Publications
183 results found
Sancho A, Cox MJ, Cartwright T, et al., 2019, Effects of strain rate and temperature on ductile damage of metals, ASME Pressure Vessels and Piping Conference (PVP 2018), Publisher: Amer Soc Mechanical Engineers
Khosla G, Balint D, Farrugia D, et al., 2019, Toughness measurements of a Cr martensitic high alloy steel susceptible to clinking, Proceedings of the Institution of Mechanical Engineers, Part L: Journal of Materials: Design and Applications, Vol: 233, Pages: 63-72, ISSN: 1464-4207
'Clinking’ is an audible fracture that occurs during the cool down and reheating of as-cast high alloy materials. When this process occurs, audible fracture can be heard and observed as large transverse cracks that propagate through large slabs. This causes high material losses and major disruption to processing operations. Given the fracture is known to be brittle, this research is aimed at developing a way to predict the onset of clinking through the application of fracture mechanics. Linear elastic and elastic–plastic fracture mechanics were both used to assess the fracture behaviour. The stress state during cool down and reheating was estimated through finite element analysis using a three-dimensional finite element model. Tensile tests were conducted to obtain the stress–strain characteristics to be used in the fracture analysis. Charpy tests were completed to assess the relative toughness dependent on temperature across the temperature range for which the high alloy steel is susceptible to clinking. Four C(T) specimens were tested at a room temperature. Despite showing little ductile crack propagation on the fracture surface, the fractured samples did not meet the Linear Elastic Fracture Mechanics (LEFM) validity criterion but did meet the Jcvalidity criterion. This allows a minimum Jcvalue of 118 N/mm to be attributed to the onset of unstable fracture. Conversion into a KJcgives 164MP√m, which gives a minimum critical crack length of 138 mm for the onset of brittle fracture. Charpy tests showed a pronounced increase in the energy for fracture between 20 ℃ and 300 ℃ which is in line with practical observations, where the onset of clinking is reduced with a higher reheat temperature.
Moghaddam BT, Hamedany AM, Mehmanparast A, et al., 2019, Numerical analysis of pitting corrosion fatigue in floating offshore wind turbine foundations, 3rd International Conference on Structural Integrity (ICSI), Publisher: ELSEVIER SCIENCE BV, Pages: 64-71, ISSN: 2452-3216
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- Citations: 8
Ejaz M, Davies CM, 2019, TDFAD ANALYSIS OF CREEP CRACK INITIATION IN 0.5CMV/2.25CRMOV STEEL WELDMENTS, ASME Pressure Vessels and Piping Conference, Publisher: AMER SOC MECHANICAL ENGINEERS
Ejaz M, Ab Razak N, Morris A, et al., 2019, LONG TERM CREEP LIFE PREDICTION OF NEW AND SERVICE EXPOSED P91 STEEL, ASME Pressure Vessels and Piping Conference (PVP 2018), Publisher: AMER SOC MECHANICAL ENGINEERS
O'Connor AN, Davies CM, Garwood SJ, et al., 2019, OPTIMISING THE SAFE DESIGN OF PRESSURISED COMPONENTS, ASME Pressure Vessels and Piping Conference, Publisher: AMER SOC MECHANICAL ENGINEERS
Rait GK, Davies CM, Garwood SJ, 2019, THE EFFECT OF A LOW CONSTRAINT GEOMETRY ON MEASURED To VALUES FOR A NUCLEAR REACTOR PRESSURE VESSEL FERRITIC STEEL, ASME Pressure Vessels and Piping Conference (PVP 2018), Publisher: AMER SOC MECHANICAL ENGINEERS
El Chamaat S, Patel M, Wenman MR, et al., 2019, MULTISCALE STRESS-DIFFUSION ANALYSIS OF NOTCH-TIP HYDROGEN PROFILES IN ZIRCALOY-4, ASME Pressure Vessels and Piping Conference (PVP 2018), Publisher: AMER SOC MECHANICAL ENGINEERS
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- Citations: 1
Jones MD, Nikbin KM, Davies CM, 2019, LOAD LINE DISPLACEMENT PARTITIONING IN CREEP CRACK GROWTH ANALYSES OF 316H STAINLESS STEEL, ASME Pressure Vessels and Piping Conference (PVP 2018), Publisher: AMER SOC MECHANICAL ENGINEERS
