Imperial College London

John P. Dear MA PhD CEng FIMechE CPhys FInstP

Faculty of EngineeringDepartment of Mechanical Engineering

Professor of Mechanical Engineering
 
 
 
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Contact

 

j.dear Website

 
 
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Location

 

520City and Guilds BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
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222 results found

Zhou J, Liu H, Dear JP, Falzon BG, Kazancı Zet al., 2023, Comparison of different quasi-static loading conditions of additively manufactured composite hexagonal and auxetic cellular structures, International Journal of Mechanical Sciences, Vol: 244, Pages: 1-16, ISSN: 0020-7403

Auxetic cellular structures have the potential to revolutionise sandwich panel cores due to their potential superior energy absorption capability. Because of their negative Poisson's ratio, auxetics behave counterintuitively and contract orthogonally under an applied compressive force, resulting in a densification of material in the vicinity of the applied load. This study investigates three cellular structures and compares their compressive energy absorbing characteristics under in-plane and axial loading conditions. Three unit cell topologies are considered; a conventional hexagonal, re-entrant and double arrowhead auxetic structures. The samples were additively manufactured using two different materials, a conventional Nylon and a carbon fibre reinforced composite alternative (Onyx). Finite element simulations are experimentally validated under out of and in-plane loading conditions and the double arrowhead (auxetic) structure is shown to exhibit comparatively superior energy absorption. For the carbon fibre reinforced material, Onyx, the specific energy absorbed by the double arrowhead geometry was 125% and 244% greater than the hexagonal (non-auxetic) and re-entrant (auxetic) structures respectively.

Journal article

Liu H, Liu J, Hall ZEC, Brooks RA, Crocker JWM, Joesbury AM, Harper LT, Blackman BRK, Kinloch AJ, Dear JPet al., 2023, Modelling the effects of patch-plug configuration on the impact performance of patch-repaired composite laminates, Composites Science and Technology, Vol: 233, Pages: 1-22, ISSN: 0266-3538

The patch-plug configuration has been widely used to repair composite structures and restore the structural integrity of damaged composites. In the present research, single-sided CFRP patch-repaired panels, with different patch-plug configurations, are prepared. This is where a circular-shaped damaged area has been removed and a CFRP patch has been adhesively-bonded onto the panel. In some cases, a CFRP plug is inserted into the hole, caused by removal of the damaged area, before the patch is applied. Such patch-repaired panels, and the pristine CFRP panel, are subjected to a low-velocity impact at an energy of 7.5 J. These impacted pristine and repaired panels are then examined using ultrasonic C-scan and optical microscopy to inspect the impact-associated permanent indentation, interlaminar and intralaminar damage. A finite element analysis (FEA) model, which significantly extends a previously validated elastic-plastic (E-P) numerical damage model, has been developed to predict the impact behaviour of the pristine CFRP panel and the various designs of patch-repaired CFRP panels. The comparison between the experimental and numerical results for all the studied cases shows the maximum deviations for the loading response and the damage area are 12% and 15%, respectively. The good agreement between the experimentally-measured impact properties and those predicted using the numerical model demonstrates that the model is a useful design tool.

Journal article

Tüfekci M, Özkal B, Maharaj C, Liu H, Dear JP, Salles Let al., 2023, Strain-rate-dependent mechanics and impact performance of epoxy-based nanocomposites, Composites Science and Technology, Vol: 233, Pages: 1-17, ISSN: 0266-3538

Strain-rate-dependent mechanical properties and impact performance of manufactured epoxy-based nanocomposites are investigated. As reinforcements, fumed silica (FS) and halloysite nanotube (HNT) are used alongside Albipox 1000 and Nanopox F700. First, the internal structures of the composites are visualised using scanning electron microscopy (SEM). To identify the strain-rate-dependent mechanical properties, three-point bend tests are conducted at three different strain rate levels. For the impact resistance, Charpy impact tests are performed. For further investigations of the mechanical properties of the composites, mean-field homogenisation (MFH) and finite element (FE) analyses on the representative volume elements (RVE) are performed for each type of composite material. Overall, the modelling and experiments are in good agreement and account for the mechanical behaviour of these epoxy-based nanocomposites.

Journal article

Irven G, Carolan D, Fergusson A, Dear JPet al., 2023, Fracture performance of fibre-reinforced epoxy foam, Composites Part B: Engineering, Vol: 250, Pages: 1-12, ISSN: 1359-8368

Low density aramid and carbon fibre-reinforced epoxy foam has been synthesised with the aim of improving mechanical properties, principally fracture performance. The foam properties measured were fracture energy, compressive strength, and density. The influence of fibre type, loading, and length was investigated. In addition, composite face-sheet bond tests were performed to ascertain how effective toughness transferred from individual component to composite structure. In general, the addition of fibres improved the mechanical performance of reinforced samples compared to the control foam. Increases in compressive strength were moderate whilst fracture energy was increased by up to 107% from 124 J/m2 to 256 J/m2 by the addition of 0.75 mm aramid fibres. Increased fracture energy of the foam and the presence of fibres on the foam surface, caused an increase in face-sheet bond propagation fracture toughness of 50% from 277 J/m2 to 416 J/m2.

