Publications
114 results found
Pham MS, Dovgyy B, Hooper PA, 2017, Twinning induced plasticity in austenitic stainless steel 316L made by additive manufacturing, Materials Science and Engineering A: Structural Materials: Properties, Microstructure and Processing, Vol: 704, Pages: 102-111, ISSN: 0921-5093
Additively manufactured (AM) 316L steel exhibits extraordinary high yield strength, and surprisingly good ductility despite the high level of porosity in the material. This detailed study sheds light on the origins of the observed high yield strength and good ductility. The extremely fine cells which are formed because of rapid cooling and dense dislocations are responsible for the macroscopically high yield strength of the AM 316L (almost double of that seen in annealed 316L steel). Most interestingly, twinning is dominant in deformed samples of the AM316. It is believed that twinning-induced plasticity (TWIP) behaviour to be responsible for the excellent ductility of the steel despite the high level of porosity. The dominant twinning activity is attributed to Nitrogen gas used in 3D printing. Nitrogen can lower the stacking fault energy of the steel, leading to the disassociation of dislocations, promoting the deformation twinning. Twinning induces large plasticity during deformation that can compensate the negative effect of porosity in AM steel. However, twinning does not induce significant hardening because (1) the porosity causes a negative effect on hardening and (2) twinning spacing is still larger than extremely fine solidification cells.
Rolfe E, Kaboglu C, Kelly M, et al., 2017, Evaluating the blast performance of sandwich structures with novel carbon and glass construction, 21 st International Conference on Composite Materials
© 2017 International Committee on Composite Materials. All rights reserved. The brittle nature of composite materials results in the overdesign of composite components which counteracts their benefits. Naval vessels undergo a spectrum of loading from static to dynamic due to wave slamming, impact and blast loads. Experiments investigating the performance of composite laminates and composite sandwich panels under static and dynamic conditions are the focus of the research presented in this paper. By hybridizing composites, researchers seek to improve their ductility. Interlaminar glass-/carbon-fiber hybrids are investigated in this study as they can be manufactured in large volumes at a reasonable cost. Three interlaminar hybrids along with a glass-fiber and a carbon-fiber laminate were manufactured and tested under quasi-static tension and flexure. In flexure, one hybrid had greater strain to failure than the purely glass-fiber laminate and another dissipated more energy than the glass-fiber laminate. All hybrids demonstrated greater strain to failure than the carbon-fiber laminate and greater flexural strength than either constituent materials. In tension, the elastic moduli and the tensile strength of the hybrids lay between that of the glass- and carbon-fiber laminates. Once again all hybrids had greater strain to failure in tension than the carbon-fiber laminates, however, none exceeded the glass-fiber laminates. Composite sandwich panels with glass-fiber, carbon-fiber and hybrid laminate skins were subjected to impact testing using a gas gun and aluminium projectile. The carbon-fiber skins demonstrated brittle failure. The addition of glass-fiber layers into carbon-fiber skins improves their impact resilience and the stacking sequence of the fibers has an effect on panel deflection and rear skin strain.
Samieian MA, Cormie D, Smith D, et al., 2017, Delamination in blast resistant laminated glass, 21st International Conference on Composite Materials
© 2017 International Committee on Composite Materials. All rights reserved. Laminated glass is used in structures for protection against blast loads. Laminated glass facilitates its safety mechanism through delamination of the glass from the interlayer. However, the amount of delamination is important. An experimental study has taken place to quantify the delamination energy in laminated glass composites. Through-cracked tensile tests on laminated glass and also tensile tests on the polymer interlayer alone have been carried out. The results were used to calculate an adhesive fracture energy. The tests were carried out at a temperature range of 20-60°C for three different interlayer thicknesses. The adhesive fracture energy was found to be independent of temperature and interlayer thickness for the range of temperatures tested.
