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)

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)

Correlation between K+-Na+ diffusion coefficient and flexural strength of chemically tempered aluminosilicate glass

The correlation between K+-Na+ diffusion coefficient and mechanical properties of chemically tempered and hybridly tempered (thermally plus chemically tempered) aluminosilicate glass is investigated. First, the profile of the potassium ion concentration is experimentally measured, and the diffusion coefficient is calculated according to the Boltzmann-Matano approach. Second, the flexural strength and the Weibull modulus are determined using a method combining experimental (coaxial double ring) and finite element analysis. 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 the diffusion coefficient and the Weibull modulus. The diffusion coefficient on the air side shows a higher value than that on the tin side. With the same diffusion coefficient, the flexural strength of chemically tempered glasses is found to be higher than that of hybridly tempered glasses. The effect of the diffusion coefficient on the modulus of rupture (MOR) for the hybridly tempered glass is more remarkable. These results would be useful for designing the glass strength and guiding the strengthening process by chemical or hybrid tempering

A novel underdetermined source recovery algorithm based on k-sparse component analysis

Sparse component analysis (SCA) is a popular method for addressing underdetermined blind source separation in array signal processing applications. We are motivated by problems that arise in the applications where the sources are densely sparse (i.e. the number of active sources is high and very close to the number of sensors). The separation performance of current underdetermined source recovery (USR) solutions, including the relaxation and greedy families, reduces with decreasing the mixing system dimension and increasing the sparsity level (k). In this paper, we present a k-SCA-based algorithm that is suitable for USR in low-dimensional mixing systems. Assuming the sources is at most (m−1) sparse where m is the number of mixtures; the proposed method is capable of recovering the sources from the mixtures given the mixing matrix using a subspace detection framework. Simulation results show that the proposed algorithm achieves better separation performance in k-SCA conditions compared to state-of-the-art USR algorithms such as basis pursuit, minimizing norm-L1, smoothed L0, focal underdetermined system solver and orthogonal matching pursuit