Journal article

Samieian MA, Cormie D, Smith D, Wholey W, Blackman B, Dear J, Hooper Pet al., 2022, A study on the bending of laminated glass under blast loading, Experimental Mechanics, Vol: 63, Pages: 385-400, ISSN: 0014-4851

Background:The bending behaviour of laminated glass plays an important role in determining its overall response to blast loading. It is costly and difficult to characterise the bending behaviour by carrying out full-scale blast tests, therefore an alternative method is required.Objective:The objective of this study is to understand the response of laminated glass under high-rate bending in the laboratory at rates representative of blast loading.Methods:In this paper a novel testing method is presented in which laminated glass strips of 700 mm long by 60 mm wide are tested up to speeds of 10 m/s in the laboratory. The laminated glass is accelerated to speeds comparable to blast loading and then brought to rest at its edges to mimic impulsive blast loading conditions. Different interlayer thickness, impact speeds, and boundary conditions were explored. Additionally, modelling methods were used to study the flexural rigidity of post-cracked laminated glass.Results:From the experiments it was found that the interlayer thickness plays a key role in determining whether the dominant failure mechanism is de-bonding of interlayer from the glass or interlayer tearing. In addition, it was found that by allowing the frame to bend under loading, the laminated glass can carry greater loads without failure. Finally, an iterative method was used to quantify the flexural rigidity of post-cracked laminated glass depending on the speed of travel. This is a novel finding as it is usually assumed that laminated glass behaves like a membrane in the post-cracked phase of the response.Conclusion:In modelling and design of laminated glass structures under blast loading, post-crack flexural rigidity must be taken into account. Additionally, having novel frame designs to add further load bearing capacity to the framing members, plays a key role in reducing the load intensity on the laminated glass structure.

Journal article

Irven G, Carolan D, Fergusson A, Dear JPet al., 2022, Fracture performance of epoxy foam: Low density to bulk polymer, Polymer, Vol: 261, Pages: 1-13, ISSN: 0032-3861

Epoxy foams with densities ranging from 180 to 500 kg/m3 were prepared and mechanically tested incompression, tension, and single-edge notched bending (SENB) configurations. Fracture results revealed amarked transition in behaviour at a critical density, between 227 kg/m3 and 249 kg/m3. Lower density foamsfailed at low SENB displacement, producing low toughness and fracture energy results, whereas higher densityfoams failed at higher SENB displacements, with correspondingly higher values of toughness and fracture energy.The stress-intensity factor increased monotonically with density, from 0.1 to 0.79 MPa m1/2. The fracture energy,GIc, of the foams reached values of up to 3.5 times that of the bulk polymer, 268 J/m2. Lower density foamsbelow the transition in fracture behaviour exhibited a small number of large cells, caused by cell coalescence, anda wider cell size distribution than the denser foams. This distribution appears linked to the transition in fracturebehaviour. The behaviour revealed in this paper raises the point whether in future design criteria, where foamsare now often used in composite sandwich structures, allowance should be made for denser foams to be used asappreciable increases in fracture energy of the foam core are achievable.

Journal article

Liu H, Brooks R, Hall Z, Liu J, Crocker J, Joesbury A, Harper L, Blackman B, Kinloch A, Dear Jet al., 2022, Experimental and numerical investigations on the impact behaviour of pristine and patch-repaired composite laminates, Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol: 380, Pages: 1-15, ISSN: 1364-503X

The present paper investigates the impact behaviour of both pristine carbon-fibre reinforced- plastic (CFRP) composite laminates and repaired CFRP laminates. For the patch-repaired CFRP specimen, the pristine CFRP panel specimen has been damaged by cutting out a central disc of the CFRP material and then repaired using an adhesively-bonded patch of CFRP to cover the hole. Drop-weight, impact tests are performed on these two types of specimens and a numerical elastic-plastic (E-P), three-dimensional (3-D) damage model is developed and employed to simulate the impact behaviour of both types of specimen. This numerical model is meso-scale in nature and assumes that cracks initiate in the CFRP at a nano-scale, in the matrix around fibres, and trigger sub-micrometre intralaminar matrix cracks during the impact event. These localised regions of intralaminar cracking then lead to interlaminar, i.e. delamination, cracking between the neighbouring plies which possess different fibre orientations. These meso-scale, intralaminar and interlaminar, damage processes are modelled using the numerical finite-element analysis (FEA) model with each individual ply treated as a continuum. Good agreement is found between the results from the experimental studies and the predictions from the numerical simulations.

