112 results found
Domun N, Kaboglu C, Paton KR, et al., 2019, Ballistic impact behaviour of glass fibre reinforced polymer composite with 1D/2D nanomodified epoxy matrices, Composites Part B: Engineering, Vol: 167, Pages: 497-506, ISSN: 1359-8368
In this paper, experimental studies on the ballistic impact behaviour of nanomodified glass fibre-reinforced polymer (GFRP) are reported. The epoxy matrix of the GFRP was modified by the addition of graphene platelets (GNPs), carbon nanotubes (CNTs), combined hybrid hexagonal boron nitride nanosheets (BNNS)/CNT, and combined boron nitride nanotubes (BNNTs)/GNPs nanoparticles.Ballistic impact tests were carried out on GFRP laminates at two projectile velocities of 76 ± 1 m s−1 for full-field deformation measurements and 134.3 ± 1.7 m s−1 for perforation tests. The behaviour of the plates during impact was recorded using digital image correlation (DIC), in order to monitor strain and out-of-plane deformation in panels with nanoreinforced matrices. Following penetrative impact tests, pulse thermography was used to characterise the delamination of impacted plates. The results of full-field deformation, exit velocity and energy absorption measurements from the ballistic tests show significant improvements in impact resistance for the panels made from nanomodified epoxies relative to laminates with the unmodified epoxy matrix. The highest absolute absorbed energy was observed for the GFRP panels fabricated using the epoxy matrix loaded with BNNT/GNP at 255.7 J, 16.8% higher than the unmodified epoxy matrix.
Samieian M, Cormie D, Smith D, et al., On the bonding between glass and PVB in laminated glass, Engineering Fracture Mechanics, ISSN: 0013-7944
In blast protective design, laminated glass is used to facilitate the safety of building occupants. Laminated glass provides its safety through the maintenance of the bond between the glass and the interlayer, and also through the deformation of the interlayer. The amount of deformation is related to the stretching of the interlayer, which is related to the amount of adhesion between the glass and the interlayer. An experimental and modelling study has taken place on the bond between the glass and the interlayer at different testing rates and temperatures. Tensile tests on cracked laminated glass and pure PVB were carried out. These tests were coupled with fracture mechanics methods to calculate a bond fracture toughness. This bond fracture toughness was used to develop a finite element model to predict the separation between the glass and the interlayer. From the experimental studies it was found that the adhesion between the glass and the interlayer is temperature independent in the range of 20oC-60oC at a constant testing rate. In contrast, at a constant temperature the adhesion was found to be loading rate dependent. The finite element model developed showed good consistency with experimental data for a range of testing rates and temperatures.
Liu H, Falzon B, Li S, et al., Compressive failure of woven fabric reinforced thermoplastic composites with an open-hole: an experimental and numerical study, Composite Structures, ISSN: 1879-1085
This research presents a detailed experimental and numerical study on the compressive failure of woven fabric reinforced thermoplastic composites, with an open-hole and with a pinned open-hole. The experimental evaluations are performed on the composite specimens using the Combined Loading Compression (CLC) evaluation method. Experimental results, including load response and damage morphology, are obtained and analysed. A novel meso-scale damage model is developed, based on Continuum-Damage-Mechanics (CDM), for predicting damage in woven fabric reinforced composites. The developed model, which can capture fibre fracture and matrix cracking, as well as the nonlinear response within the woven composite materials, is employed to conduct virtual Combined Loading Compression (CLC) tests. Numerical simulation results are compared with the extracted experimental results for model validation. Good correlation is achieved between experimental and computational results for both the open-hole and the pinned open-hole, with a two-stage failure process being observed for the pinned open-hole.
