103 results found
Gaikwad A, Williams RJ, de Winton H, et al., 2022, Multi phenomena melt pool sensor data fusion for enhanced process monitoring of laser powder bed fusion additive manufacturing, Materials and Design, Vol: 221, ISSN: 0264-1275
Finding actionable trends in laser-based metal additive manufacturing process monitoring data is challenging owing to the diversity and complexity of the underlying physical interactions. A single monitoring solution that captures a particular process phenomenon, such as a photodiode that tracks melt pool intensity, is not alone capable of evaluating process stability or detecting flaw formation with sufficient precision for routine application in industry. In this work, to improve flaw detection performance, we adopted a data fusion approach that captures multiple process phenomena. To demonstrate this, we acquired data from laser powder bed fusion (LPBF) builds of cylindrical specimens produced with different laser spot sizes, emulating defocusing due to process faults such as thermal lensing. The resulting specimens had porosity of varying types and severity, quantified by post-build non-destructive X-ray computed tomography, Archimedes density measurements, and destructive metallographic characterization. During the build, the melt pool state was monitored with two coaxial high-speed video cameras and a temperature field imaging system. Physically intuitive low-level melt pool signatures, such as melt pool temperature, shape and size, and spatter intensity were extracted from this high-dimensional, image-based sensor data. These process signatures were subsequently used as input features in relatively simple machine learning models, such as a support vector machine, which were trained to detect laser defocusing, and in addition, predict porosity type and severity. The results show that the data fusion approach significantly enhanced system performance by reducing the overall false positive rate from ∼ 0.1 to ∼ 0.001 without sacrificing the true positive rate (∼0.90). These results were at par with a black-box, deep machine learning approach (convolutional neural network).
Larsen S, Hooper PA, 2022, Deep semi-supervised learning of dynamics for anomaly detection in laser powder bed fusion, Journal of Intelligent Manufacturing, Vol: 33, Pages: 457-471, ISSN: 0956-5515
Highly complex data streams from in-situ additive manufacturing (AM) monitoring systems are becoming increasingly prevalent, yet finding physically actionable patterns remains a key challenge. Recent AM literature utilising machine learning methods tend to make predictions about flaws or porosity without considering the dynamical nature of the process. This leads to increases in false detections as useful information about the signal is lost. This study takes a different approach and investigates learning a physical model of the laser powder bed fusion process dynamics. In addition, deep representation learning enables this to be achieved directly from high speed videos. This representation is combined with a predictive state space model which is learned in a semi-supervised manner, requiring only the optimal laser parameter to be characterised. The model, referred to as FlawNet, was exploited to measure offsets between predicted and observed states resulting in a highly robust metric, known as the dynamic signature. This feature also correlated strongly with a global material quality metric, namely porosity. The model achieved state-of-the-art results with a receiver operating characteristic (ROC) area under curve (AUC) of 0.999 when differentiating between optimal and unstable laser parameters. Furthermore, there was a demonstrated potential to detect changes in ultra-dense, 0.1% porosity, materials with an ROC AUC of 0.944, suggesting an ability to detect anomalous events prior to the onset of significant material degradation. The method has merit for the purposes of detecting out of process distributions, while maintaining data efficiency. Subsequently, the generality of the methodology would suggest the solution is applicable to different laser processing systems and can potentially be adapted to a number of different sensing modalities.
Williams RJ, Davies CM, Hooper PA, 2021, In situ monitoring of the layer height in laser powder bed fusion, Material Design and Processing Communications, Vol: 3
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.
