Military Bulletproof Helmet Manufacturing Process -Introduction

The factors considered in engineering design of military composite bulletproof helmets for modern society have a much broader spectrum, focusing not only on the ballistic resistance, coverage and curvature, but also on the comfort to the wearers, and the compatibility with more and more desirable accessory devices. These will make the design a complex issue, while providing more freedom for designers. Most of all, achieving a helmet shell with sufficient ballistic resistance was and still is the priority task.

To improve the ballistic resistance, a wide range of experimental and simulation works across various disciplinary sections have been done in literature. These works could be subcategorized into the studies of material impact failure mechanisms, fibre surface frictional treatments fibre architecture parametric analysis, draping (forming) and ply-ply sliding in preforming, hybrid materials, resin enhancement and manufacturing method optimization. These material-related engineering results were validated by the investigation on the interaction between helmet shell and human brain, where the blunt traumatic injuries and bone fracture was simulated based on different types of head forms, or tested by the way of energy absorption. To sum up, the protective ability improvement is largely dependent on the tensile strength and modulus of the fibres used, along with the type of resin which is used to adhere these fibres together.

Some reviews in the literature investigated the manufacturing methods for thermoset which was closely related to the resin filling process taking account for variability of dry prepregs and the uncertainty in draping. While the review on forming include the knowledge of material mechanical properties verification. There is also review on the link between the helmet and head injuries as well . However, these works have not covered the fabric types correlate to the manufactures in specific mechanisms. This review will based on the interdependency of materials properties, fibre architectures, preforming and manufacturing methods. The manufacturing rules in the fields of complex shaped composite parts (i.e. hemispherical, domed), will be covered.

Despite of the fact that more and more accessory devices are integrated to functionalize a ballistic helmet system, its core ballistic protective function needs to be improved with weight reduction was and still is the main course in engineering design. The two major generic classes of synthetic fibres for ballistic composites are Ultra High Molecular Weight Polyethylene (UHMWPE) fibre (0.97 g/cm3) and aramid fibre (1.44 g/cm3). In the area of military helmets, these fibres are constructed into different topologies, draping/forming into double-curvature geometric shape in multiple plies, serving as reinforcement for composite shell. The preforming ways influence the subsequent impregnation / solidification and curing step in manufacture, in terms of the fibre orientation and fibre volume fraction. The inherent structural heterogeneity thus leads to scatter in permeability and composite thickness, and have further impact in generating process-induced defects. During the processing, the fibre continuity without wrinkles, together with voids-free are determinative factors to a quality final part. The aim of this paper is to review the manufacturing technologies characterised by thermo-mechanical forming and Liquid Composite Moulding (LCM), relating their processing parameters respectively to the properties of reinforcements in one dimension (1D), two dimensions (2D) and three dimensions (3D), along with that of the matrix in dry or wet phase, interdependency of them are sought.