Samieian M, Cormie D, Smith D, et al., Prediction of blast response in laminated glass, Engineering Structures, ISSN: 0141-0296
Toolabi M, Blackman BRK, 2018, Guidelines for selecting the dimensions of adhesively bonded end-loaded split joints: An approach based on numerical cohesive zone length, Engineering Fracture Mechanics, Vol: 203, Pages: 250-265, ISSN: 0013-7944
Recent attempts to measure the mode II fracture resistance of structural adhesive joints employing highly toughened adhesives have been hindered by the formation of extensive damage zones ahead of the crack tip. This can lead to strongly rising resistance curves being generated and to the steady-state not being attained. In this paper, a cohesive zone element formulation has been implemented in Abaqus v6.14 via a user element subroutine to determine the length of fracture process zone. It has been demonstrated that the existing analytical expressions cannot predict the length of fracture process zone accurately; hence a formulation for the length of the fracture process zone in an end-loaded split (ELS) joint specimen has been derived using a numerical parametric method. The results obtained from the proposed formulation correlate very well with those from numerical results. The results have allowed guidelines to be developed for selecting the dimensions of an ELS test specimen, comprising fibre-composite substrates bonded with a toughened adhesive, which ensure the attainment of the steady-state and hence a plateau in crack growth resistance curve is ensured. This removes a significant obstacle in the definition of a successful standard method test.
Kaboglu C, Yu L, Mohagheghian I, et al., 2018, Effects of the core density on the quasi-static flexural and ballistic performance of fibre-composite skin/foam-core sandwich structures, Journal of Materials Science, Vol: 53, Pages: 16393-16414, ISSN: 0022-2461
Polymeric foams are extensively used as the core materials in sandwich structures and the core material is typically bonded between relatively thin fibre-composite skins. Such sandwich structures are widely used in the aerospace, marine and wind-energy industries. In the present work, various sandwich structures have been manufactured using glass-fibre-reinforced polymer (GFRP) skins with three layers of poly(vinyl chloride) foam to form the core, with the densities of the foam layers ranging from 60 to 100 kg/m3. This study has investigated the effects on the quasi-static flexural and high-velocity impact properties of the sandwich structures of: (a) the density of the polymeric-foam core used and (b) grading the density of the foam core through its thickness. The digital image correlation technique has been employed to quantitatively measure the values of the deformation, strain and onset of damage. Under quasi-static three-point and four-point bend flexural loading, the use of a low-density layer in a graded-density configuration reduced the likelihood of failure of the sandwich structure by a sudden force drop, when compared with the core configuration using a uniform (i.e. homogenous) density layer. The high-velocity impact tests were performed on the sandwich structures using a gas-gun facility with a compliant, high-density polyethylene projectile. From these impact experiments, the graded-density foam core with the relatively low-density layer located immediately behind the front (i.e. impacted) GFRP skin was found to absorb more impact energy and possess an increased penetration resistance than a homogeneous core structure.
Alvarez D, Guild FJ, Kinloch AJ, et al., Partitioning of mixed-mode fracture in adhesively-bonded joints: experimental studies, Engineering Fracture Mechanics, ISSN: 0013-7944
The fracture resistance, Gc, of adhesively bonded joints is known to vary significantly with loading mode and because joints are usually subject to mixed-mode loading in service, it is essential to be able to partition Gc into its I/II components so the appropriate failure criterion can be derived. Various partitioning schemes have been proposed in the literature but they give different results and which is most suitable has been an open question. In this work, joints were tested under a wide range of I/II loading ratios and the various partition schemes were assessed. The singular and global schemes were found to define the extremes, while Davidson’s non-singular field scheme represents an intermediate case. The semi-analytical scheme proposed by Conroy et al. exhibited a gradual transition from the singular to the global solutions and was shown to be the best approach.
