We investigate phenomena resulting from oblique impact of a right circular cylinder (RCC) and a steel sphere onto laminate targets consisting of multiple, cross-plied sheets of UHMWPE fiber in a polymer matrix at 10-20% mass fraction. Laminates were produced in both an un-pressed and hot-pressed condition using resin matrices differing widely in stiffness. Ballistic tests under both normal and 30°oblique impact, were conducted at multiple velocities to determine both V50 and the depth of penetration (DOP) below V50. Results were compared across all test conditions: RCC vs. sphere, oblique vs. normal, stiff vs. flexible matrix, and pressed vs. un-pressed. Pressed targets with 50% higher thickness and an intermediate stiffness matrix were also tested using an RCC. A numerical model was developed to simulate normal and oblique RCC impact into the various laminate targets. Simpler RCC oblique impact into a single yarn was also studied under varying sliding friction. The simulation model embodied a large array of coupled, mass-spring-damper yarn and matrix elements arranged to mimic the laminate geometric structure and mechanical properties, including both RCC and sheet on sheet, frictional slip. For a given target thickness, the impact velocity was the median experimental V50 for the set. Although not allowing yarn failure, the model still provided details on the evolution of layer-to-layer yarn displacements and strains, including their variability, allowing prediction of: (i) yarns most likely to fail on impact, (ii) target versions likely to have a higher V50 than others, and (iii) effects of obliquity on both V50 and DOP. Model predictions broadly agreed with experimental results, both demonstrating that a lower V50 under RCC oblique vs. normal impact strongly depends on target thickness where its increase reversed this trend while its reduction amplified it. Oblique RCC impact on a single yarn under sliding friction, reveals a strain concentration on the trailing side, however, this does not explain behavior seen in laminated targets as the full model uncovers far more intricate mechanisms.

We investigated the effects of finite lengths of polymer chains on the structural and mechanical properties of polyethylene (PE) fibers with atomistic molecular dynamics simulations, PE fiber models containing long but finite chains with different distributions of chain-end defects were prepared from the orthorhombic crystalline PE configuration. In our main PE fiber model, chain-end defects were uniformly spaced along the chain direction with chain ends located at boundaries of multiple crystalline regions, which is consistent with the distribution of chain ends found in semicrystalline PE. At the early stage of tensile deformations before the yield point, a mismatch of conformational preferences in the chain-end region of a chain and chain segments away from the chain-end regions in neighboring chains developed gradually. Near the yield point, this mismatch in conformational preferences triggered a yield behavior associated with chain slippage near the chain-end region and caused significant changes in the structural and mechanical properties. After the yield point, the pattern of changes in effective lattice parameters estimated with the PE fiber model with finite chains was significantly different from that estimated with the crystalline PE without chain ends but was in an excellent agreement with those observed in recent experiments on PE fibers and preoriented high-density PE films. Rotations of chains in specific directions in response to the conformational mismatch with neighboring chains caused the observed pattern of changes in lattice parameters.

We investigate the molecular features of high-performance gel-spun ultrahigh-molecular-weight polyethylene UHMWPE fibers (SK75, invented and manufactured by DSM) and propose a multiscale structural model that describes the organization of molecules from the unit cell to the filament level, based on X-ray diffraction in static and dynamic conditions (during tensile testing). The model emphasizes the discontinuous nature of the crystalline phas e, which is embedded in a percolating amorphous phase and connected by tie molecules running through the amorphous phase. The tie molecules play a critical role in the tensile properties (e.g., Young's modulus and sonic modulus) of the material. We analyze the micromechanics of the material during tensile deformation and show that, in the elastic regime, the stress-transfer mechanisms (e.g., tie molecules) are so efficient to realize a homogeneous stress distribution through the various length scales (from filament level to unit cell level). Plastic deformation of filaments begins with shear break-up of crystals that triggers or is triggered by an unusual, not well explored, deformation mode of the orthorhombic unit cell (contraction of the a-axis with simultaneous expansion of the b-axis). We also show that the morphological model with discontinuous crystalline phase provides a logical base for the interpretation of the sonic modulus of UHMWPE fibers. Realignment of molecules in the noncrystalline regions of the material can explain the remarkable increase of the sonic modulus measured during tensile tests.

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.

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.

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.

