The use of polymer layers to alter the impact response of metallic plates has emerged recently as an effective and economical means to enhance perforation resistance. However, the function of the polymer in such laminate systems remains unclear. In this investigation we aim to identify, through systematic experiments, the influence of a polymer layer on the perforation mechanisms and energy absorption of laminated plates. In particular, we consider the combination of a polymer with a thin metallic plate in a bi-layer configuration, subjected to either quasi-static or impact loading by a blunt indenter. Bi-layers are investigated which comprise an aluminium alloy layer (6082-T6) and a polyethylene layer (LDPE, HDPE and UHMWPE). It is found that the energy required to perforate the bi-layer plate can significantly exceed that of the bare metallic substrate (showing the potential for polymer coatings as an effective retro-fit solution) when the polymer is on the impacted face. Furthermore, bi-layer configurations are also shown to outperform the equivalent mass of monolithic metal if the correct thickness ratio of polymer and metal is selected. The effectiveness of a polymer layer in enhancing perforation energy is connected to its large ductility, allowing extensive deformation of the polymer under the indenter, which in turn suppresses plugging and diffuses plastic deformation in the metal layer. In this way the energy absorbed by the metal layer can be maximised. The thickness of the polymer layer is found to be a crucial parameter in maximising the effectiveness of the bi-layer target. An optimum polymer thickness is observed which maximises energy absorption per unit mass of bi-layer target (for a fixed substrate thickness). The synergy between metal and polymer layers also depends on the polymer type and the rate of loading. A polymer with high strain hardening performs best under impact conditions. However, under quasi-static loading, the bi-layer performance is less sensitive to the yield strength and strain hardening of the polymer.
Recent research has established that polymer?metal laminates are able to provide enhanced impact perforation resistance compared to monolithic metallic plates of the same mass. A number of mechanisms have been proposed to explain this benefit, including the dissipation of energy within the polymer itself, and the polymer deformation enhancing dissipation within the metallic layer. This understanding of the layer interactions and synergies informs the optimisation of the laminate. However, the effect of the nose shape geometry of the projectile on perforation resistance of a particular laminate configuration has not been established. An optimal laminate configuration for one projectile may be sub-optimal for another. This investigation aims to clarify this nose shape sensitivity for both the quasi-static and impact perforation resistance of light-weight polymer?metal laminates. Bi-layer plates are investigated, with a polyethylene layer positioned on either the impacted or distal face of a thin aluminium alloy substrate. Three contrasting nose shapes are considered: blunt, hemi-spherical and conical. These have been shown to induce distinctly different deformation and fracture modes when impacting monolithic metallic targets. For all projectile nose shapes, placing a polyethylene layer on the impacted (rather than distal) face of the bi-layer plate results in an increase in perforation resistance compared to the bare substrate, by promoting plastic deformation in the metal backing. However, the effectiveness of the polymer in enhancing perforation resistance is sensitive to both the thickness of the polymer layer and the nose shape of the projectile. For a thin polyethylene layer placed on the impacted face, the perforation resistance is greatest for the blunt projectile, followed by the hemi-spherical and conical nose geometries. As the thickness of the polymer facing layer approaches the projectile radius, there is a convergence in both failure mode and perforation energy for all three nose shapes. Bi-layer targets can outperform monolithic metallic targets on an equal mass basis, though this is sensitive to the type of polyethylene used, the polymer layer thickness and the projectile nose shape. The greatest benefit of bi-layer construction (on an equal mass basis) is seen for blunt projectiles, using a polyethylene that maintains a high degree of strain hardening at high strain rates (i.e. UHMWPE), and a polymer thickness just sufficient to switch the failure mode in the metal layer from discing (failure at the projectile perimeter) to tensile failure at the plate centre.