de Winton H, Cegla F, Hooper P, 2021, A method for objectively evaluating the defect detection performance of in-situ monitoring systems, Additive Manufacturing, Vol: 48, Pages: 1-13, ISSN: 2214-8604
In-situ monitoring systems have the potential to assess material quality in additive manufacturing processes on-the-fly, paving the way for accelerated component qualification using defect digital twins. However, current systems vary widely in sensor technology and data analysis methods, leading to a lack of consensus in how the performance of these systems should be measured and compared. This work proposes a methodology and set of metrics, specifically Receiver Operating Characteristic (ROC) and Probability of Detection (POD) curves, to allow objective comparison of performance between any system, regardless of its underlying technology. We demonstrate this approach by comparing the ability to detect increases in part-wide porosity using two of the most common co-axial monitoring techniques in laser powder bed fusion; photodiodes and high-speed cameras. Using ROC curves, we show that melt pool metrics extracted from the camera offer a better trade off between detection and false alarms compared to the photodiodebased system when discriminating between samples at a 0.5% porosity threshold. POD curves were used to characterise detection capability across all porosity levels. It was found that the camera-based system can detect 43% of compromised parts (0.5% porosity), while the photodiode system detects 20%. However, for significantly compromised parts (5% porosity), the camera based method reaches 100%, while the photodiode only achieves 85%. The developed methodology shows that while the camera-based system is measurably superior, further improvement is needed before commercial implementation can be realised. Ultimately, the ROC-POD methodology allows objective assessments of detection performance, enabling quantifiable progress in the development of defect detection systems based on in-situ monitoring.
Lan B, Wang Y, Liu Y, et al., 2021, The influence of microstructural anisotropy on the hot deformation of wire arc additive manufactured (WAAM) Inconel 718, Materials Science and Engineering A: Structural Materials: Properties, Microstructure and Processing, Vol: 823, Pages: 1-14, ISSN: 0921-5093
Hybrid additive manufacturing, incorporating additive manufacturing (AM) and other thermo-mechanical processes, has been developed to improve AM mechanical properties by modifying the as-deposited microstructure and eliminating defects. Additive manufactured parts present strong anisotropic properties, as shown by the anisotropic columnar grain morphology and texture. Samples of AM Inconel 718 were tested at high temperature and under uniaxial compression over a range of conditions. The evolution of microstructural anisotropy and the viscoplastic behaviour under these hot deformation processes was studied. The microstructure and texture evolution were characterised with optical microscopy (OM), scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The results show that the initial anisotropic microstructure had a negligible effect on flow stress and slip system activation during the hot deformation. The shape of original grains did, however, play a predominant role in determining the final microstructure. When the compression direction was perpendicular to the longitudinal of columnar grains, a more uniform microstructure was obtained under high-flow-stress conditions. This preferred compression direction provides guidance for hot deformation in hybrid additive manufacturing practice. Furthermore, for the nickel alloy studied, controlling the deformation direction to achieve a fine grain structure at a lower temperature (950 °C, lower than δ-solvus) brings practical benefits in the form of possible further δ grain refinement and less demanding thermal conditions during subsequent deformation processes.
Yavari R, Williams R, Riensche A, et al., 2021, Thermal modeling in metal additive manufacturing using graph theory ? Application to laser powder bed fusion of a large volume impeller, ADDITIVE MANUFACTURING, Vol: 41, ISSN: 2214-8604
Dong P, Vecchiato F, Yang Z, et al., 2021, The effect of build direction and heat treatment on atmospheric stress corrosion cracking of laser powder bed fusion 316L austenitic stainless steel, ADDITIVE MANUFACTURING, Vol: 40, ISSN: 2214-8604
Hopper C, Pruncu CI, Hooper PA, et al., 2021, The effects of hot forging on the preform additive manufactured 316 stainless steel parts, Micron, Vol: 143, Pages: 103026-103026, ISSN: 0968-4328
Additive Manufacture (AM) offers great potential for creating metallic parts for high end products used in critical application i.e. aerospace and biomedical engineering. General acceptance of AM within these fields has been held back by a lack of confidence in the consistency of the mechanical properties of AMed parts associated by the occurrence of porosity, large columnar grains and texture. In this research, to counters this problem we have combined hot forging and subsequent heat treatment. Although, perhaps not best suited to components featuring fine detail, this technique should be well suited to the manufacture of forged components such as fan blades. Here, AM is able to create a near net-shape blank which is then hot forged to size, eliminating intermediate production stages and generating good mechanical properties in the final component. The material used in the current study is AM 316 L Stainless Steel. By altering the printing parameters of the AM machine, two batches of samples were built, each displaying a different porosity content. This allowed the influence of initial build quality to be illustrated. By comparing the two sample batches, it was possible to gain an insight into the possibilities of controlling porosity and material microstructure. The success of the proposed hot forging and heat treatment technique was validated by mechanical testing (i.e. tensile and hardness experiments) and microstructure evolution characterization (i.e. optical microscopy observation and electron backscatter diffraction (EBSD) techniques). The results revealed that the post processing strategy reduced material porosity and enabled the creation of a more robust microstructure, resulting in improved mechanical properties of the AM material.