Kaboglu C, Mohagheghian I, Zhou J, et al., 2017, High-velocity impact deformation and perforation of fibre-metal laminates, Journal of Materials Science, Vol: 53, Pages: 4209-4228, ISSN: 0022-2461
The quasi-static flexural and impact performance, up to projectile impact velocities of about 270 m s−1, of fibre metal laminates (FMLs), which consist of relatively thin, alternately stacked, layers of an aluminium alloy and a thermoset glass fibre epoxy composite, have been investigated. The effects of varying (a) the yield strength, tensile strength and ductility of the aluminium alloy layer, (b) the surface treatment used for the aluminium alloy layers and (c) the number of layers present in the FML have been studied. It was found that increasing the strength of the aluminium alloy increases the quasi-static flexural strength of the FML, providing that good adhesion is achieved between the metal and the composite layers. Further, increasing the number of alternating layers of the aluminium alloy and fibre composite also somewhat increases the quasi-static flexural properties of the FML. In contrast, increasing the strength of the aluminium alloy had relatively little effect on the impact perforation resistance of the FML, but increasing the number of alternating layers of aluminium alloy and fibre composite did significantly increase the impact perforation resistance of the FML. The degree of adhesion achieved between the layers had only a negligible influence on the impact perforation resistance.
Samieian MA, Cormie D, Smith D, et al., 2017, Temperature effects on laminated glass at high rate, International Journal of Impact Engineering, Vol: 111, Pages: 177-186, ISSN: 0734-743X
The load bearing capacity of a laminated glass pane changes with temperature. In blast protection, laminated glass panes with a Polyvinyl Butyral (PVB) interlayer are usually employed. The post-crack response of the laminated pane is determined by the interlayer material response and its bond to the glass plies. An experimental study has been performed to determine the effects of temperature on the post cracked response of laminated glass at a test rate of 1 m/s for PVB thicknesses of 0.76 mm, 1.52 mm and 2.28 mm. Tensile tests were carried out on single cracked and randomly cracked samples in a temperature range of 0 °C–60 °C. Photoelasticity observation and high speed video recording were used to capture the delamination in the single cracked tests. Competing mechanisms of PVB compliance and the adhesion between the glass and PVB, were revealed. The adhesion showed an increase at lower temperatures, but the compliance of the PVB interlayer was reduced. Based on the interlayer thickness range tested, the post-crack response of laminated glass is shown to be thickness dependent.
Xiong W, Wang X, Dear JP, et al., 2017, The effect of protrusion density on composite-metal joints with surfi-sculpt reinforcement, Composite Structures, Vol: 180, Pages: 457-466, ISSN: 1879-1085
The effect of protrusion density on the static mechanical properties of composite-metal joints strengthened by surfi-sculpt protrusions has been experimentally studied with single lap joints. The CFRP composite adherends were constant thickness with a quasi-isotropic layup. The metallic adherends were Ti-6Al-4V alloy with a variable number of protrusions per unit area, manufactured by electron beam surfi-sculpt. Digital image correlation was used to measure the debonding on the overlap during the tests. Although the surfi-sculpt protrusions did not significantly affect the onset of debonding, they did resist the initial unstable failure mechanism and converted it into stable growth. The analysis indicated that the efficiency of the surface protrusions was different at the metal and composite ends of the overlap. This finding opens the possibility to vary the protrusion density across the overlap to meet specific damage tolerance criteria and optimise joint efficiency. Increasing the protrusion density significantly increased the ultimate failure load, joint extension and hence absorbed energy.
Kaboglu C, Liu J, Blackman BRK, et al., The effects of different types of architecture of composite and matrix on the impact behaviour of Carbon Fibre Reinforced Composites, ICAS2017 - International Conference on Advances in Science
Kamaludin MA, Patel Y, Williams JG, et al., 2017, A fracture mechanics approach to characterising the environmental stress cracking behaviour of thermoplastics, Theoretical and Applied Fracture Mechanics, Vol: 92, Pages: 373-380, ISSN: 0167-8442
Environmental stress cracking (ESC) is known to affect certain thermoplastics and occurs under simultaneous exposure to both applied stress and a hostile environment. The combination of these can cause a crack to form from a flaw in the material; upon reaching a critical size, the crack may accelerate thus causing catastrophic failure in the component. Various tests have been utilised to measure the resistance of different polymers to ESC, but these are often material- and application-specific and overlook the different stages of the failure process. In the present work, a fracture mechanics approach has been developed and applied, with a view to developing a test method that has wide applicability and provides both insight into the failure mechanisms as well as information for engineering design. Experimental results are presented for the following polymer-environment combinations: linear low-density PE in Igepal solution, HIPS in sunflower oil, and PMMA in methanol. It is shown that the representation of the results in the form of G versus crack velocity and G versus time can distinguish between materials of varying ESC resistance, identify the important regions of the failure process, and enable component life prediction.