Ali AM, Newman S, Hooper P, et al., 2017, The effect of implant position on bone strain following lateral unicompartmental knee arthroplasty. A biomechanical model using digital image correlation, Bone and Joint Research, Vol: 6, Pages: 522-529, ISSN: 2046-3758
ObjectivesUnicompartmental knee arthroplasty (UKA) is a demanding procedure, with tibial component subsidence or pain from high tibial strain being potential causes of revision. The optimal position in terms of load transfer has not been documented for lateral UKA. Our aim was to determine the effect of tibial component position on proximal tibial strain.MethodsA total of 16 composite tibias were implanted with an Oxford Domed Lateral Partial Knee implant using cutting guides to define tibial slope and resection depth. Four implant positions were assessed: standard (5° posterior slope); 10° posterior slope; 5° reverse tibial slope; and 4 mm increased tibial resection. Using an electrodynamic axial-torsional materials testing machine (Instron 5565), a compressive load of 1.5 kN was applied at 60 N/s on a meniscal bearing via a matching femoral component. Tibial strain beneath the implant was measured using a calibrated Digital Image Correlation system.ResultsA 5° increase in tibial component posterior slope resulted in a 53% increase in mean major principal strain in the posterior tibial zone adjacent to the implant (p = 0.003). The highest strains for all implant positions were recorded in the anterior cortex 2 cm to 3 cm distal to the implant. Posteriorly, strain tended to decrease with increasing distance from the implant. Lateral cortical strain showed no significant relationship with implant position.ConclusionRelatively small changes in implant position and orientation may significantly affect tibial cortical strain. Avoidance of excessive posterior tibial slope may be advisable during lateral UKA.
Rolfe E, Kelly M, Arora H, et al., 2017, Failure analysis using X-ray computed tomography of composite sandwich panels subjected to full-scale blast loading, Composites Part B: Engineering, Vol: 129, Pages: 26-40, ISSN: 1359-8368
The tailorable mechanical properties and high strength-to-weight ratios of composite sandwich panels make them of interest to the commercial marine and naval sector, however, further investigation into their blast resilience is required. The experiments performed in this study aimed to identify whether alterations to the composite skins or core of a sandwich panel can yield improved blast resilience both in air and underwater. Underwater blast loads using 1.28 kg TNT equivalent charge at a stand-off distance of 1 m were performed on four different composite sandwich panels. Results revealed that implementing a stepwise graded density foam core, with increasing density away from the blast, reduces the deflection of the panel and damage sustained. Furthermore, the skin material affects the extent of panel deflection and damage, the lower strain to failure of carbon-fibre reinforced polymer (CFRP) skins reduces deflection but increases skin debonding. A further two panels were subjected to a 100 kg TNT air blast loading at a 15 m stand-off to compare the effect of a graded density core and the results support the underwater blast results. Future modelling of these experiments will aid the design process and should aim to include material damage mechanisms to identify the most suitable skins.
Rolfe E, Kelly M, Arora H, et al., 2017, Full-scale blast testing of composite sandwich structures with novel skin and core constructions, 14th International Conference on Fracture, Pages: 167-168
© 2017 Chinese Society of Theoretical and Applied Mechanics. All Rights Reserved. A range of composite sandwich panels with different polymeric foam cores and face-sheets were subjected to full-scale air and underwater blast testing. The panel with a styrene acrylonitrile (SAN) foam core had the lowest deflection and suffered from the least damage. By implementing a stepwise graded density SAN foam core, a smoother deflection profile during air blast can be achieved. Underwater blast testing revealed that implementing a stepwise graded density core reduces the panel deflection. Additionally, there is a trade-off between reduced panel deflection and damage depending on the skin material selected.
