Structural and mechanical characteristics of aramid fibres for bullet-proof vests

Abstract

The creation of high-strength, high-modulus aramid fibres is related to the notion of using them as reinforcing f'dlers for polymer composites designed for manufacture of light construction elements in aircraft. The study of the elastic-strength properties of maximally oriented aramid fibres, their structural features, and the character of failure allowed predetermining the high efficiency of their use as energy-absorbing materials for different applications: construction materials with elevated damping properties capable of withstanding intense vibrational and acoustic loads for a long time (construction .elements of aircraft and helicopter airframes); construction materials for engine parts exposed as protective shields to high-energy mechanical effects with a kinetic energy of 30-50 kJ/m 2 (retaining fragments of fan blades weighing up to 8 kg with a linear velocity of 300-400 m/see); energy-absorbing materials in combined armor withstanding the effect of high-velocity ballistic impact (technical bulletproofing elements and individual protection).The analysis of the experience in creating energy-absorbing materials for different applications showed that the best results are obtained when the high potentials incorporated in the supermolecular structure of maximally oriented high-strength aramid fibre can be realized.We report the results of an electron-microscopic study of the character of failure of SVM para-aramid fibre based on the depth of penetration of a bullet in a multilayer pack of aramid cloth. The structural studies suggested three basic zones which are distinguished by the character of failure of SVM fibre.The zone of direct impact of the pack with the discrete bullet (deformed bullet, fragments) can be designated as the zone of adiabatic local heating and thermomechanical failure of the fibre, accompanied by perturbation and reorientation of its supermolecular structure. In this zone, under the effect of high-speed impact of the bullet and the associated wave processes and local high-temperature tribological effects, the fibres are partially split and are quasi-plastically deformed due to slipping of fibriUar fibre bundles. A totally atypical fracture zone is formed (Fig. 1).In the inferior layer of aramid fabric deformation develops, accompanied by the maximum possible extension of the fibres in the fabrics forming the protective pack in the direction of active radial stresses. The rapidity of the "protective" reactions of the fibres in these conditions, i.e., "exit" from failure, is determined to a significant degree by the structure of the fibres and fabrics and the surface state of the fibres, i.e., their friction relative to each other. When a critical level of tensile stresses acting in the zone of involvement of the fabric in work is attained, the fibres break. An extensive classic fracture surface is formed due to axial splitting of the fibres and avalanche-like breaking of fibrillar fragments (Fig. 2).All-Russian Scientific-Research Institute of Polymer Fibres, Mytishchi.

Photoacoustic methods of investigating fibre materials

“…[1][2][3][4]. Thin monofilaments based on polymers with d(([erent maclvmolecular chain rigidity were investigated.…”