176 results found
Al-Lami J, Hoang P, Davies C, et al., 2023, Plastic inhomogeneity and crack initiation in hybrid wrought - additively manufactured Inconel 718, Materials Characterization, Vol: 199, Pages: 1-12, ISSN: 1044-5803
Directed energy deposition (DED) holds great promise for repair applications involving site-specific deposition that creates hybrid components. However, it has been reported that failure in hybrid components occurs in the additively manufactured (AM) side despite its higher strength and excellent consolidation. To unravel the underlying mechanisms responsible for the observed behaviour, we carried out both ex-situ and in-situ mechanical testing complemented with detailed microstructural characterisation of the wrought, AM and bond interface regions in hybrid Inconel 718. The role of the local microstructure in the spatial strain development and crack initiation in the hybrid wrought-AM Inconel 718 is revealed. Most importantly, it is shown that severe inhomogeneous plastic deformation quickly developed in the AM side of the hybrid sample, and this is primarily attributed to few, but very large columnar grains that were preferentially oriented for dislocation slip under external loading. Furthermore, the AM region of the hybrid Inconel 718 had a lower work-hardening rate that caused a higher thinning rate during deformation, promoting strain localisation in the AM side. The degradation in the ductility of AM Inconel 718 is shown to have occurred not because of defects as widely reported for AM components, but rather as a consequence of the interaction between the brittle Laves phase with the localised slip bands which were rapidly intensified by the plastic inhomogeneity. This interaction resulted in early crack initiation in the AM side, primarily leading to the final fracture of the hybrid wrought-AM Inconel 718. The study also demonstrates that the bond interface is the strongest region of the hybrid component under uniaxial loading, and reveals the origins of the strength of the bond interface. The insights revealed advance the understanding of the mechanical performance and the direct failure mechanism in hybrid wrought-AM Inconel 718 components, and opens new p
Zhang W, Guo D, Wang L, et al., 2023, X-ray diffraction measurements and computational prediction of residual stress mitigation scanning strategies in powder bed fusion additive manufacturing, ADDITIVE MANUFACTURING, Vol: 61, ISSN: 2214-8604
Sandmann P, Keller S, Kashaev N, et al., 2022, Influence of laser shock peening on the residual stresses in additively manufactured 316L by Laser Powder Bed Fusion: A combined experimental-numerical study, Additive Manufacturing, Vol: 60, Pages: 1-14, ISSN: 2214-8604
Detrimental subsurface tensile residual stresses occur in laser powder bed fusion (LPBF) due to significant temperature gradients during the process. Besides heat treatments, laser shock peening (LSP) is a promising technology for tailoring residual stress profiles of additively manufactured components. A multi step process simulation is applied aiming at predicting the residual stress state after applying LSP to a cuboid shaped specimen manufactured by LPBF in two different building directions as well as comparing it with a post-build heat treatment. The validity of the numerical simulation is evaluated based on comparisons of residual stresses determined by incremental hole drilling technique within different stages of the multi step process: in the as-build condition, after subsequent heat treatment as well as after applying LSP to the as-build and heat treated specimens, showing overall a good experimental–numerical agreement throughout each of the process stages. Applying a heat treatment to the as-build LPBF sample at 700 °C for 6 h showed not to be effective in eliminating the surface tensile stress entirely, reducing the tensile residual stresses by 40%. However, the application of LSP on LPBF components showed promising results: LSP was able even to convert the detrimental near surface tensile residual stresses in the LPBF component into compressive residual stresses next to the surface, which is known to be beneficial for the fatigue performance.
