This research aims to quantify the contribution of welding residual stress and microstructural embrittlement to the toughness variation of a set of submerged arc weldments. To determine these contributions, the fracture toughness of several ferritic steel plate butt-weldments was measured using the method for J-integral estimation detailed in ISO 12135.Welding residual stress was mapped in the transverse-perpendicular plane to the weld longitudinal direction using the contour method. Spatial variation in microstructure in the weld longitudinal direction was evaluated by testing sub-standard sized Charpy specimens.Analysis suggests no relevant correlations exist between Charpy impact fracture energy, welding residual stress or spatial position. Further, Charpy impact fracture energy and welding residual stress are individually inadequate estimators of weld fracture toughness variability.Investigating the joint responsibility of either inhomogeneity using principal component analysis suggests that microstructural embrittlement accounts for the vast majority of the variability in weld fracture toughness data.

The influence of orientation on octet-truss lattice static properties has been extensively studied. Fatigue failure is one of the main forms of structural failure of lattices. The fatigue properties of lattice structures, such as crack growth direction and crack growth rate, depend on orientation. Hence, it is important to explore the orientation effects on octet-truss lattice fatigue. Differently oriented octet-truss lattices in this research were additively manufactured into compact tension specimen geometry using photopolymer. From the experiment, the fatigue crack paths for differently oriented lattices were observed and compared with each other. The compliance method described in ASTM E647 was used to calculate the fatigue crack growth rates of lattices in different orientations. It is found from experiment that orientation-ZX lattices have the greatest resistance to fatigue crack propagation. Analytical formulae to predict the lattice fatigue life in different orientations were derived by simplifying individual lattice struts as pin-jointed bars. These findings can help us understand the orientation dependence of lattice fatigue failure.

The octet-truss lattice is a three-dimensional lattice with high strength and low density. Practical use of these lattices requires a knowledge of their resistance to fatigue crack growth. Lattice specimens manufactured by laser powder bed fusion technique were prepared with different orientations to explore the orientation effects on lattice fatigue behaviour. X-ray tomography was used to measure the surface roughness of the lattice struts, and also used to validate the accuracy of the crack length measurement by compliance method. The experimental results show that the fatigue crack growth rate and crack growth path depend on the orientation of the lattice. A finite element prediction of the fatigue life of the lattice was carried out, based on measurements of the fatigue life of single struts cut from the lattice. The finite element predictions were in good agreement with the experimental results. A procedure for predicting the crack growth path was proposed. These findings can help designers optimize the orientation of lattice structures in structural applications.

Octet-truss lattice materials have excellent potential for use as lightweight structures due to their high strength and stiffness, but low relative density. Octet-truss lattice specimens fabricated by stereolithography technique with a photopolymer resin were studied in this research. The unit cell orientation effects on the compressive fatigue behaviors of octet-truss lattices were studied using experimental analysis and computer simulation. A detailed comparison was made between the failure modes of static and fatigue failures, and the deformation mechanisms of lattices in different orientations under compressive cyclic load were determined. Both the static mechanical properties and fatigue properties of octet-truss lattices are highly dependent on the lattice orientation. When normalized with respect to their orientation-dependent Young's moduli, the fatigue endurance of lattices in different orientations conforms very well (R-2 = 0.88) to a single S-N curve described by a power law. Finally, the differences in fatigue performance between lattices in different orientations were explained and a simple compression-compression fatigue mechanism was determined.

Orientation effects on fatigue crack growth rates of octet-truss lattice were studied using experimental method in this research. In order to perform the fatigue test, octet-truss lattices in different orientations were 3D printed into compact tension specimen geometry using photopolymer. The fatigue crack paths for different orientations lattices were obtained from the experiment and compared with each other. The fatigue crack growth rates of lattices in different orientations were calculated using compliance method suggested by ASTM E647. The experimental results show that orientation-Z lattice has the greatest resistance to fatigue crack propagation.

Octet-truss lattices printed by photopolymer have been used in this research to investigate the orientation effects on fracture behaviours. Octet-truss lattices were printed into CT samples in three orthogonal orientations. The fracture resistances and crack paths for different orientations lattices were obtained and compared. The finite element method with beam element was used in the present study to simulate the fracture response of this structure. Both the load reaction and crack paths were obtained in simulations and compared with the experimental results. The numerical results agree well with the test results.

A new test configuration called Holed-Cracked Square Plate (HCSP) is proposed to investigate I/II mixed-mode fracture of brittle and quasi-brittle materials. This specimen is a square plate containing a central hole with two radial cracks emanating from its circumference. The finite element method was used to calculate mode I and mode II stress intensity factors and T-stress solutions for various crack lengths and hole diameters. The numerical results show that a full range of fracture mode mixities can be realized by changing the orientation angle of two radial cracks, while maintaining a very simple specimen geometry and loading requirement. A series of fracture tests were conducted on polymethylmethacrylate (PMMA) to study the practical capabilities of the HCSP specimen. These yielded fracture toughness values that are consistent with other experimental results. Other observed quantities including fracture initiation angles and fracture resistance also agree very well with mixed-mode fracture theories.