Ballistic protective performance of fiber composites correlates highly with fiber tensile strength. The mechanical tensile properties of fibrous UHMWPE cross-ply armor like Dyneema® are a challenge to assess. The reason is the high stiffness and strength combined with low interlaminar shear strength and low friction coefficient. When evaluating the strength of a hard-pressed composite plate using the standard ISO or ASTM dogbone shapes and pneumatic grips an inverse relation of apparent strength to sample thickness is found. This is an artefact of the test not being able to load the whole cross section cooperatively. High quality fiber, yarn and flexible ply tests are possible exploiting the capstan principle. We report typical measurements and discuss which strength to use in hydrocode models.

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.

A general plasticity-based hydrocode model for the Dyneema (R) HB26 UHMWPE (ultra-high molecular weight polyethylene) armor grade was derived from an extensive experimental static, dynamic and shock characterization to account for the main energy absorption mechanisms during impact. This model is presented along with successful validation in terms of depth of penetration and residual velocity in the high-velocity regime. With this validated model, we could investigate the relevant parameters for improved ballistic performance. New UHMWPE fibers from DSM Dyneema with exceptional strength were produced and manufactured into cross-plied unidirectional composites. The experimental ballistic limits of these grades are reported and discussed. It is shown that with the new generation of Dyneema (R) fibers and composites, very significant weight reductions of up to 25 % or more can be obtained compared to state-of-the-art armor materials.

This paper presents detailed experimental characterization of quasi-static anisotropic directional strength properties as well as the shock behavior of ultra-high molecular weight polyethylene (UHMWPE) for the development of an advanced material model for this class of materials. Specifically, we consider Dyneema® HB26 – pressed from uni-directional (UD) tapes in a 0/90° stacking sequence. A material model based on a constitutive law with orthotropic, non-linear strength, shock response, composite failure and softening criteria is presented. A set of material parameters is derived for applications in hydrocodes (here: ANSYS AUTODYN). High- and hypervelocity impact tests with different impact velocities are used for preliminary validation and discussion of the predictive capabilities in view of future application.

Numerical models of unidirectional panels of ultra-high-molecular weight polyethylene (UHMWPE), like Dyneema (R) HB80 or Spectra (R), have been a difficult challenge. The problem arises from the intimate structure of the material. It is a huge collection of fibers loosely held together by a matrix. For example, for HB80 the fibers are very small in diameter (similar to 17 mu m) but extremely strong (similar to 3.5 GPa). The amount of matrix is small (similar to 20% in volume) and very weak.The "natural" scale to use in the numerical model for this material is the fiber scale (similar to 17 mu m). This scale would provide the right sound speed, transverse wave speed, failure strain, strength of the laminate, deflection, slippage between the fibers/layers, etc. which are all essential details if we want to have predictive capability. But even with the very powerful computers available nowadays we are still very far from being able to simulate a real-size laminated target (for example 30 cm wide and 1 or 2 cm thick) at the fiber scale.This paper proposes an "intermediate" scale that bundles or consolidates many fibers together so that the numerical problem is solvable in a few hours (or maximum one or two days for thick targets) of computation in a single board (24 processors). The scale is chosen precisely so that the essential physics of the ballistics problem is kept and the material properties used are the ones measured in the lab for the fiber and matrix. A similar approach was presented in a previous paper for fabrics (Kevlar, Dyneema, and PBO) where a yarn-level model was able to reproduce both the wave propagation patterns and the ballistic limits.This paper first describes new material test results (shear and compression at low and high confinement pressures). These were essential in properly capturing the delamination of the material. Then, for completeness, transverse wave propagation and ballistics results in Dyneema published elsewhere are briefly presented. Finally the numerical model in LS-DYNA is developed and comparison with deflection history and ballistic limits is conferred. (C) 2014 Elsevier Ltd. All rights reserved.

This paper is an extended abstract of a paper that was accepted to be published in the journal Composites Science and Technology [1]. For details the reader should refer to the full paper.

A detailed very basic experimental study of ballistic depth of penetration tests on hard pressed ultra-high molecular weight polyethylene Dyneema(R) HB26 plates impacted by non-deforming steel spheres is reported. The depth of penetration shows surprisingly an almost linear relation with the impacting kinetic energy suggesting no significant change in penetration mechanisms for the reported test series. The absorbed energy per penetrated thickness has a tendency to increase for higher velocities and thinner plates up to 40 J/(kg/m(2)). From the kinetic energy and depth of penetration an apparent force and stress on the projectile can be worked out. The apparent stress is in the order of 1-1.6 GPa.