Ian G. Crouch

Ulverston Armour Solutions, Ulverston, LA12 7BJ, Cumbria, UK

Keywords:Interfaces Body armour Body armor Ceramic armour UHMWPE Aramids Ballistic fibres Fabrics Interfacial materials Hard armour plates Soft vests

ABSTRACT The ballistic performance,and behaviour,of an armour system is governed by two major sets of variables,geometrical and material. Of these, the consistency of performance, especially against small arms ammunition, will depend upon the consistency of the properties of the constituent materials. In a body armour system for example,fibre diameter,areal density of woven fabric,and bulk density of ceramic are examples of critical parameters and monitoring such parameters will form the backbone of associated quality control procedures. What is often overlooked, because it can fall into the User’s domain, are the interfaces that exist between the various products;the carrier,the Soft Armour Insert(SAI),and the one or two hard armour plates (HAP1 and HAP2). This is especially true if the various products are sourced from different suppliers.

1. Introduction

The ballistic performance, and impact behaviour, of an armour system is governed by two major sets of variables:geometrical and material. Of these, the consistency of performance, especially against small arms ammunition, will depend very much upon the consistency of the properties of the constituent materials.In a body armour system,for example,fibre diameter,areal density of woven fabric, and the local bulk density of a ceramic, are all examples of critical parameters,and monitoring such parameters will form the backbone of any quality control program. However, there are also numerous interfaces and interfacial materials which can be equally important. For example, the interface/interlayer between an armour-grade ceramic and its substrate are of popular interest, as are the inter-ply spaces within a soft armour pack. There are also numerous internal interfaces within a Hard Armour Plate (HAP)and a Soft Armour Insert(SAI).What is often overlooked,however,are the interfaces that exist between the various products; the carrier,the SAI,and the one or two HAPs that may make up a body armour system.At the end of a very recent review of body armour systems and materials [1], the following advice was offered:

“The advent of stacked systems introduces yet another interface within a body armour system, and a final word of caution is required. Controlling the quality, and magnitude, of interfaces,not only between the layers within a soft armour pack,and the gap between soft vest and HAP,but now also between HAP1 and HAP2, is very crucial in guaranteeing the reproducible quality and ballistic performance of a body armour system-it is highly recommended, therefore, that more attention be given to this topic since it is not a trivial exercise.”

In principle, every interface and interfacial material within a body armour system should/could, in some way or another,contribute to the protection offered by that system.This paper aims to provide a User-friendly overview of all interfaces and offer unique guidance as to their criticality and influence.In so doing,it is hoped to steer future research in this important area since inadequate attention has previously been given to the topic of interfaces and interlayers within body armour systems.

2. General overview

The global defence market is served by a plethora of advanced body armour systems from suppliers like Tencate [2] and Morgan Defence Systems [3], to name but two. Numerous types are available including a new variant called a stacked system,as defined in a recent review [1]. This type of body armour system involves the most interfaces/interlayers because it includes a second HAP. For this reason, it represents an extreme case with maximum number of products and therefore maximum number of interfaces. Fig. 1 illustrates a deconstructed cross-section of such a system.

In any system, however, whether it be modular, tiered, standalone or stacked, there are between 30 and 150 layers, as listed below:

· 2-4 layers, at least, in the load-carriage system

· 2 in the soft armour carrier, encapsulating the SAI

· 10-40 layers in the SAI itself

· 20-80 plies in the backing layer of a HAP

· 4-8 secondary layers in the HAP itself, including the bond between ceramic tile and backing layer

· The air-gap between HAP1 and HAP2, in the case of a stacked system.

Also note that in a real system there are often individual pouches within the load-carriage system, in order to contain the individual protective elements. HAP1, for example, is normally contained within a separate pouch just forward of the SAI pouchso,there exists other materials between the HAP and the SAI.One of the questions posed is whether the existences of these pockets affect the system’s ballistic performance in any way?

