Publications
189 results found
Whitehouse AD, Medeau V, Mencattelli L, et al., 2023, A novel profiling concept leading to a significant increase in the mechanical performance of metal to composite joints, Composites Part B: Engineering, Vol: 261, Pages: 1-15, ISSN: 0961-9526
Traditional adhesive joints with straight edged adherends suffer from a significant stress concentration in the composite coincident with the edge of the metal adherend, which can lead to accelerated translaminar failure of the substrate. In this work, we developed a novel profiling concept which improves the mechanical performance of adhesive joints between metallic adherends and composite substrates. We conducted quasi-static four-point bending (4PB) tests which showed that profiling the edge of the metallic adherend could improve the peak load by at least 27%, and that the stability of failure was simultaneously improved. We investigated varying the profile parameters and were able to conclude that further significant mechanical performance gains could be achieved by increasing any of the profile: amplitude, frequency, or number of fractal length-scales. By analysing in-situ acoustic emission (AE) monitoring data we were able to observe that profiling of the metallic adherend results in failure initiation occurring at higher loads, which suggests that the concept is successful in providing better stress distributions and lowering peak stresses. By analysing the fracture surfaces, it is apparent that the profiling concept is successful in deflecting the translaminar fracture path; and additionally that a debonding mechanism occurs at the profile tips which is thought to be an important additional mechanism for creating damage tolerant joints.
Sun F, Penchev P, Pruncu CI, et al., 2023, On enhancement of fracture resistance of adhesive joints by surface micropatterning using a femtosecond laser, Journal of Materials Processing Technology, Vol: 315, Pages: 1-12, ISSN: 0924-0136
This study focuses on the influence of surface micropatterns, including uniform and nonuniform grooves fabricated by selective removal of a designed volume from aluminum alloy substrates using a femtosecond laser, on the mode I fracture behavior of adhesively bonded interfaces. The morphology, wettability, chemistry and microstructure of the patterned surfaces have been analyzed. The mode I fracture behavior of adhesive joints was characterized by measuring the fracture resistance using a J-integral approach, and the fracture process in the joint was investigated numerically using a continuum damage model. The results show that the laser patterning has modified the surface roughness, wettability and surface chemistry such that the fracture resistance could be greatly increased. It also reveals the significance of patterning uniformity across the surfaces and the existence of a limiting effective patterning ratio (the ratio of the patterned area to the flat bonding area) on enhancing the fracture resistance. Local plastic deformation that occurred in the adhesive at the patterned structures due to stress concentration was found to be one toughening mechanism although it tended to induce crack growth close to one substrate-adhesive interface.
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.
Brunner AJ, Blackman B, 2023, CURRENT ISSUES IN FRACTURE AND FATIGUE FRACTURE OF FIBER-REINFORCED POLYMER COMPOSITES: AN OVERVIEW
Obtaining sufficiently reproducible quasi-static or fatigue fracture data for fiber-reinforced polymer (FRP) composites is an essential requirement for the design of damage-tolerant FRP components and structures allowing for controlled crack growth in service rather than imposing a "no growth" design requirement. Despite long-term research and development efforts, the different fracture test procedures for FRP composites proposed or standardized remain unsatisfactory. Selected examples will be presented and discussed. Among the issues not yet considered in the standardized testing procedures are fiber-bridging and multiple delaminations, the service environment, possible synergistic effects from material modifications, as well as scatter and sources of scatter in fracture testing and in modelling. Potential approaches to improve testing and reduce scatter are also discussed.
Blackman B, Sun F, Teixeira de Freitas S, et al., 2023, Understanding fracture mode-mixity and its effects on bond performance, Advances in Structural Adhesive Bonding, Second Edition, Pages: 579-613, ISBN: 9780323984379
This chapter discusses the mixed-mode loading of adhesive joints. The importance of mixed-mode loading is first introduced and then test methods commonly used to measure the mixed-mode fracture resistance of adhesive joints are presented and briefly discussed. The approaches to determine the fracture resistance are briefly reviewed and then the partitioning of mixed-mode fracture energies is discussed. The limitations of the local singular field and global approaches to mixed-mode partitioning are discussed and the use and application of a semianalytical cohesive zone analysis partitioning scheme is evaluated. The limitations of the global partitioning approach are further discussed in the context of developing a scheme to design and analyze adhesive joints with dissimilar adherends (a bi-material interface). A longitudinal strain criterion is proposed in addition to the matching of flexural rigidities and the approach is validated numerically. Finally, the practical issues of crack stability, failure path selection, and the use of mixed-mode failure envelopes is considered.