Journal article

Brooks R, Wang H, Ding Z, Xu J, Song Q, Liu H, Dear J, Li Net al., 2022, A review on stamp forming of continuous fibre-reinforced thermoplastics, International Journal of Lightweight Materials and Manufacture, Vol: 5, Pages: 411-430, ISSN: 2588-8404

Continuous fibre-reinforced thermoplastics (FRTPs) are replacing metals in certain applications in the aerospace industry due to their superior properties e.g., high strength-to-weight ratio and good fatigue resistance. Adopting these lightweight materials in vehicles is a solution for improving vehicle efficiency across the transport industry. Among various manufacturing techniques for FRTP parts, stamp forming is one of the most advantageous when small structures and mass production are targeted. However, a significant barrier for this technique is the quality control of manufacturing. The current paper reviews the development of stamp forming technology, benefits of using such technology and the typical quality issues in stamp forming of FRTP parts. First, advantages of stamp forming, compared to other thermoforming techniques, are discussed, followed by a review of the historical development of the process. Second, deformation mechanisms of FRTPs during stamp forming are examined, with particular focuses on the frictional behaviour and testing thereof. Third, the main defects associated with stamp forming are considered, alongside suggestions towards reducing their presence. Finally, an extensive survey of the effect of process parameters on the mechanical properties of formed parts is included, with generally expected trends highlighted and methodologies for finding optimum conditions presented. Based on the thorough review of state-of-the-art stamp forming, future trends and research gaps to be tackled for widening the applicability of FRTP stamp forming are suggested.

Journal article

Hall Z, Liu J, Brooks R, Liu H, Crocker J, Joesbury A, Harper L, Blackman B, Kinloch A, Dear Jet al., 2022, The effectiveness of patch repairs to restore the impact properties of carbon-fibre reinforced-plastic composites, Engineering Fracture Mechanics, Vol: 270, ISSN: 0013-7944

The present paper studies the low-velocity impact testing of carbon-fibre reinforced-plastic (CFRP) pristine and patch-repair CFRP panels. Firstly, the effect of repeated impacts on the pristine CFRP damage growth is considered at impact energies of 7.5, 10.5 and 30 J. Secondly, such tests lead to a single-sided, patch-repair panel being manufactured by removing a 40 mm diameter central hole, to act as the ‘damaged area’, from the parent CFRP panel and then adhesively-bonding a circular CFRP patch-repair over the hole so generated. Various diameters and thicknesses for the CFRP patch-repair are employed and, in some cases, a CFRP circular plug is also used to fill the hole created by removal of the parent composite. The measured load versus time, and load versus displacement, traces are compared. Further, the extent and location of any interlaminar damage, i.e. delaminations between the plies of the CFRP, caused by the impact event are mapped using an ultrasonic C-scan technique. It is shown that single-sided patch repairs can be very effective in restoring the impact performance of damaged CFRP panels.

Journal article

Zheng J, Maharaj C, Liu J, Chai H, Liu H, Dear Jet al., 2022, A comparative study on the failure criteria for predicting the damage initiation in fibre-reinforced composites, Mechanics of Composite Materials, Vol: 58, Pages: 125-140, ISSN: 1573-8922

In this research, a review is performed to explore the advantages and disadvantages of different failure criteria for fibre reinforced composites. Widely-used failure criteria, such as the Maximum stress criterion, Hashin criterion, Puck’s criterion, LaRC03 and Northwestern University (NU) criteria are reviewed based on the relevant literature. A comparison is performed of these failure criteria, using the analytical results obtained from a MATLAB programme and numerical results obtained from an Abaqus finite element model. The applicability and reliability of these failure criteria for predicting damage in thermoplastic laminates, i.e. AS4 carbon fibre reinforced Polyether-ether-ketone (PEEK), are evaluated based on the analytical and numerical results. Thenumerical results reveal that the Maximum Stress criterion provides the most conservative prediction, whilst the Hashin and Northwestern University (NU) criteria give reasonable and sensible results with an acceptable running time. Puck and LaRC03 criteria deliver more accurate predictions, but with longer running times.

