Fish Scale-Inspired Stab-Resistant Body Armour ^†

Abstract

While commercially available lightweight “stab-proof” apparel exists, it offers little resistance to true stabbing as it is primarily designed to withstand slash attacks. Yet, crimes involving the use of a knife or sharp instrument have consistently been rising in the UK over several decades. For the most part, the various proposed solutions to stab-proofing are based on speciality textiles and while these have shown success in slash-proofing, their utility for stab-proofing is still somewhat of a misnomer. Nature showcases a plethora of puncture-resisting materials and structures. At the macro-scale, these include carapaces, egg cases, toughened skin, and more. One of the most effective protective mechanisms known comes through surface scaling, present on animals such as reptiles and fish. Scaled protective armours present in extant fish species include overlapping elasmoid scales, interlocking ganoid scales, placoid scales, tessellating carapace scutes, and interlocking plates. Here, we research overlapping and interlocking scaled structures to ascertain the stab penetration resistance of biomimetic scaled structures against continuum material to obtain the force–time relationship of the impact event as well as ascertaining the penetration depth. We use additive manufacturing methods to manufacture biomimetic armour made of nylon, a common protective artificial material used in slash-proofing textiles. Stab testing to the closely replicated HOSDB body armour standard 2017, we find that biomimetic scales made of nylon offer greater protection against direct stabbing than continuum nylon material sheets. This can be attributed to (a) the heightened flexibility in an interlocked fish scale structure that does not exist in a continuum sheet of the same material; (b) the effect of overlapping of the fish scales, resulting in a greater penetration depth requirement before the structure undergoes perforation; and (c) segmentation into smaller armour plates (of the same thickness) rather than continuum sheets provides a lower span-to-depth ratio, therefore leading to a smaller deflection of the plate upon impact and a greater deceleration and, hence, a greater impact force.

1. Introduction

Body armour has evolved significantly from its early origins in the Bronze Age, where basic leather and metal plates were employed, to modern designs incorporating cutting-edge materials. The increasing threat of knife attacks in civilian settings has heightened the importance of stab-proof body armour in recent decades [1]. Despite the availability of lightweight “stab-proof” apparel, many of these solutions primarily provide defence against slashing rather than true stabbing attacks, exposing a critical gap in protective gear. In nature, various animals have developed sophisticated systems of armour to protect themselves against predators. These natural armours, such as the overlapping scales of fish or the segmented plates of armadillos, protect and also have other essential characteristics such as flexibility and a thermoregulatory ability. Nevertheless, natural armours also face challenges in terms of the optimization of damage tolerance, often balancing protective strengths with other factors like mobility and thermoregulation. For instance, armadillos exhibit higher thermal conductivity in their armour but at the cost of a reduced force of fracture, as compared to the giant girdled lizard, which prioritises protection over thermoregulation [2]. Recent research has focused on applying the principles of natural armour to human body armours. Miranda et al. (2019), for example, demonstrates that segmented armours effectively localise damage from repeated stab attacks by containing the damage area to one segment, allowing multiple stab attacks across different segments [3]. This segmentation, often seen in hexagonal panels in nature, such as in boxfish and armadillos, enhances the overall durability. Additionally, 3D-printed biomimetic designs inspired by natural structures like crocodile skin show promise as they effectively distribute impact energy, reducing damage [4]. Critical thickness studies are crucial in understanding the protective capacity of materials used in body armour. Cicek et al. (2022) assessed various materials and found that the critical thickness of PA6/66 (Nylon) was 11 mm to pass the KR1-E1 stab impact energy of 24 J [5]. Similarly, Johnson et al. (2018) identified the critical thickness for DuraForm EX to be 11 mm for effective stab resistance [6]. These studies highlight the importance of material thickness in enhancing stab resistance, providing a benchmark for developing more effective protective gear by more closely examining the mechanisms of impact. Additive manufacturing has emerged as a viable method for prototyping complex body armours using polymers based on naturally occurring body armour [7]. Elasmoid and ganoid scales, for instance, provide optimal strength, penetration resistance, and flexibility [8,9]. This research aims to investigate the stab penetration resistance of biomimetic scaled structures made of nylon, commonly available for additive manufacturing, carry out a novel comparison of their effectiveness against continuum nylon sheets, and closely examine the mechanical phenomena through which they fail. By replicating the HOSDB body armour standard 2017, this study will determine whether or not there are any mechanical advantages in using biomimetic scales over continuum sheets in terms of providing superior protection to stabbing [10].

2. Materials and Methods

Samples were manufactured from 2.85 mm black Nylon filament, a PA 6/66 co-polymer from Ultimaker (Utrecht, The Netherlands), using an Ultimaker 3 printer. Nylon 6/66 was chosen for its excellent mechanical properties, such as high tensile strength, abrasion resistance, and toughness, making it suitable for additively manufacturing armour samples.

The fish scale (FS) samples shown in Figure 1a,b were individually fabricated and interlocked manually. The FS samples were compared against a plate control sample. The dimensions of the plates were 60 × 60 mm², with varying thicknesses ranging from 4 mm to 8 mm. The thickness of the plate is crucial to prevent complete perforation. These varying thicknesses were used to observe the critical thickness required to prevent complete perforation of the armour and to verify the validity of the rapid prediction model developed using hot-pressed Nylon plates by Guo et al. [11] for additively manufactured samples. The critical thickness was calculated as 4 mm for 5J of impact energy, and greater-thickness plates were fabricated, as hot-pressed plates are generally denser than additively manufactured plates as they are processed under high pressure and temperature, resulting in a denser micro-structure with fewer voids.