O'Connor AN, Davies CM, Garwood SJ, 2022, The influence of constraint on fracture toughness: Comparing theoretical T0 shifts in master curve analyses with experimental data, ENGINEERING FRACTURE MECHANICS, Vol: 275, ISSN: 0013-7944
Ab Razak N, Davies CM, 2022, Numerical simulation of creep notched bar of P91 steel, FRATTURA ED INTEGRITA STRUTTURALE-FRACTURE AND STRUCTURAL INTEGRITY, Pages: 261-270
Hales T, Rønneberg T, Hooper PA, et al., 2022, Ductile damage model development and validation of 316l laser powder bed fusion steel under multiaxial stress conditions, ISSN: 0277-027X
Laser powder bed fusion (LPBF) is an additive manufacture technique which builds components up in layers from a powder feedstock, using a scanning laser to selectively melt the powder into the required shape. The process of LPBF can often introduce defects into the structure of a part, since the powder may not fully melt and leave holes, or pores, in the sample. Excessive laser power may also cause the powder to vaporise and create pores. In whatever manner these pores are formed, they can significantly impact the properties of the finished component. Since pores and small defects already exist in LPBF components, the void growth and ductile fracture behaviour of LPBF components under multiaxial stress conditions needs to be characterised and predicted. In this work, notched bar tensile tests have been performed on samples with a range of notch acuities and hence multiaxial stress states. These tests have enabled ductile damage models to be calibrated and finite element (FE) simulations of the notched bar tests performed. The model was validated by comparison to the experimental results. The model agrees well with the results in many cases assessed in this work, but sometimes suffers from discrepancies and premature failure due to variability in material tensile properties, emphasising the need for sensitivity studies.
Liu Y, El Chamaa S, Wenman MR, et al., 2021, Hydrogen concentration and hydrides in Zircaloy-4 during cyclic thermomechanical loading, Acta Materialia, Vol: 221, Pages: 1-16, ISSN: 1359-6454
Hydride formation in Zircaloy-4 under cyclic thermomechanical loading has been investigated using characterized notched beam samples in four-point beam testing, and microstructurally-representative crystal plasticity modelling of the beam tests which incorporates an atomistically-informed equilibrium-state model for hydrogen concentration. The model provided the locations within the microstructure of high hydrogen content, above that required for saturation, hence predicting the anticipated locations of hydride observations in the experiments. The strain rate sensitivity of this alloy over the temperature range considered led to considerable intragranular slip and corresponding stress redistribution, and cyclic strain ratcheting leading to high hydrostatic stresses and in turn hydrogen concentrations, which explains the locations of experimentally observed hydride formation. The interstitial hydrogen interaction energy as well as the intragranular geometrically necessary dislocation density were shown to be important in controlling the spatial distributions of observed hydrides.
Williams RJ, Davies CM, Hooper PA, 2021, In situ monitoring of the layer height in laser powder bed fusion, Material Design & Processing Communications, Vol: 3, Pages: 1-5, ISSN: 2577-6576
In situ process monitoring has frequently been cited as an critical requirement in certifying the performance of laser powder bed fusion (LPBF) components for use in high integrity applications. Despite much development in addressing this need, little attention has been been paid to monitoring the layer thickness during the process. In this paper, a laser displacement sensor has been integrated into the build chamber of an LPBF machine, and the height of the top surface layer of a component has been monitored during a build. This has permitted the deposited layer thickness to be measured throughout the build, and the effect on this of a change in processing conditions is characterised. The thermal contraction of the top layer in between successive laser scans has also been evaluated. This demonstrates the potential of utilising laser displacement sensory as a process monitoring tool in LPBF and provides insightful data for implementation in detailed process models.
