Military Bulletproof Helmet Manufacturing Process - Materials
Among all the synthetic fibres, para-aramid (such as Kevlar, Twaron) fibre (1.44 g/cm3) gained prevalence in helmet practical use for several decades. Recently, market is shifting to the domination of the crystallized fibrils olefin ethylene UHMWPE (such as Dyneema, Spectra and Endumax) fibres (0.97 g/cm3), which supersedes para-aramid in both V50 increment and weight reduction. The UHMWPE having high strength to weight ratio characteristic, low coefficient of friction, good cut resistance and creep effect, but low melting point (around 145–153 °C). Creep provides the chance for the fibre being stretched and extended to a certain degree subjecting to some magnitude of strains. However, the UHMWPE based material features higher back face deformation value than that of aramid.
Ascribing to their unique chemical structures where the atoms are held by the strong covalent bonds, the traits of excellent tensile, high modulus and inertia to most of the environments are presenting [30]. Thereof, the ballistic helmets currently can be subcategorized into three types on material-based, namely UHMWPE-based helmet, aramid-based helmet and hybrid helmet combining both of these two materials or others, depicted in Fig. 2.
Fibre Architectures
In application for military bulletproof helmets, these fibres are constructed into different topologies widely known as woven, unidirectional (UD), braided, stitched, and tufted fabrics. The pores, fibre orientation and fibre volume fraction distribution inside these fabric types present various interconnections and patterns, giving different fluid flow resistance. Besides, the crimps (curvature) level determines the suitability for ballistic application, simultaneously influencing the flow pattern [34] in later resin filling process or response to compression force. These structural heterogeneities lead to scatter in permeability and composite thickness, having further impact in process-induced defects. In this review, the fibre architectures are broken down into the form of one dimensional (1D), two dimensional (2D) (referring to 2D woven, UD shield and films here) and three dimensional (3D) (referring to 3D solid woven, braided, stitched and tufted), to better describe their flexural properties.
Matrices
As a primary constituent out of two in composite materials, the matrix provides adhesion to hold the fibres in place. The bonding between fibre and matrix offers the interfacial shear strength to lessen the risk of delamination. In the impact event, it also works as a transient medium to transfer the load into the reinforcing fibres. The matrices are characterized into thermoset (rigid) and thermoplastic (flexible) matrix.