Journal article

Liu H, Blackman B, Kinloch AJ, Dear J, Dear J, Liu H, Liu J, Kaboglu C, Zhou J, Kong X, Li S, Blackman B, Kinloch Aet al., 2022, Modelling the quasi-static flexural behaviour of composite sandwich structures with uniform- and graded-density foam cores, Engineering Fracture Mechanics, Vol: 259, Pages: 1-188, ISSN: 0013-7944

In-service, composite sandwich structures, which consist of fibre-composite skins (also termed face-sheets) adhesively bonded to a polymeric foam core, can encounter extreme quasi-static flexural loading that may cause serious damage to the sandwich structure. The ability to model the flexural behaviour of such structures can lead to improved designs and more efficient maintenance procedures. In the present research, a three-dimensional finite-element analysis (FEA) model is developed to predict the flexural behaviour of such sandwich structures using a commercial software package (i.e. Abaqus/Explicit). The high-fidelity FEA simulation combines an elastic–plastic (E-P) damage model of the composite skins together with a crushable foam-core damage model. The E-P damage model is implemented with a user subroutine to capture the damage, such as plastic deformation of the matrix and matrix cracking, fibre fracture and delamination cracking of the composite skins. The crushable foam model is used to predict (a) the mechanical response of the crushed foam core, (b) the induced damage from ductile fracture due to growth, coalescence and fracture of the cells and (c) the induced damage from shear fracture of the foam due to plastic shear-band localisation. Results from the modelling studies, such as the loading response and the damage mechanisms, are discussed and compared with the experimental results obtained from the sandwich structures manufactured with both uniform- and graded-density foam cores but which all have the same average core density. Good agreement is achieved between the experimental results and the predictions from the numerical modelling simulations.

Journal article

Andrews D, Bourne N, Brown E, Dear J, Dickson P, Freeman C, Goveas S, Gray III G, Hauser H, Huntley J, Hutchings I, Leighton T, Matthewson J, Meyers M, Rae P, Siviour C, Swain M, Townsend D, van der Zwaag S, Walley S, Williamson Det al., 2021, Contributions to Dynamic Behaviour of Materials Professor John Edwin Field, FRS 1936–2020, Journal of Dynamic Behavior of Materials, Vol: 7, Pages: 353-382, ISSN: 2199-7446

Professor John Edwin Field passed away on October 21st, 2020 at the age of 84. Professor Field was widely regarded as a leader in high-strain rate physics and explosives. During his career in the Physics and Chemistry of Solids (PCS) Group of the Cavendish Laboratory at Cambridge University, John made major contributions into our understanding of friction and erosion, brittle fracture, explosives, impact and high strain-rate effects in solids, impact in liquids, and shock physics. The contributions made by the PCS group are recognized globally and the impact of John’s work is a lasting addition to our knowledge of the dynamic effects in materials. John graduated 84 Ph.D. students and collaborated broadly in the field. Many who knew him attribute their success to the excellent grounding in research and teaching they received from John Field.

Journal article

Kaboglu C, Liu J, Liu H, Russo P, Simeoli G, Lopresto V, Dear J, Maharaj Cet al., 2021, The effect of a coupling agent on the impact behavior of flax fibre composites, Journal of Engineering Materials and Technology, Vol: 143, Pages: 031008-1-031008-10, ISSN: 0094-4289

The effects of a coupling agent on the behavior of flax fiber-reinforced composites have been investigated by testing the specimens under both quasi-static (QS) indentation and high-velocity impact loading. The specimens are manufactured embedding a commercial flax fiber fabric in a polypropylene (PP) matrix, neat and premodified with a maleic anhydride-grafted PP, the latter acting as a coupling agent to enhance the interfacial adhesion. QS compressive tests were performed using a dynamometer testing machine equipped with a high-density polyethylene indenter having the same geometry of the projectile employed in the impact tests. The impact tests were conducted setting three different impact velocities. Digital image correlation maps of out-of-plane displacement were employed to compare the specimens with and without the coupling agent. The QS testing results indicate that the coupling agent has an enhancing influence on the bending stiffness of tested flax composites. The testing results show that the coupling agent improves the mechanical behavior by decreasing the out-of-plane displacement under impact loading. This approach gives rise to new materials potentially useful for applications where impact performance is desired while also providing an opportunity for the incorporation of natural fibers to produce a lightweight composite.

Journal article

Irven G, Duncan A, Whitehouse A, Carolan D, Fergusson A, Dear Jet al., 2021, Impact response of composite sandwich structures with toughened matrices, Materials and Design, Vol: 203, ISSN: 0264-1275

The mechanisms of failure of a composite sandwich structure subjected to a projectile impact have been investigated. The results reveal the complex interplay between the various damage dissipation mechanisms. The effects of modifying the matrix of the skins with polysiloxane core–shell rubber (CSR) nanoparticles and silica nanoparticles were investigated. Single cantilever beam specimens were tested to evaluate skin-core debonding. The addition of CSR nanoparticles to the matrix beyond 3 wt% causes a change in failure mechanism from sub-interface foam failure to interfacial failure when 6 and 9 wt% CSR are added. The sandwich structures were impacted with an aluminium projectile at 130 m/s. High speed cameras were used to obtain 3D digital image correlation of the back-face. Sectioning and imaging of the panels revealed damage in the form of front skin perforation and delamination, crushing and fracture of the core and back-face skin-core debonding. The impacted specimens also exhibited a transition in failure mechanism relating to rear face skin-core debonding between 3 and 6 wt%. Panels containing low amounts of CSR resulted in increased core cracking, while beyond the transition point, widespread rear face skin-core debonding was observed. At 3 wt% CSR, optimum back face deflection is achieved, and lower front skin delamination is experienced.