O'Connor AN, Davies CM, Nikbin KM, 2019, FRACTURE TOUGHNESS OF DEFECTS ORIENTATED PARALLEL TO A DISSIMILAR METAL WELD BOUNDARY, ASME Pressure Vessels and Piping Conference (PVP 2018), Publisher: AMER SOC MECHANICAL ENGINEERS
Davies CM, Zhou R, Withnell O, et al., 2019, FRACTURE TOUGHNESS BEHAVIOUR OF 316L STAINLESS STEEL SAMPLES MANUFACTURED THROUGH SELECTIVE LASER MELTING, ASME Pressure Vessels and Piping Conference (PVP 2018), Publisher: AMER SOC MECHANICAL ENGINEERS
Ibrahim Y, Li Z, Davies C, et al., 2018, Acoustic resonance testing of additive manufactured lattice structure, Additive Manufacturing, Vol: 24, Pages: 566-576, ISSN: 2214-8604
Additive manufacturing (AM) allows engineers to design and manufacture complex weight saving lattice structures with relative ease. These structures, however, present a challenge for inspection. A non-destructive testing and evaluation method used to assess material properties and quality is the focus of this paper, namely acoustic resonance (AR) testing. For this research, AR testing was conducted on weight saving lattice structures (fine and coarse) manufactured by powder bed fusion. The suitability of AR testing was assessed through a combined approach of experimental testing and FE modelling. A sensitivity study was conducted on the FE model to quantify the influence of element coarseness on the resonant frequency prediction and this needs to be taken into account in the application and analysis of the technique. The analysis was extended to extract effective modulus values for the lattice structures and the solid materials from every detected overtone, allowing for multiple measurements from a single AR test without the need to carefully isolate the fundamental. The AR and FE modelling modulus of elasticity values were validated using specimens of known properties. There was fair agreement between the FE and compression test extracted values of effective modulus for the coarse lattice. For the fine lattice, there was agreement in the values of effective modulus extracted from AR, 3-point bend, and compression experimental tests carried out. It was found that loose powder fusing from AM resulted in the fine lattice structure having a higher density (at least 1.5 times greater) than calculated due to the effect of loose powder adhesion. This effect resulted in an increased stiffness of the fine lattice structure. AR can be used as a measure of determining loose powder adhesion and other unique structural characteristics resulting from AM.
Sancho A, Cox MJ, Aldrich-Smith G, et al., 2018, Experimental methodology for the measurement of plasticity on metals at high strain-rates, DYMAT 2018 - 12th International Conference on the Mechanical and Physical Behaviour of Materials under Dynamic Loading, Publisher: EDP Sciences, ISSN: 2101-6275
An experimental methodology has been developed for the tensile characterisation of ductile isotropic metals at high strain-rate. This study includes the region beyond plastic instability or necking, which is rarely analysed for conventional applications. The research explores an imaging technique used to track the geometry of the specimen during tensile tests and calculate true local values of stress and strain by applying Bridgman theory [1]. To improve the quality of the images taken at high strain-rate an in-situ high speed shadowgraph technique has been developed, and to obtain better results from the images a sub-pixel accuracy edge detection algorithm has been implemented. The technique has been applied to an austenitic stainless steel. Its tensile behaviour has been assessed by testing round samples at strain-rates ranging from quasi-static to ~103 s-1. The results obtained with the proposed methodology have been validated by comparison with more conventional techniques such as video-extensometer and digital image correlation in the pre-necking region and good performance even at the highest strain-rate tested has been proved.
