234 results found
Ding Y, Liu J, Hall Z, et 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.
Irven G, Carolan D, Fergusson A, et 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.
Zhang W, Zhang D, Zhou J, et al., 2023, Damage mechanisms of composite laminates under impact loading including the effect of pre-load, Thin Walled Structures, 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.
Rolfe E, Quinn R, Irven G, et 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.
Irven G, Whitehouse A, Carolan D, et 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.
Kong X, Zhou H, Zheng C, et 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.
Hall Z, Liu J, Brooks R, et 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.
Zhou J, Liu H, Dear JP, et 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.
Tüfekci M, Özkal B, Maharaj C, et 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.
Liu H, Liu J, Hall ZEC, et 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.
Irven G, Carolan D, Fergusson A, et 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.
Samieian MA, Cormie D, Smith D, et 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.
Irven G, Carolan D, Fergusson A, et 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.
Liu H, Brooks R, Hall Z, et 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.
Brooks R, Wang H, Ding Z, et 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.
Siu D, 2022, Characterising Plastic Deformation in Metallic Materials using Uniaxial Tensile Tests and Microstructural Investigations
Hall Z, Liu J, Brooks R, et 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.
Zheng J, Maharaj C, Liu J, et 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.
Brooks RA, Liu J, Hall ZEC, et 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.
Liu H, Blackman B, Kinloch AJ, et 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.
Andrews D, Bourne N, Brown E, et 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.
Kaboglu C, Liu J, Liu H, et 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.
Irven G, Duncan A, Whitehouse A, et 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.
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.
Zhang P, Kong X, Wang Z, et 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.
Tufekci M, Rendu Q, Yuan J, et 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.
Liu H, Liu J, Ding Y, et 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.
Liu H, Liu J, Ding Y, et 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.
Rolfe E, Quinn R, Irven G, et 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
Liu H, Liu J, Ding Y, et 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.
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