Dear JP, Rolfe E, Kelly M, et al., 2017, Blast performance of composite sandwich structures, 11th International Symposium on Plasticity and Impact Mechanics (IMPLAST), Publisher: Elsevier, Pages: 471-478, ISSN: 1877-7058
A range of composite sandwich panels with different polymeric foam cores and face-sheets were subjected to full-scale air and underwater blast testing. The air blast panels had glass fiber reinforced polymer (GFRP) face-sheets with three different polymeric foam cores: styrene acrylonitrile (SAN), polyvinylchloride (PVC) and polymethacrylimide (PMI). The panels were subjected to 100 kg TNT equivalent charge from a stand-off of 15 m. The SAN panel had the lowest deflection and suffered from the least damage. The underwater blast panels had either a single density or graded density SAN foam core and either glass fiber reinforced polymer or carbon fiber reinforced polymer (CFRP) face-sheets. The research revealed that there is a trade-off between reduced panel deflection and damage. All the blast research that has been performed is part of a program sponsored by the Office of Naval Research (ONR).
Sancho A, Hooper PA, Davies CM, 2017, DUCTILE DAMAGE ASSESSMENT USING CONTINUUM DAMAGE MECHANICS AND METHODOLOGY FOR HIGH STRAIN-RATE DAMAGE ANALYSIS, ASME Pressure Vessels and Piping Conference, Publisher: AMER SOC MECHANICAL ENGINEERS
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Davies CM, Thomlinson H, Hooper PA, 2017, FATIGUE CRACK INITIATION AND GROWTH BEHAVIOUR OF 316L STAINLESS STEEL MANUFACTURED THROUGH SELECTIVE LASER MELTING, ASME Pressure Vessels and Piping Conference, Publisher: AMER SOC MECHANICAL ENGINEERS
Davies CM, Garg P, Hooper PA, 2017, DEFORMATION AND FRACTURE BEHAVIOUR OF 316L STAINLESS STEEL MANUFACTURED THROUGH SELECTIVE LASER MELTING, ASME Pressure Vessels and Piping Conference, Publisher: AMER SOC MECHANICAL ENGINEERS
Rolfe E, Kelly M, Arora H, et al., 2016, X-ray CT analysis after blast of composite sandwich panels, Procedia Engineering, Vol: 167, Pages: 176-181, ISSN: 1877-7058
Four composite sandwich panels with either single density or graded density foam cores and different face-sheet materials were subjected to full-scale underwater blast testing. The panels were subjected to 1kg PE4 charge at a stand-off distance of 1 m. The panel with graded density core and carbon fiber face-sheets had the lowest deflection. Post-blast damage assessment was carried out using X-ray CT scanning. The damage assessment revealed that there is a trade-off between reduced panel deflection and panel damage. This research has been performed as part of a program sponsored by the Office of Naval Research (ONR).
Sancho A, Cox MJ, Cartwright T, et al., 2016, Experimental techniques for ductile damage characterisation, Procedia Structural Integrity, Vol: 2, Pages: 966-973, ISSN: 2452-3216
Ductile damage in metallic materials is caused by the nucleation, growth and coalesce of voids and micro-cracks in the metal matrix when it is subjected to plastic strain. A considerable number of models have been proposed to represent ductile failure focusing on the ultimate failure conditions; however, only some of them study in detail the whole damage accumulation process. The aim of this work is to review experimental techniques developed by various authors to measure the accumulation of ductile damage under tensile loads. The measurement methods reviewed include: stiffness degradation, indentation, microstructure analysis, ultrasonic waves propagation, X-ray tomography and electrical potential drop. Stiffness degradation and indentation techniques have been tested on stainless steel 304L hourglass-shaped samples. A special interest is placed in the Continuum Damage Mechanics approach (CDM) as its equations incorporate macroscopic parameters that can represent directly the damage accumulation measured in the experiments. The other main objective lies in identifying the strengths and weaknesses of each technique for the assessment of materials subjected to different strain-rate and temperature conditions.