Ahn J, He E, Chen L, et al., 2018, In-situ micro-tensile testing of AA2024-T3 fibre laser welds with digital image correlation as a function of welding speed, International Journal of Lightweight Materials and Manufacture, Vol: 1, Pages: 179-188, ISSN: 2588-8404
In this paper, the influence of welding speed on tensile properties of AA2024-T3 fibre laser welds was investigated by monitoring the deformation behaviour during tensile loading. In-situ micro-tensile testing combined with a high-resolution optical microscope and DIC was used to measure strain distribution in narrow weld regions showing characteristics of fibre laser beam welding with limited metallurgical modifications. A chemical etching technique was used to generate a micro-scale random speckle pattern by revealing the weld microstructure. Such pattern enabled a sufficient spatial resolution of strain while keeping the weld seam visible during deformation. The results of microstructural and mechanical properties determined under numerous welding speeds indicated that increasing the welding speed led to the transition of weld pool shape from circular to elliptical to teardrop with a greater fraction of equiaxed dendrites. The weaker strength of the weld, as measured by local lower micro-hardness values, constrained significant plasticity development locally within the weld. Tensile tests revealed that increasing the welding speed resulted in greater yield strength and ultimate tensile strength, whereas, total elongation to failure dropped. The tensile properties improved with increasing welding speed as the fraction of equiaxed dendrites increased.
Williams RJ, Hooper PA, Davies CM, 2018, Finite element prediction and validation of residual stress profiles in 316L samples manufactured by laser powder bed fusion, 22nd European Conference on Fracture (ECF) - Loading and Environmental Effects on Structural Integrity, Publisher: ELSEVIER SCIENCE BV, Pages: 1353-1358, ISSN: 2452-3216
Davies CM, Withnell O, Ronnerberg T, et al., 2018, Fracture Analysis of 316L Steel Samples Manufactured by Selective Laser Melting, 22nd European Conference on Fracture (ECF) - Loading and Environmental Effects on Structural Integrity, Publisher: ELSEVIER SCIENCE BV, Pages: 1384-1389, ISSN: 2452-3216
Khosla G, Balint D, Farrugia D, et al., 2018, Analysis of an as-cast high Si slab to elucidate fundamental causes of the fracture mechanism: Clinking, 22nd European Conference on Fracture (ECF) - Loading and Environmental Effects on Structural Integrity, Publisher: ELSEVIER SCIENCE BV, Pages: 1447-1452, ISSN: 2452-3216
Williams R, Davies C, Hooper P, 2018, A pragmatic part scale model for residual stress and distortion prediction in powder bed fusion, Additive Manufacturing, Vol: 22, Pages: 416-425, ISSN: 2214-8604
Parts manufactured by laser powder bed fusion contain significant residual stress. This stress causes failures during the build process, distorts parts and limits in-service performance. A pragmatic finite element model of the build process is introduced here to predict residual stress in a computationally efficient manner. The part is divided into coarse sections which activate at the melting temperature in an order that imitates the build process. Temperature and stress in the part are calculated using a sequentially coupled thermomechanical analysis with temperature dependent material properties. The model is validated against two sets of experimental measurements: the first from a bridge component made from 316L stainless steel and the second from a cuboidal component made from Inconel 718. For the bridge component the simulated distortion is within 5% of the experimental measurement when modelled with a section height of 0.8 mm. This is 16 times larger than the 50 μm layer height in the experimental part. For the cuboid component the simulated distortion is within 10% of experimental measurement with a section height 10 times larger than the experiment layer height. These results show that simulation of every layer in the build process is not required to obtain accurate results, reducing computational effort and enabling the prediction of residual stress in larger components.
Ahn J, Chen L, He E, et al., 2018, Optimisation of process parameters and weld shape of high power Yb-fibre laser welded 2024-T3 aluminium alloy, Journal of Manufacturing Processes, Vol: 34, ISSN: 1526-6125
A novel approach to welding crack sensitive 2024 aluminium alloy was made by using fibre laser. Bead on plate welding of 3 mm thick sheets of 2024-T3 was performed to determine the optimum sets of welding parameters including laser power, welding speed, power density and focal position, which meet the quality and specification requirements of aircraft structures. A correlation between these parameters and weld shape, microstructure, and defects was found. The weld quality was assessed in terms weld-seam geometry, root to width ratio, surface appearance, penetration depth, microstructure and defects. Microstructural analysis was performed using optical microscopy, scanning electron microscopy and energy dispersive spectroscopy. The parametric optimisation was conducted to obtain crack and porosity free full penetration welds with ideal sized face and root width or weld shape, and a minimal amount of undercut, underfill and reinforcement. While high-quality welds were produced, in some cases, micro-cracks less than 0.5 mm were observed in the weld metal as optimising the parameters only had a limited effect on completely shifting the crack sensitive composition. The addition of filler metal with a different chemistry was found to be also necessary to adjust the composition to a less crack sensitive range.