The effect of residual stress on the fracture of chemically strengthened thin aluminosilicate glass was investigated. The large deflection problem on the flexure of thin chemically strengthened glass was solved through finite element analysis. The relationship among compressive stress (CS), central tension (CT), and the modulus of rupture of chemically strengthened thin glass was also discussed. High CS and low CT improved the flexural strength of chemically strengthened glass. However, the effect of residual stress was more complex on Weibull modulus than on strength. The effect of residual stress on the fractography of chemically strengthened thin glass was analyzed. Transparent and opaque zones were observed on the fracture surface of chemically strengthened glass. The relative thickness of the opaque zone (dOpaque/d0), which is a constant in the same fracture zone, linearly decreased with increasing fracture zone. This result indicates that the stored elastic strain energy was released with the number of transverse cracks. These results provide useful information on the failure analysis of chemically strengthened thin glass.
Perforation resistance is an important design consideration for thin-walled metallic structures. However, the perforation energy of thin metallic plates is known to be sensitive to the nose shape of the indenter. This poses a challenge for predictive modelling, both analytical and numerical, as the material deformation and state of stress at the onset of failure can vary significantly from one indenter geometry to the next. Effective design requires an understanding of the key modelling parameters, and their influence on the predicted perforation response, across the widest range of possible indenter geometries. This paper aims to investigate systematically the indenter nose shape sensitivity of the quasi-static perforation of a 1 mm thick plate of aluminium alloy 6082-T4, and the modelling of the conditions at failure. The nose shape of the indenter is gradually changed from flat (i.e. blunt) to hemi-spherical either by (i) introducing a chamfer at the edge of the indenter or (ii) by changing the indenter frontal nose radius. This allows a wide range of states of deformation at the onset of failure to be spanned. The problem is investigated by both analytical and numerical methods. The results of both modelling techniques are compared with quasi-static perforation experiments, and the conditions necessary to achieve good agreement are obtained. Careful consideration of (i) material anisotropy, (ii) indenter-plate friction and (iii) boundary compliance is necessary for accurate prediction of the perforation energy. A Lode angle-dependent model for the onset of failure in the metal is found to be essential for predicting the perforation response for a range of indenter chamfer radii.
In this paper, firstly the quasi-static bending performance of chemically strengthened alumina silicate glass plates is investigated for different glass thicknesses: 2.2, 4.0 and 6.0 mm. The flexural strength is measured using coaxial double ring experiments. The 3D Digital Image Correlation (DIC) technique is employed to measure the strain at failure. The failure probability is then assessed using the Weibull statistical distribution.
Secondly, the performance of the laminated glass windows made of these chemically strengthened glass plates is evaluated quasi-statically under concentrated and distributed loadings. The effects of polymer interlayer thickness, glass and polymer type and multi-layering the polymer interlayer on the structural performance are investigated. The type and thickness of the polymer interlayer, as well as the type of loading are found to influence the fracture sequence in the glass plates and consequently the post fracture safety of the structure. The response of laminated glass specimens is then assessed under low velocity soft impacts, for velocities up to 3.3 m s?1, using a drop tower facility. Laminated glass with a polyvinyl butyral (PVB) interlayer shows the greatest improvement in terms of peak force and absorbed energy.