When designing body armour systems,it is standard practice to consider the material requirements(e.g.which grade of ceramic or fabric is best?) and the geometrical requirements (e.g. how thick the ceramic should be, or how many layers of a particular fabric should be used?), as well as appropriate choices of manufacturing routes. There are also well-established design rules to assist the armour technologist in finding optimal solutions, and threatdependent solutions are now becoming well established. However, especially the End User and the suppliers of single armour products,like the soft armour pack,for example,or the HAP,often overlook the role that interfaces play between the various products.

The principles of layered structures and interfacial materials were reviewed recently,within Chapter 4 of The Science of Armour Materials[4].As illustrated in Fig.1,these interfaces can be divided into those that exist within a defined product (internal interfaces)and those that exist between defined products(external interfaces).The defined products are the soft armour pack(SAI plus carrier)and the HAP(s).The body armour system consists of all these products layered together within the Load Carriage System.

In a layered/laminated structure subjected to an impact from a high velocity projectile the interfaces, between the layers, will be required to function in several different ways:

a) transmit or reflect sound/pressure waves

b) transfer the interlaminar shear forces that are generated from bending loads experienced by the system

c) slide over, or separate from, each other during delamination events.

Surface topology will affect each of these functions.For example,it is easy to imagine that the surface finish of adjacent plies of a woven fabric will greatly affect the dynamic friction between the two surfaces. A great deal of research has been focussed upon the internal sliding interfaces, within a woven fabric, such as that reported by Moure et al., in 2019 [5], and the friction between the yarns has been shown to be extremely important in controlling the in-plane properties of single-ply fabrics. However, little attention appears to have been given to the frictional effects between the plies in a multi-ply pack.The use of macro-scale numerical models is to be encouraged in this area,since there are a number of ways of controlling the contact surfaces between layers and simulating the sliding frictional effects that will be tabled in this paper. Some worked examples on pages 540-560 of reference [6] illustrate different approaches to modelling contact surfaces within an armour system.

3. Review of specific interfaces

3.1. Internal interfaces within the soft armour pack

Fig.1. Schematic cross-section of a deconstructed, Stacked Body Armour System.

These are the least defined of all the interfaces since they consist of dry,loosely bound,plies of either aramid or Ultra High Molecular Weight Polyethylene(UHMWPE)fabrics.Within each ply,as in,for example, a cross-plied UD UHMWPE fabric, the interface between the fibres and the polymeric matrix material is critical in preventing defibrillation of the fibres and misalignment/displacement of the fibre tows. However, the interface between these well-formed plies, is thought to be even more crucial in controlling the flexural behaviour of the pack itself and, in turn, the ballistic performance of the soft armour.

3.1.1. Inter-ply surfaces

When SAI packs are impacted, especially by a relatively slow knife or spike weapon,the impact energy appears to be absorbed in two distinct phases [7], as shown in Fig. 2: a physical deflection stage (in which plies can slide over each other), and a cutting/piercing stage(in which fibres/fabrics are actually being cut).In this case,the displacement of the penetrator was tracked using a highspeed, digital video camera. In Fig. 2, which is based upon three independent 50J knife(KR2)or spike(SP2)tests,one can observe a transition region between these two stages after about 25 mm of deflection. It is also interesting to note that more than 50% of the impact energy is absorbed in this first stage- that is, without any cutting/perforation. This suggests that the membrane stretching behaviour of the pack is important. Consider, for a moment, the deformation of layers of fabric. If the Interlaminar Shear Strength(ILSS)between the layers(plies)is negligible(as in a dry,unbonded pack)the plies will deform independently.Conversely,if the ILSS is sufficiently high (as in a bonded pack) then the plies will move more collectively and offer greater resistance to bending. In other words, during this initial membrane stretching stage, all plies will be forced together and could act as a single dry laminate, if the inter-ply surfaces were locked together in some way. So, even in a totally dry,unbonded laminate the inter-ply friction between those plies is critical in determining the flexural properties of the entire pack. In other words, surface finish of the individual plies may be important. The amount of stitching and quilting will also affect a pack’s ability to deform“as one”during this first deformation stage.

