Impact mitigating capabilities of a spray-on elastomer coating applied to concrete

Structural protection against the effects of a nearby explosive detonation is an area of growing importance. Spray-on elastomer coatings are of interest as a practical and low cost protective solution. Recent research has demonstrated the effectiveness of such coatings for blast mitigation. However, there are two loading scenarios of concern for these applications: blast pressures and fragment impacts. To date, there remains a need to understand the merits of this protective solution for impact indentation of concrete structural elements. In this work, we examine whether, and by what mechanism, an elastomer coating can offer protection in this case. A series of quasi-static indentation and dynamic impact experiments are performed using a 0.1 kg circular cylindrical (i.e. blunt) projectile. It is demonstrated that the coating displays a significant protective capability over the full range of impact velocities considered, c. 45 - 150 m/s. The coating remains intact until impacted at a velocity of c. 120 m/s when it fails by a ductile, tearing mechanism, forming a plug which undergoes large elastic contraction after projectile penetration. A finite element model of the impact indentation of uncoated and coated concrete cubes is developed and validated against the experiments. Focusing on the early time steps and damage initiation in the concrete, the numerical model is used to interrogate the mechanism by which the elastomer achieves its mitigating effect. It is found that the way in which the elastomer alters the stress distribution in the concrete, and its time evolution, is key to its performance. These findings provide a basis for optimising protective coatings for concrete structural elements.George and Lillian Schiff Foundation, University of Cambridg

Impact perforation of polymer-metal laminates: Projectile nose shape sensitivity

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 authors are grateful for joint financial support from the Engineering and Physical Sciences Research Council (EPSRC) and the Defence Science and Technology Laboratory (DSTL) through project EP/G042756/1 (Polymer Nanocomposites for Light Armour Applications). We acknowledge the EPSRC instrument loan pool for the use of the high speed camera (Vision Research Phantom V710).This is the final version of the article. It first appeared from Elsevier via https://doi.org/10.1016/j.ijsolstr.2016.01.01

Impact perforation of monolithic polyethylene plates: Projectile nose shape dependence

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.The authors are grateful for joint financial support from the Engineering and Physical Sciences Research Council (EPSRC) and the Defence Science and Technology Laboratory (DSTL) through project EP/G042756/1 (Polymer Nanocomposites for Light Armour Applications).This is the final published version. It first appeared at http://www.sciencedirect.com/science/article/pii/S0734743X15000184#

Predicting indenter nose shape sensitivity for quasi-static perforation of thin metallic plates

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.The authors are grateful for joint financial support from the Engineering and Physical Sciences Research Council (EPSRC) and the Defence Science and Technology Laboratory (DSTL) through project EP/G042756/1 (Polymer Nanocomposites for Light Armour Applications)

Novel stacked folded cores for blast-resistant sandwich beams

Recent research has established the effectiveness of sandwich structures with metallic cellular cores for blast mitigation. The choice of core architecture can enhance sandwich performance, dissipating energy through plastic core compression and exploiting fluid-structure interaction effects to reduce the momentum imparted to the structure by the blast. In this paper we describe the first analysis of a novel sandwich core concept for blast mitigation: the Stacked Folded Core. The core consists of an alternating stacked sequence of folded sheets in the Miura (double-corrugated) pattern, with the stack oriented such that the folding kinematics define the out-of plane compressive strength of the core. It offers a number of distinct characteristics compared to existing cellular cores. (i) The kinematics of collapse of the core by a distinctive folding mechanism give it unique mechanical properties, including strong anisotropy. (ii) The fold pattern and stacking arrangement is extremely versatile, offering exceptional freedom to tailor the mechanical properties of the core. This includes freedom to grade the core properties through progressive changes in the fold pattern. (iii) Continuous manufacturing processes have been established for the Miura folded sheets which make up the core. The design is therefore potentially more straightforward and economical to manufacture than other metallic cellular materials. In this first investigation of the Stacked Folded Core, finite element analysis is used to investigate its characteristics under both quasi-static and dynamic loading. A dynamic analysis of an impulsively loaded sandwich beam with a stacked folded core reveals the versatility of the concept for blast mitigation. By altering the fold pattern alone, the durations of key phases of the dynamic sandwich response (core compression, beam bending) can be controlled. By altering both fold pattern and sheet thickness in the core, the same is achieved without altering the density of the core or the mass distribution of the sandwich beam.This is the author's accepted manuscript. The final version is available from Elsevier at: http://www.sciencedirect.com/science/article/pii/S0020768314003035