The correlation between K
diffusion coefficient and mechanical
properties of chemically tempered and hybrid tempered (chemically tempered
subsequent to thermally tempered) aluminosilicate glass are investigated. Firstly, the
potassium ion concentration profiles are experimentally measured and the diffusion
coefficient is calculated according to the Boltzmann-Matano approach. Secondly, the
flexure strength and Weibull modulus are determined using a combined experimental
(coaxial double ring) and finite element analysis method. The results indicate that the
flexural strength decreases with the diffusion coefficient of the air side for both types
of glass samples while there is no significant relationship between diffusion
coefficient and Weibull modulus. Diffusion coefficient on air side shows a higher
value than that of the tin side. With the same diffusion coefficient, the flexural strength of chemically tempered glasses are found to be higher than that for hybrid
tempered glasses. The effect of diffusion coefficient on modulus of rupture (MOR) for
hybrid tempered glass is more remarkable. These results would be useful in designing
the strength of glass and guiding the strengthening process by chemical or hybrid
Ductile thermoplastics, for example Ultra High Molecular Weight Polyethylene (UHMWPE), are of interest for their impact energy absorbing capabilities. While the impact perforation mechanisms of metallic targets have been investigated in some detail, far less progress has been made towards understanding the impact resistance of ductile polymers. The aim of this investigation is to identify the relationship between the projectile tip geometry and impact energy absorption of semi-crystalline thermoplastics. The focus of the study is light-weight monolithic plates of extruded polymer impacted normally by rigid projectiles at velocities up to 100 ms?1. Three polymers will be considered: Low Density Polyethylene (LDPE), High Density Polyethylene (HDPE) and Ultra High Molecular Weight Polyethylene (UHMWPE). Polyethylene provides a convenient test material, as variations in microstructure provide a contrast in mechanical properties, without significant variations in density. Three distinct projectile nose shapes are considered: blunt, hemi-spherical and conical. For a conical tip, perforation occurs by ductile hole expansion. For this nose shape the high yield strength and strain rate sensitivity of HDPE offers an advantage over the other two polyethylenes. Perforation by blunt and hemi-spherical projectiles is more sensitive to deformation localisation. The high strain hardening of UHMWPE, which increases with strain rate, results in a significantly greater impact resistance than either HDPE or LDPE. The perforation mechanisms and energy absorption of these PE plates are contrasted with those of thin aluminium alloy targets that have the same total mass. UHMWPE outperforms these metallic targets for all three projectile nose shapes. Finally, the influence of target thickness on the impact perforation of LDPE is considered. All three nose shapes show a linear increase in perforation energy with target thickness.
Bird strike can cause serious risks to the safety of air travel. In this paper, the aim is to improve design by determining deformation and damage mechanisms of laminated glass windows when subjected to high velocity soft impacts. To achieve this, laboratory-scale impact experiments using bird substitute materials were performed in the velocity range of 100?180 m s?1. An important step forward is that high-speed 3D Digital Image Correlation (DIC) has effectively been employed to extract the full-field deformation and strain on the back surface of the specimens during impact. The finite element simulations were performed in Abaqus/explicit using Eulerian approach and were able to represent successfully the experiments.
For the laminated glass structures investigated, the damage inflicted is strongly sensitive to the nose shape of the projectile and most deleterious is a flat-fronted projectile. Two threshold velocities for impact damage have been identified associated with firstly the front-facing and secondly the rear-facing glass layer breaking. The order of the glass layers significantly influences the impact performance. The findings from this research study have led to a deeper and better-quantified understanding of soft impact damage on laminated glass windows and can lead to more effective design of aircraft windshields.
The effect of residual stress on subcritical crack growth in chemically strengthened aluminosilicate glass in air and water was firstly investigated using the double torsion (DT) technique. An experimental evaluation procedure was developed based on the DT method. The research demonstrates that high compressive stress (CS) and low central tension (CT) in chemically strengthened glass are beneficial in improving crack growth index and decreasing susceptibility to fatigue. Chemically strengthened glass with high CS and low CT exhibits a smaller proof-test ratio, which indicates better survival characteristics. The results are useful in designing the strength and optimizing the strengthening process by ion exchange to obtain a more robust glass with long service lifetime.
Owing to their high strength and stiffness to mass ratio, composite sandwich structures have increasing been used for various engineering applications especially where mass has a direct influence on operating cost and performance. In sandwich structures, the core has an important role of separating and stabilising the skins as well as transferring shear and compressive stresses between the skins. Therefore, the core properties have a strong influence on the strength and perforation resistance of sandwich structures. In this study, the aim is to investigate the effect of grading the core on the structural as well as impact performance of sandwich panels made of GFRP skins and PVC foam core. Both low and high velocity impacts are considered. High speed 3D digital image correlation is employed to extract full-field displacement and strain contours during impact.