Li C, Viswanathan-Chettiar S, Sun F, et al., 2023, Effect of CFRP surface topography on the adhesion and strength of composite-composite and composite-metal joints, Composites Part A: Applied Science and Manufacturing, Vol: 164, Pages: 1-10, ISSN: 1359-835X
Manufacturing carbon-fibre reinforced polymer (CFRP) composites via different techniques often leads to contrasting surface topographies. Such differences can affect any subsequent surface pre-treatments that are performed and these can ultimately affect joint strength. In the present work, CFRP adherends made using compression moulding or autoclaving were investigated. Pre-treatment techniques of acetone cleaning, plasma treatment, and grit blasting were studied. It was found that the patterned surfaces which had resulted from the vacuum autoclave moulding resulted in improved joint performance when CFRP substrates were bonded together (homogeneous bonding) compared to joints formed with flatter surfaces following compression moulding. However, when CFRP substrates were bonded to flat aluminium alloy substrates (hybrid bonding) the patterned surfaces resulted in inferior joint performance compared to the flatter CFRP substrates. It is proposed that the dissimilarity of surface topographies on the metal and composite substrates negatively influences the strength of the joint.
Whitehouse A, Medeau V, Mencattelli L, et al., 2022, A novel profiling concept leading to a significant increase in the mechanical performance of metal to composite joints, ECCM20 - The 20th European Conference on Composite Materials, Publisher: Composite Construction Laboratory (CCLab)
In this work, we designed metal-CFRP joints with a profiled adherend termination to improve the mechanical performance. We have applied several profiles to the edge of titanium adherends which were adhesively bonded to CFRP substrates. We conducted finite element modelling and experimental 4PB (4-Point-Bend) testing to investigate how the geometry of the adherend edge profile effects the mechanical performance of the joint. This work shows that profiling of the metal adherend can result in increases of at least 27% in the peak load, and of at least 272% in the energy dissipated up to critical failure normalised by the mechanical energy.
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.
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.
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.
Sun F, Kenyon M, Pargeter C, et al., 2022, Performance of Ti/Zr and silane coating pretreatments on adhesive bonding of an automotive aluminium alloy produced using the Hot Form Quench (HFQ®) process, Applied Surface Science, Vol: 575, Pages: 1-11, ISSN: 0169-4332
Surface pretreatments of an automotive aluminium alloy delivered in F temper and subsequently processed using Hot Form Quench (HFQ®), a novel press forming technique combining solution heat treatment, press forming and in-die quenching to produce high-strength aluminium alloys, for adhesive bonding have been explored. The performance of two commercial pretreatments including Ti/Zr and silane coatings, with either acid spray or alkaline immersion cleaning, was investigated. Scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM) were employed to characterize the surface chemistry and topography of the alloy after pretreatments. Adhesively bonded aluminium-aluminium single lap shear joints and double cantilever beam specimens were tested to evaluate the performance of the pretreatments on the bonding strength and fracture resistance, respectively. The FTIR and TEM techniques show that the natural oxides and near-surface deformed layer have been successfully removed, but the surface cleanliness level was sensitive to the cleaning approach adopted. New Ti/Zr oxide layers or silane films have been built upon the aluminium surfaces, which confirms that sound surface conditions for adhesive bonding can be obtained for parts produced using HFQ® from F temper input material.
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.
Li M, Li C, Blackman BRK, et al., 2021, Energy conversion based on bio-inspired superwetting interfaces, Matter, Vol: 4, Pages: 3400-3414, ISSN: 2590-2385
Bio-inspired superwetting interfaces can realize rapid transfer of liquid mass or momentum due to their unique surface structure and wetting characteristics. Combined with a suitably electrified material, these special interfaces can further promote the generation or transmission of electrons. Herein, we summarize the latest developments in water-energy collection technologies based on these interfaces, such as piezoelectric/triboelectric/pyroelectric nanogenerators. When it comes to harvesting energy generated by salinity gradients, reverse electrodialysis based on ion channels is now being widely investigated. We review the concept of “quantum-confined superfluids” on superwetting interfaces, and the conditions required to form a superfluid in molecular and ion channels. The applications of the superfluids in energy conversion are discussed, including the charging and discharging process of lithium batteries and harvesting salinity-gradient energy. This perspective identifies advantages, current challenges, and future directions in the development of energy-conversion devices using superwetting interfaces that could open the door to their broader application.
