Performance of Fish Scale-Inspired Armour Subjected to Impact Loading by Different Impactor Shapes: A Numerical Investigation ^†

Abstract

This paper investigates the impact performance of a hybrid scale–tissue structure inspired by elasmoid fish scales. Acrylonitrile butadiene styrene (ABS) acted as the hard scales, whereas thermoplastic polyurethane (TPU) mimicked the soft tissue. Low-velocity impact scenarios were investigated using a commercially available software, ANSYS LS-Dyna. The effect of indenter shape (conical, hemispherical, and flat head) was studied at three impact energy levels (50 J, 75 J, and 100 J). The specific energy absorbed for the conical indenter was the highest. The peak force for the conical, hemispherical, and flat-head indenters at an impact energy of 100 J was 6.0 kN, 5.4 kN, and 4.5 kN, respectively. The primary failure modes were shear failure and tensile breaking of the scales. The present study highlights the effect of indenter shape on the impact behaviour of a fish scale-inspired structure.

1. Introduction

Fish scales have a variability in overlapping angle depending on their location (anterior, mid-section, or posterior). Likewise, the thickness of these scales varies from species to species and between different locations of the body [1].

A prior investigation on a 2D elasmoid fish scale-inspired design was conducted to study the influence of scale volume fraction, scale overlapping ratio, and the radius of curvature. These tests were carried out at quasi-static and low-velocity impact using a hemispherical indenter [2,3,4,5,6]. At higher scale overlapping ratios, the impact load was distributed over a wider region, resulting in a better impact resistance [2]. In comparison to the single monolithic plate, massively overlapping scales showed superior penetration resistance per unit weight. Rudykh et al. [4] investigated the design of fish scale-inspired armour with two design parameters: scale volume fraction and angle of inclination. With a lower scale inclination angle and a higher scale volume fraction, the penetration resistance was increased 40 times while sacrificing the flexibility by 5 times. According to a study by Martini and Barthelat [5], a bio-inspired armour composed of overlapping ceramic scales exhibited better damage tolerance, improved puncture resistance, and increased flexibility.

According to Ravi et al. [7], compared to a single plate of the same thickness, their bio-inspired hexagonal segmented glass armour with a soft silicone rubber substrate increased the puncture resistance by 70%. Eight distinct geometrically shaped acrylonitrile butadiene styrene (ABS) scales on a polyurethane (PU) substrate were studied by Martini et al. [8]. The puncture resistance was enhanced, and the flexural compliance was lowered by the scale-to-scale interactions. Multilayered ceramic panels inspired by conch and nacre shells were investigated by Sarvestani et al. [9]. According to their study, under quasi-static and impact loading, an optimized architecture increased energy absorption by 20% and 48%, respectively. Zhang et al. [10] investigated the biomimetic structure of two-layer fish scales—hard ceramic and ultra-high-molecular-weight polyethylene (UHMWPE). These hybrid ceramic UHMWPE plates were placed at an angle on top of the Kevlar base layer. The study found a negative correlation between the overlapping area of successive scales and the residual striker velocity. Ali et al. [11] investigated the puncture performance of segmented hard scales attached to a flexible membrane. These natural constructs provided suitable combinations of puncture resistance and flexibility. Rigid darts and hexagonal shapes gave the best result among the studied designs. The puncture resistance by the 60º inclined dart scale was about 42 times stronger than that of an isolated scale. Most of the previous works have investigated elasmoid fish scale-inspired protective structures impacted by a hemispherical impactor. The effect of the impactor’s shape was largely lacking.

It is evident that there are a paucity of data exploring the effect of impactor shape during the low-velocity impact on elasmoid fish scale-inspired protective structures [6,7,8,9,12]. This is despite the fact that a structure’s response is affected by the shape of a penetrator [12,13]. This study aims to investigate the performance of elasmoid fish scale-inspired designs when impacted by different impactor shapes at varying impact energy levels. The study highlights the application of bio-inspired design for protective structures.

2. Material and Methods

A numerical analysis was conducted using a commercially available finite-element code, ANSYS 2023 R2 LS-Dyna. Figure 1 show the test setup, three different impactor shapes, and the finite-element mesh used in the numerical model. The advantage of full symmetry was used to reduce the computational cost. The conical indenter had an indenter tip inclined at 45°, and the diameter of the indenters was 10 mm (see Figure 1b). The indenter and supports were modelled with a single-layer rigid element to reduce the computational cost. The density of the impactor was modified to simulate the actual mass of the impactor in a drop tower setup. The sample measured 80 mm (length) × 80 mm (width) × 10 mm (height). The sample included two regions: a 5 mm hybrid scale–tissue region and the bottom 5 mm of a tissue-only layer. The ABS (yellow) with a higher elastic modulus was used as a scale material, and TPU (red) with a lower elastic modulus was used as a tissue material. The size of each scale measured 12 mm in width and 100 mm in the radius of curvature, with an inclination of 10°.

Hexahedral 8-node solid elements were used to mesh all the components. A mesh element size with a 0.5 mm edge length was used, as it provided good balance between the computational time and the result accuracy [13]. *MAT20 (a rigid material model) was used for the steel supports and the impactor. The ABS material showed clear yield and ultimate and fracture strains [14]. For this, the ABS material was modelled using a *MAT03 (piecewise linear plasticity) material model. The TPU material did not exhibit a clear yield stress value at quasi-static loading. Thus, it was modelled with the *MAT89 (plasticity polymer) material model [15]. The material properties of the ABS and TPU were obtained from uniaxial tests and the literature [16], as referenced.

