Elsevier

Composite Structures

Volume 237, 1 April 2020, 111863
Composite Structures

Impact analysis of PEEK/ceramic/gelatin composite for finding behind the armor trauma

https://doi.org/10.1016/j.compstruct.2020.111863Get rights and content

Highlights

  • Impact and Failure analysis of a complex PEEK/Ceramic/Gelatin composite system.

  • Behind the Armor Trauma.

  • Large 3-dimensional coupled thermo-elasto-plastic deformations.

  • Sensitivity Analysis.

  • Effect of reduced versus full integration rule.

Abstract

A ceramic-based armor system must arrest bullet penetration and dissipate a large amount of impact energy. Taking the total linear momentum transferred to the wearer of a ceramic armor as a measure of the behind-armor ballistic/blunt trauma (BABT), we study the effect on the BABT of adding a thin polyether-ether-ketone (PEEK) layer on the face of a Silicon Carbide (SiC) ceramic target impacted at normal incidence by a full metal jacket bullet traveling at a low velocity. A human torso is simulated by a block of ballistic gelatin. Three-dimensional non-linear large deformations of the system are numerically analyzed using the commercial finite element software ABAQUS/Explicit with the full integration rule to evaluate elemental matrices. Sensitivity tests are conducted with random samples generated by the Latin Hypercube Sampling technique to delineate the influence of the static yield stress, strain hardening, Young’s modulus and the mass density of the PEEK layer as well as of the shear modulus, the mass density and the maximum hydrostatic tensile stress of the ceramic on the BABT. It is found that reducing the acoustic mismatch between the PEEK and the ceramic increased the plastic energy dissipated in the ceramic. For normal impact at 180 m/s of a bullet having kinetic energy of 130 J, the 2 mm thick surface layer decreased the total peak normal force, the normal impulse transmitted, and the maximum pressure transferred from the SiC target to the gelatin layer by 19%, 18% and 32%, respectively. Simulations using a reduced integration rule significantly under-predict the final kinetic energy and the back face displacement of the bullet as compared to those using the full integration rule. For the 2 mm thick PEEK layer located on the top, the middle and the bottom of the SiC plate, the back face displacement is the least for the layer located in the middle but the BABT increased.

Introduction

Ceramics are extensively used for lightweight armor because of their high specific hardness and dynamic compression strength [1]. Their high fracture toughness minimizes their shattering upon impact [2] and improves their performance to multiple hits [3], [4]. Commonly used ceramic materials are alumina (Al2O2), due to low cost, as well as sintered, hot pressed or reaction-bonded varieties of Silicon Carbide (SiC) and Boron Carbide (B4C), which have higher specific fracture toughness and stiffness than that of alumina [5].

A typical ballistic event begins with a full metal jacket armor piercing (AP) bullet, usually made of a copper outer jacket with a lead, steel or tungsten carbide core, ejected from either a small firearm or a heavy machine gun [6] impacting the strike face of a ceramic tile. Since the ceramic is significantly harder than the projectile, the impactor tip either shatters or deforms into a blunt mushroom shape [7]. Impact generated compressive shock waves travel in the bullet and the ceramic [8] to their rear surfaces. They are reflected back as tensile waves from the traction-free back face of the ceramic, and produce cracks near the rear surface due to low dynamic tensile strength of the ceramic. As this damage zone propagates towards the impacted face of the ceramic, shallow co-axial cracks are produced in the form of a spreading cone, known as the fracture cone or conoid [9], [10], that points to the impacted face and consists of crushed or comminuted ceramic fragments. The fracture cone helps redistribute the impact load over a larger area. Until the ceramic region right under the impactor is completely comminuted, the projectile does not penetrate the ceramic tile (known as dwell [11]). After the dwell period, the projectile advances into the ceramic rubble causing it to compact and exert a large pressure on the surrounding material, called bulking. During this phase, the projectile is eroded by the ceramic and, if not completely fragmented, can exit the back face causing plugging, and can penetrate into the wearer’s body [12].