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.
Yavari MR, Williams RJ, Cole KD, et al., 2020, Thermal Modeling in Metal Additive Manufacturing Using Graph Theory: Experimental Validation With Laser Powder Bed Fusion Using In Situ Infrared Thermography Data, JOURNAL OF MANUFACTURING SCIENCE AND ENGINEERING-TRANSACTIONS OF THE ASME, Vol: 142, ISSN: 1087-1357
Jin M, Piglione A, Dovgyy B, et al., 2020, Cyclic plasticity and fatigue damage of CrMnFeCoNi high entropy alloy fabricated by laser powder-bed fusion, Additive Manufacturing, Vol: 36, Pages: 1-15, ISSN: 2214-8604
The CrMnFeCoNi high-entropy alloy is highly printable and holds great potential for structural applications. However, no significant discussions on cyclic plasticity and fatigue damage in previous studies. This study provides significant insights into the link between print processes, solidification microstructure, cyclic plasticity and fatigue damage evolution in the alloy fabricated by laser powder bed fusion. Thermodynamics-based predictions (validated by scanning transmission electron microscopy (STEM) energy dispersive X-ray spectroscopy (EDX)) showed that Cr, Co and Fe partition to the core of the solidification cells, whilst Mn and Ni to the cell boundaries in all considered print parameters. Both dislocation slip and deformation twinning were found to be responsible for plastic deformation under monotonic loading. However, the former was found to be the single dominant mechanism for cyclic plasticity. The surface finish helped to substantially delay the crack initiation and cause lack-of-fusion porosity to be the main source of crack initiation. Most significantly, the scan strategies significantly affect grain arrangements and grain dimensions, leading to noticeable effects on fatigue crack propagation; in particular, the highest resistance crack propagation was seen in the meander scan strategy with 0° rotation thanks to the most columnar grains and the smallest spacing of grain boundaries along the crack propagation path.
Vecchiato FL, de Winton H, Hooper PA, et al., 2020, Melt pool microstructure and morphology from single exposures in laser powder bed fusion of 316L stainless steel, Additive Manufacturing, Vol: 36, Pages: 101401-101401, ISSN: 2214-8604
Croteau J-F, Kulyadi EP, Kale C, et al., 2020, Effect of strain rate on tensile mechanical properties of high-purity niobium single crystals for SRF applications, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, Vol: 797, ISSN: 0921-5093
Dovgyy B, Piglione A, Hooper P, et al., 2020, Comprehensive assessment of the printability of CoNiCrFeMn in laser powder bed fusion, Materials and Design, Vol: 194, ISSN: 0264-1275
This study assesses the printability including the consolidation, solidification microstructure, and mechanical properties of the CoCrFeMnNi high entropy alloy fabricated by laser powder bed fusion. A range of print parameters was used for a comprehensive assessment of printability, providing a basis to establish the relationship between process, microstructure, and mechanical properties. The study demonstrates a high relative density of the alloy fabricated with energy density in the range 62.7-109.8 J/mm3. It is shown that the scan strategy plays an important role in consolidation. For the same energy density, the rotation of 67° between two consecutive layers tends to yield higher consolidation than other considered strategies. Moreover, the scan strategy is found to be most influential in microstructure development. The scan strategy rotation angle controls the extent to which epitaxial growth can occur, and hence the crystallographic texture and the grain morphology. Amongst four considered strategies, the 0°- and 90°-rotation meander led to the strongest preferred texture while the 67°-rotation resulted in weaker texture. The 67°-rotation strategies led to broadened grains with lower aspect ratios. The understanding of texture and grain size provides explanations to the observed mechanical properties (such as flow stress and plastic anisotropy) of the alloy.
