Ballistic impact behaviour of glass fibre reinforced polymer composite with 1D/2D nanomodified epoxy matrices

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

On the extent of fracture toughness transfer from 1D/2D nanomodified epoxy matrices to glass fibre composites

Abstract: In this study, the effects of adding nanofillers to an epoxy resin (EP) used as a matrix in glass fibre-reinforced plastic (GFRP) composites have been investigated. Both 1D and 2D nanofillers were used, specifically (1) carbon nanotubes (CNTs), (2) few-layer graphene nanoplatelets (GNPs), as well as hybrid combinations of (3) CNTs and boron nitride nanosheets, and (4) GNPs and boron nitride nanotubes (BNNTs). Tensile tests have shown improvements in the transverse stiffness normal to the fibre direction of up to about 25% for the GFRPs using the ‘EP + CNT’ and the ‘EP + BNNT + GNP’ matrices, compared to the composites with the unmodified epoxy (‘EP’). Mode I and mode II fracture toughness tests were conducted using double cantilever beam (DCB) and end-notched flexure (ENF) tests, respectively. In the quasi-static mode I tests, the values of the initiation interlaminar fracture toughness, GICC, of the GFRP composites showed that the transfer of matrix toughness to the corresponding GFRP composite is greatest for the GFRP composite with the GNPs in the matrix. Here, a coefficient of toughness transfer (CTT), defined as the ratio of mode I initiation interlaminar toughness for the composite to the bulk polymer matrix toughness, of 0.68 was recorded. The highest absolute values of the mode I interlaminar fracture toughness at crack initiation were achieved for the GFRP composites with the epoxy matrix modified with the hybrid combinations of nanofillers. The highest value of the CTT during steady-state crack propagation was ~ 2 for all the different types of GFRPs. Fractographic analysis of the composite surfaces from the DCB and ENF specimens showed that failure was by a combination of cohesive (through the matrix) and interfacial (along the fibre/matrix interface) modes, depending on the type of nanofillers used

Experimental characterisation and numerical modelling of the influence of bondline thickness, loading rate, and deformation mode on the response of ductile adhesive interfaces

A new method for characterising the rate-dependent failure of ductile adhesively bonded structures has been developed and used to investigate the different modes of loading of representative interfaces. Furthermore, experimental observations enabled a newly developed cohesive zone model that captures all critical aspects of the observed and quantified behaviour of the adhesive under consideration. In particular, the model is capable of reproducing the conducted experiments by incorporating both the dependence of the deformation rate and the adhesive thickness. For that, computed tomography of the adhesive interface was used to resolve three-dimensionally the adhesive volume. The volume fraction of microscopic voids in the adhesive was introduced into the model to rationalise the observed dependence of the mechanical response of the adhesive upon its thickness. Finally, the cohesive zone model was proven with mixed-mode fracture experiments which demonstrate the model’s ability to simulate more complex deformation regimes

Modelling the effects of patch-plug configuration on the impact performance of patch-repaired composite laminates

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

Delamination properties of laminated glass windows subject to blast loading

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

Blast performance of silicone-bonded laminated glass

Blast resistant glazing systems typically use laminated glass to reduce the risk of flying glass debris in an explosion. Laminated glass has a bonded polymer interlayer that retains glass fragments upon fracture. With proper design, the flexibility of the interlayer in laminated glass can offer protection from significantly higher blast loads when compared to an equivalent monolithic pane. This thesis investigates the post-fracture behaviour of laminated glass under blast loading and aims to build the knowledge required to improve design methods for blast resistant glazing. Full-scale open-air blast tests were performed on laminated glass containing a polyvinyl butyral (PVB) interlayer. Test windows ranged in size from 1.5m×1.2m to 3.5m×1.8m and were bonded to robust frames using structural silicone sealant. Blast loads were produced using charge masses of 15 kg to 500 kg (TNT equivalent) and distances of 10m to 30 m. Deflection and shape measurements were obtained using high-speed digital image correlation. Measurements of loading at the joint were also made with strain gauges. The main failure mechanisms observed were the cohesive failure of the bonded silicone joint and tearing of the interlayer. These failure mechanisms were investigated further using a highspeed tensile test machine to reproduce blast loading conditions. Cracked laminated glass samples were loaded in tension at varying rates. Their response was characterised by a rate dependant plateau force which can be used to estimate the maximum load on the glazing joint. Delamination between the PVB and glass was found to play a key role in the laminate response. Thinner PVB and higher strain rates reduced the delamination area, leading to premature tearing of the interlayer. The strength of structural silicone sealant in a blast situation was also investigated. A novel test method was used to determine the bond length required to retain the laminated glass window in a blast event. A nominal strength of not greater than 1.1MPa should be used for design of conventional single-sided silicone joints. A finite element model of the laminated glass response to blast loading was developed using the results of the experimental investigations. The failure predictions of the model were compared against a single-degree-of-freedom (SDOF) model and showed good agreement. Differences in the deflected shape at maximum deflection were seen between the model and those measured in blast testing.EThOS - Electronic Theses Online ServiceArup Security Consulting and the Engineering and Physical Sciences Research Council (EPSRC)GBUnited Kingdo