A commonly used ceramic armor can shatter and arrest bullets on impact due to its high impact strength; however, a large amount of the incident projectile energy is transferred to the wearer. Stresses related to the momentum transferred to the wearer through the armor-torso interface can cause behind-armor blunt/ballistic trauma (BABT) [13], [14] in the form of internal fractures and thoracic injuries which can impair a soldier’s combat capability. Kirchner and Seretsky [15] experimentally showed that bonding a thin layer of a low elastic modulus material like vitreous petalite improved the impact resistance, by approximately 75%, of SiC and SiN ceramic plates. Sarva et al. [16] have shown that restraining the impact face of alumina and SiC ceramic tiles with a membrane of suitable tensile strength improved the ballistic efficiency, measured as the kinetic energy (KE) of the impactor absorbed by the tile. With layers of Scotch 893 glass fiber tape, Titanium alloy sheets, Carbon-fiber and E-glass/Epoxy composites bonded to the front face of the ceramic, it was observed that the flow of the ejected pulverized ceramic particles was confined to a smaller area that increased the ejecta velocity by almost 40% when compared with that for the bare tile. Since a considerable amount of the energy transferred to the target is converted to the kinetic energy of the ejecta, the restrained tiles showed nearly 20% improvement in the ballistic efficiency for 2% increase in the areal density. Furthermore, the backface deflections were delayed by the presence of the restraint which resulted in increased erosion and reduction in the projectile velocity. Nunn et al. [17] reported up to 40% improvement in the ballistic performance of B4C tiles with a polymer matrix composite facing.

Crouch et al. [18] found that using a fiber-reinforced composite layer on the strike face of B4C tiles had a minimal effect upon the deformation and erosion process of the bullet at V50 impact speeds. Similar results were reported by Rahbek et al. [19] who found minimal difference in the erosion of the projectile during sub-ballistic limit impact on bare alumina tiles when compared with those covered by low-melting temperature polyethylene terephthalate (LPET) layers. They concluded that the composite cover restrained the cracked and fragmented ceramic material from moving away from the impacted region which results in more damage to the covered tiles than that in the bare tiles.

Here, we computationally investigate effects of a thin polyether–ether–ketone (PEEK) layer bonded to the impacted surface of a ceramic target on its impact resistance. The finite element (FE) software ABAQUS/ Explicit ver. 6.14 [20] is used to numerically solve transient nonlinear initial-boundary-value problems pertaining to the ballistic impact on SiC ceramic targets by full metal jacket bullets. PEEK, due to its bio-compatibility, is often used as a matrix for composites in trauma, orthopedic and dental implants [21]. In order to quantify the momentum transferred to the wearer, we place the ceramic tile in front of a ballistic gelatin block that simulates a human torso [22]. The SiC is modeled as a Johnson-Holmquist (JH-2) material and the PEEK by the Johnson-Cook (JC) constitutive relation supplemented with the Mie-Grüneisen equation of state and the JC damage model. Islam et al. [23] have recently shown that the JH-2 and the Johnson-Holmquist-Beissel (JHB) models for the ceramic predict back surface velocity profiles during flyer plate impact tests, in better argument with the test observations than the JH-1 model. The lead core of the bullet is modeled as an elastic-perfectly-plastic material with a ductile damage initiation criteria while the copper outer jacket is modeled as a JC material. The gelatin is modeled as an elastic-plastic linearly strain–hardening material. All materials are assumed to be homogeneous and isotropic. In order to quantify the momentum transferred to the wearer of the ceramic armor (behind-armor blunt/ballistic trauma), and to study the added mitigating capability of a thin polymeric layer bonded on the impacted surface of the ceramic armor, we have modeled the sub-ballistic limit impact of a full metal jacket bullet traveling at low muzzle velocity. Rather than modeling penetration into the wearer, we have focused on characterizing the impulse and the maximum pressure transmitted from the armor to the wearer. Similar experimental studies have been conducted by Crouch et al. [18] and Rahbek et al. [19].