Pruncu CI, Hopper C, Hooper PA, et al., 2020, Study of the effects of hot forging on the additively manufactured stainless steel preforms, Journal of Manufacturing Processes, Vol: 57, Pages: 668-676, ISSN: 1526-6125
The production of wrought stainless steel components in power generators can involve a combination of many manufacturing processes. These are expensive in tooling costs and number of operations, as in the Hot Forging (HF) of stainless steel turbine blades. Additive Manufacturing (AM) techniques provide a valuable opportunity to produce near-net-shaped preforms, thus avoiding the material wastage and high tooling costs associated with the intermediate stages of HF processes. This study focuses on the proposed hybrid AM and HF method, in which AM is used to produce near-net–shape preforms which are subsequently formed into net-shaped parts by HF. The HF process is used to significantly reduce the material defects introduced by, and intrinsic to, AM processes. To understand the mechanical and microstructure changes during various AM and HF conditions, single-phase 316 L stainless steel was used as the test material. Samples were produced by AM using a laser powder-bed fusion AM machine. Two different AM build directions were used to produce samples, so, as to allow evaluation of the anisotropic properties induced by AM. These samples subsequently underwent a HF process, in which various processing conditions of plastic strain and forging temperature were applied, to study the general effects of thermal plasticity on the AM microstructure. Tensile testing, optical microscopy (OM), scanning electron microscope (SEM) together with electron backscatter diffraction (EBSD) techniques were used to characterise the evolution of mechanical properties, porosity and grain size. The HF technique was found to remove defects from the AM material, resulting in enhanced mechanical strength, ductility, and isotropy. The technique therefore offers a potential alternative to conventional forging while retaining the required level of material performance.
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.
Quinn R, Zhang LH, Cox MJ, et al., 2020, Development and validation of a Hopkinson bar for hazardous materials, Experimental Mechanics, Vol: 60, Pages: 1275-1288, ISSN: 0014-4851
Background: There are a variety of approaches that can be employed for Hopkinson bar compression testing and there is no standard procedure. Objectives: A Split-Hopkinson pressure bar (SHPB) testing technique is presented which has been specifically developed for the characterisation of hazardous materials such as radioactive metals. This new SHPB technique is validated and a comparison is made with results obtained at another laboratory. Methods: Compression SHPB tests are performed on identical copper specimens using the new SHPB procedures at Imperial College London and confirmatory measurements are performed using the well-established configuration at the University of Oxford. The experiments are performed at a temperature of 20 °C and 200 °C. Imperial heat the specimens externally before being inserted into the test position (ex-situ heating) and Oxford heat the specimens whilst in contact with the pressure bars (in-situ heating). For the ex-situ case, specimen temperature homogeneity is investigated both experimentally and by simulation. Results: Stress-strain curves were generally consistent at both laboratories but sometimes discrepancies fell outside of the inherent measurement uncertainty range of the equipment, with differences mainly attributed to friction, loading pulse shapes and pulse alignment techniques. Small metallic specimens are found to be thermally homogenous even during contact with the pressure bars. Conclusion: A newly developed Hopkinson bar forhazardous materials is shown to be effective for characterising metals under both ambient and elevated temperature conditions.
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.