Del Linz P, Wang Y, Hooper PA, et al., 2016, Determining Material Response for Polyvinyl Butyral (PVB) in Blast Loading Situations, Experimental Mechanics, Vol: 56, Pages: 1501-1517, ISSN: 1741-2765
Protecting structures from the effect of blast loads requires the careful design of all building components. In this context, the mechanical properties of Polyvinyl Butyral (PVB) are of interest to designers as the membrane behaviour will affect the performance of laminated glass glazing when loaded by explosion pressure waves. This polymer behaves in a complex manner and is difficult to model over the wide range of strain rates relevant to blast analysis. In this study, data from experimental tests conducted at strain rates from 0.01 s−1 to 400 s−1 were used to develop material models accounting for the rate dependency of the material. Firstly, two models were derived assuming Prony series formulations. A reduced polynomial spring and a spring derived from the model proposed by Hoo Fatt and Ouyang were used. Two fits were produced for each of these models, one for low rate cases, up to 8 s−1, and one for high rate cases, from 20 s−1. Afterwards, a single model representing all rates was produced using a finite deformation viscoelastic model. This assumed two hyperelastic springs in parallel, one of which was in series with a non-linear damper. The results were compared with the experimental results, assessing the quality of the fits in the strain range of interest for blast loading situations. This should provide designers with the information to choose between the available models depending on their design needs.
Samieian MA, Cormie D, Doebbel F, et al., 2016, Experimental investigation into the strength of single sided silicone glazing joints under blast loading, Challenging Glass Conference 5, Publisher: TU Delft, ISSN: 2589-8019
An experimental study has taken place to quantify the strength of single sided structural silicone glazing joints under blast loading. The structural silicone specimens in this experiment were tested using a high-speed servo-hydraulic test machine at varying rates, representative of that experienced in a blast. Tests were conducted at displacement rates of 1m/s, 2m/s and 4m/s. The load was applied at two different angles of 30° and 45°. The tests were carried out on samples with different bite depths. The load was measured and the strength of the silicone joint was calculated at different testing rates. For a given testing rate and loading angle, the strength was found to be constant for different bite depths. The strength also showed an escalation at higher displacement rates. For the loading angles tested, there was no correlation found between the angle of loading and the strength. Through the measurement of displacement during the test, the work done on the silicone joint was also calculated.
Del Linz P, Hooper PA, Arora H, et al., 2016, Delamination properties of laminated glass windows subject to blast loading, International Journal of Impact Engineering, Vol: 105, Pages: 39-53, ISSN: 1879-3509
Delamination processes absorb significant amounts of energy in laminated glass windows when they are subjected to blast loads. Blast tests were performed previously and their results had been used to calculate the loads imposed on the support systems. In this research, the delamination process at realistic deformation rates was studied to understand the reaction force response obtained. Laboratory tensile tests were performed on pre-cracked laminated glass specimens to investigate their delamination behaviour. The experiments confirmed the presence of a plateau in the force-deflection graphs, suggesting that the delamination process absorbed significant energy. The experimental results were then employed to calibrate FEA models of the delamination process with the aim of estimating the delamination energy of the polyvinyl butyral (PVB) membrane and glass layers and its relationship with deformation speed. The delamination energies obtained through this research, if used with the appropriate PVB material model, are a valuable new tool new tool in the modelling and design of laminated glass façade structures.
Corcoran J, Hooper P, Davies C, et al., 2016, Creep strain measurement using a potential drop technique, International Journal of Mechanical Sciences, Vol: 110, Pages: 190-200, ISSN: 0020-7403
This paper will demonstrate the use of a potential drop sensor to monitor strain. In particular, the suitability of the technique to high temperature or harsh environment applications presents an opportunity for monitoring strain in components operating under creep conditions. Monitoring creep damage in power station components is a long standing technological challenge to the non-destructive evaluation community. It is well established in the literature that strain rate serves as an excellent indicator of the progress of creep damage and can be used for remnant life calculations. To facilitate the use of such strain rate based evaluation methods, a permanently installed, strain sensitive, potential drop technique has been developed. The technique has very simple and robust hardware lending itself to use at high temperatures or in harsh environments. Strain inversions are presented and demonstrated experimentally; a room temperature, plastic deformation experiment is used for validation and additionally an accelerated creep test demonstrates operation at high temperature (600 °C+). Excellent agreement is shown between potential drop inverted strain and control measurements.
