Tensile behaviour of polyethylene and poly(p-xylylene) fibres

This thesis deals with the tensile behaviour of fibres prepared from high molecular weight polymers.The tensile strength of a polymeric fibre is in general much lower than the corresponding theoretical value. In case of ultra-high molecular weight polyethylene (UHMWPE), fibres can be prepared by gel-spinning and subsequent hot-drawing having a tensile strength of 7.2 GPa, whereas the theoretical tensile strength is about 30 GPa. Although such fibres are relatively perfect with respect to crystallinity and orientation of the chains, there must be intrinsic imperfections that determine the tensile strength.

Modeling and Experiments on Ballistic Impact into UHMWPE Yarns Using Flat and Saddle-Nosed Projectiles

Yarn shooting experiments were conducted to determine the ballistically-relevant, Young’s modulus and tensile strength of ultra-high molecular weight polyethylene (UHMWPE) fiber. Target specimens were Dyneema® SK76 yarns (1760 dtex), twisted to 40 turns/m, and initially tensioned to stresses ranging from 29 to 2200 MPa. Yarns were impacted, transversely, by two types of cylindrical steel projectiles at velocities ranging from 150 to 555 m/s: (i) a reverse-fired, fragment simulating projectile (FSP) where the flat rear face impacted the yarn rather than the beveled nose; and (ii) a ‘saddle-nosed projectile’ having a specially contoured nose imparting circular curvature in the region of impact, but opposite curvature transversely to prevent yarn slippage off the nose. Experimental data consisted of sequential photographic images of the progress of the triangular transverse wave, as well as tensile wave speed measured using spaced, piezo-electric sensors. Yarn Young’s modulus, calculated from the tensile wave-speed, varied from 133 GPa at minimal initial tension to 208 GPa at the highest initial tensions. However, varying projectile impact velocity, and thus, the strain jump on impact, had negligible effect on the modulus. Contrary to predictions from the classical Cole-Smith model for 1D yarn impact, the critical velocity for yarn failure differed significantly for the two projectile types, being 18% lower for the flat-faced, reversed FSP projectile compared to the saddle-nosed projectile, which converts to an apparent 25% difference in yarn strength. To explain this difference, a wave-propagation model was developed that incorporates tension wave collision under blunt impact by a flat-faced projectile, in contrast to outward wave propagation in the classical model. Agreement between experiment and model predictions was outstanding across a wide range of initial yarn tensions. However, plots of calculated failure stress versus yarn pre-tension stress resulted in apparent yarn strengths much lower than 3.4 GPa from quasi-static tension tests, although a plot of critical velocity versus initial tension did project to 3.4 GPa at zero velocity. This strength reduction (occurring also in aramid fibers) suggested that transverse fiber distortion and yarn compaction from a compressive shock wave under the projectile results in fiber-on-fiber interference in the emerging transverse wave front, causing a gradient in fiber tensile strains with depth, and strain concentration in fibers nearest the projectile face. A model was developed to illustrate the phenomenon

On the ultimate potential of high strength polymeric fibers to reduce armor weight

The capability of polymeric fiber based composites to stop ballistic projectiles at low armor weight is predominantly related to the specific tensile strength of the fibers. This paper and presentation discusses the ultimate theoretical and experimental specific tensile strength of various polymeric fibers (polyethylene, para-Aramids, rigid-rods, graphene). It is concluded that the realistic achievable strength of these fiber systems is about half the theoretical strength of single atomic bonds. Ultimate experimental strengths of most fiber systems have reached almost 50 % of this realistic achievable strength. Considering that no defect structures have yet been incorporated in this theoretical strength, it is postulated that these fibers strength can not be increased strongly anymore. Polyethylene fibers, like Dyneema®, have the highest experimental specific fibers strength of all fibers, which would enable a reduction in armor weight by 40% of the current best polyethylene fiber based armor

Numerical sensitivity study of ballistic impact on UHMWPE composites: Paper presented at the 6th International Conference on Design and Analysis of Protective Structures, DAPS 2017, November 29 - December 1, 2017, Melbourne, Australia

In recent years, ultra-high molecular weight polyethylene (UHMWPE) composite materials have frequently been in the spot-light of investigations for protective applications. As a result, a state-of-the-art material model with a material-characteristic discretization scheme was developed that enables a wide range of high-velocity impact simulations in hydrocodes [1, 2]. In particular, the back face deformation and the V50 for impact velocities above 800 m/s are reported to be well predicted when compared to ballistic impact experiments. In this study, the numerical model is applied to investigate key parameters such as sample size, strength in fiber direction and normal and shear cohesion of the bonding interfaces with regard to their influence on the back face deformation. The main part of this study is concerned with a standard 100 % crosswise layup. Chosen results for a laminate that has the back 25 % of sub-laminates helicoidally oriented with a 22.5° sequence, similar to the so-called ARL-X layup presented by [3], are compared to the results from the standard layup

Effect of consolidation pressure on the impact behavior of UHMWPE composites

For the first time, the influence of the manufacturing process on the dynamic performance of ultra-high molecular weight polyethylene (UHMWPE, Dyneema® HB26) composites is investigated. The material is significantly influenced by the hot-pressing parameters temperature and pressure. The ballistic resistance and shock wave behavior was characterized for the UHMWPE composite consolidated with three different pressures. In the case of UHMWPE composites, higher consolidation pressures result in a better ballistic performance. The shock wave behavior converges to high-density polyethylene (HDPE). Based on these observations, an analytical approach is proposed describing the equation of state as a function of consolidation pressure

Performance polymers from renewable monomers : high molecular weight poly(pentadecalactone) for fiber applications

Enzymatic ring-opening polymerization was applied to synthesize high molecular weight polypentadecalactone (PPDL). The synthetic procedure was optimized on a small-scale and subsequently transferred to 30 g scale to yield sufficient material for fiber spinning. Molecular weights (Mw) of 143 000 g mol(-1) were obtained. Mechanical and thermal properties of the non-oriented, high molecular weight PPDL were determined and are largely in agreement with the literature data. The high molecular weight PPDL was melt-processed into fibers, which were further elongated to about 9-10 times their original length. Analysis of the fibers revealed differences in crystal orientation as a function of the processing conditions. Preliminary fiber tensile measurements confirm a high strength of up to 0.74 GPa for the fiber with the highest crystal orientation

Transitioning a unidirectional composite computer model from mesoscale to continuum

Ballistic impact on composites has been a challenging problem as seen in the abundant literature about the subject. Continuum models usually cannot properly predict deflection history on the back of the target while at the same time giving reasonable ballistic limits. According to the authors the main reason is that, while continuum models are very good at reproducing the elastic characteristics of the laminate, the models do not capture the behaviour of the “failed” material. A “failed” composite can still be very effective in stopping a projectile, because it can behave very similar to a dry woven fabric. The failure aspect is much easier to capture realistically with a mesoscale model. These models explicitly contain yarns and matrix allowing the matrix to fail while the yarns stay intact and continue to offer resistance to the projectile. This paper summarizes the work performed by the authors on the computationally expensive mesoscale models and, using them as benchmark computations, describes the first steps towards obtaining more computationally effective models that still keep the right physics of the impact