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

 

+44 (0)20 7594 7086j.dear Website

 
 
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Location

 

520City and Guilds BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

206 results found

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

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, ISSN: 2199-7446

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, ISSN: 0264-1275

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

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, 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

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

Domun N, Paton KR, Blackman BRK, Kaboglu C, Vahid S, Zhang T, Dear JP, Kinloch AJ, Hadavinia Het al., 2020, On the extent of fracture toughness transfer from 1D/2D nanomodified epoxy matrices to glass fibre composites, Journal of Materials Science, Vol: 55, Pages: 4717-4733, ISSN: 0022-2461

In this study, the effects of adding nanofillers to an epoxy resin (EP) used as a matrix in glass fibre-reinforced plastic (GFRP) composites have been investigated. Both 1D and 2D nanofillers were used, specifically (1) carbon nanotubes (CNTs), (2) few-layer graphene nanoplatelets (GNPs), as well as hybrid combinations of (3) CNTs and boron nitride nanosheets, and (4) GNPs and boron nitride nanotubes (BNNTs). Tensile tests have shown improvements in the transverse stiffness normal to the fibre direction of up to about 25% for the GFRPs using the ‘EP + CNT’ and the ‘EP + BNNT + GNP’ matrices, compared to the composites with the unmodified epoxy (‘EP’). Mode I and mode II fracture toughness tests were conducted using double cantilever beam (DCB) and end-notched flexure (ENF) tests, respectively. In the quasi-static mode I tests, the values of the initiation interlaminar fracture toughness, GCIC, of the GFRP composites showed that the transfer of matrix toughness to the corresponding GFRP composite is greatest for the GFRP composite with the GNPs in the matrix. Here, a coefficient of toughness transfer (CTT), defined as the ratio of mode I initiation interlaminar toughness for the composite to the bulk polymer matrix toughness, of 0.68 was recorded. The highest absolute values of the mode I interlaminar fracture toughness at crack initiation were achieved for the GFRP composites with the epoxy matrix modified with the hybrid combinations of nanofillers. The highest value of the CTT during steady-state crack propagation was ~ 2 for all the different types of GFRPs. Fractographic analysis of the composite surfaces from the DCB and ENF specimens showed that failure was by a combination of cohesive (through the matrix) and interfacial (along the fibre/matrix interface) modes, depending on the type of nanofillers used.

Journal article

Liu H, Liu J, Kaboglu C, Zhou J, Kong X, Blackman BRK, Kinloch AJ, Dear JPet al., 2020, The behaviour of fibre-reinforced composites subjected to a soft impact-loading: An experimental and numerical study, Engineering Failure Analysis, Vol: 111, Pages: 1-18, ISSN: 1350-6307

The present paper presents experimental and numerical studies on the behaviour of composite laminates subject to impact loading by soft projectiles. In this research, gas-gun experiments are performed to study woven carbon-fibre reinforced poly (ether-ether ketone) (CF/PEEK) composites subjected to an impact by soft-gelatine projectiles. In addition, woven carbon-fibre reinforced epoxy (CF/epoxy) composite specimens are also evaluated using gelatine projectiles to investigate the effect of the matrix system on the impact response of the composites. A high-speed camera is employed to capture the deformation of the projectiles and a three-dimensional (3D) Digital Image Correlation (DIC) system is used to record the deformation of the impacted composite specimens. A Finite Element (FE) model is developed to simulate the impact by a soft projectile on the composite specimens. Good agreement is shown between the predictions from using the FE model and the experimental results.

Journal article

Liu H, Liu J, Ding Y, Zhou J, Kong X, Harper L, Blackman B, Falzon B, Dear Jet al., 2020, Modelling damage in fibre-reinforced thermoplastic composite laminates subjected to three-point-bend loading, Composite Structures, Vol: 236, ISSN: 0263-8223

It is important to account for nonlinearity in the deformation of a thermoplastic matrix, as well as fibre fracture and matrix cracking, when predicting progressive failure in unidirectional fibre-reinforced thermoplastic composites. In this research, a new high-fidelity model approach is developed incorporating elastic-plastic nonlinearity. In order to validate the model, three-point bend experiments were performed on composite specimens, with a lay-up of [03/903]2s, to provide experimental results for comparison. Digital Image Correlation (DIC) was employed to record the strain distribution in the composite specimens. The developed intralaminar damage model, which is implemented as a user defined material (VUMAT in Abaqus/Explicit) subroutine, is then combined with a cohesive surface model to simulate three-point bend failure processes. The simulation results, including the load-displacement curves and damage morphology, are compared with the corresponding experimental results to assess the predictive capability of the developed model. Good agreement is achieved between the experimental and numerical results.

