G. P. Goncharuk

The influence of water on the friction forces of fibers in aramid fabrics

Fabrics based on high-impact organic fibers have an excellent potential to dissipate the energy of a ballistic impact. That is why they are used in protective helmets and flexible armor vests. The work of friction is the main mechanism of energy absorption in fabrics during a transverse impact. The friction forces of fibers were studied via the pullout of several neighboring fibers and via the transverse hardness indentation. The influence of water on indentation forces and pullout forces of Armos and Rusar fibers during their pullout from fabrics is studied. Water enhances friction force several-fold during the pullout of fibers. Consequently, the potential to dissipate the energy of an impact changes during a transverse action. The influence of moisture is irreversible in the Armos fabrics without a water-repellent coating, and drying does not lead to complete recovery of the friction forces of fibers. In the case of Rusar 56319 fabrics with a water-repellent coating, large drops of water roll off the fabric and only small drops influence the friction forces. A substantial variation in the indentation force is detected, thereby apparently providing evidence of the instability of the density of the fabric. An analysis of the mechanisms of energy dissipation is performed. The energy of the elastic deformation in an individual fiber is three times smaller than the kinetic energy of the fiber. Friction work can exceed the sum of kinetic energy and strain energy by an order of magnitude. The estimation of the value of the increase in the temperature of a fiber during an impact is performed. Heat is not emitted during an impact on an individual fiber in the case of the formation of a transverse wave during an inelastic impact. In the process of transmission of transverse and dilatational waves, the energy dissipation is proportionate to the impact velocity raised to the power of 8/3.

Deformability of Particle-Filled Composites at Brittle Fracture

Strain at the brittle rupture of rubber-particle-filled composites based on polymeric matrices deformed via the neck propagation was found to be equal to the value for the onset of the neck propagation in the initial matrix polymer. This characteristic is important for particle-filled composites, because it determines their ultimate elongation at brittle fracture. Conventionally, caoutchouc particles are introduced into brittle polymers, for example, polystyrene, to increase their shock viscosity and deformability [1]. However, the introduction of caoutchouc or rubber particles into a plastic polymer reduces its ultimate strain [2, 3]. For a certain critical filler content, the composite becomes brittle, more precisely, quasibrittle, which is accompanied by a sharp decrease in the rupture strain by a factor of about 100 [3]. The translation from plastic to brittle fracture of the composite occurs because the formed neck loses its capability to propagate along the sample with a certain content of particles, and it ruptures during the neck formation [4, 5].

A study of yarn friction in aramid fabrics

Energy dissipation by the friction forces of yarns in aramid fabrics with different weave patterns is studied. The upper theoretical limit of the ability of fibers to absorb the energy of transverse impact is determined. The maximum pull-out force for yarns of a plain-weave fabric nonlinearly depends on the number of pulled-out yarns. In twill-weave fabrics, this dependence is linear. Changing the weave pattern is an effective way to change the pull-out forces for yarns. Owing to the slippage of the yarns, the fabric behaves as a plastic material.

Effect of transverse compression on the tensile strength of aramid yarns

115 Fabrics based on aramid fibers have a high ability to dissipate the energy of a ballistic impact. For this rea son, they are used in helmets and flexible armor vests, as well as internal layers of hard armor [1]. A transverse ballistic impact gives rise to a longitudinal wave travel ing along a fiber at the speed of sound in both direc tions away from the point of impact. By high speed photography, Rakhmatulin has detected that a trans verse impact on a rubber fiber initiates a transverse wave in the form of a triangle the sides of which increase with time while preserving angles with the impactor at the upper vertex [2, 3].

Fracture of Composites Based on Polyethylene and Elastic Particles

The composites based on low-density polyethylene with elastomer filling particles are studied. A fracture mechanism induced by the fracture of filler particles or their separation from the matrix polymer is revealed. The fracture of the composites is caused by the growth of formed rhombic pores. The natural relative elongation in a neck is shown to be an important characteristic of a polymer. If the relative elongation in a neck is lower than the strain of appearance of rhombic pores, they form at the stage of uniform tension after necking, and the composite remains plastic. If the relative elongation in a neck is higher than the strain of formation of rhombic pores, they nucleate during necking, and the material undergoes quasi-brittle fracture. Good adhesion between the matrix polymer and elastic particles hinders the appearance of rhombic pores in a neck and, thus, retains high deformation properties of the composites.

Effect of Temperature on the Stress-Strain Behavior of a Polypropylene-Particulate Rubber Composite

Composites based on polypropylene and rubber particles were studied at different temperatures. It was found that, as the temperature is elevated, the type of defects that are formed near large filler particles changes from a crack to a diamond-shaped void and, next, to an elliptical or slit-type void. The change in the defect type predetermines the change of the composite failure mechanism at a constant particulate-filler content from brittle fracture before the yield point to fracture during neck formation or propagation and, finally, to non-uniform plastic drawing with a stable neck growth.

The effect of filler content on the lower yield stress of polymer composites

The effect of the concentration of the dispersed elastic filler on the lower yield stress of matrix composites based on plastic polymers is studied. As the matrix polymers, LDPE-HDPE and LDPE-(medium-density PE) are used. The elastic filler is rubber crumb prepared by roll grinding of worn tires or by deformation grinding of ethylene-propylene-diene rubber. Irrespective of the type of filler particles and their adhesion to the polymer matrix, the lower yield stress σd of the composite is described by the linear law σd = σdm(1 − Vf), where σdm is the lower yield stress of the polymer matrix and Vf is the volume content of the filler. Analysis of the published data shows that this relationship is quite general and describes the effect of rigid inorganic particles on the lower yield stress when adhesion between the filler particles and the matrix is poor.

The brittle-ductile transition in rubber-filled polymers

Composites based on various polymers and rubber particles as a filler were studied. As the filler concentration was increased, the transition from necking to brittle fracture and then to uniform ductile yielding was observed. The criterion for the brittle-ductile transition, which is accompanied by an increase in the elongation at break, is equality between the tensile strength and the upper yield stress of the filled composite. Upon the brittle-ductile transition, the critical concentration of rubber particles is determined by two parameters: the height of the yield drop (difference between the upper and lower yield stresses of matrix polymer) and adhesive strength at the interface between the matrix polymer and filler particles (in the case of good adhesion, tensile strength of rubber particles). The larger the yield drop, the broader the concentration range corresponding to the polymer brittle fracture. The enhancement of adhesion between the matrix and the particles makes it possible to displace the brittle-ductile transition to lower filler contents and, hence, to narrow the region of brittle fracture of the composite.

Influence of the properties of the matrix polymer on the strain characteristics of dispersion-filled composites based on polyethylene and crumb rubber

Mechanical properties of highly filled composites based on polyethylene of various grades and crumb based on ethylene-propylene-diene rubber were studied. The influence of the crack resistance of the matrix polymer on the strain properties of rubber-reinforced plastics was considered. A scheme of failure of highly filled composites with the deformable filler was suggested.

The effect of temperature on the stress-strain behavior of composites based on high-density polyethylene and rubber particles

The failure behavior of composites based on HDPE, which breaks down at the necking stage, and dispersed rubber particles is studied. In was shown that the materials containing at most 8 vol % filler experience the brittle-to-ductile transition with increasing temperature. It was assumed that the ductility retained at elevated temperatures by the composites based on a polymer with unstable neck propagation is due to the interplay of two factors, the decrease in the upper yield point of the matrix polymer and the increase in the polymer draw ratio in the neck. These factors markedly reduce the sensitivity of the materials to the presence of defects and facilitate neck formation and propagation, as well as change the form of the defects from cracks to slitlike pores.