coat interface is reduced significantly, compared with that in the baseline model. Finally, the potential technical challenges and future works for the proposed steam dual pipe system are discussed.

Improving the energy efficiency of advanced ultra-supercritical power plants, by increasing steam operating temperature up to 700 degrees C, can be achieved, at reduced cost, by using novel engineering design concepts, such as coated steam pipe systems manufactured from high temperature materials commonly used in current operational power plants. This paper describes a preliminary feasibility analysis of the design concept of a novel coated dual pipe system under steady-state operation, using analytical and finite element models to evaluate the possible thermal gradients and stresses generated. The results show that the protective coating layer contributes to the effective reduction in the surface temperature of the primary steel pipe. Thermal stresses generated due to the significant difference in the thermal and mechanical properties of the coating and substrate pipe are larger than the mechanical stresses generated by the combined effects of the internal steam pressure in the primary steam pipe and external pressure from the counter-flow cooling steam during steady-state operation. Compared with the stress relaxation of the coating and substrate pipe, creep has a significant impact on the stress distribution within the coating layer. Several key factors have been identified, such as the coating thickness, conductivity, thermal expansion, heat transfer coefficient of cooling steam, cooling steam temperature and cooling steam pressure, which are found to govern thermal and stress distributions during steady-state operation.

Incremental sheet-forming (ISF) processes have been developed rapidly in the past two decades. Its high flexibility and easy operability have a significant appeal for industrial applications, and substantial progress has been made in fundamental understanding and demonstration of practical implementation. However, there are a number of obstacles including achievable accuracy and instability in material deformation, which are considered as a main contributing factor for preventing the ISF process to be widely used in industry. As a variant of the general ISF process, double-sided incremental forming (DSIF) uses an additional supporting tool in the opposite side of the workpiece, maintains the flexibility, and at the same time improves the material deformation stability and reduces material thinning. In recent years, there has been increased research interest in looking into DSIF-specific material deformation mechanisms and investigation. This paper aims to provide a technical review of the DSIF process as benchmarked with single point incremental forming (SPIF). It starts with a brief overview of the current state of the art of both SPIF and DSIF. This is followed by a comparative study between SPIF and DSIF with the key research challenges identified. This leads to a recommendation of future directions for DSIF focused research.

Double-Sided Incremental forming (DSIF) is a flexible sheet forming method with increasing research interest in the last decade. It offers improved formability and accuracy over the conventional single point incremental forming (SPIF) although it accompanies with the possibility of tools losing contact during the forming process. Mounting a pneumatic supporting tool is an efficient solution. However, the tools may squeeze and over-bend the material when forming for a high wall angle. A possible reason may be due to inaccurate prediction of material thinning or the effect of tool deflection. This work uses a modified finite element (FE) model with dedicated toolpath design incorporated with simplified stiffness of tools and machine in order to reproduce the effect of system elasticity to the DSIF process with hydraulic supporting tool and evaluate the root causes of the over-bending problem. The numerical results are compared with experimentally produced components, focusing on the thickness evaluation and development of the tool deflection throughout the process. This comparison demonstrates the effect of the tool deflection upon the formed parts, including the geometric error and excessive deformation on the wall region. The conclusion suggests a need for a compensation based approach in the design of DISF toolpath and machine systems.

This paper reports the short-term creep behaviour at elevated temperatures of a MarBN steel variant. Creep tests were performed at three different temperatures (625 degrees C, 650 degrees C and 675 degrees C) with applied stresses ranging from 160 MPa to 300 MPa, and failure times from 1 to 350 h. Analysis of the macroscopic creep data indicates that the steady-state creep exhibits a power-law stress dependence with an exponent of 7 and an activation energy of 307 kJ mol(-1), suggesting that dislocation climb is the dominant rate-controlling creep mechanism for MarBN steel. Macroscopic plastic instability has also been observed, highlighted by an obvious necking at the rupture region. All the macroscopic predictions have been combined with microstructural data, inferred from an examination of creep ruptured samples, to build up relations between macroscopic features (necking, damage, etc.), and underlying microstructural mechanisms. Analysis of the rupture surfaces has revealed a ductile fracture mode. Electron Backscatter Diffraction (EBSD) analysis near to the rupture surface has indicated significant distortion and refinement of the original martensitic substructure, which is evidence of long-range plastic flow. Dislocation pile-ups and tangles from TEM were also observed near substructure boundaries and precipitate particles. All of these microstructural observations suggest that creep is influenced by a complex interaction between several elements of the microstructure, such as dislocations, precipitates and structure boundaries. The calculated stress exponent and activation energy have been found to agree quantitatively with the highlighted microstructural features, bearing some relationships to the true observed creep microstructures.