Fielden M, Davies CM, 2021, PREDICTION OF RESIDUAL STRESSES AND STRESS INTENSITY FACTORS IN FRACTURE MECHANICS SAMPLES MANUFACTURED BY LASER POWDER BED FUSION, ASME Pressure Vessels and Piping Conference (PVP), Publisher: AMER SOC MECHANICAL ENGINEERS
Calvet T, Wang Y, Pham M-S, et al., 2021, PREDICTION OF J-INTEGRALS AT DEFECTS IN W-9CR STEEL SANDWICH-TYPE COOLING PIPES, ASME Pressure Vessels and Piping Conference (PVP), Publisher: AMER SOC MECHANICAL ENGINEERS
Burridge DJ, Davies CM, 2021, CREEP DAMAGE PREDICTIONS UNDER MULTIAXIAL CONDITIONS FOR 316L STAINLESS STEELSAMPLES MANUFACTURED BY LASER POWDER BED FUSION, ASME Pressure Vessels and Piping Conference (PVP), Publisher: AMER SOC MECHANICAL ENGINEERS
Williams RJ, Al-Lami J, Hooper PA, et al., 2021, Creep deformation and failure properties of 316 L stainless steel manufactured by laser powder bed fusion under multiaxial loading conditions, Additive Manufacturing, Vol: 37, Pages: 1-11, ISSN: 2214-8604
316 L stainless steel has long been used in high temperature applications. As a well-established laser powder bed fusion (LPBF) alloy, there are opportunities to utilise additive manufacturing in such applications. However, the creep behaviour of LPBF 316 L under multiaxial stress conditions must first be quantified before such opportunities are realised. Uniaxial and double notched bar creep tests have been performed and characterised using power-law relations to evaluate the creep strain and rupture properties of LPBF 316 L. The creep response was found to be anisotropic with specimen build orientation, with samples loaded perpendicular to the build direction (Horizontal) exhibiting 8 times faster minimum creep rates than samples built parallel to the build direction (Vertical) and significantly shorter rupture lives. This was mainly attributed to the columnar grain structure, which was aligned with the build direction of the LPBF samples. The multiaxial creep rupture controlling stress was determined and found to be a combination of the equivalent and max. principal stress. X-Ray CT measurements in selected samples illustrated that the samples were approximately 99.6% dense post-build and the quantity of damage post testing was determined. Optical and EBSD microstructural characterisation revealed intergranular creep damage present in the specimens, however rupture was ultimately trans-granular in nature and influenced by the presence and orientation of pre-existing processing defects relative to the sample build and loading direction.
Williams RJ, Vecchiato F, Kelleher J, et al., 2020, Effects of heat treatment on residual stresses in the laser powder bed fusion of 316L stainless steel: Finite element predictions and neutron diffraction measurements, Journal of Manufacturing Processes, Vol: 57, Pages: 641-653, ISSN: 1526-6125
Heat treatments are used in laser powder bed fusion (LPBF) to reduce residual stress and improve service life. In order to qualify components for service, the degree of stress relaxation under heat treatment must be known. In this work, the effect of heat treatment on residual stress (RS) in LPBF 316L stainless steel was studied. Finite element (FE) models were developed to predict the RS distribution in specimens in the as-built state and subjected to heat treatment. The models simulated the thermo-mechanical LPBF build process, sample removal from the build plate and creep stress relaxation effects from a 2 h heat treatment at 700 C. The predictions were validated by neutron diffraction measurements in as-built and heat treated samples, in both build orientations. Large tensile RS of around 450 MPa were predicted at the vertical sample's outer gauge surfaces, balanced by high compressive stresses of similar magnitude at the centre. The residual stresses in the horizontal sample were significantly lower, by around 40%. The influence of sample removal from the base plate on the RS distribution was found to be strongly dependent on the sample orientation and geometry. The heat treatment preserved the unique microstructure of the LPBF process and reduced the peak RS by around 10% in the vertical sample and 40% in the horizontal sample. The FE model predictions were found in good agreement with the experimental measurements, thus providing an effective tool for RS predictions in LPBF components and proving the effectiveness of the heat treatment on RS relaxation.