Journal article

Hu P, Yang S, Zheng F, Yuan Y, Wang T, Li S, liu H, Dear Jet al., 2021, Accurate and dynamic 3D shape measurement, Measurement Science and Technology, Vol: 32, ISSN: 0957-0233

Phase shifting profilometry (PSP) has been widely used in structured-light (SL) system for three-dimensional (3D) shape measurements, but the speed of PSP technique is limited by the increased phase-shifting patterns. This paper proposes an accurate and dynamic 3D shape measurement method by projecting only four patterns including three-step phase-shifting patterns and one speckle pattern. Three-step phase-shifting images are used to obtain the initial unwrapped phase map with phase ambiguity. Based on the principle of digital image correlation (DIC) and multi-view geometry, the absolute phase can be recovered reliably without requiring any embedded features or pre-defined information of the object. To improve the measurement accuracy, the projector coordinate is used as the measuring coordinate to establish a novel stereo structured-light system model. By solving a least square solution using the triple-view information, accurate 3D surface data can be reconstructed. The experimental results indicate that the proposed method can perform high-speed and accurate 3D shape measurements with an accuracy of 10.64 μm, which is superior to conventional methods and has certain instructive significance for 3D profilometry and measurement engineering.

Journal article

Zhang P, Kong X, Wang Z, Zheng C, Liu H, Shi G, Dear JP, Wu Wet al., 2021, High velocity projectile impact of a composite rubber/aluminium fluid-filled container, International Journal of Lightweight Materials and Manufacture, Vol: 4, Pages: 1-8, ISSN: 2588-8404

When penetrated by a high-velocity projectile, a fluid-filled container can be severely damaged and ruptured due to the intense impact loading from Hydrodynamic Ram (HRAM), which causes a primary shock wave, and then a subsequent loading phase when a cavity evolves in the fluid. In the design of fuel tanks for aircraft, and other transport vehicles, the HRAM pressure is a major concern for the reliability of the structure. In this paper, experiments of high-velocity projectiles impacting two different types of fluid-filled containers, including an aluminium wall and a composite aluminium/rubber wall, were performed to study the mitigation effect of the rubber layer on the damage of the structure and the impact loading from Hydrodynamic Ram. A high-speed camera was employed to record the formation process of the cavity, and the shock wave pressure-time histories in the fluid were also obtained by pressure transducers. By comparing and analysing the experimental results, it is shown that the rubber layer of the composite wall container was able to reduce the reflected shock pressure and the deformation of the structure.

Journal article

Tufekci M, Rendu Q, Yuan J, Dear JP, Salles L, Cherednichenko AVet al., 2021, Stress and modal analysis of a rotating blade and the effects of nonlocality, ASME Turbo Expo 2020: Turbomachinery Technical Conference and Exposition, Publisher: American Society of Mechanical Engineers, Pages: 1-12

This study focuses on the quasi-static stress and modal analyses of a rotor blade by using classical and nonlocal elasticity approaches. The finite element method with an additional numerical integration process is used to evaluate the integral equation of nonlocal contionuum mechanics. The blade is assumed to be made of a linear elastic material of weak nonlocal characteristic. Such materials can be composites, metallic foams, nanophased alloys etc. A full-scale fan blade model is chosen as the test case to represent the rotor blade for a modern high bypass ratio turbofan engine. The boundary conditions and loads are chosen based on the steady-state cruising operating conditions of such blades. The nonlocal stresses are calculated by processing the calculated local stresses. To calculate the nonlocal stresses, the integral form of nonlocal elasticity is employed in the discretised domain. The results of the two cases are compared and discussed.

Conference paper

Liu H, Liu J, Ding Y, Hall Z, Luo L, Zhou J, Kong X, Blackman BRK, Kinloch AJ, Dear JPet al., 2020, Investigations on the impact behaviour of fibre-reinforced composites: effect of impact energy and impactor shape, Procedia Structural Integrity, Vol: 28, Pages: 106-115, ISSN: 2452-3216