A miniature two-material testing method has been developed for material property characterization. Elastic-plastic and creep damage models have been applied for a semi-analytical model and a finite element (FE) model to simulate the miniature specimen tensile and creep tests on a two-material system. An inverse optimization algorithm has been developed to extract the creep properties of the unknown material in the two-material system from the experimental results. Single-material miniature specimen tensile tests have been performed for an aluminum alloy at room temperature and 400 degrees C and full stage creep tests have been performed for a P91 steel at 650 degrees C. The miniature specimen testing results are comparable to the results from the corresponding conventional uniaxial tensile and creep tests on the same materials and under the same test conditions. This miniature specimen testing technique and the developed inverse method have the potential to become a new approach for determining the elastic-plastic and the full-stage creep properties until rupture for a two-material system at high temperature, e.g. a coating-substrate system.

This work is focused on the determination of elastic-plastic material properties from indentation loading-unloading curves using optimisation techniques and experimental data from instrumented indentation tests. Three different numerical optimisation methods (namely, FE analysis, dimensional mathematical functions and simplified mathematical equations approaches) have been used to determine three material properties; Young's modulus, yield stress and work-hardening exponent. The predictions of the material properties from the three approaches have been validated against the values obtained from uniaxial tensile tests and compared to the experimental loading-unloading curves. In general, the elastic-plastic material properties predicted from these three proposed optimisation methods estimate the Young's modulus to within 6% and the yield stress and work-hardening exponent to within 12%, compared to the values obtained from the uniaxial tensile tests.

A combined experimental and numerical approach was adopted to investigate the focused-tungsten inert gas welding process by producing bead-on-plate welds in Inconel 718 plates. Experimental investigations were carried out by means of thermocouple measurements and optical macrographs of the weld cross-section. Three-dimensional finite element simulations were conducted using the commercial specialized finite element software Sysweld in order to predict the thermal field induced by the process in the plates. The work presents an approach to investigate the process efficiency and calibrate the heat source model in order to produce a full thermal characterization of the plasmatron welding apparatus.

The interaction of weld induced residual stresses is an important issue for multi-pass and repair welding operations; effective prediction of the magnitude and location of peak residual stresses can lead to improved lifing predictions and greater understanding of the performance of components in service; however, the interaction of existing residual stress field with new ones imposed by fusion welding processes has received little attention to date. This study presents a numerical and experimental investigation into the interaction of bead-on-plate welds in thick plates of IN718 with the aim of evaluating the effects of pre-existing weld residual stresses on the final global residual stress distribution. Sequentially coupled thermo-mechanical finite element (FE) models, which have been validated through temperature measurements, optical macrography and residual stress measurements, performed using the neutron diffraction technique, have been used to investigate the interactions. The results show good correlation between the experimental and model residual stress fields, demonstrating that the FE models are capable of predicting the redistribution of existing residual stress fields subjected to fusion welding processes and that this can be achieved with the use of parent material properties throughout, without considering any material property modifications which may occur due to microstructural changes in and around the weld region. It can also be seen from the results that, even with the relatively thick plates used in the study, a plane-stress state exists in the plates with a normal stress of approximately zero in all cases.

The burst pressure of commonly used ductile steel pipes in oil and gas industries, i.e. X52 and X60, is measured under internal pressure loading. The pipes were machined with circular and boxed defects at different orientations to simulate actual metal loss defects. Defect shapes and orientations were investigated in detail to study how they affect the failure behaviour of interacting defects. The experimental burst pressure results were compared with those obtained using existing analytical methods from Design Codes. Comparison of the results showed conservatism in the existing analytical methods which may potentially lead to unnecessary plant shutdowns and pipe repairs. The outcome of the experimental tests revealed that the shapes of the defects have very small influence on the defect interaction behaviour. The burst tests interestingly showed that the defect orientation has an important effect on defect interaction. Defects oriented in the hoop and diagonal directions showed no defect interaction even when spaced by a distance of one wall thickness, while defects oriented in the longitudinal directions showed that defects interact even when the spacing is up to six wall thickness but the interaction fades away for defects spaced at longer distances.