Moghaddam BT, Hamedany AM, Taylor J, et al., 2020, Structural integrity assessment of floating offshore wind turbine support structures, OCEAN ENGINEERING, Vol: 208, ISSN: 0029-8018
Ibrahim Y, Davies C, Maharaj C, et al., 2020, Post-yield performance of additive manufactured cellular lattice structures, Progress in Additive Manufacturing, Vol: 5, Pages: 211-220, ISSN: 2363-9512
In energy absorption applications, post-yield behaviour is important. Lattice structures, having low relative densities, are an attractive way to obtain effective material properties that differ greatly from that of the parent material. These properties can be controlled through the manipulation of the cellular geometry, a concept that has been made significantly more attainable through the use of additive manufacturing (AM). Lattice structures of various geometries were designed, additively manufactured and tested to assess their structural integrity as well as to investigate the effect of varying the cell geometry on the overall performance of the structures. Uniaxial tensile and compressive tests were carried out on bulk material AM samples made of 316L, followed by tests on the lattice structures. Finite element (FE) analysis was also carried out and the results compared to the experimental data. The FE simulations were able to accurately predict the elastic response of both structures; however, the post-yield behaviour did not closely match the experimental data due to inadequate beam contact resolution in the FE model. The FE model yield stress was also overestimated in the regular lattice due to the presence of manufacturing defects found only in the manufactured test samples. The stochastic structure, both experimentally and in the FE model, displayed a transition in the elastic stiffness from a lower to a higher stiffness in the elastic region. This is due to changing load paths within the lattice from the beams in contact with the compression platens to the rest of the structure. This phenomenon did not occur within the regular structure.
Ronneberg T, Davies C, Hooper P, 2020, Revealing relationships between porosity, microstructure and mechanical properties of laser powder bed fusion 316L stainless steel through heat treatment, Materials and Design, Vol: 189, ISSN: 0264-1275
The understanding of relationships between processing, microstructure and mechanical properties in laser powder bed fusion is currently incomplete. Microstructure-property relations in 316L stainless steel are revealed in this study using isothermal heat treatments as an investigative tool. As-built material was heat treated to selectively remove microstructural features such as melt pool boundaries, microsegregations and the as-built grain structure to evaluate their influence on yield and failure behaviour. Anisotropic yield behaviour was found to be caused by microstructural features alone and not influenced by porosity. However, ductility and failure were dominated by lack of fusion porosity. The alignment of pores between tracks along layer boundaries was found to cause anisotropic ductility. Three strengthening mechanisms in as-built material were identified as grain boundaries, chemical segregation and dislocation density. Heat treatments were categorised into three regimes: recovery, homogenisation and annealing. The findings of this study show that the shape, size, orientation and distribution of pores are crucial parameters for evaluating the structural integrity of parts produced by laser powder bed fusion.
Corcoran J, Davies CM, Cawley P, et al., 2020, A quasi-DC potential drop measurement system for materials testing, IEEE Transactions on Instrumentation and Measurement, Vol: 69, Pages: 1313-1326, ISSN: 0018-9456
Potential drop measurements are well established for use in materials testing and are commonly used for crack growth and strain monitoring. Traditionally, the experimenter has a choice between employing direct current (DC) or alternating current (AC), both of which have strengths and limitations. DC measurements are afflicted by competing spurious DC signals and therefore require large measurement currents (10’s or 100’s of amps) to improve the signal to noise ratio, which in turn leads to significant resistive Joule heating. AC measurements have superior noise performance due to utilisation of phase-sensitive detection and a lower spectral noise density, but are subject to the skin-effect and are therefore not well suited to high-accuracy scientific studies of ferromagnetic materials. In this work a quasi-DC monitoring system is presented which uses very low frequency (0.3-30 Hz) current which combines the positive attributes of both DC and AC while mitigating the negatives. Bespoke equipment has been developed that is capable of low-noise measurements in the demanding quasi-DC regime. A creep crack growth test and fatigue test are used to compare noise performance and measurement power against alternative DCPD equipment. The combination of the quasi-DC methodology and the specially designed electronics yields exceptionally low-noise measurements using typically 100-400 mA; at 400mA the quasi-DC system achieves a 13-fold improvement in signal to noise ratio compared to a 25A DC system. The reduction in measurement current from 25A to 400mA represents a ~3900 fold reduction in measurement power, effectively eliminating resistive heating and enabling much simpler experimental arrangements.