Grading the core is found to result in a more stable failure process under quasi-static loading which can sustain larger deflections before final failure. Under impact loading, grading the core can improve the perforation resistance if a lower density core layer is immediately behind the impacted skin. The efficiency of grading, however, is influenced by the deformability of the impactor.
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.
Mohagheghian Iman, Wang Y., Zhou J., Guo X., Charalambides M., Yan Y., Dear J.P. (2015) Soft impact response of laminated glass plates,ICCM 20 - 20th International Conference on Composite Materials
International Committee on Composite Materials (ICCM)
A laminated glass typically consists of two layers of glass and one layer of polymer. It is utilised in
many applications in which the glazing is exposed to external threats like impact or blast. In this paper,
damage development of laminated glass plates by soft impact is investigated in both low and high
velocity regimes. Low velocity impacts (up to 4 ms-1
) were conducted using a drop tower. Soft impact
was achieved by attaching a silicon rubber cylinder to a flat steel impactor, with a diameter larger than
that of the rubber, which itself is backed by a 16.9 kg weight. Different velocities were obtained by
dropping the weight from various heights. For high velocity impacts (up to 220 ms-1
), a gas gun
apparatus was used. The silicon rubber cylinder was fired, using a sabot, in a 25 mm diameter barrel.
High speed photography was employed to monitor the deformation and damage development in the
laminated glass samples. Laminated glasses with different types of polymer interlayer were tested. The
results show a better impact resistance for laminated glass with a stiffer polymer interlayer at both low
and high velocity regimes.
Composite sandwich panels are well known for their relatively high stiffness over weight ratio and
have been increasing utilized in various applications where the weight of the structure is a key design
concern, e.g. in aircraft and aerospace components. However, these structures are vulnerable when
subjected to a transverse impact loading. In this paper, the impact performance of composite sandwich
structures with foam core is investigated. In particular, the idea of multi-layering the core by foam
layers of different density and its effect on the energy absorption under low and high velocity impact
is of interest. In this study, composite sandwich panels made of Glass Fibre Reinforced Polymers
(GFRP) for skins and PVC foam for the core are used. Two different arrangements of foam core are
considered: uniform core (80/80/80 kg/m3
) and graded core (100/60/100 kg/m3
). Both of these core
arrangements have the same areal density. Low (up to 5 ms-1
) and high (up to 200ms-1
) velocity impact
tests were performed using a drop tower and a gas gun respectively. In-plane and out-plane properties
of composite sandwich samples were measured employing 2-D and 3-D Digital Imaging Correlation
(DIC) methods. The results indicate that the composite sandwich structure with graded foam core has
a better energy absorption capability compare to the one with uniform foam core.
The high velocity impact resistance of fibre metal laminates (FMLs) based on combinations of three
different aluminium alloys (6161-O, 6061-T6, 7075-T6) and a glass fibre reinforced epoxy resin have
been investigated both experimentally and numerically. A series of perforation tests on multilayer
configurations, ranging from a simple 2/1 lay-up to a seven ply 4/3 laminate. High velocity impact was
conducted using a projectile gas-gun launcher, operating in the velocity range between 119 m/s and
252 m/s. The impact response of fibre metal laminates samples was characterised by determining
the energy required to perforate the panels. A stereoscopic Digital Image Correlation (DIC) method
was adopted to measure full-field deformations and strain for FMLs which providing the full field
strain history and 3D measurements up to sample perforation. The perforation resistance of the panels
was predicted using the finite element analysis package Abaqus/Explicit. A vectorized user-defined
material subroutine (VUMAT) was employed to define Hashin?s 3D rate-dependant damage criteria
for the composite layers. The subroutine was implemented into the commercial finite element software
ABAQUS/Explicit to simulate the deformation and failure of FMLs. Agreement between the
predictions of the finite element models and the experimental data was good across the range of
configurations. Ballistic limit of those FMLs was obtained from both the experimental tests and
The effect of ion-exchange on the fracture behavior and the threshold load is investigated for radial crack initiation resulting from cube-corner indentation. Both tin and air sides of the sodium aluminosilicate float glass are considered. The threshold load and mechanical properties are experimentally measured by nanoindentation. A qualitative explanation of crack initiation is developed by analyzing the stresses at the indentation site. The ion-exchanged glasses show a lower threshold load for radial crack initiation with a cube-corner indenter than the raw glass, and this is due to a higher crack driving stress for ion-exchanged glasses. However, the compressive stress on the surface of the ion-exchanged glasses can inhibit the expanding of the radial cracks. The air side always shows higher values for the threshold load than the tin side before and after ion-exchange, which is in accordance with the calculated crack driving stress results.