Del Linz P, Liang X, Hooper PA, et al., 2016, An analytical solution for pre-crack behaviour of laminated glass under blast loading, Composite Structures, Vol: 144, Pages: 156-164, ISSN: 0263-8223
Laminated glazing is often employed to minimise damage and injuries during blast events. In this work, the von Karman theory for large deflections of plates was used to simulate the effect of large explosions on laminated glazing. Linear material properties were assumed for both the glass and Polyvinyl Butyral layers. The glass and PVB layers were assumed to act fully compositely during the pre-crack phase of the deformation. A higher order deflection function was employed to represent the complex deformed shape observed in DIC blast test data collected by Hooper et al. [1]. The deflection results showed that the method developed could produce accurate estimates of the glazing deformation history during a blast event. The analytical solution was also used to compute the reaction forces acting on the window supports, which were found to be of a similar magnitude to those calculated from experimental data. In addition, crack densities were predicted, which were found to follow a pattern similar to those seen in blast experiments. The analytical approach developed is valuable for risk assessment engineers and façade designers who much prefer analytically based models over full-scale FE analysis, as FEA is often too time consuming for design assessments.
Del Linz P, Hooper PA, Arora H, et al., 2015, Reaction forces of laminated glass windows subject to blast loads, Composite Structures, Vol: 131, Pages: 193-206, ISSN: 1879-1085
Several blast trials on laminated glass windows have been performed in the past, using both full field 3D Digital Image Correlation and strain gauges located on the supporting structure to collect information on the glass pane behaviour. The data obtained during three blast experiments were employed to calculate reaction forces throughout the perimeter supports both before and after the fracture of the glass layers. The pre-crack experimental data were combined with finite element modelling results to achieve this, whilst solely experimental results were employed for post-cracked reactions. The results for the three blast experiments were compared to identify similarities in their behaviour. It is intended that the results can be used to improve the existing spring–mass systems used for the design of blast resistant windows.
Kelly M, Arora H, Worley A, et al., 2015, Sandwich panel cores for blast applications: materials and graded density, Experimental Mechanics, Vol: 56, ISSN: 1741-2765
Sandwich composites are of interest in marine applications dueto their high strength-to-weight ratio and tailorable mechanical properties, but their resistance to air blast loading is not well understood. Full-scale 100 kg TNT equivalent air blast testing at a 15 m stand-off distance wasperformed on glass-fibre reinforced polymer (GFRP) sandwich panels withpolyvinyl chloride (PVC); polymethacrylimid (PMI); and styrene acrylonitrile(SAN) foam cores, all possessing the same thickness and density. Further testingwas performed to assess the blast resistance of a sandwich panel containinga stepwise graded density SAN foam core, increasing in density away from theblast facing side. Finally a sandwich panel containing compliant polypropylene(PP) fibres within the GFRP front face-sheet, was subjected to blast loadingwith the intention of preventing front face-sheet cracking during blast. Measurementsof the sandwich panel responses were made using high-speed digital image correlation (DIC), and post-blast damage was assessed by sectioning thesandwich panels and mapping the damage observed. It was concluded that allcores are effective in improving blast tolerance and that the SAN core wasthe most blast tolerant out of the three foam polymer types, with the DIC resultsshowing a lower deflection measured during blast, and post-blast visualinspections showing less damage suffered. By grading the density of the core itwas found that through thickness crack propagation was mitigated, as well asdamage in the higher density foam layers, thus resulting in a smoother backface-sheet deflection profile. By incorporating compliant PP fibres into thefront face-sheet, cracking was prevented in the GFRP, despite damage beingpresent in the core and the interfaces between the core and face-sheets.
