Modelling the structural response of GLARE panels to blast load

This paper deals with the structural response of fully-clamped quadrangular GLARE panels subjected to an intense air-blast load using the commercial finite element software, LS-DYNA. A cohesive tie-break algorithm is implemented to model interfacial debonding between adjacent plies. The blast loads was simulated using a ConWep blast algorithm and a multi-material ALE formulation with fluid–structure interaction to determine the performance of each method. Numerical model validation have been performed considering case studies of GLARE panels subjected to spherical explosive charges of C-4, for which experimental data on the back face-displacement and post-damage observations were available. Excellent agreement of mid-point deflections and evidence of severe yield line deformation were presented and discussed against the performed blast tests.     

Blast loading response of reinforced concrete panels reinforced with externally bonded GFRP laminates

The behavior of reinforced concrete panels, or slabs, retrofitted with glass fiber reinforced polymer (GFRP) composite, and subjected to blast load is investigated. Eight 1000 × 1000 × 70 mm panels were made of 40 MPa concrete and reinforced with top and bottom steel meshes. Five of the panels were used as control while the remaining four were retrofitted with adhesively bonded 500 mm wide GFRP laminate strips on both faces, one in each direction parallel to the panel edges. The panels were subjected to blast loads generated by the detonation of either 22.4 kg or 33.4 kg ANFO explosive charge located at a 3-m standoff. Blast wave characteristics, including incident and reflected pressures and impulses, as well as panel central deflection and strain in steel and on concrete/FRP surfaces were measured. The post-blast damage and mode of failure of each panel was observed, and those panels that were not completely damaged by the blast were subsequently statically tested to find their residual strength. It was determined that overall the GFRP retrofitted panels performed better than the companion control panels while one retrofitted panel experienced severe damage and could not be tested statically after the blast. The latter finding is consistent with previous reports which have shown that at relatively close range the blast pressure due to nominally similar charges and standoff distance can vary significantly, thus producing different levels of damage.     

Response of flexible sandwich-type panels to blast loading

This paper reports on experimental and numerical investigations into the response of flexible sandwich-type panels when subjected to blast loading. The response of sandwich-type panels with steel plates and polystyrene cores are compared to panels with steel face plates and aluminium honeycomb cores. Panels are loaded by detonating plastic explosive discs in close proximity to the front face of the panel. The numerical model is used to explain the stress attenuation and enhancement of the panels with different cores when subjected to blast induced dynamic loading. The permanent deflection of the back plate is determined by the velocity attenuation properties (and hence the transmitted stress pulse) of the core. Core efficiency in terms of energy absorption is an important factor for thicker cores. For panels of comparable mass, those with aluminium honeycomb cores perform “better” than those with polystyrene cores.     

Effect of impact damage on fatigue performance of structures reinforced with GLARE bonded crack retarders

Fibre-Metal Laminates (FML) such as GLARE are of interest as bonded crack retarders (BCR) to improve the fatigue performance of aircraft structures. The degradation of the performance of the crack retarder in service if subjected to damage is a critical factor in designing with this concept. Bonded assemblies of an aluminium alloy substrate reinforced with a GLARE strap were prepared, and were subjected to low velocity impact damage onto the GLARE, with impact energies ranging from 10 to 60 J. The thermal residual stresses developed during the bonding process of the GLARE to the aluminium were determined using neutron diffraction, and the change in the thermal residual stresses owing to impact damage onto the GLARE was evaluated. Pre- and post-impact fatigue performance of the BCR assemblies has been investigated. The results show that the BCR provides an improvement in fatigue life, but the reduction is impaired following impact damage. The results show that monitoring of impact damage will be critical in the damage tolerance assurance for aerospace structures containing bonded crack retarders.     