The fatigue behaviour of a 9Cr power plant steel at a temperature of 600 C was studied by uniaxial fatigue tests and microstructural analysis using electron microscopy. A continuum damage mechanics apporach was coupled to the constitutive equations of the Chaboche elasto-visco-plastic model to describe the fatigue behaviour of the 9Cr power plant steel at 600 C. A stress partition method is introduced to understand the fatigue softening behaviour, and used to give an initial estimate of the material constants in the Chaboche model. Further optimisation procedures with plastic strain range dependency of the material constants were introduced in the optimisation procedure in order to accurately predict the material behaviour, especially after damage initiation. An ABAQUS UMAT subroutine was coded to allow the full life cyclic softening behaviour of a power plant component to be accurately predicted in a finite element calculation. The multiaxial capability of the coding is validated against notched bar test data.

This work presents the application of a finite element (FE) model developed to simulate the repair process in the case of components with a pre-existing stress state. The approach is tested in the case of a repair of a laser beam weld in a stainless steel pipe with the region of repair located in the heat affected zone of the original weld. The area of the repair is removed and refilled testing different approaches in terms of the number, and direction of the repair passes. The comparison between the refilling procedures is presented with the aim of evaluating the effects on the final residual stress distribution.

This paper presents a review of the mechanical characterisation of thin film and coated systems using indentation tests. The potential in assessing mechanical properties of films and coatings using indentation tests has received a great deal of attention since this knowledge is vital for predicting their performance. The relevant theoretical background is discussed. Experimental work, numerical studies and data interpretation techniques for indentation on single bulk materials and thin films are discussed. Surface conditions, indentation depths and indentation size effects for indentation tests on thin films and coated systems are discussed. Data interpretation methods for indentation on films and coated systems are reviewed with a discussion on their limitations. Other studies in this field concerning the substrate effects and critical indentation depth ratios are also discussed. Suggestions for future experimental work and data interpretation are provided.

An innovative finite element modelling approach has been tested to investigate the effects of weld repair of thin sheets of titanium alloy, taking into account a preexisting stress field in the components. In the case study analysed, the residual stress fields due to the original welds are introduced by means of a preliminary sequentially-coupled thermo-mechanical analysis and considered as preexisting stress in the sheets for the subsequent repair weld simulation. Comparisons are presented between residual stress predictions and experimental measurements available from the literature, with the aim of validating the numerical procedure. As a destructive sectioning technique was used in the reference experimental measurements, an investigation is also presented on the use of the element deactivation strategy when adopted to simulate material removal. Although the numerical tool is an approximate approach to simulate the actual material removal, the strategy appears to predict a physical strain relaxation and stress redistribution in the remaining part of the component. The weld repair modelling strategy and the element deactivation tool adopted to simulate the residual stress measurement technique are shown to predict residual stress trends which are very well correlated with experimental findings from the literature.

Two computational procedures have been developed in the commercial finite element (FE) software codes Sysweld and Abaqus to analyse and predict the residual stress state after the repair of small weld defects in thin structural components. The numerical models allow the effects of the repair to be studied when a pre-existing residual stress field is present in the fabricated part and cannot be relieved by a thermal treatment. In this work the modelling strategies are presented and tested by simulating a repair of longitudinal welds in thin sheets of Inconel 718 (IN718). Although the numerical strategies in the two codes are intrinsically different, the results show a significant agreement, predicting a notable effect imposed by the initial residual stress.

A total of two computational procedures have been developed in the commercial finite element software codes Sysweld and ABAQUS to analyse and predict the residual stress state after the repair of small weld defects in thin structural components. The numerical models allow the effects of the repair to be studied when a pre-existing residual stress field is present in the fabricated part and cannot be relieved by a thermal treatment. In this work, the modelling strategies are presented and tested by simulating a repair of longitudinal welds in thin sheets of Inconel 718. Although the numerical strategies in the two codes are intrinsically different, the results show a significant agreement, predicting a notable effect imposed by the initial residual stress.