The elastic-plastic deformation in raw and ion-exchanged aluminosilicate glass is investigated by loading rate dependent nanoindentation. The nanohardness and Young's modulus of the raw and ion-exchanged glasses at different loading rate (100?20,000 ¼N.s?1) are explored up to a maximum load of 9000 ¼N. The nanoindentations are scanned with AFM to observe the morphology of the indents. The results show that Young's modulus of the aluminosilicate glass increases after ion exchange and that the nanohardness of the raw and ion-exchanged aluminosilicate glass increases linearly with the loading rate, when plotted on a double logarithmic axes. However, the nanohardness of the raw glass is more sensitive to the variation of loading rate. The compressive stress on the ion-exchanged glass can inhibit plastic deformation. These results are explained in terms of shear stress underneath the indenter and the number of the flow lines in the nanoindentations. These findings are useful for better understanding the dynamic contact-induced damage growth mechanisms of ion-exchanged glass.
The choice of the polymer interlayer is a key consideration for laminated aircraft windshields. Such windshields often employ chemically strengthened glasses and are required to withstand impact by birds, hail-stones and other foreign bodies. In the present study, windshields employing three different polymer interlayer materials were investigated under high-velocity impact by a soft projectile: Thermoplastic Polyurethane (TPU), Polyvinyl Butyral (PVB) and Ionoplast interlayer-SentryGlas® Plus (SGP). Parameters such as the polymer interlayer type and thickness, multi-layering the interlayer and the sensitivity of the behaviour of the windshield to the environmental temperature were studied. The performance was assessed through a series of laboratory-scale impact experiments (using a bird-substitute material) and modelled via finite element simulations (using a smoothed particle hydrodynamics approach). The experimental and numerical results were found to be in good agreement for the three polymer interlayers investigated. The polymer interlayer type was found to have the most significant effect on both the deformation and the failure of the laminated glass windows at room temperature, i.e. 25°C. However, the influence of the polymer interlayer type became less pronounced at lower temperatures. The novel modelling that has been developed assists in the choice of the best polymer interlayer, including the multi-layering of interlayers, for complex windshield designs.
This paper outlines preliminary work developing graphene modified thermoplastic inserts to be used for the toughening of CFRP. The paper outlines laminate
manufacture, mechanical testing and fracture analysis of graphene modified CFRP.
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.
Two polymeric foams have been characterised to develop a simple calibration process for extracting the parameters which are required by the material
model available in the commercial FE package Abaqus. Indentation tests with DIC were conducted to study the validity of the proposed method.
The choice of the materials used for the core and skin of a sandwich structure plays an extremely important role in the skin-core interfacial behaviour. In
this paper, three PMI foams are used as core material and the effect of foam type in the skin-core interfacial response is examined.
This paper investigates the influence of the interlayer material on the low velocity impact performance of laminated glass. The effect of temperature (50°C, 23°C, 0°C and -30°C) has been explored to observe damage mechanisms and the associated impact resistant properties of the laminated glass. The four interlayer materials investigated were: SGP?Ionoplast as employed in Sentry Glas® Plus, TPU- Thermo-plastic polyurethane, PVB-Polyvinyl butyral and a TPU/SGP/TPU hybrid interlayer. The impact resistance was measured in terms of load peak, absorbed energy, ultimate deformation and crack patterns. The low velocity impact results indicated that both the type of the interlayer materials and testing temperature have great influence on the impact resistant properties of the laminated glass. The laminated glass with SGP interlayer exhibited best impact resistant properties amongst the four structures at room temperature. However, as the temperature was varied, the TPU/SGP/TPU hybrid interlayer performed the best over the entire range of temperatures tested, which can better ensure the safety of the occupants in the vehicle. This is because the elastic and viscous properties of the interlayer materials greatly changes with the temperature caused by the different glass transition temperatures of the interlayer materials.