In the present research, a detailed experimental study of the impact behaviour of CFRP composites is performed. To investigate the effects of impactor velocity, a round-nosed steel impactor is employed to strike the composite specimens at two impact velocities (i.e. 2.40 m.s-1 and 4.16 m.s-1). To investigate the effects of the geometry of the head of the impactor, a flat-faced steel impactor is also employed to strike the composite specimens at a velocity of 2.40 m.s-1. After the impact experiments, all the tested composite specimens are inspected using a C-scan device to assess the damage due to the different types of impact. The experimental results, including the loading response and impact-induced damage, are employed to analyse the effects of impact velocity and impactor shapes on the impact behaviour of the composite laminates. The results indicate that, at the higher impact velocity (i.e. 4.16 m.s-1), delamination is more extensive near the rear face of the composite, whilst the delamination near the front face is less sensitive to the increase in the impact velocity. For the lower impact velocity (i.e. 2.40 m.s-1), the area of the damage footprint from the round-nosed steel impactor and the flat-faced steel impactor are similar in extent, but the shape of the damage footprint is very different. The round-nosed steel impactor causes a centrally symmetric damage area, whilst the flat-faced steel impactor causes damage in which the central area shows much less damage.

Journal article

Liu H, Liu J, Ding Y, Zheng J, Luo L, Kong X, Zhou J, Blackman B, Kinloch A, Dear Jet al., 2020, Modelling the effect of projectile hardness on the impact response of a woven carbon-fibre reinforced thermoplastic-matrix composite, International Journal of Lightweight Materials and Manufacture, Vol: 3, Pages: 403-415, ISSN: 2588-8404

In the present paper numerical modelling results are described to predict the effects of the hardness of a projectile impacting a woven carbon-fibre reinforced thermoplastic-matrix composite. The projectiles are prepared from either relatively soft gelatine or hard high-density polyethylene (HDPE) materials, of the same mass, and are fired from a gas-gun at about 60 m s−1 to impact a woven carbon-fibre reinforced poly(ether-ether ketone) (woven CF/PEEK) composite. A two-dimensional, elastic, finite-element analysis (FEA) model is developed to simulate the gas-gun impact experiments and study the impact damage processes, and this numerical model is relatively computationally efficient. This FEA model makes predictions for the plastic flow for the gelatine projectile and the elastic deformation of the polyethylene projectile. In addition, the model predicts the effects of the hardness of the projectile on (a) the deformation of the impacted composite specimens and (b) the location and extent of damage in the composites. Very good agreement between the predictions from the model and the experimental measurements is observed. This research is of key importance in studying the behaviour of thermoplastic-matrix composites under impact loading by various types of threat such as relatively soft bodies, e.g. birds and hard objects, e.g. dropped-tools and runway debris.

Journal article

Rolfe E, Quinn R, Irven G, Brick D, Dear JP, Arora Het al., 2020, Underwater blast loading of partially submerged sandwich composite materials in relation to air blast loading response, International Journal of Lightweight Materials and Manufacture, Vol: 3, Pages: 387-402, ISSN: 2588-8404

The research presented in this paper focusses on the underwater blast resilience of a hybrid composite sandwich panel, consisting of both glass-fibre and carbon-fibre. The hybrid fibres were selected to optimise strength and stiffness during blast loading by promoting fibre interactions. In the blast experiment, the aim was to capture full-field panel deflection during large-scale underwater blast using high-speed 3D Digital Image Correlation (DIC). The composite sandwich panel was partially submerged and subjected to a 1 kg PE7 charge at 1 m stand-off. The charge was aligned with the centre of the panel at a depth of 275 mm and mimicked the effect of a near-field subsurface mine. The DIC deflection data shows that the horizontal cross-section of the panel deforms in a parabolic shape until excessive deflection causes core shear cracking. The panel then forms the commonly observed “bathtub” deformation shape. DIC data highlighted the expected differences in initial conditions compared to air-blast experiments, including the pre-strains caused by the mass of water (hydrostatic pressure). Furthermore, water depth was shown to significantly influence panel deflection, strain and hence damage sustained under these conditions. Panel deformations and damage after blast was progressively more severe in regions deeper underwater, as pressures were higher and decayed slower compared to regions near the free surface.An identical hybrid composite sandwich panel was subjected to air blast; one panel underwent two 8 kg PE7 charges in succession at 8 m stand-off. DIC was also implemented to record the panel deformations during air blast. The air and underwater blast tests represent two different regimes of blast loading: one far-field in air and one near-field underwater. The difference in deflection development, caused by the differing fluid mediums and stand-off distances, is apparent from the full-field results. During underwater blast the panel underwent peak pres

Journal article

Liu H, Liu J, Ding Y, Hall ZE, Kong X, Zhou J, Blackman BRK, Kinloch AJ, Dear JPet al., 2020, A three-dimensional elastic-plastic damage model for predicting the impact behaviour of fibre-reinforced polymer-matrix composites, Composites Part B: Engineering, Vol: 201, Pages: 1-23, ISSN: 0961-9526