Wang X, Ahn J, lee J, et al., 2017, Investigation on failure modes and mechanical properties of CFRP-Ti6Al4V hybrid joints with different interface patterns using digital image correlation, Materials & Design, Vol: 101, Pages: 188-196, ISSN: 0261-3069
An advanced hybrid joining technology for joining metal and composite is introduced. Protrusions formed on the surface of the metal by electron beam are embedded into carbon fibre reinforced polymer layers and thus forms an integrated joint. The performance of a joint produced using such method was found to be better than traditional joints. In this paper, the properties of two different patterns of protrusions, including a linear pattern and a cylindrical pattern were studied by uniaxial tensile testing of double lap composite structures using digital image correlation. The distributions of strains in the composites tested varied and were found to be influenced by the shape of the protrusions which also resulted in different failure modes. The joints with a linear pattern failed between laminate layers, whereas the joints with a cylindrical pattern fractured at the interface of metal and composite. Furthermore, the tensile properties such as the ultimate tensile strength and elongation to failure of the joint with the linear pattern were around twice the value of the joint with the cylindrical pattern. Consequently, the performance of hybrid joints can be improved significantly by optimising the protrusion pattern.
Jia L, Yu L, Zhang K, et al., 2016, Combined modelling and experimental studies of failure in thick laminates under out of plane shear, Composites Part B - Engineering, Vol: 105, Pages: 8-22, ISSN: 1359-8368
A multi-scale model validated with out-of-plane shear testing is presented to analyse thick composite structural failure. Key features of this multi-scale analysis approach are inclusion of shear non linearity and modelling the response at a sub-laminate level whilst the structural failure is predicted at a ply level. Based on this multi-scale approach, a user-defined FORTRAN subroutine (VUMAT) has been written for ABAQUS/EXPLICIT solver and is used to model the shear nonlinearity and intra-laminar failure. In addition, a cohesive zone model is used to predict the inter-laminar delamination. The modelling has been employed to predict the failure processes for Iosipescu shear test specimens with different fibre orientations. The results show that both the failure mode and the load-displacement trace for finite element simulations agree closely with the experimental findings. This demonstrates the validity of this multi-scale, nonlinear, three-dimensional model for thick laminates. In particular, for the Iosepescu shear test, the effect of the fibres being aligned along the length of the specimen or out-of-plane is investigated as well as different dimensions of the specimen. These simulations are validated by experiments using Digital Image Correlation (DIC).
Kamaludin MA, Patel Y, Blackman BRK, et al., 2016, Fracture mechanics testing for environmental stress cracking in thermoplastics, Procedia Structural Integrity, Vol: 2, Pages: 227-234, ISSN: 2452-3216
Under the combined influence of an aggressive environment and applied stress, engineering thermoplastics may undergo a phenomenon known as environmental stress cracking (ESC). This can result in adverse effects such as embrittlement and premature failure in service, due to the growth of environmentally-induced cracks to critical sizes, with little to no fluid absorption in the bulk material. Fracture mechanics is proposed as a suitable scheme to study and quantify ESC, with the aim being to obtain characterising data for different polymer-fluid combinations of interest, as well as to develop a reliable fracture mechanics test protocol. In the proposed method, slow crack growth is monitored to assess the effect of a range of applied crack driving forces (K, or alternatively G) on observed crack speeds, as opposed to simply measuring time-to-failure. This paper presents the results of experiments performed on the following materials: linear low density polyethylene (LLDPE) in Igepal solution and high impact polystyrene (HIPS) in sunflower oil. A discussion of the various issues surrounding the data analysis for these long-term tests is also included, as the attainment of consistent and repeatable results is critical for a method to be internationally standardised, which is a goal of the European Structural Integrity Society (ESIS) Technical Committee 4 from whose interest this work is drawn.