Fish scale samples at 45° were tested under two different conditions, underarm and overarm, as shown in Figure 2a,b, respectively, in order to simulate the behaviour of an underarm and overarm stab attack.

A bespoke drop-weight tower was manufactured for the tests, and data were recorded using a high-speed camera. Drop-weight tests were performed using this setup. A HOSDB P1/B blade was affixed to the impactor. Tests were conducted at a controlled energy level of 5J, with a drop height of 235 mm ± 1 mm. Soft non-drying sulphur-free Plasteline (NSP) by Chavant™ (Macungie, PA, USA) was used as the backing material to minimise deflection after impact, providing a cost-effective solution for repeated tests.

The experimental setup, shown in Figure 3, included a high-speed camera recording at 2111.62 FPS and a Kistler three-axis accelerometer with a range of ±500 g. Halogen lamps were used to eliminate flickering caused by the lab’s fluorescent lights, and a lighting panel was used to block sunlight from clerestory windows, creating a controlled environment for filming high-speed videos.

To prepare for the impact test, clay was moulded to accommodate the sample with sufficient clearance, followed by placing the sample inside a container. The container was securely fastened onto a wooden block. The knife’s orientation was adjusted to the weakest orientation (parallel to the fibres or 90°). Once the knife orientation was confirmed, the impactor was set to the fixed height by using a pull pin mechanism, and the pin was pulled to commence the impact test.

3. Results and Discussion

The experiment aimed to comprehensively compare the forces and failure modes between the fish scales and the control samples. Penetration depth measurements were taken using a digital vernier caliper post testing by carefully inserting the knife through the plate sample until the cross-section of the knife matched the footprint on the sample without applying any force. Figure 4a shows the average penetration depth against the average sample thickness. The 4 mm and 5 mm thick samples showed complete perforation on average, i.e., penetration through the whole thickness of the sample as per the measurements. Additionally, the penetration depth was between 5 and 6 mm for all the samples except for 4 mm due to a higher span-to-depth ratio, which meant that the sample underwent more significant bending and deflection. This bending concentrates the stress into a localised area rather than spreading it across a broader surface, leading to greater perforation.

Figure 4b shows the average impact force against the sample thickness, showing that the average force is proportional to the sample thickness except for the 7 mm thick sample, which is an anomaly. The reason for this anomaly is that the first knife became blunt after testing the 8 mm thick samples (the first sample to be tested). Watson et al. (2002) found a much larger impact force with a blunter knife compared to a sharp knife, as the area of the knife-point is larger; hence, entry into the sample is no longer as smooth, leading to a faster deceleration and a larger impact force [12]. Following this deviation, the knives were replaced regularly during the testing to allow for more accurate results.

Flat tests for FS show a greater average impact force compared to 4mm thick plates, as shown in Figure 5, even though the individual scales are 4 mm thick as well. This could be due to the individual fish scales having a smaller span-to-depth ratio and, as such, less deflection of the individual scale upon impact and a lower stress concentration at the contact point as the stress can be spread out through out the structure, leading to a greater requirement of force on impact. Additionally, a significant difference is noted between the average forces of the two sample sets, with p < 0.05 (using a one way ANOVA test at a 5% level of significance).

Figure 5a,b contrast the different samples with a layer thickness of 4 mm. FS overarm and FS underarm both have insignificant differences between them, which may indicate that they act similarly under knife impact. Overall, the results show that the average impact force is dependent on the span-to-depth ratio, as indicated by the increase in average impact force with sample thickness and increased average impact force for fish scales samples, which at an individual scale had a smaller span as such a lower span-to-depth ratio.

Tests for the fish scales at 90° only showed penetration from the top layer of the scale and did not penetrate completely through the overlapping structure, which we presume is due to at least a greater overall thickness. Tests for 4 mm with a 90° and 45° orientation plate showed a similar impact force, with an insignificant difference found between the two, with both being penetrated fully. A flat plate only has a normal load; however, when angled, it will also have a transverse load, as shown in Figure 6a,b. However, as both of the samples were fully constrained, there was little difference found with regard to the penetration. Tests for the fish scales at 45° showed no penetration through even a singular scale, even though the scales were 4 mm thick. For fish scales at a 45° orientation, a transverse load, as shown in Figure 6c, enabled a freer deformation and, as such, caused the knife to slip rather than to penetrate through the scales.

4. Conclusions

In conclusion, we found that the penetration depth is similar across all thicknesses except for the 4 mm sample, which was found as the critical thickness as per the rapid prediction model. Additionally, we found that the average impact force is directly proportional to the sample thickness. The average impact force is greater for fish scales when compared to 4 mm plates, as they are stiffer. Flat FS plates were advantageous due to their overlapping nature, doubling the thickness compared the monolithic plate. Moreover, the fish scales at 45° showed no penetration, which means that an angled segmented armour is superior in performance compared to a monolithic plate of the same sample thickness.