Fig. 2. Generated curves of residual KE of impactor, as a function of time, from highspeed video capture of three separate 50J stab and spike events against an SAI [7]showing two distinct stages of perforation, with a clearly defined transition zone.

Roy et al. recently confirmed [8] that natural rubber-coated aramid fabrics can perform better than untreated fabrics. However, this work, like all previous works [9,10], did not mention the effect of inter-ply friction. So, for stab and spike packs, the surface condition of the carrier(e.g.a nylon bag),as well as the individual plies within the SAI,may be key to maximising the energy absorbed during this membrane stretching/bending stage.It should be noted that the family of woven aramids, with the inclusion of shear thickening fluids, are now in vogue and being actively researched by numerous centres[11],especially those concerned with energyabsorbing mechanisms [12]. However, even though STF mechanisms are well understood,it was recently pointed out[13]that any associated increase in ballistic performance is probably due to an increase in frictional resistance rather than a shear thickening effect per se. Nevertheless, utilising fluids like the STFs, to actively influence not only inter-fibre and inter-tow surfaces but also interply surfaces, is to be encouraged.

3.1.2. Air-gaps

It is well recognised that the air-gaps that develop during testing of a soft armour panel are not desirable and ballistic test standards are written to include the need for both smoothing and flattening between each impact in a multi-hit assessment.This is certainly the case within the most recent ballistic test standard,NIJ 0101.06[14]at Section 7.7.2.2 which states that “The armor panel shall be manipulated by hand so that any deformations in the armor are smoothed out”.This is done to improve the consistency of the pack and decrease variability in the results.It is hoped that this practice will still be upheld in the next version of this standard,NIJ 0101.07[15].

Fig. 3 represents some general advice offered by DuPont in the 2000s,based upon many decades of in-depth,in-house experience.More specifically, the V50of an SAI constructed of 24 layers of a woven aramid designated S363F and tested against a 0.357”handgun round was reduced by 7% when well-defined air-gaps were present in the pack[16].

It is very easy to ensure that an SAI, when first tested, or a soft armour pack, when first fitted into a body armour system, is very flat, as in case A in Fig. 3. However, conditions like in B or C may arise during wearing and Condition C certainly occurs during testing, even if the pack is allowed to be flattened, and smoothed,between impacts. For example, in packs made up of fairly rigid fabrics,like some of the latest UHMWPE materials(e.g.DSM SB71 at 190 gsm), produce very stiff and well-formed ridges upon first impact (see Fig. 4), which are very difficult to flatten and smooth out.Subsequent impacts,adjacent to this initial damage, may well have inferior performance. In earlier work, by van Es [17], the location of these air gaps, in a UHMWPE pack, does influence the result. He reported that gaps near the strike face can reduce performance, whilst gaps towards the edge of the pack may improve performance. But as recently highlighted [1], the ballistic performance of such packs does also vary with both size of the pack, as well as shot location.

Tacking, or stitching, may assist a pack’s ability to remain flat and smooth during impacts from multiple strikes. For example, it may be advantageous to actually preform the SAI into a singlecurved structure by hot-tacking the edges of the pack during manufacture. This is certainly one distinct advantage of having a thermoplastic matrix material. Quilting, covered in detail elsewhere [18], also has the advantage of keeping the pack flat and ridge-free [19]. However, there have been mixed results from quilted packs, and it appears that any effect may be dependent upon the type of threat.

3.2. External interfaces adjacent to the soft armour pack

3.2.1. Between soft armour pack and HAP

Fig. 3. A schematic cross-section of a typical soft armour insert: (A) without any airgaps, (B) uniform airgap, and (C) random airgaps.

Fig. 4. Rear view of an impacted SAI consisting of layers of UHMWPE fabric showing formation of wrinkles.