Del Linz P, Wang Y, Hooper PA, et al., 2016, Determining Material Response for Polyvinyl Butyral (PVB) in Blast Loading Situations, Experimental Mechanics, Vol: 56, Pages: 1501-1517, ISSN: 1741-2765
Protecting structures from the effect of blast loads requires the careful design of all building components. In this context, the mechanical properties of Polyvinyl Butyral (PVB) are of interest to designers as the membrane behaviour will affect the performance of laminated glass glazing when loaded by explosion pressure waves. This polymer behaves in a complex manner and is difficult to model over the wide range of strain rates relevant to blast analysis. In this study, data from experimental tests conducted at strain rates from 0.01 s−1 to 400 s−1 were used to develop material models accounting for the rate dependency of the material. Firstly, two models were derived assuming Prony series formulations. A reduced polynomial spring and a spring derived from the model proposed by Hoo Fatt and Ouyang were used. Two fits were produced for each of these models, one for low rate cases, up to 8 s−1, and one for high rate cases, from 20 s−1. Afterwards, a single model representing all rates was produced using a finite deformation viscoelastic model. This assumed two hyperelastic springs in parallel, one of which was in series with a non-linear damper. The results were compared with the experimental results, assessing the quality of the fits in the strain range of interest for blast loading situations. This should provide designers with the information to choose between the available models depending on their design needs.
Del Linz P, Hooper PA, Arora H, et al., 2016, Delamination properties of laminated glass windows subject to blast loading, International Journal of Impact Engineering, Vol: 105, Pages: 39-53, ISSN: 1879-3509
Delamination processes absorb significant amounts of energy in laminated glass windows when they are subjected to blast loads. Blast tests were performed previously and their results had been used to calculate the loads imposed on the support systems. In this research, the delamination process at realistic deformation rates was studied to understand the reaction force response obtained. Laboratory tensile tests were performed on pre-cracked laminated glass specimens to investigate their delamination behaviour. The experiments confirmed the presence of a plateau in the force-deflection graphs, suggesting that the delamination process absorbed significant energy. The experimental results were then employed to calibrate FEA models of the delamination process with the aim of estimating the delamination energy of the polyvinyl butyral (PVB) membrane and glass layers and its relationship with deformation speed. The delamination energies obtained through this research, if used with the appropriate PVB material model, are a valuable new tool new tool in the modelling and design of laminated glass façade structures.
Wang X, Ahn J, Kaboglu C, et al., 2015, Characterisation of composite-titanium alloy hybrid joints using digital image correlation, Composite Structures, Vol: 140, Pages: 702-711, ISSN: 0263-8223
An advanced Comeld hybrid joining technology for joining Ti6Al4V and carbon fibre reinforced polymer (CFRP) is presented. The Comeld joining method utilises protrusions formed on the surface of the metal by electron beam surface treatment which are then embedded into composite layers to form an integrated joint by co-curing. In this paper, joints were formed with wedge profiled protrusions on the metal surface and during tensile testing the protrusions resisted fracture in all tests, leading to failure in the composite part. A digital image correlation (DIC) technique was used to monitor full field, in-plane strain during the tests and to also identify the development of failure modes. The results from specimens with protrusions exposed on the side surface by grinding showed that the embedded protrusions are effective in reducing stress concentration near the metal composite interface and therefore contributed to improved mechanical properties of the hybrid joints.