Numerical simulation and validation of impact response of axially-restrained steel–concrete–steel sandwich panels

Steel–concrete–steel (SCS) sandwich panels are an effective means for protecting personnel and infrastructure facilities from the effects of external blast and high-speed vehicle impact. In conventional SCS construction, the external steel plates are connected to the concrete infill by welded shear stud connectors. This paper describes a programme of research in which the non-composite SCS panels with axially restrained connections were studied experimentally and numerically. High fidelity finite element models for axially restrained steel–concrete–steel panels subjected to impact loading conditions were developed using LS-DYNA. The simulation results were validated against the dynamic testing experimental results. The numerical models were able to predict the initial flexural response of the panels followed by the tensile membrane resistance at large deformation. It was found that the strain rate effects of the materials and the concrete material model could have significant effect on the numerically predicted flexural strength and tensile membrane resistance of the panels.     

Some Inspection Methods for Quality Control and In-service Inspection of GLARE

Quality control of materials and structures is an important issue, also for GLARE. During the manufacturing stage the processes and materials should be monitored and checked frequently in order to obtain a qualified product. During the operation of the aircraft, frequent monitoring and inspections are performed to maintain the quality at a prescribed level. Therefore, in-service inspection methods are applied, and when necessary repair activities are conducted. For the quality control of the GLARE panels and components during manufacturing, the C-scan method proves to be an effective tool. For in-service inspection the Eddy Current Method is one of the suitable options. In this paper a brief overview is presented of both methods and their application on GLARE products.     

Localised blast loading of fibre–metal laminates with a polyamide matrix

This paper presents the results of localised blast tests on fully clamped square fibre–metal laminate panels, manufactured using sheets of 2024-O aluminium alloy, woven glass–fibre reinforced polyamide and a polypropylene adhesive. The fracture properties of the composite–metal interface were determined using the single cantilever beam geometry and the measured interfacial fracture toughness was between values in the literature for thermosetting composites and aluminium/glass fibre polypropylene. Observations from blast experiments performed on panels with different stacking configurations are reported. Diamond and circular back face damage were observed, along with pitting, global displacement and tearing of the front face. Examinations of sectioned panels are presented and multiple debonding, plastic deformation and fibre fracture were identified within the panels.     

Composite and non-composite behaviors of foam-insulated concrete sandwich panels

The structural behaviors of foam-insulated concrete sandwich panels subjected to uniform pressure have been evaluated. This study showed that the interface conditions such as composite and non-composite had a significant effect on the response of foam-insulated concrete sandwich panels, indicating that the simulated shear tie resistance should indeed be incorporated in numerical analyses. Finite element models were developed to simulate the detailed shear resistance of connectors and the nonlinear behaviors of concrete, foam and rebar components. The models were then validated using data from static tests performed at the University of Missouri. The modeling approach used here was compatible with the American Concrete Institute (ACI) Code and existing design practices. The results of this study will therefore provide improved methodology for the analysis and design of foam-insulated sandwich panels under both static and blast loadings.     

Experimental and numerical investigation of carbon fiber sandwich panels subjected to blast loading

The objective of this paper is to investigate the structural response of carbon fiber sandwich panels subjected to blast loading through an integrated experimental and numerical approach. A total of nine experiments, corresponding to three different blast intensity levels were conducted in the 28-inch square shock tube apparatus. Computational models were developed to capture the experimental details and further study the mechanism of blast wave-sandwich panel interactions. The peak reflected overpressure was monitored, which amplified to approximately 2.5 times of the incident overpressure due to fluid-structure interactions. The measured strain histories demonstrated opposite phases at the center of the front and back facesheets. Both strains showed damped oscillation with a reduced oscillation frequency as well as amplified facesheet deformations at the higher blast intensity. As the blast wave traversed across the panel, the observed flow separation and reattachment led to pressure increase at the back side of the panel. Further parametric studies suggested that the maximum deflection of the back facesheet increased dramatically with higher blast intensity and decreased with larger facesheet and core thickness. Our computational models, calibrated by experimental measurements, could be used as a virtual tool for assessing the mechanism of blast-panel interactions, and predicting the structural response of composite panels subjected to blast loading.     