We find that when compared with impact on a bare ceramic target, using a thin surface layer reduces the bullet back face displacement and dissipates a larger fraction of the incident bullet KE through a combination of a greater average resisting force on the bullet, plastic energy dissipated in the surface layer, and distribution of the impact load on a wider area on the ceramic top face. For a bullet with incident velocity vi=180 m/s and KE of 130.4 J, the 2 mm thick PEEK layer reduced the initial pressure at the initial contact point by a factor of 11.5 and the initial high pressure at the center of the ceramic top face by 33% when compared with the impact on the bare target. Furthermore, the surface layer reduced the BABT by lowering the peak normal contact force, the impulse transmitted and the maximum pressure transferred from the SiC target to the gelatin layer by 19%, 18%, and 32%, respectively.

For the 2 mm thick PEEK layer located at the top, the middle and the bottom surfaces of the ceramic layer, the maximum total contact force and the impulse transmitted to the gelatin layer increased when the PEEK layer was bonded to the plate bottom surface.

We also studied effects on the numerical results of using a reduced integration rule to evaluate elemental matrices. We found that while qualitative trends are similar for the reduced and the full integration rules, the quantitative results significantly differ. The energy dissipated in the structure due to hour-glassing with the reduced integration rule noticeably decreased the bullet back face displacement and its final KE.

Finally, using the Latin Hypercube sampling method [24], [25] to construct trials that are representative of the variability (or uncertainty) in material properties, we study effects of varying Young’s modulus, mass density, static yield stress and strain hardening characteristics of the PEEK layer as well as the shear modulus, the mass density and the maximum hydrostatic tensile stress of the ceramic on the ballistic performance of the armor. We develop relations between material properties and the plastic dissipation in the PEEK surface layer and the ceramic target as well as the peak normal contact force and the impulse transmitted to the top face of the gelatin block. Increasing the mass density of the PEEK layer and lowering that of the ceramic to reduce the acoustic impedance mismatch between the two materials increased the plastic energy dissipated in the armor and decreased the BABT.

Section snippets

Problem description

A schematic sketch of the problem studied is exhibited in Fig. 1. SiC ceramic plates, 100 mm × 100 mm × 10 mm, with a major surface coated with either tv=1 or 2 mm thick PEEK layer were impacted at normal incidence at the major surface centroid by a 9 mm diameter and 13.3 mm long 8.03 g full metal jacket bullet with a lead core and a 0.5 mm thick copper jacket at an axial velocity vi=180 m/s and KE  = 130.4 J. The target plates shielded a 100 mm × 100 mm × 20 mm block of ballistic gelatin (10%

Dependence of results on the finite element mesh

In order to study the dependence of numerical results on the FE mesh used, we varied the mesh size as follows: for the central regions of 5 mm radius of the ceramic plate and of the surface layer, and of 10 mm radius of the gelatin block, the element size was varied from 0.11 × 0.11 × 0.11 mm3 to 0.09 × 0.09 × 0.09 mm3. For the bullet, the element size was varied from 0.14 × 0.14 × 0.14 mm3 to 0.1 × 0.1 × 0.1 mm3. Thus, the total number of elements in the structure varied from 1,238,000 to

Sensitivity analysis with respect to PEEK and ceramic material parameters

In order to ascertain the influence of the static yield stress A, the strain hardening parameter B, Young’s modulus E and the mass density ρ0 of the PEEK layer on the ballistic performance of the PEEK/SiC target, we conduct sensitivity tests using the Latin Hypercube Sampling (LHS) method to generate random trials. Anticipating the influence of the interaction between the material properties of the SiC plate and the PEEK surface layer, we also consider effects of shear modulus Gc, mass density ρ

Conclusions

The three-dimensional (3D) non-linear initial-boundary value problem of the ballistic impact of full metal jacket projectiles on Silicon Carbide ceramic targets with and without a thin energy-absorbing polyether–ether–ketone (PEEK) surface layer bonded to their top surfaces is numerically studied using the commercial finite element software ABAQUS/ Explicit with full integration to evaluate elemental matrices. The SiC target is placed in front of a ballistic gelatin block that simulates a human

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the US Army grant W91CRB-17-C-0041 to Virginia Polytechnic Institute and State University. Views expressed in the paper are those of the authors and neither of the US Army nor of Virginia Tech.

References (40)

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