Finite Elemente hoher Ordnung für materiell und geometrisch nichtlineare Probleme

For the simulation of geometric and material nonlinear problems implicit high-order (p-version) displacement-based finite elements are applied. Besides hyperelastic materials a finite strain viscoplasticity model with internal variables is considered. We apply the combination of Backward-Euler integration and Multilevel-Newton algorithm to solve the system of differential-algebraic equations resulting from the space-discretized weak form. For an efficient modeling of the cold isostatic pressing (CIP) process an axisymmetric finite strain element, reaction forces and follower loads are derived in the p-version context. We demonstrate that the p-version can overcome volumetric locking also in the finite strain case. An adaptive time-stepping algorithm is presented to perform simulations of metal powder compaction. We report applications to die-compaction and isostatic pressing (CIP,RIP), and a complex validation example where good agreement to experimental data is achieved.Finite Elemente hoher Ordnung (p-Version) werden zur Simulation von geometrisch und materiell nichtlinearen Problemen angewandt. Neben hyperelastischen Materialien wird ein viskoplastisches Modell mit inneren Variablen verwendet. Zur Lösung des Algebro-Differentialgleichungssystems, das aus der räumlichen Diskretisierung der schwachen Form entsteht, wird die Backward-Euler Methode zusammen mit dem Mehrebenen-Newton-Verfahren angewandt. Um den Prozess des kalt-isostatischen Pressens effizient abzubilden, wurden ein axialsymmetrisches Element für große Dehnungen, Reaktionskräfte und Folgelasten für die p-Version abgeleitet. Analytische Vergleichslösungen zeigen, dass die p-Version Locking auch für große Dehnungen überwindet. Die effiziente Anwendung der entwickelten Methoden auf uni-axiales und isostatisches Pressen von Metallpulvern wird demonstriert. Ein komplexes Validierungsbeispiel zeigt gute Übereinstimmung mit dem Experiment

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

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

Strategic positioning of carbon fiber layers in an UHMwPE ballistic hybrid composite panel

The effects of inter-ply stacking sequences on the ballistic and structural performance of ultra-high molecular weight polyethylene (UHMwPE) fiber/carbon fiber hybrid composite hard ballistic panels have been studied. Unexpected effects were observed simply by varying the positions of small amounts of carbon layers at the front, middle, back and front-back of the UHMwPE-based panels. The most dramatic positive hybrid effect was observed for a front-facing hybrid configuration resulting in a significant 30% reduction in back-face signature (BFS) with a more than two times improvement in flexural yield strength. Interesting secondary hybrid effects such as the mitigation of bulging and buckling of the UHMwPE component of the panel as well as bullet deformation upon ballistic impact by the carbon layer was observed. These results indicate that strategic positioning of the carbon layer in UHMwPE panels can boost the ballistic performance of applications where low BFS and structural performances are key.Economic Development Board (EDB)They also acknowledge DSM Singapore Industrial Pte Ltd-Economic Development Board of Singapore for their grant support (Grant Ref Number M4062094.070)