The influence of core height and face plate thickness on the response of honeycomb sandwich panels subjected to blast loading

This paper reports on an investigation into the behaviour of circular sandwich panels with aluminium honeycomb cores subjected to air blast loading. Explosive tests were performed on sandwich panels consisting of mild steel face plates and aluminium honeycomb cores. The loading was generated by detonating plastic explosives at a pre-determined stand-off distance. Core height and face plate thickness were varied and the results are compared with previous experiments. It was observed that the panels exhibited permanent face plate deflection and tearing, and the honeycomb core exhibited crushing and densification. It was found that increasing the core thickness delayed the onset of core densification and decreased back plate deflection. Increasing the plate thickness was also found to decrease back plate deflection, although the panels then had a substantially higher overall mass.     

Optimising curvature of carbon fibre-reinforced polymer composite panel for improved blast resistance: Finite-element analysis

Numerical studies were conducted to investigate the optimum curvature of a carbon fibre-reinforced polymer (CFRP) panel that would provide an improved blast resistance. A dynamic finite-element (FE) model that incorporates fluid–structure interaction was developed to evaluate the response of these panels to blast in commercial finite-element software ABAQUS/Explicit. Previously reported experimental data by authors were utilised to validate a FE model, where a shock-tube apparatus was utilised to apply a controlled shock loading to quasi-isotropic composite panels with different radii of curvature. A three-dimensional digital image correlation (DIC) technique coupled with high-speed photography was employed to measure out-of-plane deflections and velocities, as well as in-plane strains at the back face of panels. Macroscopic post-mortem analysis was performed to compare the deformation in these panels. The numerical results were compared to the experimental data and demonstrated a good agreement. The validated FE model was further used to predict the optimal curvature of CFRP panel with the aim to improve its blast-mitigation characteristics.     

圆孔对GLARE层合板抗冲击性能的影响

为获得圆孔对玻璃纤维增强铝合金（GLARE）层合板抗冲击性能的影响规律,采用40 J的冲击能量对无孔和含圆孔GLARE层合板进行了落锤低速冲击试验,获得了冲击载荷、挠度和能量-时间曲线。应用ABAQUS/Explicit有限元分析软件对试验进行模拟,并预测了圆孔直径对GLARE层合板抗冲击性能的影响。结果显示:在低速冲击下,GLARE层合板纤维层的失效模式以分层损伤和纤维断裂为主;随着圆孔边缘至冲击中心距离的增加,层合板的冲击载荷峰值提高,而挠度峰值减小;数值模拟结果与试验结果的比较验证了模型的合理性;随着圆孔直径的增大,GLARE层合板的抗冲击性能逐步劣化。      

Simulation of the response of fibre–metal laminates to localised blast loading

The response of Fibre–Metal Laminates (FML) to localised blast loading is studied numerically in order to interpret the deformation mechanism due to highly localised pressure pulses causing permanent deformations and damage observed experimentally in FML panels comprising different numbers of aluminium alloy layers and different thickness blocks of GFPP material [Langdon GS, Lemanski SL, Nurick GN, Simmons MS, Cantwell WJ, Schleyer GK. Behaviour of fibre–metal laminates subjected to localised blast loading: part I – experimental observations and failure analysis. International Journal of Impact Engineering 2007;34:1202–22; Lemanski SL, Nurick GN, Langdon GS, Simmons MS, Cantwell WJ, Schleyer GK. Behaviour of fibre–metal laminates subjected to localised blast loading: part II – quantitative analysis. International Journal of Impact Engineering 2007;34:1223–45; Langdon GS, Nuric GN, Lemanski SL, Simmons MS, Cantwell WJ, Schleyer GK. Failure characterisation of blast-loaded fibre–metal laminate panels based on aluminium and glass-fibre reinforced polypropylene. Composite Science and Technology 2007;67:1385–405]. The influence of the loading and material parameters on the final deformation characteristics is examined. Particular attention is paid to the transient deformation process by using finite element and analytical models to analyse the panel behaviour. It is shown that the response of the FML panels is extremely sensitive to the spatial and temporal distribution variation of the pressure caused by the blast loading. The study reveals that the properties of GFPP in the through-thickness direction play an essential role in the velocity transfer, which influences considerably the failure and final deformed shape of the FML panel. Good agreement between the experimental and numerical results is observed. Comparisons between the responses of relatively thin FML panels, monolithic aluminium alloy plates of equivalent mass and a foam-core panel to localised blast are also presented and discussed.     