Li M, Li C, Blackman BRK, et al., 2021, Mimicking nature to control bio-material surface wetting and adhesion, International Materials Reviews, Vol: 67, Pages: 1-24, ISSN: 0950-6608
Nature has developed unique strategies to refine and optimise structural performance. Using surfaces designed at multiple length scales, from micro to nano levels, combined with complex chemistries, different natural organisms can exhibit similar wetting but different adhesion to liquids under specific environments. These biological surfaces have inspired researchers to develop new approaches to control surface wetting and liquid behaviour via surface adhesion. Here we review natural strategies to control the interaction of liquids with solid surfaces and the efforts to implement these strategies in synthetic materials designed to work in either atmospheric or underwater environment. Particular attention is paid to droplet behaviour on the special-adhesion surfaces in nature and artificial smart surfaces. We highlight recent progress, identify the common threads, and discuss the fundamental differences in a way that can help formulate rational approaches towards surface engineering, and identify current challenges as well as future directions for the field.
Sun F, Blackman BRK, 2021, Using digital image correlation to automate the measurement of crack length and fracture energy in the mode I testing of structural adhesive joints, Engineering Fracture Mechanics, Vol: 155, Pages: 1-15, ISSN: 0013-7944
In this study, the crack lengths in adhesively bonded double cantilever beam (DCB) test specimens have been determined using the digital image correlation technique in combination with an elastic foundation model. This method facilitates the continuous measurement of the crack length and offers significant advantages over conventional visual observation including improved accuracy and the potential for automation. This method has been applied to three joints bonded with different structural adhesives, and fracture energy Gc values calculated with the effective crack lengths determined by this method have been shown to be accurate by comparison to Jc values. Finally, a new method is proposed to correct the crack lengths that are visually measured and the Gc values determined using the standard analysis. This scheme is shown to improve the accuracy of the Gc values appreciably.
Li C, Li M, Ni Z, et al., 2021, Stimuli-responsive surfaces for switchable wettability and adhesion, Journal of the Royal Society Interface, Vol: 18, ISSN: 1742-5662
Diverse unique surfaces exist in nature, e.g. lotus leaf, rose petal and rice leaf. They show similar contact angles but different adhesion properties. According to the different wettability and adhesion characteristics, this review reclassifies different contact states of droplets on surfaces. Inspired by the biological surfaces, smart artificial surfaces have been developed which respond to external stimuli and consequently switch between different states. Responsive surfaces driven by various stimuli, e.g. stretching, magnetic, photo, electric, temperature, humidity and pH, are discussed. Studies reporting on either atmospheric or underwater environments are discussed. The application of tailoring surface wettability and adhesion includes microfluidics/droplet manipulation, liquid transport and harvesting, water energy harvesting and flexible smart devices. Particular attention is placed on the horizontal comparison of smart surfaces with the same stimuli. Finally, the current challenges and future prospects in this field are also identified.
Sun F, Zhang R, Blackman BRK, 2021, Determination of the mode I crack tip opening rate and the rate dependent cohesive properties for structural adhesive joints using digital image correlation, International Journal of Solids and Structures, Vol: 217-218, Pages: 60-73, ISSN: 0020-7683
The present work addresses two key issues relating to the study of rate effects in adhesively bonded joints. Firstly, the accurate determination of the crack tip strain rate and secondly the accurate determination of cohesive zone length. The rate-dependent fracture behaviour of adhesive joints bonded with either a toughened epoxy or a ductile polyurethane adhesive was investigated under mode I loading rates ranging from 0.1 mm/min to 1.0 m/s with digital image correlation (DIC) analysis. The traction-separation laws (TSLs) were determined by measuring the J-integral values and the crack tip opening displacements simultaneously. An analytical method is proposed to correlate the crack tip opening velocity with the external loading rate. The lengths of the cohesive zones were measured, and the values were compared with results obtained from thickness-independent and -dependent models. The cohesive properties of the two adhesives exhibited very different rate dependences. The analytical tool developed using the DIC approach successfully determines the strain rates for the TSLs investigated.