During development of ultra-lightweight HAPs in the early 2010s [1,20], it became clear that the performance of such plates was dependent upon the type of SAI used.On an areal density basis,for example, DuPont’s non-woven, stitched fabrics (initially designated XP fabrics) were found to be more “supporting” than conventional woven aramid plies/packs,leading to reduced BFS values.This interdependence between HAP and SAI was studied in depth by Jaitlee [21], who determined a number of features of this important interface, especially about the effects of cladding the ceramic(see Section 3.3.3).In real systems,of course,this interface is further complicated by the existence of both a carrier bag(encapsulating the SAI)and the HAP’s covering material,whether it be ballistic nylon or a polymeric dipped/sprayed coating. Because the optimal ballistic performance of the HAP is dependent upon the structure and style of the SAI,it is suggested that these interlaying materials at this interface should be as thin as possible so that intimate contact is maintained between the SAI and the HAP.

3.2.2. Between soft armour pack and wearer

Body armour standards have always tried to cover aspects of possible air-gaps between the HAP and the SAI,and the most recent UK standard from CAST [22] emphasises this point when, for example, testing of female body armour systems. But little work appears to have been carried out to examine whether air-gaps between the soft armour pack and the wearer are critically important. In the only recent study that this author could find,Tilsley, and co-workers at Cranfield [23], carried out some welldesigned experiments to investigate possible effects.

Panels of HG1/A + KR1 body armour, typical of police body armour, were assessed using 9 mm FMJ rounds, at both 0°and 30°obliquities. The panels are assumed (by this author) to be of a woven aramid-based design.When tested,a controlled thickness of air-gap, ranging from 0 to 15 mm, was created behind the panel using plywood spacers. Their results were well analysed, and the team reached very clear conclusions:

“For 0°impacts,a critical air-gap size of 10mm is detrimental to armour performance for the armour/ammunition combination assessed in this work. Specifically, the incidences of pencilling were more common with a 10mm air-gap and resulted in BFS depth:volume ratios ≥1.0. For impacts at 30°the armour was susceptible to perforation irrespective of air-gap.”

In summary, the researchers concluded that an air-gap behind body armour might result in an increased likelihood of injury.This point is especially salient since, in the past, some manufacturers have claimed improvements in thermo-physiological burden by having an air-gap designed into the wearer’s system.It should also be noted that this particular interface is also the favoured location of trauma packs, compressible interlayer materials which are designed to reduce Behind Armour Blunt Trauma (BABT), [15].

3.3. Internal interfaces within a hard armour plate

Hard Armour Plates are getting lighter, especially those containing 5th generation UHMWPE fibres. Why is this? Well, this latest generation of fibres are smaller in diameter and, as a result,are stiffer and stronger [1,24]. As recommended by the suppliers,the pre-impregnated fibres (in the form of cross-plies) are also processed under much higher applied pressures than conventional autoclaving/hydroclaving - see Table 1. Work reported by Vargas-Gonzalez in 2015 [25], and more recently by Lassig et al. [26], has shown that ballistic limit velocities increase with processing pressure.These significant improvements(both in grade of fibre,and in processing cycles) have led to measurable improvements in ballistic performance which is normally realised by a substantial reduction in weight of product [1]. This major advance in armour material technology has significantly influenced the global armour market. In UHMWPE-rich HAPs there has been about a 30%reduction in weight over the past five years or so [1]. However,what it has also brought about is the arrival of stacked systems -that is, systems containing two HAPs. Why is this?

To maximise the ballistic performance of a HAP,designed to stop armour-piercing (AP) rounds, it is now considered best practice to use a ceramic tile in combination with a highly compressed UHMWPE backing material. This, however, presents an enormous technical challenge and prevents a one-step process from being used (namely an autoclave or hydroclave in which the ceramic is bonded to the backing using much lower consolidation pressures).It is extremely challenging to incorporate brittle ceramics when compacting, and bonding, UHMWPE material under extremely high, non-hydrostatic pressures in a one-step procedure. This leaves the manufacturer one other option-to produce the backing separately and then to bond the ceramic tile to the backing in a secondary operation.This latter step,however,is a lot more difficult to get right since one needs to marry-up two very rigid surfaces.If these are multi-curved surfaces, this operation needs to be extremely precise to achieve a consistently thin bond line across the whole product.One solution is therefore to produce two HAPs:HAP1 which is exclusively UHMWPE,and HAP2,which is ceramicrich.

