Imperial College London

John P. Dear FREng

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
Year
to

238 results found

Evans E, Brooks R, Liu J, Hall Z, Liu H, Lowe T, Withers P, Kinloch A, Dear Jet al., 2024, Comparison of X-ray computed tomography and ultrasonic C-scan techniques and numerical modelling of impact damage in a CFRP composite laminate, Applied Composite Materials, Vol: 31, Pages: 249-264, ISSN: 0929-189X

Two different experimental techniques are employed to visualize the impact damage generated by a low-velocity impact on a carbon-fibre reinforced-polymer (CFRP) composite laminate. At the relatively low impact-velocity of 1.69 m.s-1, and a corresponding impact energy of 7.5 J, used in the present work the damage induced in the CFRP panel is barely visible to the naked eye but the techniques of ultrasonic C-scan and X-ray computed tomography (CT) can detect the damage that has occurred. This damage is mostly interlaminar damage, i.e. delaminations, between the plies due to a change in modulus from one ply to the next in the laminate. This interlaminar damage is usually accompanied by intralaminar damage, e.g. matrix cracking, in the ply itself. The type and extent of damage detected from using these two techniques is discussed and the relative merits of these techniques are compared. In general, the CT gave the better resolved picture of damage but the lateral extent of the damage was underestimated relative to C-scan which was more sensitive to very fine delamination cracks. In addition, a numerical approach, based on a finite-element analysis model, is employed to predict the type, location and extent of damage generated by the impact event and the modelling predictions are compared to the experimental results.

Journal article

Tufekci M, Sun Y, Yuan J, Maharaj C, Liu H, Dear J, Salles Let al., 2024, Analytical Vibration Modelling and Solution of Bars with Frictional Clamps, Journal of Sound and Vibration, ISSN: 0022-460X

Journal article

Tufekci M, Dear J, Salles L, 2024, Forced Vibration Analysis of Beams with Frictional Clamps, Applied Mathematical Modelling

Journal article

Tufekci M, Baytak T, Bulut O, Pir İ, Acarer S, Özkal B, Liu H, Dear J, Salles Let al., 2023, Nonlinear behaviour of epoxy and epoxy-based nanocomposites: an integrated experimental and computational analysis, Mechanics Based Design of Structures and Machines, ISSN: 1539-7734

The focus of this study is on the nonlinear mechanical properties of epoxy and epoxy-based nanocomposites, exploring frequency and strain amplitude dependency.Nanocomposite samples of epoxy are reinforced with fumed silica (FS), halloysite nanotubes (HNT) and Albipox 1000 rubber (Evonik) nanoparticles. Considering these particles have different geometries and stiffnesses, they are expected to have significantly different influences on the mechanics of the resulting composite. To enhance the reliability of the results and to reveal the impact of nanofillers on the mechanics of the material more distinctly, the manufacturing process is designedto be the same for all the specimens within the same material groups to eliminate the effects of the manufacturing process. The comprehensive characterisation process consists of Fourier-Transform InfraRed Spectroscopy (FTIR), Scanning Elec-tron Microscopy (SEM) and Dynamic Mechanical Analysis (DMA). The DMA tests are designed so that the material properties are measured depending on the vibration frequency and strain amplitude. Finally, the characterised nonlinear dynamic properties of these nanocomposites are used as the input material properties into anumerical model. In this simulation, a cantilever beam with representative nonlinear material properties, for these nanocomposites, is created, as example and its forced response is plotted under the same levels of excitation in the frequency domain. Key effects of the different nanofillers are identified using the resonance behaviour, primarily focusing on the stiffness and damping of the epoxy-based nanocomposites.These experimental and numerical procedures followed show the significant impact of the nanoparticle reinforcements on the nonlinear nature of the seepoxy-based composites.

Journal article

Ding Y, Liu J, Hall Z, Brooks R, Liu H, Kinloch A, Dear Jet al., 2023, Damage and energy absorption behaviour of composite laminates under impact loading using different impactor geometries, Composite Structures, Vol: 321, Pages: 1-13, ISSN: 0263-8223