Journal article

Carolan D, Mayall A, Dear JP, Fergusson ADet al., 2020, Micromechanical modelling of syntactic foam, Composites Part B: Engineering, Vol: 183, Pages: 1-10, ISSN: 1359-8368

A combined numerical-experimental method that enables accurate prediction of not only the elastic moduli and tensile failure strengths of syntactic foams, but also accounts for the experimentally observed scatter in these measurements is presented. In general, for the systems studied, an increase in microsphere content resulted in an increase in tensile modulus and a decrease in tensile strength. At low particle loading ratios, the variance in the measured experimental strength can be almost entirely attributed to the distribution of inter-particle distances between the microspheres, whilst at high particle loadings, geometric variance in the microstructure is shown to be only partially responsible for the observed scatter in strength data. Thus, for the first time, a direct link between the underlying microstructure and the experimentally observed scatter in fracture strength is drawn and substantiated with modelling.

Journal article

Rolfe E, Arora H, Hooper PA, Dear JPet 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.

Conference paper

Tufekci M, Rendu Q, Yuan J, Dear JP, Salles L, Cherednichenko AVet al., 2020, Stress and modal analysis of a rotating blade and the effects of nonlocality

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

Rolfe E, Arora H, Hooper PA, Dear JPet 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.

Conference paper

Liu H, Falzon BG, Dear JP, 2019, An experimental and numerical study on the crush behaviour of hybrid unidirectional/woven carbon-fibre reinforced composite laminates, International Journal of Mechanical Sciences, Vol: 164, ISSN: 0020-7403

In composite aircraft structures, woven carbon-fibre reinforced plies are often used as the surface plies of a monolithic composite panel, made from unidirectional plies, to mitigate damage during drilling and provide a measure of impact damage resistance. This research presents, for the first time, a detailed experimental and numerical study on the crush behaviour of hybrid unidirectional/woven carbon-fibre reinforced composite laminates. Quasi-static crush tests are performed on composite specimens with two different trigger geometries; a bevel-trigger and a steeple-trigger. A computational model, which accounts for both interlaminar and intralaminar damage in hybrid unidirectional (UD)/woven composite laminates, implemented as a user subroutine in Abaqus/Explicit, was used. A comparison between experimental and numerical results confirms the computational tool's accuracy in predicting the energy absorption and damage mechanisms of hybrid specimens. The proposed approach could significantly reduce the extent of physical testing required in the development of crashworthy structures.

Journal article

Liu J, Liu H, Kaboglu C, Kong X, Ding Y, Chai H, Blackman B, Kinloch T, Dear Jet al., 2019, The Impact Performance of Woven-Fabric Thermoplastic and Thermoset Composites Subjected to High-Velocity Soft- and Hard-Impact Loading, Applied Composite Materials, Vol: 26, Pages: 1389-1410, ISSN: 0929-189X

The present paper investigates the impact performance of woven-fabric carbon-fibre composites based upon both thermoplastic- and thermoset-matrix polymers under high-velocity impact loading by conducting gas-gun experiments at impact velocities of up to 100 m.s-1. The carbon-fibre reinforced-polymers (CFRPs) are impacted using soft- (i.e. gelatine) and hard- (i.e. aluminium-alloy) projectiles to simulate either a soft bird-strike or a hard foreign-body impact (e.g. runway debris), respectively, on typical composites employed in civil aircraft. The out-of-plane displacements of the impacted composite specimen are obtained by means of a three-dimensional Digital Image Correlation (DIC) system for the soft-projectile impact on the composites and the extent of damage is assessed both visually and by using portable C-scan equipment. The perforation resistance and energy absorbing capability of the composites are also studied by performing high-velocity impact experiments using the hard-projectile and the resulting extent and type of damage are identified. In addition, a Finite Element (FE) model is also developed to investigate the interaction between the projectile and the composite target.