Tarnowski K, Nikbin K, Dean DW, et al., 2018, A unified potential drop calibration function for common crack growth specimens, Experimental Mechanics, Vol: 58, Pages: 1003-1013, ISSN: 0014-4851
Calibration functions, used to determine crack extension from potential drop measurements, are not readily available for many common crack growth specimen types. This restricts testing to a limited number of specimen types, typically resulting in overly conservative material properties being used in residual life assessments. This paper presents a unified calibration function which can be applied to all common crack growth specimen types, mitigating this problem and avoiding the significant costs associated with the current conservative approach. Using finite element analysis, it has been demonstrated that Johnson’s calibration function can be applied to the seven most common crack growth specimen types: C(T), SEN(T), SEN(B), M(T), DEN(T), CS(T) and DC(T). A parametric study has been used to determine the optimum configuration of electrical current inputs and PD probes. Using the suggested configurations, the error in the measurement of crack extension is <6% for all specimen types, which is relatively small compared to other sources of error commonly associated with the potential drop technique.
Zheng J, Lin J, Pan R, et al., 2018, A novel constitutive model for multi-step stress relaxation ageing of a pre-strained 7xxx series alloy, International Journal of Plasticity, Vol: 106, Pages: 31-47, ISSN: 0749-6419
A novel set of unified constitutive equations has been developed and validated to describe stress relaxation ageing (SRA) behaviour of 7xxx series aluminium alloys. The model, based on dynamic ageing and power-law creep relations, can predict the stress relaxation, age hardening response and their interactions at different temperatures, through considering the microstructure evolutions (precipitate radius, volume fraction and dislocation density) during SRA. In addition, the model newly incorporates the effects of prior plastic strain. This model was verified through T74 multi-step SRA experiments for different pre-strain conditions. Excellent agreement was achieved between the predicted and experimental results for stress relaxation and yield strength variation. The evolution of micro-internal variables (e.g. normalised precipitate radius) within the model were calibrated by observing transmission electron microscopy (TEM) images performed in this work and available in literature. The advanced constitutive model developed predicts the mechanical properties and residual stresses in components after ageing. Therefore, the model provides a valuable tool to optimise manufacturing processes leading to many benefits including reduced scrap rates and financial losses.
Pan R, Shi Z, Davies CM, et al., 2018, An integrated model to predict residual stress reduction by multiple cold forging operations in extra-large AA7050 T-section panels, Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture, Vol: 232, Pages: 1319-1330, ISSN: 0954-4054
A finite element (FE) model has been developed to determine the effectiveness of acold compression technique to reduce the large residual stresses generated fromquenching solution heat treated T-Section components of aluminium alloy AA7050. Tocompress long components, a multi-step process is required with some amount ofoverlap. A parametric study has been performed to determine the effect of thecompression ratio, friction coefficient, length of overlap and length of the T-sectioncomponent on the residual stress distribution post quenching and after subsequent coldcompression. Generally a percentage reduction in the peak residual stress of over 90%was found. The optimal parameters for residual stress relief by cold compression havebeen suggested from the cases considered.
Corcoran J, Davies C, 2018, Monitoring power-law creep using the failure forecast method, International Journal of Mechanical Sciences, Vol: 140, Pages: 179-188, ISSN: 0020-7403
Creep is considered to be the life limiting damage mechanism in many load bearing high temperature components. A range of different parameters determine the creep life of a component, many of which are unlikely to be known to sufficient accuracy to enable satisfactory estimation of remnant life. Instead,the integrity of a component shouldbe established through direct measurement of the response of the component to the operating conditions. Creep deformation is shown to be a positive feedback mechanism; anincreasein strain leadsto an increasing strain rate. It has recently been shown that as a consequence of positive feedback the Failure Forecast Method, a generalised framework for predicting time to criticality based on rates of change of damage, may beapplied for remnant life calculations. A range of strain rate based assessments have been proposed in the literature but it is proposed that the Failure Forecast Method unifies many of these techniques and provides additional insight into creep behaviour by virtue of the underlying positive feedback. The methodology has been demonstrated using experiment datasets that are pertinent to creep in high temperature pressure vessels and piping; it is shown that failure times are accurately predicted shortly after the minimum creep strain rate.