Hooper P, Li N, JIANG JUN, et al., 2020, Method of creating a component using additive manufacturing, US20200055121A1
Images (4) Classifications B22F3/24 After-treatment of workpieces or articlesView 19 more classificationsUS20200055121A1
Pham M-S, Dovgyy B, Hooper P, et al., 2020, The role of side-branching in microstructure development in laser powder-bed fusion, Nature Communications, Vol: 11, ISSN: 2041-1723
In-depth understanding of microstructure development is required to fabricate high quality products by additive manufacturing (i.e. 3D printing). Here we report the governing role of side-branching in the microstructure development of alloys by laser powder bed fusion. We show that perturbations on the sides of cells (or dendrites) facilitate crystals to change growth direction by side-branching along orthogonal directions in response to changes in local heat flux. While the continuous epitaxial growth is responsible for slender columnar grains confined to the centreline of melt pools, side-branching frequently happening on the sides of melt pools enables crystals to follow drastic changes in thermal gradient across adjacent melt pools, resulting in substantial broadening of grains. The variation of scan pattern can interrupt the vertical columnar microstructure, but promotes both in-layer and out-of-layer side-branching, in particular resulting in the helical growth of microstructure in a chessboard strategy with 67 rotation between layers.
Rolfe E, Arora H, Hooper PA, et al., 2020, Comparative assessment of hybrid composite sandwich panels under blast loading
© CCM 2020 - 18th European Conference on Composite Materials. All rights reserved. The use of composite sandwich structures across multiple disciplines, including the naval sector, is ever increasing. A combination of high specific strength, corrosion resistance and low radar signature make composite sandwich structures an attractive material choice. However, the brittle behaviour of the composite skins results in overdesign of composite sandwich components, counteracting their weight saving benefits. Since naval vessels must withstand a range of loads including blast loading, representative materials need to be tested under real blast conditions in order to avoid unnecessary safety factors and overdesign. The research detailed here is concerned with full-scale air blast testing of two composite sandwich panels with different glass-fibre/carbon-fibre hybrid face-sheets. The panels were subjected to a 100 kg nitromethane charge at 15 m stand-off distance. High speed 3D digital image correlation was used to record the displacement of the rear skins of the sandwich panels during the blast event. Strain gauges were adhered to the front skins of the panels to enable comparison between the front and rear skins at certain locations. Overall the two sandwich panels demonstrated similar deflection and strain. Under blast loading the presence of both types of fibres is the key factor not the position of each fibre fabric layer.
Yavari R, Williams R, Cole K, et al., 2020, THERMAL MODELING IN METAL ADDITIVE MANUFACTURING USING GRAPH THEORY: EXPERIMENTAL VALIDATION WITH IN-SITU INFRARED THERMOGRAPHY DATA FROM LASER POWDER BED FUSION, 15th ASME International Manufacturing Science and Engineering Conference (MSEC), Publisher: AMER SOC MECHANICAL ENGINEERS
Davies CM, Sandmann P, Ronneberg T, et al., 2020, Residual stress measurements in a 316L uniaxial samples manufactured by laser powder bed fusion, ISSN: 0277-027X
Uniaxial samples have been manufactured for tension/compression testing from 316L stainless steel by laser powder bed fusion (LPBF). Samples manufactured by LPBF are known to contain high levels of residual stresses. These uniaxial samples were built from a solid cylindrical rod and subsequently machined to reduce the central cross section of the sample to the required gauge diameter and improve the surface finish. Finite element (FE) models have been developed to simulate the LPBF process of the rods, their removal from the build plate and subsequent machining into the tension/compression samples. High tensile residual stresses were predicted at the surface of the samples, balances by similar magnitude compressive stresses along their axis. Post machining however, these stresses were reduced by around 80% or more. Residual stress measurements were performed on the samples post machining using the neutron diffraction techniques. These measurements confirmed that negligible residual stresses remained in the samples post removal from the build plate and machining.
Rolfe E, Arora H, Hooper PA, et al., 2020, Comparative assessment of hybrid composite sandwich panels under blast loading
The use of composite sandwich structures across multiple disciplines, including the naval sector, is ever increasing. A combination of high specific strength, corrosion resistance and low radar signature make composite sandwich structures an attractive material choice. However, the brittle behaviour of the composite skins results in overdesign of composite sandwich components, counteracting their weight saving benefits. Since naval vessels must withstand a range of loads including blast loading, representative materials need to be tested under real blast conditions in order to avoid unnecessary safety factors and overdesign. The research detailed here is concerned with full-scale air blast testing of two composite sandwich panels with different glass-fibre/carbon-fibre hybrid face-sheets. The panels were subjected to a 100 kg nitromethane charge at 15 m stand-off distance. High speed 3D digital image correlation was used to record the displacement of the rear skins of the sandwich panels during the blast event. Strain gauges were adhered to the front skins of the panels to enable comparison between the front and rear skins at certain locations. Overall the two sandwich panels demonstrated similar deflection and strain. Under blast loading the presence of both types of fibres is the key factor not the position of each fibre fabric layer.