Zheng J-H, Pan R, Lin J, et al., 2020, FE Simulation of the residual stress reduction in industrial-sized T-section component during a newly proposed manufacturing process, 18th International Conference on Metal Forming, Publisher: ELSEVIER SCIENCE BV, Pages: 492-497, ISSN: 2351-9789
Reali L, El Chamaa S, Balint DS, et al., 2020, Deformation and fracture of zirconium hydrides during the plastic straining of Zr-4, MRS ADVANCES, Vol: 5, Pages: 559-567, ISSN: 2059-8521
Jung H-W, Kim Y-J, Takahashi Y, et al., 2020, THE EFFECT OF CYCLIC HARDENING MODEL ON DEFORMATION BEHAVIOR OF CRACKED BODY UNDER CREEP-FATIGUE LOADING CONDITION, ASME Pressure Vessels and Piping Conference (PVP), Publisher: AMER SOC MECHANICAL ENGINEERS
de Andres J, Jones MD, Davies CM, 2020, APPLICATION OF A NOVEL CRACK MOUTH OPENING DISPLACEMENT PARTITIONING TECHNIQUE TO CREEP CRACK GROWTH TESTS ON SEN(T) GEOMETRIES OF TYPE 316H STAINLESS STEEL, ASME Pressure Vessels and Piping Conference (PVP), Publisher: AMER SOC MECHANICAL ENGINEERS
Jones MD, Dean DW, Hughes D, et al., 2020, A novel method for load line displacement rate partitioning in creep crack growth tests on Type 316H stainless steel, Engineering Fracture Mechanics, Vol: 223, Pages: 1-19, ISSN: 0013-7944
Characterising the creep crack growth behaviour of Type 316H stainless steel is vital in obatining accurate predictions for the lifetime of high temperature components, for example in UK advanced gas cooled reactors. The correlation between creep crack growth rates and the fracture mechanics parameter C*, considered to govern the crack growth process, is obtained from creep crack growth tests. The C* parameter is experimentally determined using an expression which requires knowledge of the load line displacement rate due to creep. Historically this has been calculated by subtracting values for the elastic and plastic contributions to the load line displacement, obtained from available solutions, from the total experimentally measured load line displacement. However, the solutions available to determine the plastic contribution rely on generating a power-law fit to uniaxial tensile data, which is difficult to accomplish accurately over a large stress range. In addition, these expressions cannot account for strain history effects during crack growth. Consequently the elastic and plastic contributions are often erroneously large and can even be in excess of the experimental total load line displacement. A novel technique has been proposed to provide improved estimates of the creep contribution to the load line displacement rates during creep crack growth tests. This technique employs finite element analysis that incorporates material specific uniaxial tensile test data to simulate crack growth in an experimental test. A single elastic-plastic-creep simulation is used to determine the separate elastic-plastic and creep contributions to the load line displacement, meaning that, unlike historic analyses, creep stress relaxation and strain history effects can now be accounted for. The results have demonstrated that advanced predictions of the creep contributions to the load line displacement can be obtained using this technique.
Williams R, Ronneberg T, Piglione A, et al., 2019, In-situ thermography for laser powder bed fusion: effects of layer temperature on porosity, microstructure and mechanical properties, Additive Manufacturing, Vol: 30, ISSN: 2214-8604
In laser powder bed fusion(LPBF)the surface layer temperature is continually changing throughout the build process. Variations in part geometry, scanned cross-section and number of parts all inffluence the thermal field within a build. Process parameters do not take these variations into account and this can result in increased porosity and differences in local microstructure and mechanical properties, undermining confidence in the structural integrity of a part. In this paper a wide-field in-situ infra-redimaging system is developed and calibrated to enable measurement of both solid and powder surface temperatures across the full powder bed. The influence of inter-layer cooling time is in-vestigated using a build scenario with cylindrical comp onents of differing heights. In-situ surface temperature data are acquired through out the build process and are compared to results from porosity, microstructure and mechanical property investigations. Changes in surface temperature of u to 200°C are attributed to variation in inter-layer cooling time and this is found to correlate with density and grain structure changes in the part. This work shows that these changes are significant and must be accounted for to improve the consistency and structural integrity of LPBF components.