The paper extends the notion of steady-state cutting of polymers with a sharp tool to scratching. The analysis assumes there is separation at the tool tip (fracture) and the removed layer undergoes plastic shear. Results are presented for three polymers: PMMA, PC and PBT. For the tougher polymer, PC, smooth scratches were obtained and the modified cutting analysis works well provided that the wear on the initially sharp tip is accounted for. For the more brittle polymers, PMMA and PBT, rougher scratches were obtained and this is consistent with the notion that the polymers exhibited micro-cracking ahead of the tool tip, which led to rough surfaces being generated. The results demonstrate that the fracture toughness and the yield stress are controlling parameters in the scratching process and that a sufficiently high value of crack opening displacement COD (greater than about 10 μm) ensures that smooth scratches are obtained, as was the case for PC.
Blackman BRK, Steininger H, Williams JG, et al., 2015, The fatigue behaviour of ZnO nano-particle modified thermoplastics, Composites Science and Technology, Vol: 122, Pages: 10-17, ISSN: 0266-3538
The present paper describes an investigation into the effects on the fatigue behaviour of adding up to 1.00 vol.% ZnO nano-rods with a diameter of 13 nm and an aspect ratio of three to two amorphous styrene acrylonitrile copolymers. Two acrylonitrile contents, i.e. 24% and 34% respectively, were studied. Fracture mechanics based fatigue tests were conducted at 5 Hz and a Paris Law analysis approach was followed. The fatigue threshold increased notably with the addition of the ZnO nano-rods. The increases in toughness were analysed using an analytical model of plastic void growth around the particles and this enabled the surface energy per unit area for the particle-matrix debonding process to be deduced. Good particle distributions were achieved only for very small volume fractions (<0.3%) above which the occurrence of agglomerated particles and consequential toughness declines were observed. In the fatigue threshold region, surface micrographs showed clear evidence of debonding and plastic void growth and the average measured void diameter agreed closely with that predicted by the model.
Del Linz P, Hooper PA, Arora H, et al., 2015, Reaction forces of laminated glass windows subject to blast loads, Composite Structures, Vol: 131, Pages: 193-206, ISSN: 1879-1085
Several blast trials on laminated glass windows have been performed in the past, using both full field 3D Digital Image Correlation and strain gauges located on the supporting structure to collect information on the glass pane behaviour. The data obtained during three blast experiments were employed to calculate reaction forces throughout the perimeter supports both before and after the fracture of the glass layers. The pre-crack experimental data were combined with finite element modelling results to achieve this, whilst solely experimental results were employed for post-cracked reactions. The results for the three blast experiments were compared to identify similarities in their behaviour. It is intended that the results can be used to improve the existing spring–mass systems used for the design of blast resistant windows.
Xiong W, Blackman B, Dear J, et al., 2015, The effect of composite orientation on the mechanical properties of hybrid joints strengthened by surfi-sculpt, Composite Structures, Vol: 134, Pages: 587-592, ISSN: 1879-1085
Advanced hybrid joints strengthened by surfi-sculpt (manufactured by electron beam surfi-sculpt (EBS)) have been developed to address the challenges in joining fibre reinforced polymer (FRP) composites to metals. In the present work, the effect of composite orientation on the mechanical properties of the joints is experimentally studied. In the study, the thickness of the composite adherend is kept constant and the volume content of ±45° ply is increased from 11.1% to 88.9%. Joints without surfi-sculpts are manufactured as reference joints. The result indicates that surfi-sculpt is able to delay the damage initiation and improve the joints ultimate failure load, failure strain, and absorbed energy. Composite orientation is able to vary joints mechanical properties significantly. With the volume content of ±45° ply increase, joints damage mechanisms change from bare composite matrix crush to the combination of surfi-sculpts bending, surfi-sculpts breakage and composite compression. With optimum composite orientation, the joints damage initiation load is increased by 24.84%, the joints ultimate failure load is increased by 134.5% and the joint energy absorption is increased by 257.39%. Scanning electron microscope (SEM) observation shows that surfi-sculpts experience three damage stages.