Fluid-structure interactions for the air blast loading of elastomer-coated concrete

The fluid-structure interaction (FSI) effect experienced by an elastomer-coated concrete slab subjected to blast loading in air is studied numerically. The aim is to establish whether a flexible coating alters blast-structure interactions and whether this can explain the apparent blast mitigating capability of this retrofit solution as reported in published experimental investigations. Numerical models for a typical concrete and spray-on elastomer coating are established and a Coupled Eulerian-Lagrangian (CEL) model is employed to predict the air blast response. A 1D FSI analysis suggests that the elastomer coating increases the peak compressive stress in the concrete during short timescale pressure wave interactions. But the effect on the total imparted momentum is small, across a range of target mass and blast intensity. However, due to momentum sharing, the impulse imparted to the concrete plate is reduced in the coated configuration. By extending the analysis into 2D, it is found that the displacement of a concrete slab is marginally reduced when coated on either the blast-receiving or non-blast-receiving face. Thus, it is postulated that the elastomer contributes a small, beneficial mechanical effect. Finally, the need for a fully coupled (CEL) approach to model the blast-structure interaction is interrogated. For a wide range of cases, the results suggest that using a purely Lagrangian approach, in which a pressure-time history is directly applied to the structure (thereby neglecting full representation of FSI effects), is sufficient to capture the deflection behaviour of coated concrete plates. However, it is shown that the significance of the error associated with this simplification depends on the blast intensity and slab geometry under consideration.George and Lillian Schiff Foundation, University of Cambridg

Static and Dynamic Properties of Semi-Crystalline Polyethylene.

Properties of extruded polymers are strongly affected by molecular structure. For two different semi-crystalline polymers, low-density polyethylene (LDPE) and ultra-high molecular weight polyethylene (UHMWPE), this investigation measures the elastic modulus, plastic flow stress and strain-rate dependence of yield stress. Also, it examines the effect of molecular structure on post-necking tensile fracture. The static and dynamic material tests reveal that extruded UHMWPE has a somewhat larger yield stress and much larger strain to failure than LDPE. For both types of polyethylene, the strain at tensile failure decreases with increasing strain-rate. For strain-rates 0.001⁻3400 s-1, the yield stress variation is accurately represented by the Cowper⁻Symonds equation. These results indicate that, at high strain rates, UHMWPE is more energy absorbent than LDPE as a result of its long chain molecular structure with few branches.This work was partially sponsored by Foundation of State Key Laboratory of Explosion Science and Technology of China under Grant No.KFJJ13-1Z, No. YBKT15-02 and Natural Science Foundation of China under Grant No.11102023. The authors would like to thank Chunmei Liu of the First Research Institute of the China Ministry of Public Security for assistance with the static tensile tests.This is the final version of the article. It first appeared from the Multidisciplinary Digital Publishing Institute via http://dx.doi.org/10.3390/polym804007

Perforation resistance of aluminum/polyethylene sandwich structure

© 2016 Elsevier Ltd. Ballistic tests were performed on two types of polyethylene core sandwich structures (AA6082/LDPE/AA6082 and AA6082/UHMWPE/AA6082) to investigate their perforation resistance. Bulging and dishing deformation of layered plates were compared under low-velocity impact by hemispherical-nosed projectiles. Different impact failure mechanisms leading to perforation were revealed for laminates composed of a pair of aluminum alloy face sheets separated by a polyethylene interlayer. Using the finite element code Abaqus/Explicit, the perforation behavior and distribution of energy dissipation of each layer during penetration were simulated and analysed. The deformation resistance and anti-penetration properties of polyethylene core sandwich structures were compared with those of monolithic AA6082-T6 plates that had the same areal density. Although the polyethylene interlayer enlarged the plastic deformation zone of the back face, the polyethylene core sandwich structure was a little less effective than the monolithic Al alloy target at resisting hemispherical-nosed projectile impact.The authors gratefully acknowledge the Foundation of State Key Laboratory of Explosion Science and Technology of China under Grant No. KFJJ13-1Z, and Natural Science Foundation of China under Grant No. 11102023, 11172071

Quasi-static and impact perforation of polymer-metal bi-layer plates by a blunt indenter

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.The authors are grateful for joint financial support from the Engineering and Physical Sciences Research Council (EPSRC) and the Defence Science and Technology Laboratory (DSTL) through project EP/G042756/1 (Polymer Nanocomposites for Light Armour Applications). We acknowledge the EPSRC instrument loan pool for the use of the high speed camera (Vision Research Phantom V710)