Tarnowski KM, Nikbin KM, Dean DW, et al., 2018, Improvements in the measurement of creep crack initiation and growth using potential drop, International Journal of Solids and Structures, Vol: 134, Pages: 229-248, ISSN: 0020-7683
To predict the residual life of components operating in the creep regime, it is vital to accurately identify crack initiation, and measure subsequent crack growth, in laboratory tests. Potential drop (PD) measurements, used for this purpose, are susceptible to errors caused by the accumulation of creep strain. For creep ductile materials, this can result in highly conservative crack initiation models and the implementation of unnecessary inspection and maintenance programmes that can cost millions of pounds in lost revenue. Conversely, the crack growth models can be non-conservative.Using a novel combination of interrupted creep crack growth (CCG) tests and sequentially coupled structural-electrical finite element analyses a new method of interpreting PD data has been developed and validated. It uses an increase in gradient on a plot of PD vs. load-line displacement to accurately identify crack initiation. This has been compared to the current method in ASTM E1457-15 by reanalysing data from CCG tests performed on a range of materials at various temperatures and loads. The initiation times, measured using the current ASTM method, were underestimated by factors of up to 23 and the subsequent crack growth rates were underestimated by factors of up to 1.5.
Ab Razak N, Davies CM, Nikbin KM, 2018, Testing and assessment of cracking in P91 steels under creep-fatigue loading conditions, ENGINEERING FAILURE ANALYSIS, Vol: 84, Pages: 320-330, ISSN: 1350-6307
El Chamaa S, Patel M, Davies C, et al., 2018, The effect of grain boundaries and second-phase particles on hydride precipitation in zirconium alloys, MRS Advances, Vol: 3, Pages: 1749-1754, ISSN: 2059-8521
Understanding the precipitation of brittle hydride phases is crucial in establishing a failure criterion for various zirconium alloy nuclear fuel cladding. Accordingly, it is important to quantify the sensitivity of hydride precipitation to the component microstructure. This experimental investigation focuses on two microstructural characteristics and their role as hydride nucleation sites: The grain size and the alloy chemical composition. Samples of commercially pure zirconium (Zr-702) and Zircaloy-4, each with a wide range of grain sizes, were hydrided to 100 ppm and micrographs of the hydride distribution were optically analyzed for inter-granular and intra-granular precipitate sites. For most grain sizes, it was found that a significantly lower fraction of the precipitated hydrides nucleated at grain boundaries in Zircaloy-4 than in Zr-702, suggesting that a higher SPP content encourages the formation of intra-granular hydrides. Moreover, this effect became more prominent as the grain size increased; large-grain specimens contained a higher fraction of intra-granular hydrides than small-grain specimens of both Zr-702 and Zircaloy-4, highlighting the potency of grain boundaries as nucleation sites and how SPPs can influence the hydride distribution profile.
Ahn J, He E, Chen L, et al., 2018, FEM prediction of welding residual stresses in fibre laser welded AA 2024-T3 and comparison with experimental measurement, International Journal of Advanced Manufacturing Technology, Vol: 95, Pages: 4243-4263, ISSN: 0268-3768
Welding generates a considerable amount of residual stresses which affect the structural integrity of welded components. It is often assumed that the magnitude of residual stresses around the welded joint is as high as the yield stress of the material. In this investigation, such assumption was found to be overly conservative and failed to accurately represent the distribution of residual stresses in fibre laser-welded aluminium alloy 2024-T3 sheets. Welding simulation based on the finite element method was used to reliably determine the distribution and magnitude of transient residual stress fields and distortions in thin sheets welded using three different sets of welding parameters. The accuracy of the finite element models was checked by calibrating with experimentally measured weld pool geometries and temperature field prior to conducting parametric studies. X-ray and neutron diffraction measurements were performed on the surface and in the bulk of the welded components, respectively, and compared with numerical results. The influence of weld metal softening, welding parameters and restraints on residual stresses and distortion were investigated systematically by numerically simulating ideal conditions which eliminate the practical limitations of conducting experimental studies, for process optimization as well as evaluation of in-service structure integrity and failure modes of the welded sheets.