Thermal shocks are an important incident in operation of a pressure vessel which can have a significant impact on the structural integrity of the vessel. Often experiments that consider the state of the vessel before and after the thermal shock are used to evaluate the effects of the thermal shock. The studies can be complemented by time-resolved numerical simulations, which may be validated against the final state of the vessel obtained experimentally, to infer the transient response of the material. The transient response is important as the material experiences the highest level of stress in a short period which can induce catastrophic failure. This paper reports time-resolved experimental quantification of strain in reactor pressure vessel material during thermal shock measured by in-situ synchrotron diffraction. Specimens were extracted from a plate of nuclear pressure vessel steel with a nickel alloy cladding deposited by overlay welding. The specimens, with and without cracks, were subjected to thermal loading by heating then rapidly quenching the cladding in cold water. Strains were measured during thermal loading at a point near the crack tip from which the stress state around the crack tip was calculated and compared with a transient finite element model of the experiment. It was found that the peak near-tip stress occurred within the first second after the onset of rapid cooling. It was demonstrated from experimental measurements that the peak stress intensity factor occurred during thermal shock, rather than under steady conditions before or after the thermal shock. It was shown that although the finite element simulation predicts the steady state condition of the material after thermal shock, its transient response dependents significantly on a number of inputs with high uncertainty, making its time-resolved results unreliable for high-fidelity integrity assessments.

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 work the mixed mode I/III fracture of sandstone has been studied experimentally and numerically. The experimental work used three-point bending specimens containing pre-existing cracks, machined at various inclination angles so as to achieve varying proportions of mode I to mode III loading. Dimensionless stress intensity factors were calculated using the extended finite element method (XFEM) for and compared with existing results from literature calculated using conventional finite element method. A total of 28 samples were used to conduct the fracture test with 4 specimens for each of 7 different inclination angles. The fracture load and the geometry of the fracture surface were obtained for different mode mixities. Prediction of the fracture loads and the geometry of the fracture surface were made using XFEM coupled with a cohesive zone model (CZM) and showed a good comparison with the experimental results.

Three-point bending specimens have been used to investigate the mixed mode fracture of green sandstone. Dimensionless stress intensity factors and T-stresses were calculated first by using the finite element method for various crack lengths, crack angles and span to length ratios. It is shown that three-point bending specimens can provide the whole range of mode mixities from pure mode I to pure mode II, provided suitable values are chosen for the crack angle and span to length ratio. The fracture test results were also used to compare with predictions of different criteria. These comparisons show that modified criteria including the influence of the T-stress agree better with experiment than the conventional criteria but that no one criterion matches perfectly the test results.

A new test configuration called Holed-Cracked Square Plate (HCSP) is proposed to investigate mixed mode I/II fracture of brittle and quasi-brittle materials. This specimen is a square plate containing a central hole and two radial cracks initiated from the circumference of the internal hole. The finite element method was used to calculate mode I, mode II intensity factors and T-stress for various crack lengths and hole diameters. The numerical results show that a full range of fracture mode mixities from pure mode I to pure mode II can be realized by changing the orientation angle of two radial cracks in this specimen. The simple geometry and loading set up of this specimen make it a good substitute for other fracture tests.

Octet-truss lattice structures can be used for lightweight structural applications due to their high strength-to-density ratio. In this research, octet-truss lattice specimens were fabricated by stereolithography additive manufacturing with a photopolymer resin. The mechanical properties of this structure have been examined in three orthogonal orientations under the compressive load. Detailed comparison and description were carried out on deformation mechanisms and failure modes in different lattice orientations. Finite element models using both beam elements and three-dimensional solid elements were used to simulate the compressive response of this structure. Both the load reaction and collapse modes obtained in simulations were compared with test results. Our results indicate that three-dimensional continuum element models are required to accurately capture the behaviour of real trusses, taking into account the effects of finite-sized beams and joints.

Understanding of the through-thickness distribution of assembly stresses caused by manufacture process-induced distortion (PID) in thick composite components is essential for safety and reliability assessment. Inspired by deep-hole drilling (DHD) method which is widely used to reconstruct residual stresses in thick metallic components, current research attempts to extend and modify the DHD technique to measure the manufacture assembly stresses in composite components. It is observed that the modified DHD technique can capture the global deformation profiles induced by the assembly stresses, but the previous DHD stress calculation method produces large stress errors. An integrating stress calculation method is then proposed by combining the homogeneous global and the layer-wise antistrophic stress-strain solutions. It is demonstrated that the assembly stresses calculated by the proposed integrating calculation method are significantly improved and it is feasible to measure the manufacture assembly stresses of thick composite components with the modified DHD technique.