Kelly M, Arora H, Worley A, et al., 2015, BLAST PERFORMANCE OF COMPOSITE SANDWICH STRUCTURES, 20th International Conference on Composite Materials (ICCM), Publisher: AALBORG UNIV PRESS
Arora H, Kelly M, Worley A, et al., 2014, Compressive strength after blast of sandwich composite materials, Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol: 372, Pages: 1-27, ISSN: 1364-503X
Composite sandwich materials have yet to be widely adopted in the construction of naval vessels despite their excellent strength-to-weight ratio and low radar return. One barrier to their wider use is our limited understanding of their performance when subjected to air blast. This paper focuses on this problem and specifically the strength remaining after damage caused during an explosion. Carbon-fibre-reinforced polymer (CFRP) composite skins on a styrene-acrylonitrile (SAN) polymer closed-cell foam core are the primary composite system evaluated. Glass-fibre-reinforced polymer (GFRP) composite skins were also included for comparison in a comparable sandwich configuration. Full-scale blast experiments were conducted, where 1.6×1.3 m sized panels were subjected to blast of a Hopkinson-Cranz scaled distance of 3.02 m kg(-1/3), 100 kg TNT equivalent at a stand-off distance of 14 m. This explosive blast represents a surface blast threat, where the shockwave propagates in air towards the naval vessel. Hopkinson was the first to investigate the characteristics of this explosive air-blast pulse (Hopkinson 1948 Proc. R. Soc. Lond. A 89, 411-413 (doi:10.1098/rspa.1914.0008)). Further analysis is provided on the performance of the CFRP sandwich panel relative to the GFRP sandwich panel when subjected to blast loading through use of high-speed speckle strain mapping. After the blast events, the residual compressive load-bearing capacity is investigated experimentally, using appropriate loading conditions that an in-service vessel may have to sustain. Residual strength testing is well established for post-impact ballistic assessment, but there has been less research performed on the residual strength of sandwich composites after blast.
Hunt G, Mitzalis F, Alhinai T, et al., 2014, 3D Printing with Flying Robots, IEEE International Conference on Robotics and Automation (ICRA), Publisher: IEEE, Pages: 4493-4499, ISSN: 1050-4729
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Yang H, Davies CM, Hooper P, et al., 2014, A NOVEL IMAGE PROCESSING METHOD FOR ARCMAC POINT TO POINT OPTICAL STRAIN MEASUREMENT, ASME Pressure Vessels and Piping Conference (PVP-2013), Publisher: AMER SOC MECHANICAL ENGINEERS
Del Linz P, Giversen S, Hooper PA, et al., 2013, Delamination of laminated glass during blast loading events, Pages: 6651-6661
Arora H, Hooper PA, Del Linz P, et al., 2012, Modelling the behaviour of composite sandwich structures when subject to air blast loading, International Journal of Multiphysics, Vol: 6, Pages: 199-218, ISSN: 1750-9548
Dear JP, Hooper PA, Arora H, 2012, Lightweight materials to their limit, Chengdu, China
Hooper PA, Blackman BRK, Dear JP, 2012, The mechanical behaviour of poly(vinyl butyral) at different strain magnitudes and strain rates, Journal of Materials Science, Vol: 47, Pages: 3564-3576
Hooper PA, Sukhram RA, Blackman BRK, et al., 2012, On the blast resistance of laminated glass, International Journal of Solids and Structures, Vol: 49, Pages: 899-918
Arora H, Hooper PA, Dear JP, 2011, The Effects of Air and Underwater Blast on Composite Sandwich Panels and Tubular Laminate Structures, Experimental Mechanics
Arora H, Hooper PA, Dear JP, 2011, Dynamic response of full-scale sandwich composite structures subject to air-blast loading, Composites Part A: Applied Science and Manufacturing
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