The present paper compares the damage and energy absorption behaviour of composites subjected to low-velocity impact using different frontal geometries for the impactor, with the composites possessing a layup of [02/902]2s. In this study, the rigid impactors with either round-nosed or flat-ended frontal geometry are employed to perform drop-weight tests at various impact energies ranging from 10 to 30 J. The measured loading response and energy absorption are analysed and compared. Additionally, the types and extent of impact-induced damage in the composite specimens are assessed via ultrasonic C-scan, optical microscopy (OM) and scanning electron microscopy (SEM) studies. It is shown that the impact energy threshold for damage initiation is greater than 20 J when using the flat-ended impactor but is less than 10 J when using the round-nosed impactor. In both cases, delamination initiates between the plies in the composite laminate. However, for the flat-ended impactor, the damage behaviour of the fibres exhibits kinking fracture, which differs from the pull-out fibre-fracture caused by the round-nosed impactor. These differences in behaviour are attributed to impactor/composite contact geometry effects which leads to different extents of indentation damage, which in turn directly affects the degree of delamination and fibre damage in the composite.

Journal article

Zhang W, Zhang D, Zhou J, Qi J, Zhao W, Yang S, Dear J, Liu Het al., 2023, Damage mechanisms of composite laminates under impact loading including the effect of pre-load, Thin Walled Structures, Vol: 191, ISSN: 0263-8231

The focus of the present research is on the damage modelling of composite laminates subjectedto Low Velocity Impact (LVI). This research also shows how this model can be employed toassess the effect of pre-load. The composite damage model, integrating a VUMAT subroutineand a cohesive zone model, is first validated against the results obtained from conventionalimpacts on laminates without pre-load. The validated model is then employed to predict theimpact response of pre-loaded composite laminates. It was found that the pre-load influencesthe load response, damage behaviour and energy absorption mechanisms of compositelaminates. This is with pre-tension increases the maximum load during the impact event,whereas pre-compression has the opposite effect. Matrix cracking and delamination are foundto be reduced slightly due to pre-tension, whilst matrix cracking and delamination areconsiderably increased by pre-compression. The developed model can provide guidance to thedesign and maintenance of composite components subjected to pre-load and impact.

Journal article

Irven G, Carolan D, Fergusson A, Dear JPet al., 2023, Digital image correlation of cross-ply laminates in tension to reveal microcracking, Composite Structures, Vol: 319, Pages: 1-12, ISSN: 0263-8223

The use of Digital Image Correlation (DIC) to reveal microstructural damage in cross-ply laminates was investigated. Matrix toughness plays a key role in governing microcracking at the tow level in near-surface plies. Experiments revealed that using a tough epoxy polymer as the matrix of the laminate resulted in increased laminate moduli in the principal directions. DIC provides insights into cross-ply laminate failure; the increase in modulus is attributed to microcrack formation in transverse plies. Early onset of matrix cracking around the tows is revealed by variations in the strain along the gauge length. The use of a tough epoxy polymer delays the load at which this cracking occurs. When an untoughened epoxy polymer is used as the matrix, microcracking can be observed at the beginning of the test, suggesting processing induced damage. The use of toughened polymers as the matrix of composite laminates is recommended to mitigate against this.

Journal article

Irven G, Whitehouse A, Carolan D, Fergusson A, Dear JPet al., 2023, Toughening of face-sheet core bonds in sandwich structures, Engineering Fracture Mechanics, Vol: 290, Pages: 1-14, ISSN: 0013-7944

Methods of improving the toughness of the bond between a foam core and a carbon fibre face-sheet in a sandwich structure were investigated. The Single Cantilever Beam (SCB) on a travelling platform was identified as an appropriate mode-I dominant test-method. The introduction of machined grooves in the foam resulted in a 50% improvement in the measured toughness of the face-sheet-core bond. Toughening of the face-sheets via core–shell rubber particles resulted in a change in fracture locus away from the interface and into the foam. The use of aramid fibre-reinforced foam as the core of the sandwich was also found to improve the interface bond toughness by up to 50%. The fibre-reinforced foams promoted the emergence of R-curve behaviour as the crack propagated.

Journal article

Rolfe E, Quinn R, Irven G, Brick D, Johnston RE, Dear JP, Arora Het al., 2023, Experimental investigation of the air blast performance of hybrid composite skinned sandwich panels with X-ray micro-CT damage assessment, Thin-Walled Structures, Vol: 188, Pages: 1-17, ISSN: 0263-8231