Bird strike on aircraft remains a serious threat to flight safety. Experimental investigations employing real birds are associated with high cost and low reproducibility. Therefore, physical substitute materials are often used instead of real birds. This study investigates the soft impact loading on aluminium and laminated glass targets from ballistic gelatine and rubber projectiles. The two targets simulate strike on the aircrafts' fuselage and windshield respectively. The full field out of plane displacements of the targets were recorded for velocities 110 to 170 m s?1 using digital image correlation during gas gun experiments. A simulation model based on Smoothed Particle Hydrodynamics was developed and validated against the experimental data from all four projectile-target material combinations. It was shown that for the same momentum, a rubber projectile exerts a higher pressure on a target as compared to gelatine, even though the out of plane displacements and in-plane strains are similar. This led to fractures in the impacted laminated glass when rubber was used. The study offers new experimental data as well as efficient design modelling tools to mitigate damage imposed during bird strike. The models provide a way towards enabling the optimisation of real, large scale aircraft structures and components.
Polymeric foams are used extensively as the core of sandwich structures in automotive and aerospace industries. Normally, several experiments are necessary to obtain the required properties to model the response of crushable foams using finite element analysis (FEA). Hence, this research aims to develop a simple and reliable calibration process for extracting the physical parameters which are required by the material model available in the commercial FE package Abaqus. To do this, a set of experimental tests, including uniaxial compression, uniaxial tension and shear punch tests, is proposed. All the experimental tests were also simulated, and generally, good correlations between experiments and numerical models were obtained. The validity of the overall approach was finally demonstrated using an indentation test in which the foam was subjected to a more complex mixed mode loading. During these indentation tests, digital image correlation was used to observe full-field strain distribution in the foam under the indenter. Good agreement between the experimental results and the numerical predictions was found for load?displacement response, failure mode and strain distribution.
Debonding between the skins and the core in a sandwich structure is a critical failure mode in automotive applications; once debonding occurs, the load carrying capacity of a sandwich structure drastically decreases. In the present paper, the effect of using three core materials, with different cell characteristics, on the interfacial strength between the foam cores and a CFRP skin is investigated through mechanical testing and numerical modelling. A key finding is that foams with a coarse cellular structure favour a high resin uptake at the interface during the manufacturing process, which results in a stronger interfacial bonding between the foam and the CFRP. During Mode I loading, the thick resin layer at the interface postpones crack initiation and kinking in the core, whilst under Mode II, this resin layer delays the collapse in compression of foam cells under the crack tip. Thus, the importance of including this thick resin layer in the FE modelling was demonstrated. Finally, as the CZM was shown to be unable to predict the unstable crack propagation within the core, an alternative approach was suggested which has the significant benefit of not requiring experimental testing of the interface between the skin and the core.
Debonding between the skins and the core in a sandwich structure is a critical failure mode in automotive applications where the energy absorption capability is a key property for crashworthiness. Once debonding occurs, the load carrying capacity of a sandwich structure, and therefore its energy absorption capability, drastically decreases. Debonding between a Carbon Fibre Reinforced Polymer (CFRP) skin and a foam core depends on many factors, including skin layup, foam material properties and skin-core thickness ratio.
This research investigates the effect of using various core materials, with different cell characteristics, on the interfacial strength between a cellular foam and a CFRP skin. A key finding was that foams with a coarse cellular structure favoured a high resin uptake at the interface during the manufacturing process. The role of this resin layer was well-defined with the experimental tests, where it was shown to postpone crack initiation and kinking on samples subjected to Mode I loading and to delay the collapse of foam cells under the crack tip in compression on samples under Mode II loading.