Establishing a predictive method for blast induced masonry debris distribution using experimental and numerical methods

When subjected to blast loading, fragments ejected by concrete or masonry structures present a number of potential hazards. Airborne fragments pose a high risk of injury and secondary damage, with the resulting debris field causing major obstructions. The capability to predict the spatial distribution of debris of any structure as a function of parameterised blast loads will offer vital assistance to both emergency response and search and rescue operations and aid improvement of preventative measures. This paper proposes a new method to predict the debris distribution produced by masonry structures which are impacted by blast. It is proposed that describing structural geometry as an array of simple modular panels, the overall debris distribution can be predicted based on the distribution of each individual panel. Two experimental trials using 41 kg TNT equivalent charges, which subjected a total of nine small masonry structures to blast loading, were used to benchmark a computational modelling routine using the Applied Element Method (AEM). The computational spatial distribution presented good agreement with the experimental trials, closely matching breakage patterns, initial fragmentation and ground impact fragmentation. The collapse mechanisms were unpredictable due to the relatively low transmitted impulse; however, the debris distributions produced by AEM models with matching collapse mechanisms showed good agreement with the experimental trials.     

Crack closure in fibre metal laminates

GLARE is a fibre metal laminate (FML) built up of alternating layers of S2-glass/FM94 prepreg and aluminium 2024-T3. The excellent fatigue behaviour of GLARE can be described with a recently published analytical prediction model. This model is based on linear elastic fracture mechanics and the assumption that a similar stress state in the aluminium layers of GLARE and monolithic aluminium result in the same crack growth behaviour. It therefore describes the crack growth with an effective stress intensity factor (SIF) range at the crack tip in the aluminium layers, including the effect of internal residual stress as result of curing and the stiffness differences between the individual layers. In that model, an empirical relation is used to calculate the effective SIF range, which had been determined without sufficiently investigating the effect of crack closure. This paper presents the research performed on crack closure in GLARE. It is assumed that crack closure in FMLs is determined by the actual stress cycles in the metal layers and that it can be described with the available relations for monolithic aluminium published in the literature. Fatigue crack growth experiments have been performed on GLARE specimens in which crack growth rates and crack opening stresses have been recorded. The prediction model incorporating the crack closure relation for aluminium 2024-T3 obtained from the literature has been validated with the test results. It is concluded that crack growth in GLARE can be correlated with the effective SIF range at the crack tip in the aluminium layers, if it is determined with the crack closure relation for aluminium 2024-T3 based on actual stresses in the aluminium layers.     

Transfer of impulsive loading on cladding panels to the fixing assemblies

The fixing assemblies for prefabricated cladding panels on buildings, are often designed on an empirical basis, to support the dead weight of the panel, wind loads and possibly temporary loads arising during construction. If, subsequently, the design loads are accidentally exceeded but the panel is undamaged; it may be necessary to check the condition of the fixings. A physical check would require costly dismantling and reconstruction of the panel but, as an alternative, this paper presents an analysis of the force transferred to cladding fixing assemblies when the panel is subjected to impulsive loading, using the finite element analysis program, DYNA3D. The results were validated with experimental results from impact tests on a steel plate supported on four steel bars. Having obtained this validation, a finite element analysis was then carried out to determine the response of fixings for panels under different levels of blast loading using two different models for the panel-fixing assemblies. The first model considers only the effect of out-of-plane fixing, whilst in the second model both the out-of-plane fixings and in-plane fixings are considered. It was observed that forces transferred to fixings include an axial force, shear forces, and bending moments and these force components can vary along the length of the fixing. Inspections of cladding panels, after being subjected to impulsive loads, often show that the connection between the fixing and the panel and between the fixing and the support structure are more vulnerable to impulsive loading than the panel itself. The FE analysis has shown that forces in the fixings are related to the dynamic response of the cladding panel; hence damage to the fixings could be deduced from damage to the panel.     