Fig. 5 shows a cross-section through a typical modular body armour system and illustrates all possible materials,interlayers and interfaces that might be present in such a system.Of all the possible constituents,the main two elements are,of course,the ceramic tile and the backing material and these two sub-elements control about 90%of the ballistic efficiency of the system.Furthermore,there are a number of methods used to produce such HAPs - these have recently been compared [24] and Table 1 shows the range of applied pressures that are used for each of these methods. For a given lay-up of UHMWPE backing material,therefore,the resultant properties of the laminate will differ with processing method because, as pressure increases, the volume fraction of fibres increases as more matrix material is squeezed out. The resultant interfaces will therefore also have different characteristics and properties, as further discussed below.

3.3.1. Interfaces within the backing material

The interfaces between each of the plies in a hot-pressed UHMWPE laminate have radically changed since their first use in the 1980s.At that time,using first generation materials,it was very easy to perform a four-point bend test simply between one’s fingers: the ILSS values were immeasurably low (leading to poor structural properties) but the ballistic performance was exceptionally good, especially against steel fragments and handgunammunition.Measures of G1c,K1cor ILSS are key to understanding how UHMWPE behave. However, as recent studies with 3rd generation materials have shown [29], an ILSS value is an extremely difficult parameter to measure in UHMWPE laminates. There is clearly, therefore, a need to carry out further research in this area using 5th generation materials since these latest grades are providing even better ballistic performance than earlier grades and are now quite capable of defeating mild-steel-cored ammunition[30].

Table 1 Range of applied pressures for different consolidation methods.

Fig.5. Schematic cross-section of a HAP showing numerous sub-elements of a typical modular body armour system [27,28].

More public research is certainly required to determine what real significance consolidation pressure has upon ballistic performance. With little doubt, the ILSS has significantly improved from 1st generation to 5th generation UHMWPE, mainly due to significant developments in matrix materials and fibre wetting. And, as consolidation pressure increases so will volume fraction of fibres which, in turn, will lead to increased stiffness moduli and specific strength values in the finished laminate. However, how does ILSS,or G1cor K1c, change with increases in consolidation pressure and has this a direct influence upon ballistic performance and/or impact behaviour?Further research will hopefully answer these intriguing questions.

3.3.2. Interfaces between ceramic and backing material

To maximise the ballistic performance of ceramic-based systems,when designing to defeat armour-piercing ammunition, it is critically important that the ceramic gains maximum support from the substrate behind it. In fact, the perfect support would be a metallic substrate in direct and intimate contact with the ceramic.This, of course, is not at all practical and so some kind of adhesive interlayer is essential, together with a mouldable polymeric backing material.

Structural engineers, especially in the aerospace industry have recognised the significance and importance of bond line thicknesses for decades - see original work by Kinloch [31], amongst others. The thickness of the bond line (the adhesive layer) affects delamination behaviour with fracture toughness and peel strengths varying with bond line thickness. And this principle applies to all types of substrates including ceramic joints, [32]. Recognition within the armour community has been a little slower to be realised,although there is now clear evidence,even in heavier armour systems (see recent paper by Weiss et al. [33]), that bond-line thicknesses are critically important. Most recently, work by Bao et al. [34], has also shown that the constitution of this critical interface plays a significant role in controlling the degree of collateral damage in silicon carbide - aluminium lightweight armour systems.