The research has matched physical experiments with Finite Element (FE) modelling. Initially, the work presented in this thesis focused on gaining a deep understanding of two of the most used numerical techniques to model interfacial failure: the Cohesive Zone Method (CZM) and the Virtual Crack Closure Technique (VCCT). However, limitations of these two techniques were found when modelling skin-core debonding in sandwich specimens. Experimentally, debonding in a foam-cored pre-cracked specimen was seen to initiate at the interface but to propagate within the foam core, growing in an unstable manner under both Mode I and Mode II loading. Both CZM and VCCT were not capable of capturing the stick-slip behaviour resulting from the unstable crack propagation. Therefore, a new method was suggested in order to accurately simulate debonding; this included the modelling of foam failure with an element deletion mechanism.
This was the first time a failure criterion-based method was used to model debonding in foam cored sandwich specimen. The suggested modelling technique was also validated comparing the FE predictions to the experimental results obtained performing three-point bending tests on sandwich panels with three different configurations. As a result, the work has shown that debonding in foam-cored sandwich structures can be modelled as foam failure instead of as interface failure, decreasing the computational time and the pre-processing efforts.
There has been a growing interest in the last decade for the use of composite materials in the automotive industry, due to their low weight and high energy absorption capabilities. Predicting the response under impact conditions of these lightweight materials is crucial for both design and reduction of manufacturing and testing during the development of new vehicles. However, the complexity of composites makes it difficult to predict their behaviour using Finite Element Analysis (FEA). Hence, this research looks into the development of a reliable Finite Element methodology to simulate lightweight automotive crash structures, combining composite materials with polymeric foams in a sandwich panel.
To achieve that goal, firstly a calibration process of a constitutive model for crushable foams was developed, including the proposed experimental tests to characterise the polymer. The approach was then validated by correlating a mixed-mode indentation tests with its corresponding Finite Element model and results from Digital Image Correlation (DIC).
Secondly, different configurations of the crash structure were tested under mixed-mode loading conditions at quasi-static rates, based on the side-pole impact test. These experiments provided a better understanding with regard to the performance of each configuration. Furthermore, they were used to develop a robust FE methodology to accurately predict their behaviour, including good correlations in terms of load-displacement and failure mechanisms.
Finally, the methodology was employed to design an impact test on a similar crash structure. The tests were conducted, and the outcomes were used to improve the Finite Element models, showing good agreement in terms of loads and energy absorption capability. It was found that the structures that were tested did not experience composite crushing, highlighting the need of further optimisation of the sandwich structure. Therefore, having validated the numerical methodology, FEA can now be used to assess the influence of different design approaches to improve the performance of this type of crash structures, prior to the manufacturing and testing of new full-size components.
This study describes a novel and versatile method of incorporating graphene nano-platelets (GNP) into compos-ite laminates to investigate its effect on mode I and mode II interlaminar fracture toughness (ILFT). Non-woventhermoplastic veil interleaves have been modified by spray deposition with a GNP dispersion to give either a con-tinuous or strip-patterned distribution. The coated interleaves were used to modify the interlaminar region ofcarbonfibre reinforced polymer (CFRP) laminates. The fracture surfaces were characterised by scanning electronand optical microscopy.The continuous GNP distribution in mode I prevented the formation of carbonfibre bridging, resulting in similarinitiation and propagation values. In mode II, the increased thickness of the interlaminar region, coupled with theuneven fracture surface showed the highest increase in the mode II ILFT for the continuous GNP distribution. Thestrip-patterned GNP distribution showed reduced carbonfibre bridging compared to the baseline CFRP and ther-moplastic interleave laminates. This may be due to small scalefibre bridging between the deposited GNP-stripswhich also lead to peaks and troughs in the load-displacement response. In mode II, it is suggested that the de-posited GNP-strips were sufficiently tough to re-direct the propagating crack from the modified interlaminar re-gion to the adjacent ply.