Sancho A, Cox M, Cartwright T, et al., 2019, An experimental methodology to characterise post-necking behaviour and quantify ductile damage accumulation in isotropic materials, International Journal of Solids and Structures, Vol: 167-177, Pages: 191-206, ISSN: 0020-7683
The development of ductile damage, that occurs beyond the point of necking in a tensile test, can be difficult to quantify. An experimental methodology has been developed to accurately characterise the post-necking deformation response of a material through continuous monitoring of the specimens shape up until rupture. By studying the evolution of the neck geometry, the correct values of the local stress and strain have been determined in samples of grade 304L stainless steel and C110 copper. Notched bar specimens of various notch acuities were examined enabling the effects of stress triaxiality on ductile fracture to be determined. The methodology developed has provided a robust framework for macroscopic measurements of ductile damage during the necking process. To characterise the material degradation process, the elastic modulus reduction method was employed on hourglass-shaped specimens of the same materials. Stiffness degradation was measured using a small gauge extensometer during uninterrupted tensile tests with partial elastic unloadings. A metallographic study was conducted on progressively damaged specimens in order to validate the macroscopic damage measurements. A new non-linear ductile damage accumulation law has been developed and calibrated, which provides an advanced representation of the experimental results, and a significant improvement compared to linear accumulation models frequently employed. This realistic modelling approach considers the degradation of the material when it has undergone severe plastic deformation, and provides a more accurate representation of the near failure behaviour by considering the effects of stress triaxiality. The methodology provides accurate data for damage model development and calibration, to improve the predictions of remnant life from ductile damage in engineering components.
Zheng J-H, Pan R, Wimpory RC, et al., 2019, A novel manufacturing process and validated predictive model for high-strength and low-residual stresses in extra-large 7xxx panels, Materials and Design, Vol: 173, ISSN: 0264-1275
A novel manufacturing process, enabling the production of high quality (i.e. with low and controllable residual stress distributions and good mechanical properties) T-section 7xxx panels, has been established. This process provides a solution to residual stress induced distortion problems, which greatly concerns a range of industries and especially the aircraft industry. This process consists of three sequential steps — water quenching (WQ), cold rolling (CR) and constrained ageing (CA). The effectiveness of this process was experimentally verified, through applying this process to laboratory sized 7050 T-section panels. The RS was measured by neutron diffraction and X-ray techniques, in addition to deflections and hardness at each processing stage. An integrated Finite Element (FE) model, including all three steps, was developed to simulate this manufacturing process and predict both the RS and the final strength distributions. It has been concluded that this novel process can effectively reduce the residual stresses from ±300 MPa to within ±100 MPa and produce T-section panels with required mechanical properties (i.e. hardness: ~159 HV10). A cold rolling level of 1.5% was found most appropriate. The residual stress and yield strength distributions were accurately predicted by FE, providing a valuable prediction tool to process optimization for industrial applications.
Pan R, Pirling T, Zheng J, et al., 2019, Quantification of thermal residual stresses relaxation in AA7xxx aluminium alloy through cold rolling, Journal of Materials Processing Technology, Vol: 264, Pages: 454-468, ISSN: 0924-0136
Residual stresses (RS) are often induced through quenching of aluminum alloys and present a potential risk of developing crack or distortion in subsequent manufacturing processes. Study of methods to minimise the RS in quenched components is of practical importance. In this paper, cold rolling (CR) has been carried out to remove the RS in quenched AA7050 blocks. The CR effect on relaxing RS in quenched AA7050 blocks has been evaluated via the neutron diffraction (ND), X-ray diffraction (XRD) and contour techniques. The results reveal that although CR transforms near-surface residual stresses from large compression to large tension along the rolling direction, it results in remarkable RS relief in the core part of the material. An integrated finite element model for RS evolution through the CR process was put forward and has been validated by the experimental results.
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
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
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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