Dynamic response of thin targets under ballistic impact

Dynamic response of thin targets under ballistic impact</p

Dynamic behavior of polymer-matrix composite laminates

This chapter provides an overview of behavior of composites under dynamic conditions by assessing a relationship between microscopic properties of fibers and macroscopic parameters of fiber-reinforced plastics (FRPs). So, commonly used fiber materials for dynamic applications are considered (including glass, carbon, aramid, and UHMWPE), while the mechanical behavior and dynamic performance of two major reinforcement configurations (unidirectional (UD) and bidirectional (plain weave)) are assessed. Following a brief introduction, the discussion starts with highlighting the rate-/temperature-dependent mechanical properties of fibers and anisotropic properties of FRPs, extending subsequently to damage mechanisms observed in FRPs. It is followed by the analysis of the response of laminates to impact-induced transverse loading together with factors affecting the ability of composites to absorb energy during dynamic events in Section 4. The final section provides an in-depth synopsis of the methodologies involved in numerical simulations of dynamic behavior of FRPs. The emphasis here is on macroscopic behavior and models proven most robust for 3D stress states in the second World Wide Failure Exercise (WWFE II). The complexity of such approaches and the importance of a balance between computational complexity and performance-applicability is outlined.</p

Mechanics of ballistic impact with non-axisymmetric projectiles on thin aluminium targets. Part II: Energy considerations

In Part I of this study, the ballistic performance and failure mechanisms of a thin aluminium target impacted by plate-like, non-axisymmetric projectiles with three different geometries, were considered using the gas-gun experiments. Failure mechanisms of the target material were found to depend strongly on the projectile’s geometrical features. Also, the local target’s response was not a reliable predictor of the critical kinetic energy for perforation or penetration. In order to improve the prediction of ballistic performance of thin targets, a nonlocal target behaviour should also be accounted for. In this part of the study, the global target response is examined, and a heuristic approach is introduced for the energy balance. The approach involves a new factor that represents additional energy-dissipating mechanisms between the projectile’s critical energy and the local work in target defeat. The methodology is based on the identified strong correlation of the normalised impact-induced peak kinetic energy in the target, KETmax, and the normalised target impact-induced nonlocal internal energy, UT(PL+KE), associated with plastic deformation and increase in kinetic energy. A physically-based semi-analytical formulation for KETmax  is developed by decomposition of the contributions by the inclined and blunt projectile sections, succeeding statistical analysis of key projectile’s geometrical and impact parameters. The developed approach was assessed for 17 projectile geometries implementing a total of 119 finite-element simulations with experimentally validated numerical models at subcritical (50) and critical/supercritical (69) projectile velocities. The obtained predictions had an average absolute error of less than 6.5% for impact velocities at and above the ballistic limit, while at subcritical velocities a higher scatter was observed as a result of secondary interaction effects between the projectile and the target. Overall, the methodology yielded results for the ballistic limit with an error below 5%, improving the otherwise derived results nearly twofold. The method was proven to be a practical and accurate way to improve the projectile’s critical energy prediction for thin targets, and, even though developed for non-axisymmetric projectile cases, its framework is directly transferable to axisymmetric projectiles.</p

Ballistic performance of polyurea-coated thin aluminium plates: numerical study

Polyurea elastomer exhibits desirable characteristics for impact mitigation, with varying stoichiometric-dependent properties that can be tailored for specific applications and applied to reinforce existing and new structural components. This numerical study aims to investigate the ballistic performance of polyurea-aluminium laminate targets, employing a user-defined material model for polyurea elastomer developed in a finite-element (FE) framework. The model consists of a rigid spherical projectile impacting the considered target plate. A linear increase in the ballistic performance with a growing thickness of polymer coating was observed and is consistent with previously conducted experimental work. The ballistic limit is increased by some 5% per millimetre of polymer coating thickness, when compared to the monolithic metallic plate. The presence of the polymer layer significantly affects the dynamic response mechanisms of the component during bending due to impact. The result is a more localised deformation compared to global bending of the target

Effect of nose geometry on penetration capability of non-axisymmetric thin projectiles

Hitherto, studies on impact by non-axisymmetric projectiles with high cross-sectional aspect ratios are limited, yet such cases may occur in a wide range of civil and military applications. A target response to projectiles with such geometry differs from that of the extensively studied case of impact with axisymmetric projectiles. The focus of this study is to examine the penetration capabilities of plate-like projectiles with various sharpness upon impact on thin metallic targets. To that end, attention is drawn to the projectile’s geometrical parameters and their effect on penetration characteristics. An experimentally-validated model was developed within the finite-element-method (FEA) framework to study the projectile-induced fracture in the target and the projectile’s velocity profiles, with the intention to better understand the occurring dissipation mechanisms and identify the contribution of key geometrical parameters. A change in the expected target’s fracture response is reported, while the linear section of the projectile’s velocity drop is associated through a linear relationship with the half-cone angle of the projectile. The target showed decreased resistance upon impact by non-axisymmetric, high-aspect-ratio projectiles as compared to their axisymmetric equivalents.</p