Failure characterisation of blast-loaded fibre–metal laminate panels based on aluminium and glass–fibre reinforced polypropylene

Previous work on the blast response of aluminium/glass–fibre reinforced polypropylene fibre–metal laminates (FMLs) presented observations and quantitative analysis on panels of varying thickness and stacking configuration. Diamond and cross-shaped back face damage was observed and dimensionless analysis showed that the front and back face displacement fell within one plate thickness of a linear trend line [Langdon GS, Lemanski SL, Nurick GN, Simmons MC, Cantwell WJ, Schleyer GK. Behaviour of fibre–metal laminates subjected to localised blast loading: Part I – experimental observations. Int J Impact Eng, in press; Lemanski SL, Nurick GN, Langdon GS, Simmons MC, Cantwell WJ, Schleyer GK. Behaviour of fibre–metal laminates subjected to localised blast loading: Part II – quantitative analysis. Int J Impact Eng, in press]. This paper presents failure characterisation of the blast-loaded square fibre–metal laminate panels, identifying trends and failure modes for each panel type. Multiple debonding, large plastic displacement, fibre fracture and matrix cracking are identified as damage mechanisms within the panels. Back face spalling, debonding failure and front face buckling are also discussed.     

Manufacturing of GLARE Parts and Structures

GLARE is a hybrid material consisting of alternating layers of metal sheets and composite layers, requiring special attention when manufacturing of parts and structures is concerned. On one hand the applicable manufacturing processes for GLARE are limited, on the other hand, due to the constituents and composition of the laminate, it offers new opportunities for production. One of the opportunities is the manufacture of very large skin panels by lay-up techniques. Lay-up techniques are common for full composites, but uncommon for metallic structures. Nevertheless, large GLARE skin panels are made by lay-up processes. In addition, the sequences of forming and laminating processes, that can be selected, offer manufacturing options that are not applicable to metals or full composites. With respect to conventional manufacturing processes, the possibilities for Fibre Metal Laminates in general, are limited. The limits are partly due to the different failure modes, partly due to the properties of the constituents in the laminate. For machining processes: the wear of the cutting tools during machining operations of GLARE stems from the abrasive nature of the glass fibres. For the forming processes: the limited formability, expressed by a small failure strain, is related to the glass fibres. However, although these manufacturing issues may restrict the use of manufacturing processes for FMLs, application of these laminates in aircraft is not hindered.     

Wet-sand impulse loading of metallic plates and corrugated core sandwich panels

We have utilized a combination of experimental and modeling methods to investigate the mechanical response of edge-clamped sandwich panels subject to the impact of explosively driven wet sand. A porthole extrusion process followed by friction stir welding was utilized to fabricate 6061-T6 aluminum sandwich panels with corrugated cores. The panels were edge clamped and subjected to localized high intensity dynamic loading by the detonation of spherical explosive charges encased by a concentric shell of wet sand placed at different standoff distances. Monolithic plates of the same alloy and mass per unit area were also tested in an identical manner and found to suffer 15-20% larger permanent deflections. A decoupled wet sand loading model was developed and incorporated into a parallel finite-element simulation capability. The loading model was calibrated to one of the experiments. The model predictions for the remaining tests were found to be in close agreement with experimental observations for both sandwich panels and monolithic plates. The simulation tool was then utilized to explore sandwich panel designs with improved performance. It was found that the performance of the sandwich panel to wet sand blast loading can be varied by redistributing the mass among the core webs and the face sheets. Sandwich panel designs that suffer 30% smaller deflections than equivalent solid plates have been identified.