Mulakkal M, Castillo Castillo A, Taylor A, et al., 2021, Advancing mechanical recycling of multilayer plastics through finite element modelling and environmental policy, Resources, Conservation and Recycling, Vol: 166, ISSN: 0921-3449
Plastics are attracting negative publicity due to the scale of current pollution levels, yet they are irreplaceable in several applications such as food packaging, where different types of plastics are combined in laminate form to produce multilayered packaging (MLP) materials which extend the life of food items packaged within them. Increased plastic recycling is urgently needed, however for MLP it is particularly difficult. For the first time, this study combines engineering tools with environmental policy towards developing solutions for current single use plastic packaging. This study investigates recycling challenges for MLP and emerging melt-blending based mechanical recycling solutions as this is the main current method for material recovery of conventional plastics. Melt-blending of MLP with compatibilisers is explored, and the current lack of models addressing the influence of compatibilisers is identified. This gap in knowledge is addressed using novel engineering models based on the finite element (FE) micromechanical modelling technique to estimate the mechanical properties of recycled blends. Our model output is compared with experimental data available in literature and the good agreement highlights its predictive ability, providing a fast and cost-effective novel method for optimising recycled plastics. The policy aspect proposes the introduction of twenty policies based on mission-oriented innovation strategy to enable deployment of the recycling technologies studied whilst improving the viability of recycling of material currently not recycled. Implementation of these measures by the stakeholders will enable adoption of new MLP recycling techniques, create demand for recycled materials from MLP and incentivise MLP collection to mitigate pollution.
Baldi F, Agnelli S, Andena L, et al., 2021, Determination of the fracture resistance of ductile polymers: The ESIS TC4 recent experience, Materials Performance and Characterization, Vol: 9
Round-robin (RR) tests carried out under the direction of the Technical Committee 4, "Polymers, Polymer Composites and Adhesives," of the European Structural Integrity Society (ESIS TC4) showed that the multispecimen methodology employed for the construction of the crack growth resistance curve (J vs crack extension, Δa) of polymers often does not provide reliable data because of the uncertainties associated with the measurement of Δa. With this in mind, the ESIS TC4 attention has been more recently focused on the analysis of a testing scheme based on the load separation criterion, which does not require the measurement of Δa.With the aim to employ this new approach into a standardized procedure, the degree of reproducibility of the results obtainable with the application of this testing scheme to ductile polymers has been assessed by means of multilaboratory RR testing exercises that started in 2011. An ESIS TC4 reference draft protocol was prepared and 10 laboratories participated in the RR activities. The present work describes the load separation criterion-based testing procedure recently examined by ESIS TC4 and gives a summary of the results obtained in the RR activities, which appear encouraging.
Brunner AJ, Warnet L, Blackman BRK, 2021, 35 years of standardization and research on fracture of polymers, polymer composites and adhesives in ESIS TC4: Past achievements and future directions, Pages: 443-455
Since its first meeting in 1985, ESIS TC4 has held regular semiannual meetings with between 15 and 35 participants, has organized a series of conferences (the first in 1994, then triennial since 1999) and has developed six ISO test standards on the fracture of polymers, polymer composites and adhesives with another two currently going through ISO standardization and ballots, and several more under development. The activities have also resulted in publications, including two books and two review papers. Initial activities focused on round robins providing test methods for determination of fracture properties for, e.g., technical data sheets, quality assurance, materials selection, or materials development and optimization and materials modelling. These procedures defined standard specimens, test rigs and test conditions. For polymers, standards for specific ranges of loading rate and for composites and adhesively bonded joints, procedures for different loading modes and mode mixes were developed. Recently, standard composite specimens with unidirectional fiber orientation were shown to overestimate the delamination resistance of multidirectional laminates under cyclic fatigue loading. First round robin data from the environmental stress cracking tests show the potential for discriminating between the different susceptibilities of polymers to environmentally induced fracture. Future activities will include elastomeric materials, simulation and modelling in combination with experiments or prediction of fracture behavior. Another topic of recent interest concerns digital tools, e.g., image analysis, automated data acquisition, data fitting and analysis. Guidelines on how to best reduce extrinsic scatter and eliminate human errors will improve the data quality.
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.
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.
Sun F, Pruncu CI, Penchev P, et al., 2020, Influence of surface micropatterns on the mode I fracture toughness of adhesively bonded joints, International Journal of Adhesion and Adhesives, Vol: 103, Pages: 1-11, ISSN: 0143-7496
Surface patterning has been used to enhance the fracture toughness of adhesive joints. In this study, the effect of the variable bondline thickness introduced by the patterns and the effect of pattern geometry on the fracture behaviour of adhesive joints were analysed. Surface patterns including longitudinal grooves, transverse grooves, dimples and grids were fabricated by means of laser texturing. The patterned surfaces were bonded using a tough structural adhesive and the mode I fracture toughness was measured using the J-integral method. The toughness of the patterned joints was compared with the results from bonding with control surfaces and with grit blasted in combination with chromic acid etched (GB-CAE) surfaces. It was shown that both longitudinal and transverse grooves led to the highest value of toughness. Grids patterns yielded a toughness close to the GB-CAE treatment, both of which were higher than the dimple patterns. It was also shown that the variable bondline thickness due to the existence of surface patterns, which influences the size of the plastic zone, reduced rather than increased the fracture toughness.