Impact of polyurea-coated metallic targets: computational framework

Polyurea elastomer is known to exhibit advantageous impact-mitigation characteristics and thus can improve the dynamic performance of various components and structures. This study identifies the mechanisms of dynamic response of thin metallic plates, covered by a frontal polyurea layer, using a physically verified, custom material model for two-part polyurea implemented within a finite-element-method framework. A linear increase in the ballistic performance of a target with polymer coating is consistent with experimental work captured for the first time in a numerical study. A reported ballistic-limit improvement of 7.4 m s–1 per millimetre increase of polyurea thickness for frontal-layer thicknesses higher than 4 mm on the thin monolithic plate was established. In contrast, the application of polyurea coating thinner than 4 mm resulted in a diminished ballistic performance of the target. These outcomes are attributed to significant alterations in the energy-absorbing capacity of thin plates with the introduction of the polyurea layer that strongly depend on the impact velocity, polymer thickness, and interfacial interactions

Mechanics of ballistic impact with non-axisymmetric projectiles on thin aluminium targets. Part I: Failure mechanisms

The ballistic performance of thin aluminium targets under normal and oblique impacts with platelike projectiles is investigated with emphasis on the local projectile-induced target response. Three projectiles of 5 mm thickness with decreasing bluntness were considered, with the ratio of the projectile’s equivalent diameter to the target’s thickness (deq/hT ) within the range of 0 to 4.89. The obtained results suggest that defeat mechanisms, and target resistance measures were different from those observed in impact with equivalent axisymmetric projectiles, as a result of a high projectile’s cross-sectional aspect ratio. Projectiles with inclined nose sections inflicted a shear-dominant failure that was locally energetically expensive and depended on the product of length (LN) and half-angle (α) of the projectile’s nose. On the other hand, projectiles with blunt sections were associated with retardation of their penetration capacity due to dynamic effects, followed by a low-energy mechanism associated with membrane stretching and tensile failure of targets. A total of 48 experiments were performed at normal-impact conditions, to estimate the critical velocity of perforation/penetration and examine the real-time deformation patterns at sub-critical velocities, by employing the digital image correlation technique. Overall, the critical velocity showed a quadratic dependence on deq/hT , where the benefit of ballistic performance decreased with an increase in this ratio. The observed non-monotonic behaviour of the critical velocity with increasing impact obliquity in some cases and the distinction in failure mechanisms highlight the importance of the projectile’s geometrical parameters for the energy transfer mechanisms. Also, the lack of correlation between the critical velocity and the local work in the target defeat term suggests that the resultant energy transfer mechanisms considerably contributed to the dissipation of the projectile’s kinetic energy. Experimental data were utilised for calibrating the material model and separate experimental results for validating the numerical (finite-element) model. A total of 119 simulations were carried out for normal and oblique impact incidences to examine the role of the projectile’s (i) rotation, (ii) geometrical features, and (iii) obliquity on the target’s dynamic performance and induced defeat mechanisms. The conclusions of this study form the basis for considering the role of the projectile’s geometrical parameters on the energy transfer mechanisms in the target through statistical and semi-analytical approaches presented in the second part of this work. </p

Polyurea-coated glass-fibre-reinforced laminate under high-speed impact: experimental study

One of the promising methods to increase the resistance of polymer-matrix composite materials to impact damage is the use of protective coatings. In this work, the effect of polyurea coating on impact-performance parameters of a woven glass-fibre-reinforced laminate is studied. The study was performed on a specially developed ballistic experimental test rig employing a pneumatic gun. Eleven polymer composite targets with dimensions 200 mm x 300 mm x 8 mm were impacted orthogonally with a steel projectile with 23.8 mm diameter and weight 54.7 g in the range of the impact speed up to 150 m/s. A comparative assessment of the ballistic limit for targets with a 1.2 mm protective coating on the front and rear faces of the target, as well as for samples without any protective coating, was performed. The impact process was captured using two high-speed cameras for filming the front and top views at 25,000 frames per second. Experimental data on the ballistic limit for uncoated and polyurea coated fiberglass plates on the front and back surfaces were obtained. It was shown that 1.2 mm thick coating on the face surface increases the ballistic limit by 20%. The nature of the damage of the GRP base plate and coating has been analyzed. The obtained data can be used for validation of numerical models of ballistic impacts of polyurea-coated laminates