Others like Lopez-Puente et al. [35], and this author [4], have also shown this to be the case within body armour systems. For toughened epoxy interlayers, the critical thickness is between 0.3 and 0.5 mm,[36].The toughened epoxy Hysol 9309.3(NA)has been used, in this thickness range, by this author for two decades or more,following early characterisation work[37].It was successfully used in the development,and delivery,of many thousands of HAPs for The Metropolitan Police during the 2000s, following practices described in a patent application by Klintworth and Crouch [38].When using UHMWPE backing materials, however, it is common practice these days to use a compatible thermoplastic adhesive film between the ceramic and the backing laminate. This not only results in a naturally tougher interlayer but also enables stricter control over bond line thickness.

3.3.3. Ceramic surfaces and cladding layers

Ceramics are extremely hard materials with high elastic moduli but very low elongation to fracture. They are also extremely notch sensitive which makes surface finish critically important.In a plate bending scenario,the rear tensile surface is likely to be a favoured site for cracks to initiate.Therefore,in an impact situation,ceramics not only require a stiff backing material to optimise ballistic performance but also an excellent surface finish, which is why most armour-grade tiles are ground and/or polished. However, with multi-curved,monolithic ceramics commonly used in Hard Armour Plates,and especially those manufactured by reaction sintering,the surfaces cannot be worked very easily, and grinding is not an economic option. This calls for an alternative approach - surface coating. A carefully-selected, polymeric layer, like an epoxy adhesive, as described in Section 3.3.2., is known to be beneficial by levelling out the asperities and reducing surface roughness [39].Better still, the practice of cladding the ceramics with a fibrereinforced polymeric layer can not only improve surface finish but also introduce a load-carrying layer adjacent to the surface of the tile.

These cladding layers are used to increase the robustness and multi-hit resistance of a ceramic-containing HAP [39], and [40],even though, as reported by Crouch in 2014 [39], the number of radial cracks actually increases. Their presence forces the ceramic to remain in the path of the projectile.However,has the importance of this interlayer been fully realised?In some recent work by Jaitlee et al. [21], at RMIT University in Melbourne, Australia, the researchers reported that a degree of pre-tension in the fabric used to clad the ceramic led to a reduction in back face signature(see Fig.6)against mild-steel-cored ammunition. Whilst this is an isolated piece of research, it does indicate the importance of secondary interfaces/interlayers and indicates the need for more research in this interfacial area.

3.4. External interfaces between HAP1 and HAP2

This is a special case and is the latest type of interface to evolve within body armour systems. It only applies to stacked systems in which a second HAP (HAP2) is placed in front of an existing HAP(HAP1).Such systems are just starting to emerge with limited takeup, so far. It is assumed, for the sake of this paper, that HAP1 consists of highly compressed UHMWPE fabrics,since these are known to have excellent ballistic performance against both lead-filled,and now mild-steel-cored, small arms ammunition [30]. However,monolithic UHMWPE plates do not perform at all well against armour-piercing threats. A second plate, HAP2, placed in front of the first, is therefore required in order to defeat this threat. HAP2 plates are designed to be ceramic-rich.

This author’s concern is the external interface between HAP1 and HAP2 because it is extremely challenging to marry-up two external, multi-curved surfaces: the convex, impact surface of HAP1 and the concave,distal surface of HAP2.Unless this interface is perfectly matched, it will not, by definition, be as ballistically effective as a conventional ceramic-based HAP, as shown in Fig. 5.Solutions have been offered in order to guarantee a perfect fit(magnetic devices,geometrical links,etc)but such a combination of HAP1 and HAP2 will always be thicker than a well-designed,single,AP-resistant HAP. Adoption of such stacked systems will therefore be totally down to logistics and perceived benefits by the end User.They will not provide the lightest solution against AP rounds.