Zheng JH, Pan R, Li C, et al., 2018, Experimental investigation of multi-step stress-relaxation-ageing of 7050 aluminium alloy for different pre-strained conditions, Materials Science and Engineering A: Structural Materials: Properties, Microstructure and Processing, Vol: 710, Pages: 111-120, ISSN: 0921-5093
A novel insight into the whole two-step stress relaxation ageing process during T74 multi–step ageing treatment (120 °C for 6 h and subsequently 177 °C for 7 h), which is typically experienced by extra-large aircraft components that contain high residual stresses, has been established. Stress relaxation ageing (SRA) tests, tensile tests and transmission electron microscopy (TEM) were performed on AA7050 samples to determine the relationship between internal microstructure and macroscopic behaviour during the stress relaxation and precipitate evolution process. Samples were subjected to SRA at different initial stresses (220–360 MPa) after being pre-strained to different extents (i.e. 0%, 1%, 3%). Room temperature tensile tests were then performed on interrupted SRA test specimens to examine the corresponding strengthening phenomenon. TEM was performed on a selection of peak–aged and T74 over–aged samples to study the precipitate distribution. At 120 °C typical stress relaxation behaviour was observed and the data follow ed a logarithmic curve. Subsequently at 177 °C, dislocation–creep dominated stress relaxation behaviour, with no apparent threshold stress, was observed. The absence of a threshold stress at 177 °C may be attributed to the continuous over-ageing phenomenon. The effect of pre-deformation levels and initial stresses on SRA has also been investigated. Pre-stretching, which creates uniformly distributed dislocations, promotes stress relaxation and ageing. No significant influence of initial stress level on SRA was observed at 120 °C, but noticeable effects were seen at 177 °C. The calculated stress exponent n at 177 °C is found independent of the initial stresses. These findings provide clear scientific guidance for residual stress reduction during the multi-step ageing process of AA7050 and provide the basis for residual stress prediction models.
Ahn J, He E, Chen L, et al., 2017, Determination of residual stresses in fibre laser welded AA2024-T3 T-joints by numerical simulation and neutron diffraction, Materials Science and Engineering: A, Vol: 712, Pages: 685-703, ISSN: 0921-5093
This paper presents an experimental and numerical study of residual stress states and deformation in fillet welded AA2024-T3 T-joints produced using a high-power fibre laser. Welded sheets with one and three stiffeners were prepared, respectively, to determine changes in microstructure, residual stress, distortion and micro-hardness. 3D sequentially coupled thermo-mechanical finite element models were developed to analyse welding temperature fields, and accurately simulate welding residual stresses and deformation. The simulated results were calibrated using the experimental database on weld pool geometry obtained from optical metallography and temperature fields measured using thermocouples. Residual stress measurements were made using neutron diffraction techniques and sheet distortions were measured using a coordinate measuring machine. The influence of various mechanical boundary conditions on angular and cambering sheet distortions was examined to optimise the restraint parameters. The application of element death and rebirth and dummy element techniques were studied and compared to incorporate the effect of filler metal deposition during welding. The level of residual microstrain was evaluated by diffraction peak width analysis, which indicated the maximal values in the weld metal. The effect of grain growth with respect to strength was of minor importance, whereas, considerable softening in the weld metal was observed.
Larrosa NO, Ainsworth RA, Akid R, et al., 2017, 'Mind the gap' in fitness-for-service assessment procedures-review and summary of a recent workshop, INTERNATIONAL JOURNAL OF PRESSURE VESSELS AND PIPING, Vol: 158, Pages: 1-19, ISSN: 0308-0161
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