Table 1. Material properties of ABS and TPU.

Properties	Tissue | TPU	Scale | ABS
Density (kg/m3)	1210	1070
Elastic modulus (MPa)	56.7	1998
Poisson’s ratio	0.49	0.35
C [s−1], P (for Cowper–Symonds equation)	971, 0.98	678, 3.23

lists the detailed material properties.

A penetration or perforation process was simulated by defining an eroding contact algorithm using SOFT2 between the impactor and the specimens [15]. Contact surfaces were automatically redefined after the elements subjected to extreme stresses and pressures were deleted. Failure strains of 0.12 and 0.26 were assigned for ABS and TPU, respectively [6]. The static and dynamic frictional coefficients between the indenter and the specimen were set to 0.5 and 0.4, respectively [17,18].

3. Results and Discussion

The numerical investigation included the influence of three impactor shapes and three energy levels. The impact velocities at 50.0 J, 75.0 J, and 100.0 J were 4.09 m/s, 5.01 m/s, and 5.78 m/s, respectively.

3.1. Result Validation

The numerical details discussed in Section 2 were employed to simulate the impact process and compared with the experimental study. Figure 2 shows the numerical and experimental results for a hemispherical indenter impacted at 100 J of impact energy. The comparison of the numerical and experimental results up to 6 ms shows close resemblance.

During the elastic part of the impact process (up to 1.2 ms), the experimental response was stiffer than the numerical results. The sharp peaks and valleys in the FEA results indicated the erosion of the failed elements [6]. The maximum force in the numerical results was higher than in the experimental results. Similarly, both the numerical and experimental results show abrupt failure at around 3.5 ms, indicating the significance of the numerical results.

3.2. Force Histories and Energy Absorption

Figure 3 shows the force histories for three impactors at three different energy levels. All the numerical simulations were conducted with the numerical details validated in Section 3.1. For a given impact energy, the lowest peak resistance was observed for the flat-head impactor. Similarly, the highest peak force was provided for the impactor with a cone tip. The sharp drop in force histories (at around 2 ms) represented the initial breaking of the scales as early major damage. At a higher impact energy level, the samples failed early. The effect of the impact energy was significant for the impactor with cone and flat heads (see Figure 3).

Figure 4 shows the energy absorption history plotted for three different indenters at three different impact energy levels. During the non-perforated scenarios, the impact energy was fully absorbed by the target. Consequently, higher energy absorption resulted in the cases of higher impact velocities and, therefore, impact energies. However, this was not consistent with the perforated cases [19]. Figure 4 represents the energy (internal and kinetic) absorbed history of the uneroded material during the impact process. At 100 J, the energy absorbed was the highest for the samples impacted by the conical impactor and the lowest for the flat-head indenter. The piercing by the conical indenter through the material resulted in localized failure, which eventually resulted in the highest energy absorption. However, the failure was abrupt for the flat-head impactor, leading to low energy absorption by the uneroded materials. In the linear part of the energy absorption history, the energy absorption was directly proportional to the impact energy level. For the hemispherical impactor, the energy absorption indicated direct correlation with the impact energy. The perforation was gradual with the conical and hemispherical impactors, resulting in higher energy absorption.

With the flat impactor shape, the energy absorbed after 2 ms was highest at an impact energy of 75 J. Similarly, at 100 J of impact energy (i.e., highest impact speed), the sample was perforated early and, hence, provided the least resistance. The force and energy absorbed for the flat-head impactor were the lowest because of the large contact area upon the initiation of contact between the impactor and the specimen. At the onset of the impact, the dominant shear mode led to premature failure with the flat-head impactor, which eventually led to lower energy absorption.

3.3. Stress Field

The stress contour plots were plotted to investigate the damage evolution during the impact process. The contour plot at 1 ms presented the global elastic response of the target. At 2 ms, the sample plastically deformed, and, at 4 ms, the samples were fully perforated (see Figure 5). The cone impactor pierced through the specimen without much global deformation. The present results were consistent with previous studies by Islam et al. [12]. The piercing by the cone impactor had the maximum contact surface with the specimen during penetration, which also correlated to having a higher sustained peak force in Figure 3.

The scales inclined towards the left were pushed down, resulting in the large pushback of tissue materials towards the right (see Figure 5c at t = 4.0 ms). The impact process of the conical indenter resulted in piercing of the specimen from the onset of the impact process. However, the first cracks on the back side of the specimens were observed when impacted by the flat-head impactor. The large contact area between the flat impactor and the specimen resulted in a large global deformation and, consequently, the early initiation of cracks on the back face.

4. Conclusions

The following conclusions were made based on numerical studies of low-velocity impact on fish scale-inspired structures:

• During elastic deformation (approximately up to 2 ms), increases in the impact energy resulted in higher energy absorption for all the impactor shapes;
• For the perforated cases, the hemispherical indenter was the only shape of impactor that resulted in a positive correlation between the impact energy and the energy absorbed;
• Progressive cracks on the bottom surface were first seen with a flat-head impactor, whereas the sharp impactor pierced the specimen with a minimal damage area;
• The energy (internal + kinetic) absorbed by non-eroding elements was highest when impacted by the conical impactor. A longer contact duration and a gradual failure of the elements led to higher energy absorption.

This study emphasizes the possible application of fish scale-inspired protective structures for providing protection against differing impactors.