Blast performance of silicone-bonded laminated glass

Blast resistant glazing systems typically use laminated glass to reduce the risk of flying glass debris in an explosion. Laminated glass has a bonded polymer interlayer that retains glass fragments upon fracture. With proper design, the flexibility of the interlayer in laminated glass can offer protection from significantly higher blast loads when compared to an equivalent monolithic pane. This thesis investigates the post-fracture behaviour of laminated glass under blast loading and aims to build the knowledge required to improve design methods for blast resistant glazing. Full-scale open-air blast tests were performed on laminated glass containing a polyvinyl butyral (PVB) interlayer. Test windows ranged in size from 1.5m×1.2m to 3.5m×1.8m and were bonded to robust frames using structural silicone sealant. Blast loads were produced using charge masses of 15 kg to 500 kg (TNT equivalent) and distances of 10m to 30 m. Deflection and shape measurements were obtained using high-speed digital image correlation. Measurements of loading at the joint were also made with strain gauges. The main failure mechanisms observed were the cohesive failure of the bonded silicone joint and tearing of the interlayer. These failure mechanisms were investigated further using a highspeed tensile test machine to reproduce blast loading conditions. Cracked laminated glass samples were loaded in tension at varying rates. Their response was characterised by a rate dependant plateau force which can be used to estimate the maximum load on the glazing joint. Delamination between the PVB and glass was found to play a key role in the laminate response. Thinner PVB and higher strain rates reduced the delamination area, leading to premature tearing of the interlayer. The strength of structural silicone sealant in a blast situation was also investigated. A novel test method was used to determine the bond length required to retain the laminated glass window in a blast event. A nominal strength of not greater than 1.1MPa should be used for design of conventional single-sided silicone joints. A finite element model of the laminated glass response to blast loading was developed using the results of the experimental investigations. The failure predictions of the model were compared against a single-degree-of-freedom (SDOF) model and showed good agreement. Differences in the deflected shape at maximum deflection were seen between the model and those measured in blast testing.EThOS - Electronic Theses Online ServiceArup Security Consulting and the Engineering and Physical Sciences Research Council (EPSRC)GBUnited Kingdo

35 years of standardization and research on fracture of polymers, polymer composites and adhesives in ESIS TC4: Past achievements and future directions

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

Universal slope-based J-integral methods for characterization of the mode I, mode II and mixed mode I/II fracture behaviour of adhesively bonded interfaces

Universal slope-based J-integral methods have been developed for the determination of the energy release rate for adhesively bonded joints under mode I, mode II and mixed-mode (I/II) loading conditions. The individual J components corresponding to the mode I and mode II loading were separated based on the J-integral decomposition theory. The proposed methods use the slopes of the substrates at various locations to characterize the energy release rate and thus avoid the measurement of crack lengths, which are especially suitable for characterizing the tough interfaces associated with large fracture process zones ahead of crack tips. Under linear elastic deformation, the slope-based J equations were found to be equivalent to classical G equations based on linear elastic fracture mechanics (LEFM). Both experimental and numerical testing of adhesively bonded joints were undertaken to validate the slope-based J equations. The universal slope-based J-integral methods provide a reliable alternative to the measurement of G for adhesive joints or laminated composites undergoing nonlinear or inelastic deformations where conventional LEFM is not valid. It is shown that LEFM, even when coupled with an effective crack length approach, can be inaccurate when damage occurs in a test specimen away from the fracture process zone, as was seen here in mode II. Slope-based J equations can avoid these inaccuracies with a careful selection of contour paths. Slope based methods are therefore strong candidates for selection in future test standards for mode II fracture characterisation of structural adhesive joints