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.
Samieian M, Cormie D, Smith D, et al., 2019, On the bonding between glass and PVB in laminated glass, Engineering Fracture Mechanics, Vol: 214, Pages: 504-519, ISSN: 0013-7944
In blast protective design, laminated glass is used to facilitate the safety of building occupants. Laminated glass provides its safety through the maintenance of the bond between the glass and the interlayer, and also through the deformation of the interlayer. The amount of deformation is related to the stretching of the interlayer, which is related to the amount of adhesion between the glass and the interlayer. An experimental and modelling study has taken place on the bond between the glass and the interlayer at different testing rates and temperatures. Tensile tests on cracked laminated glass and pure PVB were carried out. These tests were coupled with fracture mechanics methods to calculate a bond fracture toughness. This bond fracture toughness was used to develop a finite element model to predict the separation between the glass and the interlayer. From the experimental studies it was found that the adhesion between the glass and the interlayer is temperature independent in the range of 20oC-60oC at a constant testing rate. In contrast, at a constant temperature the adhesion was found to be loading rate dependent. The finite element model developed showed good consistency with experimental data for a range of testing rates and temperatures.
Laminated glass is often used in structures for protection against blast loads. The single-degree-of-freedom model has conventionally been used to design such structures and it continues to be widely in use today. The single-degree-of-freedom model includes mass and load transformation factors, which depend on the deflected shape of the structure. In this study, finite element models are used to derive the deflected shapes and transformation factors. The time-varying deflected shape history is taken into account in this analysis, as this is currently not included in other single-degree-of-freedom models. The analysis was conducted on a range of boundary conditions and aspect ratios along with different loading rates. For low-rate loading, the transformation factors were found not to vary during the deflected shape–time history. For high rate loading, however, the transformation factors were found to vary during the deflected shape–time history therefore requiring their inclusion in the single-degree-of-freedom design methods. These transformation factors were found to be insensitive to aspect ratios.
Whiteman G, Case S, Millett JCF, et al., 2019, Uniaxial compression of single crystal and polycrystalline tantalum, Materials Science and Engineering: A, Vol: 759, Pages: 70-77, ISSN: 0921-5093
A series of compression experiments characterising the elastic-plastic response of single crystal and polycrystalline tantalum from quasi-static to intermediate strain-rates (10−3 – 103 s−1) over a range of temperatures (233–438 K) are reported in this paper. The single crystal experiments show significant differences in the response of the three principle crystal orientations of tantalum in terms of yield, work hardening and ultimate deformed shapes. Modelling is undertaken using a dislocation mechanics based crystal plasticity finite element model giving insight into the underlying microscopic processes that govern the macroscopic response. The simulations show the importance of the dislocation mobility relations and laws governing the evolution of the mobile dislocation density for capturing the correct behaviours. The inclusion of the twinning/anti-twinning asymmetry is found to influence  orientation most strongly, and is shown to be critical for matching the relative yield strengths. In general the simulations are able to adequately match experimental trends although some specific details such as exact strain hardening evolution are not reproduced suggesting a more complex hardening model is required. 3D finite element simulations approximating the tests are also undertaken and are able to predict the final deformed sample shapes well once the twinning/anti-twinning asymmetry is included (particularly for the  orientation). The polycrystalline data in both as-received and cold rolled conditions shows the initial yield strength is highest and work hardening rate is lowest for the cold-rolled material due to the increase in mobile dislocation density caused by the prior work. The general behavioural trends with temperature and strain-rate of the polycrystalline materials are reproduced in the single crystal data however the specific form of stress versus strain curves are significantly different. This is discussed in terms of th
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