4. Conclusions and recommendations

Of the numerous interfaces present in a body armour system,this paper has focussed upon eight of them.Of these,the following summary table, Table 2, is an attempt to rank them in order of critical importance as a function of threat type.For AP threats that assume a ceramic-based solution, the most critical interface is,undoubtedly, the one between the ceramic and the backing material (see Section 3.3.2). For stacked systems, this is now closely followed, it is suggested, by the fit between HAP1 and HAP2 (see Section 3.4.0). The evidence is also clear that the cladding layer which surrounds the ceramic tile is important in multi-hit resistance (see Section 3.3.3) and some studies indicate its importance for single-strike events,especially if the cladding is pre-tensioned.Last,but by no means least,within the HAP itself,are the inter-ply surfaces of a UHMWPE laminate and the unquantified effects of consolidation pressure. High consolidation pressures may not always be required,especially in multi-layered products.When using an AP-resistant HAP,it is assumed that the inter-ply surfaces within the SAI,and possible formation of air-gaps,are not at all significant since these will be closed-up, and compressed, ahead of the impacting projectile. With HAPs, all of the interfaces associated with the SAI are considered to be of secondary importance,as long as the contact surface between the HAP and SAI is firm and continuous.

Against lead-filled,and mild-steel-cored,ammunition it is now clear that highly compacted, 5th generation UHMWPE materials perform sufficiently well,as monolithic HAPs,without the need for any hard-facing material like ceramic or steel. This is why, in Table 2,for this particular threat level,the inter-ply surfaces within a compacted UHMWPE laminate are judged to be of highest importance.This point is extremely well made in very recent work reported by Cline and Love[41]who emphasise the importance of matrix material. The use of alternative matrix materials, such as Kraton?or a polyurethane resin,results in totally different impact behaviours, and leads to different ballistic applications.

For the soft armour systems, the interface between the system and the wearer remain top priority,and a perfect,snug fit,for every User,is still a very valid ambition.However,it is also suggested that the inter-ply surfaces, within the SAI, may be more influential for stab and spike vests than for those designed to protect against hand-gun bullets.This area requires further research-see Section

3.1.1.

There is also more scope for work on load carriage materials since these are an essential, ever-present part of the whole ensemble. For example, the webbing and attachment materials,will have some resistance to soft,hand-gun rounds,it is suggested.Ashok Bhatnagar presented some work recently[42]that indicated that further savings in weight were also possible by combining UHMWPE fabrics with Cordura? within the carrier/load carriage system itself.

Fig. 6. Back Face Signature measurements as a function of impact velocity for RSSC-based targets against 7.62 mm mild-steel-cored ammunition. Targets contained 4 mm thick silicon carbide tiles clad in an aramid fabric (with, or without, pre-tension) [21].

Table 2 List of critical interfaces by threat type.

The main conclusion reached from this short review is that an holistic approach is now required from either the End User or a Prime Contractor to bring about further reductions in weight of body armour systems. Why? Because further reductions in weight can be achieved,not only by further improvements in material and/or geometrical properties [43], but also by controlling and optimising the characteristics of all interfaces. A single organisation,like a prime contractor, should now have total control over the design, manufacture and ballistic performance of the entire body armour system (i.e. load carriage system and the entire body armour system(SAI and HAP(s)since some of the critical interfaces lie outside of the individual protective elements. This is considered essential if ballistic performance is to be maximised, weight to be minimised, and the level of quality control further enhanced.

It is also concluded that future research should be considered in the following areas: (a) the ballistic properties of load-carriage materials, like the ever-present webbing, (b) a critical review of pocket materials and other secondary interfacial materials that surround the HAPs and SAIs, (c) the effects of pre-tensioning the cladding material on ballistic performance or behaviour of ceramicbased HAPs, and (d) dynamic inter-ply friction measurements of adjacent plies within an SAI, especially for stab and spike vests, as well as (e) further quantification of the effect of adding fluids, like the STFs, upon real-life ballistic performance.

Declaration of competing interest

I confirm that there is no conflict of interest.I am the sole author and the review paper is written from an independent viewpoint.

Acknowledgements

I am eternally grateful to all of my colleagues,past and present,within the global armour community,especially those from the UK and Australia,in particular my fellow researchers from the Defence Materials Technology Centre and Australian Defence Apparel during the period 2000 to 2015.