A three-dimensional (3-D) Finite Element Analysis (FEA) model incorporating an elastic-plastic (EP) damage model, which was implemented as a user-defined material (‘VUMAT’) sub-routine in a FEA code (‘Abaqus/Explicit’), is developed to simulate the impact response of carbon-fibre reinforced-plastic (CFRP) composites. The model predicts the load versus time and the load versus displacement responses of the composite during the impact event. Further, it predicts the extent, shape and direction of any intralaminar damage and interlaminar delaminations, i.e. interlaminar cracking, as a function of the depth through the thickness of the impacted CFRP test specimen, as well as the extent of permanent indention caused by the impactor striking the composite plate. To validate the model, experimental results are obtained from relatively low-velocity impact tests on CFRP plates employing either a matrix of a thermoplastic polymer, i.e. poly(ether-ether ketone), or a thermosetting epoxy polymer. The 3-D EP model that has been developed is shown to model successfully the experimentally-measured impact behaviour of the CFRP composites.

Journal article

Liu H, Liu J, Ding Y, Zhou J, Kong X, Blackman B, Kinloch A, Falzon B, Dear Jet al., 2020, Effects of impactor geometry on the low-velocity impact behaviour of fibre-reinforced composites: an experimental and theoretical investigation, Applied Composite Materials, Vol: 27, Pages: 533-553, ISSN: 0929-189X

Carbon-fibre/epoxy-matrix composites used in aerospace and vehicle applications are often susceptible to critical loading conditions and one example is impact loading. The present paper describes a detailed experimental and numerical investigation on the relatively low-velocity (i.e. <10 m/s) impact behaviour of such composite laminates. In particular, the effects of the geometry of the impactor have been studied and two types of impactor were investigated: (a) a steel impactor with a hemispherical head and (b) a flat-ended steel impactor. They were employed to strike the composite specimens with an impact energy level of 15 J. After the impact experiments, all the composite laminates were inspected using ultrasonic C-scan tests to assess the damage that was induced by the two different types of impactor. A three-dimensional finite-element (FE) model, incorporating a newly developed elastic-plastic damage model which was implemented as a VUMAT subroutine, was employed to simulate the impact event and to investigate the effects of the geometry of the impactor. The numerical predictions, including those for the loading response and the damage maps, gave good agreement with the experimental results.

Journal article

Liu H, Liu J, Ding Y, Zheng J, Kong X, Zhou J, Harper L, Blackman B, Kinloch A, Dear Jet al., 2020, The behaviour of thermoplastic and thermoset carbon-fibre composites subjected to low velocity and high velocity impact, Journal of Materials Science, Vol: 55, Pages: 15741-15768, ISSN: 0022-2461

The present paper describes the results from experimental and theoretical modelling studies on the behaviour of continuous carbon-fibre/polymer matrix composites subjected to a relatively low-velocity or high-velocity impact, using a rigid, metallic impact or. Drop-weight and gas-gun tests are employed to undertake the low-velocity and high-velocity impact experiments, respectively. The carbon-fibre composites are based upon a thermoplastic poly(ether-ether ketone)matrix (termed CF/PEEK) or a thermoset toughened-epoxy matrix (termed CF/Epoxy), which have the same fibre architecture of a cross-ply [03/903]2slay-up. The studies clearly reveal that the CF/PEEK composites exhibit the better impact performance. Also,at the same impact energy of 10.5±0.3J, the relatively high-velocity test at 54.4±1.0m.s-1 leads to more damage in both types of composite than observed from the low-velocity test where the impact or struck the composites at 2.56 m.s-1.The computationally-efficient,two-dimensional, elastic, finite-element model that has been developed is generally successful in capturing the essential details of the impact test and the impact damage in the composites, and has been used to predict the loading response of the composites under impact loading.

Journal article

Quinn R, Zhang LH, Cox MJ, Townsend D, Cartwright T, Aldrich-Smith G, Hooper P, Dear Jet 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.

Journal article

Xiang N, Zhang X, Zheng M, Ge Y, Wang T, Liu H, Maharaj C, Dear JP, Yan Yet al., 2020, Microstructure and tensile properties of injection molded thermoplastic polyurethane with different melt temperatures, Journal of Applied Polymer Science, Vol: 137, Pages: 1-12, ISSN: 0021-8995

The use of injection molding technology to prepare heterogeneous interlayer film of laminated glass holds strong applicable potential. This article aims to investigate the effects of melt temperature and melt flow on the microstructure evolution and tensile properties of thermoplastic polyurethane (TPU) specimens during the injection molding process. The tensile properties of the TPU specimens show dependency on the melt temperature and melt flow direction. The results of birefringence indicate that melt flow and lower melt temperature induce higher stretching deformation of the molecular chain network. Small‐angle X‐ray scattering analysis approves that besides the melt temperature and flow direction, the testing position on the cross section of the specimen has great influence on the microstructure of the TPU sheet. Further analysis and conclusions can be made using wide‐angle X‐ray scattering method. The above results demonstrate that both the tensile properties and microstructure of the injection molded TPU specimens tend to be isotropic with the increase of melt temperature.