Zuo K, Blackman BRK, Williams JG, et al., 2015, The mechanical behaviour of ZnO nano-particle modified styrene acrylonitrile copolymers, Composites Science and Technology, Vol: 113, Pages: 9-18, ISSN: 0266-3538
Gao T, Kinloch AJ, Blackman BRK, et al., 2015, A study of the impact properties of adhesively-bonded aluminum alloy based on impact velocity, JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY, Vol: 29, Pages: 493-499, ISSN: 1738-494X
Cui S, Blackman BRK, Kinloch AJ, et al., 2014, Durability of asphalt mixtures: Effect of aggregate type and adhesion promoters, INTERNATIONAL JOURNAL OF ADHESION AND ADHESIVES, Vol: 54, Pages: 100-111, ISSN: 0143-7496
Williams JG, Blackman BRK, Steininger H, et al., 2014, Toughening by plastic cavitation around cylindrical particles and fibres, Composites Science and Technology, Vol: 103, Pages: 119-126, ISSN: 0266-3538
Lenz J, Blackman BRK, Taylor AC, et al., 2014, Selection of test methods to examine the fracture mechanics of carbon fibre composite flywheels
Flywheels are devices that can store energy in the form of kinetic energy whilst allowing for high charge and discharge rates and providing a high efficiency. This makes them extremely suitable for applications in hybrid vehicles within the automotive industry. Yet before application in this industry can take place, the potential for failure and the associated failure mechanics of composite flywheels must be sufficiently well understood to control the risk to passengers. This paper outlines a study that aims to induce failure in CFRP flywheels such that the consequences of failure can be assessed and the implications for flywheel containment can be better understood. Following a review of flywheel failures and a consideration of the stresses in one particular composite flywheel rim, a number of defect scenarios are presented and a test programme is presented based upon these considerations.
Blackman BRK, Guild FJ, 2013, Forced air plasma treatment for enhanced adhesion of polypropylene and polyethylene, Journal of Adhesion Science and Technology, Vol: 27, Pages: 2714-2726, ISSN: 0169-4243
Alvarez D, Blackman BRK, Guild FJ, et al., 2013, Mode I fracture in adhesively-bonded joints: A mesh-size independent modelling approach using cohesive elements, Engineering Fracture Mechanics, Vol: 115, Pages: 73-95, ISSN: 1873-7315
In recent years cohesive elements, coupled with a finite-element analysis (FEA) approach, have become increasingly popular for simulating both delamination in composite materials and fracture in adhesively-bonded joints. However, the industrial application of Cohesive Zone Models to model large and complex structures has been hindered by the requirement of extremely fine meshes along the crack propagation path. In the present work two-dimensional linear and quadratic (i.e. second-order) cohesive elements to model crack initiation and growth have been implemented in Abaqus using a user subroutine. These elements, which have a modified topology that allows a user-defined number of integration points, have been employed to model the fracture response of various mode I test specimens consisting of metallic substrates bonded with a structural film-adhesive. The effects of the mesh-density, element order and number of integration points on the numerical solution have been investigated. Whilst the linear models have shown the typical mesh-size dependent behaviour, the results obtained with their quadratic counterparts have been found to be independent of the element size. Furthermore, it is shown that increasing the number of integration points improves the stability, convergence and smoothness of the solutions. The mesh-size independent response obtained with the quadratic models arises from more accurate simulation of the deformed profile of the substrates and a more accurate calculation of the energy dissipated in the process zone due to damage. Overall, it is demonstrated that the quadratic cohesive-element formulation enables the use of much coarser meshes, resulting in shorter simulation times, and will therefore allow an increase in the industrial application of Cohesive Zone Models.
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