Journal article

Kong X, Zhou H, Zheng C, Liu H, Wu W, Guan Z, Dear JPet al., 2019, An experimental study on the mitigation effects of fine water mist on confined-blast loading and dynamic response of steel plates, International Journal of Impact Engineering, Vol: 134, Pages: 1-14, ISSN: 0734-743X

The blast loading from an explosion of a condensed explosive in a confined space is quite different from that in an open environment. Due to the confinement effect, a confined-blast load usually causes more severe damage. The detonation products from a fuel-rich explosive, such as Trinitrotoluene (TNT), can react with the surrounding air under specific conditions and release additional energy, resulting in a significant increase of reflected shockwaves and quasi-static pressure in a confined space. Mitigation of the confined-blast loading is an effective way to protect structures from destruction. In this paper, a detailed experimental investigation of the influence of water mist on both the mitigation of blast load and dynamic response of the structure is presented. Six cases of experimental tests, with different charge masses of 80, 120 and 160 g of TNT explosive, with and without the presence of water mist, are performed in a confined chamber, with the shock pressure and deflection-time history curves of blast-loaded steel plates being recorded, as well as quasi-static pressure and temperature of the contained gas. By analysing the experimental data, it is found that the water mist decreases the reflected blast wave and quasi-static pressure by restraining the combustion effect and subsequent energy release of detonation products. As a consequence, both the first peak deflection and the residual deformation of the blast-loaded steel plates decrease greatly. Furthermore, based on a non-dimensional analysis of the experimental results for plates with different thickness subjected to different loading conditions, a new empirical relationship between non-dimensional deflection of the plate and a non-dimensional load parameter (for fully confined blast loading) was obtained. By employing this new empirical relation, the mitigation effect of fine water mist on confined blast load was determined quantitatively. The use of water mist is a very good alternative to mitigat

Journal article

Sancho A, Cox M, Cartwright T, Davies C, Hooper P, Dear Jet 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.

Journal article

Liu H, Liu J, Kaboglu C, Chai H, Kong X, Blackman BRK, Kinloch AJ, Dear JPet al., 2019, Experimental and numerical studies on the behaviour of fibre-reinforced composites subjected to soft impact loading, Procedia Structural Integrity, Vol: 17, Pages: 992-1001, ISSN: 2452-3216

This paper presents detailed experimental and numerical studies on the behaviour of composite laminates subject to impact loading by soft objectives. In this research, gas-gun experiments employ woven carbon fibre reinforced poly (ether-ether ketone) (CF/PEEK) and gelatine projectiles to study the response of composite laminates under soft impact loading. A high-speed camera is employed to capture the deformation of the projectiles and a three-dimensional (3D) Digital Image Correlation (DIC) system is used to record the deformation of the impacted composite specimens. A numerical finite-element (FE) model, in which Hashin’s theory is employed for composite modelling and Smooth Particle Hydrodynamics (SPH) method is used for modelling the gelatine projectile and is developed to simulate the soft impact on the composite laminates. The capability of the developed model is validated using the gas-gun experimental results. Good agreement has been shown between the predictions from using the FE model and the experimental results.

Journal article

Zhou J, Liu J, Zhang X, Yan Y, Jiang L, Mohagheghian I, Dear J, Charalambides Met al., 2019, Experimental and numerical investigation of high velocity soft impact loading on aircraft materials, Aerospace Science and Technology, Vol: 90, Pages: 44-58, ISSN: 1270-9638

Bird strike on aircraft remains a serious threat to flight safety. Experimental investigations employing real birds are associated with high cost and low reproducibility. Therefore, physical substitute materials are often used instead of real birds. This study investigates the soft impact loading on aluminium and laminated glass targets from ballistic gelatine and rubber projectiles. The two targets simulate strike on the aircrafts' fuselage and windshield respectively. The full field out of plane displacements of the targets were recorded for velocities 110 to 170 m s−1 using digital image correlation during gas gun experiments. A simulation model based on Smoothed Particle Hydrodynamics was developed and validated against the experimental data from all four projectile-target material combinations. It was shown that for the same momentum, a rubber projectile exerts a higher pressure on a target as compared to gelatine, even though the out of plane displacements and in-plane strains are similar. This led to fractures in the impacted laminated glass when rubber was used. The study offers new experimental data as well as efficient design modelling tools to mitigate damage imposed during bird strike. The models provide a way towards enabling the optimisation of real, large scale aircraft structures and components.

Journal article

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