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.
Li C, Blackman BRK, Shi Z, 2020, Surface pre-treatment methods for composites prior to adhesive bonding, Fiber Composites, Vol: 2020, Pages: 109-122, ISSN: 1003-6423
An automotive or aircraft component requires the inclusion of some light-weight high-performance materials, e.g. carbon fibre reinforced polymer (CFRP), a.luminium alloy, titanium alloy. Adhesive bonding, as a satisfactory technique for joining two materials, has attracted more and more attentions from researchers and manufacturers. Unlike welding, bolling, or riveting techniques. adhesive bonding docs not destroy the bulk struct11re and prevents galvanic corrosion in tenns of hybrid joining. Prior lo adhesive bonding, surface pre-treatments are necessary. For example, the surface of CFRPs is usually not active. Also. contaminants may affect the joint performance after adhesive bonding. Surface pre-treatments for CFRPs aim to remove any surface contaminants and also to modify the surface. They optimise surface topography and/or chemical composition to obtain a closer interaction with adhesives, i.e. mechanical interlocking and chemical bonding. In this report, surface pre-treatment methods for composites prior to adhesive bonding will be introduced, including solvent cleaning, chemical treatment, abrasion, peel-ply method. plasma treatment. and laser ablation. Relevant studies will be reviewed and summarized. The effectiveness. compatibility, economy, and enviromnental concerns of these methods will be analysed and compared. This report will also provide suggestions for further research and development of the methods, based on both findings of the research papers and the applications in the industrial production.
Liu H, Liu J, Ding Y, et 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.
Liu H, Liu J, Ding Y, et 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.
Sun F, Blackman BRK, 2020, A DIC method to determine the Mode I energy release rate G, the J-integral and the traction-separation law simultaneously for adhesive joints, Engineering Fracture Mechanics, Vol: 234, Pages: 1-13, ISSN: 0013-7944
The quasi-static Mode I fracture behaviour of joints bonded with either a brittle or toughened epoxy adhesive or a ductile polyurethane adhesive has been investigated by means of digital image correlation (DIC). A novel method to measure the crack length using DIC analysis is proposed. By measuring the crack tip separation, beam rotation and crack length, the energy release rate G and the J-integral are obtained and are compared to analyse the validity of Linear Elastic Fracture Mechanics (LEFM) methods. Simultaneously the traction-separation laws (TSLs) for the adhesive joints were measured. The TSLs were then used as input data for FE modelling to evaluate their accuracy by comparing with experimental results. It is shown that LEFM is valid for the joints bonded with either the brittle or toughened epoxy adhesives but is invalid for joints bonded with the polyurethane adhesive. The procedure proposed here to measure the crack length via DIC shows great promise and can be automated readily in practice.
Plant D, blackman B, Leevers P, 2020, Mechanical and rheological testing to develop thermoplastic elastomer-polyborodimethylsiloxane blends for personal impact protection, Polymer Testing, Vol: 86, Pages: 1-10, ISSN: 0142-9418
To claim ‘wearability’, clothing for protection against impact injury must not only act locally as a rigid shell under impact in order to satisfy drop-weight tests, but also flex freely with normal movement and offer permeability and light weight. Previous materials used carrier textiles coated with ‘dilatant’ but mechanically weak polyborodimethylsiloxane (PBDMS) compounds. We outline a procedure for developing blends of PBDMS with thermoplastic elastomers which, while minimizing the force transmitted in standard drop-weight tests on monolithic plate, can also be injection moulded into more wearable 3D shell structures. The first successful blends were developed by trial-and-error, involving the melding and testing of many plate specimens. The new procedure uses dynamic mechanical thermal analysis and time-temperature superposition to calibrate Zener-solid models for specific conditions within the service temperature and impact-speed envelope. Each model material is then subjected to a virtual drop-weight impact test to estimate the peak transmitted impact force. These results guide the selection of blends suitable for further development, correlating well with those obtained using the previous, more laborious procedure. When distributed within a suitable blend, PBDMS contributes considerably greater impact force attenuation than bulk uniaxial tests indicate.
Liu H, Liu J, Kaboglu C, et 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.
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