Journal article

Ibrahim Y, Davies C, Maharaj C, Li Z, Dear J, Hooper Pet 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.

Journal article

Liu H, Liu J, Kaboglu C, Chai H, Kong X, Blackman BRK, Kinloch AJ, Dear JPet al., 2020, Experimental investigations on the effects of projectile hardness on the impact response of fibre reinforced composite laminates, International Journal of Lightweight Materials and Manufacture, Vol: 3, Pages: 77-97, ISSN: 2588-8404

This paper presents a detailed experimental investigation on the effects of projectile hardness on the behaviour of thermoplastic composites under impact loading. In this research, gas-gun experiments employ gelatine and high-density polyethylene (HDPE) projectiles, of the same mass and diameter, to impact against woven carbon-fibre reinforced poly (ether-ether ketone) (CF/PEEK) composite specimens. During the experiments, a high-speed camera is employed to capture the deformation of the projectiles and a three-dimensional (3D) Digital Image Correlation (DIC) system is employed to record the major strain and out-of-plane displacement of the thermoplastic composite specimens. Experimental results, including the Digital Image Correlation (DIC) output and the post-impact status, are obtained and compared to show the effects of harder projectiles on increasing the impact damage. The composite specimens, impacted by gelatine and high-density polyethylene (HDPE) projectiles, presented similar major strain and out-of-plane displacement, but the high-density polyethylene (HDPE)-impacted composite specimens show more severe damage than the gelatine-impacted composite specimens.

Journal article

Zhang X, Liu H, Maharaj C, Zheng M, Mohagheghian I, Zhang G, Yan Y, Dear JPet al., 2020, Impact response of laminated glass with varying interlayer materials, International Journal of Impact Engineering, Vol: 139, Pages: 1-15, ISSN: 0734-743X

This study investigates the influence of the interlayer materials on the low velocity impact performance of laminated glass. By varying the impact velocity, with a drop-weight, the effect of impact energy levels (3, 5, 10 and 15J) has been explored on the impact resistance of laminated glass and the failure mechanisms have been assessed. The four interlayer materials investigated were: SGP–Ionoplast as employed in Sentry Glas® Plus, TPU-Thermoplastic polyurethane, PVB-Polyvinyl butyral and a TPU/SGP/TPU hybrid interlayer. The drop weight method has been employed to obtain the energy dissipation, loading and deformation of the laminated glass. The low velocity impact results indicate that both the type of the interlayer materials and the impact energy have great influence on the impact performance of the laminated glass. The laminated glass with TPU and PVB interlayer exhibited better impact resistance than the laminated glass with SGP and TPU/SGP/TPU hybrid interlayer, when impacted at energies of 3 and 5J (corresponding to impact velocities of 1.71 and 2.21 ms−1 respectively). However, the laminated glass with SGP and TPU/SGP/TPU hybrid interlayers has better load carrying capacity and better anti-deformation property than the TPU and PVB interlayers at the higher impact energies of 10 and 15J (impact velocities of 3.13 and 3.83 ms−1 respectively). The results are thought to be attributed to the differences in the viscoelastic properties of the interlayer materials with strain rates.

Journal article

Chavoshi S, Tagarielli V, Shi Z, Lin J, Wang S, Jiang J, Dear J, Nikbin Ket al., 2020, Predictions of the mechanical response of sintered FGH96 powder compacts, Journal of Engineering Materials and Technology, Vol: 142, ISSN: 0094-4289

This paper presents predictions of the response of sintered FGH96 Ni-based superalloy powder compacts at high temperature, obtained by analysis of 3D representative volume elements generated by both X-ray tomography and a virtual technique. The response ofthe material to a multiaxial state of stress/strain for porosities as large as 0.3 is explored, obtaining the yield surfaces and their evolution as well as scaling laws for both elastic and plastic properties. The two modelling approaches are found in good agreement. The sensitivity of the predictions to particle size, inter-particle friction, applied strain rate,and boundary conditions is also examined.

Journal article

Tufekci M, Mace T, Özkal B, Dear J, Schwingshackl C, Salles Let al., 2020, Nonlinear dynamic behaviour of a nanocomposite: epoxy reinforced with fumed silica nanoparticles, XXV ICTAM

This study focuses on identification and modelling of vibration characteristics of a nanocomposite; an epoxy resin as thematrix and fumed silica as the reinforcement. The resin alone is manufactured and characterised. Using the same methodology,the manufacturing and characterisation of the silica-reinforced nanocomposite are performed. Following the manufacturing and theexperimental characterisation process, a nonlinear model is built to represent characterised behaviour. The model is validated by aseparate test case which is also an experimental technique to extract the damping characteristics of a structure.

Conference paper

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