We propose a method to predict the fatigue life of triangular lattices by using fatigue data from tests on single lattice struts. The method was validated using fatigue tests on 6 identical triangular lattice plates divided into three groups with different initial crack lengths. The predicted fatigue lives of lattices agree very well with the experimental results. A three-stage mechanism of lattice fatigue was observed in the tests, and the crack propagated in the direction 30° to the horizontal axis in the upper half of lattice plate in all three groups. The fatigue crack propagation rate was also studied and calculated by using the effective mechanical parameters from the homogenized structure. Experimental fatigue crack growth rate data were compared with predictions, showing that the proposed method can also predict the crack growth rate in triangular lattices very well.

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.

This paper characterises the residual stress in nuclear reactor pressure vessel steel clad with nickel-based alloy and investigates the interaction between residual and thermal stresses during thermal shock. Residual stress measurements were made on two plates of SA508 Grade 4N steel, clad with Alloy 82 nickel-based alloy. The techniques used to measure the residual stresses were: deep hole drilling, centre hole drilling, and the contour method. One plate was as-welded, the other post-weld heat-treated. The post-weld heat-treated plate was subjected to thermal shock by heating it up and then spraying the surface of the cladding with cold water. The residual stress was measured again afterwards. A finite element simulation was made to investigate the physical mechanisms causing residual stress redistribution during thermal shock. Thermal shock caused significant residual stress redistribution in the cladding due to elastic-plastic interaction between the thermal stress and the cladding residual stress. The results demonstrate that an assessment of the safety of a reactor pressure vessel during thermal shock could be conservative for small surface defects if it is assumed that residual and thermal stresses combine elastically.

The behaviour of a crack in the centre of a plate subject to a far-field applied stress perpendicular to the crack surface has been studied. The plate contains an initial, self-equilibrated residual stress, symmetric to the central position of the crack. The component of the residual stress perpendicular to the crack at the centre of the plate can be tensile or compressive. Elastic and elastic–plastic material behaviours have been considered and crack closure effects have been included in the analyses. For elastic behaviour a series of analyses based on stress intensity factor solutions have been developed to calculate the crack opening and the stress intensity factor for cracks of different lengths relative to the size of the residual stress field. Different magnitudes of applied stress relative to the magnitude of the residual stress were applied. Crack behaviour maps have been developed that show the behaviour of the crack for different crack lengths and magnitudes of applied stress. For elastic–plastic behaviour a strip yield model has been used to develop a similar set of analyses to those for the elastic case. The results compare favourably with those produced by finite element analysis. The work provides the basis for a first estimate of the likelihood of fracture for a component containing residual stress and subject to applied load.

A method for predicting the fatigue life of triangular lattices is proposed in this paper by considering fatigue properties of single lattice struts. Fatigue tests of different sizes of lattice plates of aluminium alloy, and tests of single struts with different maximum fluctuating loads, have been conducted to validate this method. It is found that the struts in a triangular lattice break near to strut intersections, where stress and strain concentrations occur. Similar crack propagation paths were observed in different lattice plate specimens: the cracks grew at a 30° angle to the initial edge crack in the upper half of lattice plate. The mixed-mode fatigue crack propagation rate was also studied and expressed using an effective stress intensity factor. A size effect on the crack growth rate of triangular lattice plates was also observed: a fatigue crack will propagate slightly quicker in larger triangular plates than in smaller ones.

A cycle analysis has been applied to a model of a advanced ultra-supercritical steam plant with novel steam pipes. The transfer pipes proposed incorporate internal thermal coatings and are externally jacketed to enable cooling. This enables higher temperature working steam, while keeping the pipe wall temperature below the acceptable limit for more conventional steel alloys and avoiding the need to use higher cost austenitic stainless steels and nickel base alloys. The baseline design had a superheat temperature of 700 degrees C and a reheat temperature of 720 degrees C. A thermal coating thickness of 2.8 mm is sufficient to keep the wall temperatures of the steam transfer pipe after the supercritical boiler below 600 degrees C. For the transfer pipe located after the reheater a thicker coating or less ambitious reheat temperature is required to achieve acceptable pipe wall temperatures. Whereas subcritical plant has a calculated cycle efficiency of 42.1%, the elevated temperature and pressure in a customary ultra-supercritical steam boost cycle efficiency to 52.2%. Modifying this design with a thermal barrier lowers the cycle efficiency to 51.4%, still appreciably better than for subcritical plant. Alternative plant cooling arrangements might improve pipe temperatures but have minimal impact on overall cycle efficiency. (C) 2019 Elsevier Ltd. All rights reserved.