This research investigates the performance of interlaminar hybrid composites as the skins of composite sandwich panels under blast loading with the aim of promoting delamination between dissimilar plies for energy absorption. The deformation of the composite panels was captured using high-speed digital image correlation (DIC). High-speed full-field DIC enables failure to be captured at the moment it occurs across the entire panel. X-ray micro-CT imaging was used to assess the post-blast damage sustained by particular areas of interest from each panel, which were selected based on DIC results. The combination of full-field DIC and detailed X-ray micro-CT scanning enabled a unique comparison of both the global and localised blast resilience of hybrid and conventional composite sandwich panels to be performed.Following a single blast load, the extent of damage to the Hybrid-3B skinned sandwich panel was found to lie between that of GFRP and CFRP skinned sandwich panels. X-ray micro-CT scanning of these panels reveals that there is no continuous damage path through the skin thickness of Hybrid-3B, whereas the GFRP and CFRP panels sustain damage in every ply.Following repeat blast loading, the Hybrid-4 skinned sandwich panel suffered from a front skin crack spanning the length of the panel. Post-blast compressive strength testing reveals that this skin crack and resulting core crack acted as a stress relief, limiting the damage sustained elsewhere in the panel.It was concluded that Hybrid-3B results in a good trade-off between strength and stiffness and is advantageous over conventional CFRP and GFRP panels under a single blast load. Under repeated loading Hybrid-4 offers advantages over Hybrid-3B. Finally, the design of the support structure can significantly aid in blast resilience, and, a holistic approach considering both panels and support should be taken when designing for blast resilience.

Journal article

Kong X, Zhou H, Zheng C, Zhu Z, Wu W, Guan Z, Dear J, Liu Het al., 2023, Dynamic response and failure behaviour of thermoplastic fibre metal laminates subjected to confined blast load, Thin Walled Structures, Vol: 187, Pages: 1-12, ISSN: 0263-8231

Due to the confinement effect, the blast load from a confined space explosion tends to cause more severe damage to structures. When subjected to this type of confined explosion, thermoplastic fibre–metal laminate (TFML) panels would experience repeated shock waves and relatively longer durations of quasi-static pressure. A better understanding of the dynamic response and damage mechanism of TFMLs under confined blast loading can greatly benefit the design of composite structures with improved blast resistance. In the present work, TFMLs with seven different configurations were designed and fabricated. Confined-blast experiments with various masses of trinitrotoluene (TNT) were performed on these TFMLs. The experimental results, including deflection time histories and X-ray computed tomography (CT) images, have been applied to the development of a method for predicting the dynamic response of laminates under confined blast loads. The findings of this work will assist in the rapid assessment of the deformation of fibre–metal laminates and assist in the pre-design of laminate structures in confined explosions.

Journal article

Hall Z, Liu J, Brooks R, Liu H, Dear Jet al., 2023, Impact testing on the pristine and repaired composite materials for aerostructures, Applied Mechanics, Vol: 4, Pages: 421-444, ISSN: 2673-3161

Aircraft technologies and materials have been developing and improving drastically over the last hundred years. Over the last three decades, an interest in the use of composites for external structures has become prominent. For this to be possible, thorough research on the performance of composite materials, specifically the impact performance, have been carried out. For example, research of impact testing for pristine carbon-reinforced epoxy composites mentions matrix cracks, fibre fracture, and delamination as the failure modes that require monitoring. In addition, thorough testing has been carried out on composites repaired with an adhesive bond to observe the effects of conditioning on the adhesively bonded repair. The results suggest there are no major changes in the adhesive under the testing condition. By reviewing the impact testing on the pristine and repaired composite materials for aerostructures, this paper aims to illustrate the main findings and also explore the potential future work in this research scope.

Journal article

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

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

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

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

Siu D, 2022, Characterising Plastic Deformation in Metallic Materials using Uniaxial Tensile Tests and Microstructural Investigations

Thesis dissertation

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

Brooks RA, Liu J, Hall ZEC, Liu H, Blackman BRK, Kinloch AJ, Dear JPet al., 2022, IMPACT OF COMPOSITE REPAIRS: INDENTATION, PLASTICITY, INTRALAMINAR AND INTERLAMINAR DAMAGE, Pages: 182-189

The present paper explores the impact behaviour of repaired carbon fibre-reinforced plastic (CFRP) composite laminates. In particular, the relationship between indentation size and the level and type of damage formed in continuous CFRP material under low-velocity impact loading is investigated. Repairs can be performed on previously impacted CFRP composite by removing the damaged material and bonding a patch of the same CFRP over the top of the damage hole. In some cases, a plug is added to fill the hole. Generally, an increase in indentation depth appears to correlate with an increased delamination damage area. This gives the potential for a quick and economical method to identify damage level in impacted components.

Conference paper

Liu H, Blackman B, Kinloch AJ, Dear Jet 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

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