The Behaviour of Stratified Fabrics of Aramid Fibres under Stabbing Conditions.

This paper presents research on several factors influencing the stabbing behaviour of stratified panels made of aramid fabric Twaron® SRM509 Teijin Aramid BV (Arnhem, The Netherlands). The inputs in the test campaign were the number of layers, the impact energy, and the sample size. Tests were performed on small samples (130 mm × 130 mm) on an Instron® CEAST 9350 drop-tower impact system (Norwood, MA, USA) and on larger samples (400 mm × 400 mm) using a test installation with the same values of the impact energy. Knife type S1 was used, with the geometry recommended in NIJ Standard 0115.00 Stab Resistance of Body Armor SEM, and macro photography investigations revealed the failure mechanisms of panel, layers and fibres. A very important conclusion of this study regarding the stabbing performance of fabric Twaron® SRM 509 in particular, but also in general for panels for body protection is that a research study could start on small size samples, with an accurately instrumented machine, in order to establish the influence of significant factors of stab resistance (energy level, number of layers in a panel, etc.), as these samples are less expensive and less time consuming, but the study should be continued to examine larger size samples. The obtained data are useful for the prototype.

Ballistic Response of a Glass Fiber Composite for Two Levels of Threat.

This paper presents the behavior of composite panels based on glass fiber unidirectional fabrics and a bi-component epoxy resin under ballistic impacts that characterize two threat levels: FB2 and FB3, according to EN 1523:2004. The tested panels had characteristics kept in narrow ranges: thickness 18.26 ± 0.22 mm, mass ratio fabrics/panel 0.788 ± 0.015, surface density 27.51 ± 0.26 kg/m2. After testing the panels, the failure mechanisms of the panel were evidenced by scanning electron microscopy and photographs. Here the authors present a finite-element model at meso scale that was used for evaluating if the composite, initially tested at level FB2 (9 mm FMJ, v0 = 375 m/s), could withstand the higher level of impact, FB3 (projectile type 0.357 Magnum and impact velocity of v0 = 433 m/s). Simulation was performed in Explicit Dynamics (Ansys), keeping the same target but changing the projectile for the two different levels of threat. The results of the simulation were encouraging for making tests at level FB3, indicating the importance of alternating actual tests with simulations in order to achieve better protection with reduced surface weight. The simulation illustrated differences in impact duration and number of layers broken on the panel for each level. Validation of the model was based on the number of broken layers and the dimension of the delamination zone between the last two layers. Scanning electron microscopy was used for identifying failure mechanisms at the micro and meso scale. We found that damage to the composite was intensively dependent on impact velocity, this being quantitatively evaluated using the number of layers broken, the effect of delamination on separating layers and the deformation of the last layer.

The Behavior of Glass Fiber Composites under Low Velocity Impacts.

This paper presents experimental results on the behavior of a class of glass fiber composites under low velocity impacts, in order to analyze their usage in designing low velocity impact-resistant components in car and marine industries. Also, a finite element model at the meso level (considering yarn as a compact, homogenous and isotropic material) was run with the help of Ansys Explicit Dynamics in order to point out the stages of the failure and the equivalent stress distribution on the main yarns in different layers of the composite. The composites were manufactured at laboratory scale via the laying-up and pressing method, using a quadriaxial glass fiber fabric (0°/+45°/90°/-45°) supplied by Castro Composites (Pontevedra, Spain) and an epoxy resin. The resin was a two-component resin (Biresin® CR82 and hardener CH80-2) supplied by Sika Group (Bludenz, Austria). The mass ratio for the fabric and panel was kept in the range of 0.70-0.77. The variables for this research were as follows: the number of layers of glass fiber fabric, the impact velocity (2-4 m/s, corresponding to an impact energy of 11-45 J, respectively) and the diameter of the hemispherical impactor (Φ10 mm and Φ20 mm) made of hardened steel. The tests were performed on an Instron CEAST 9340 test machine, and at least three tests with close results are presented. We investigated the influence of the test parameters on the maximum force (Fmax) measured during impact, the time to Fmax and the duration of impact, tf, all considered when the force is falling to zero again. Scanning electron microscopy and photography were used for discussing the failure processes at the fiber (micro) and panel (macro) level. At a velocity impact of 2 m/s (corresponding to an impact energy of 11 J), even the thinner panels (with two layers of quadriaxial glass fiber fabric, 1.64 mm thickness and a surface density of 3.51 kg/m2) had only partial penetration (damages on the panel face, without damage on panel back), but at a velocity impact of 4 m/s (corresponding to an impact energy of 45 J), only composite panels with six layers of quadriaxial fabric (5.25 mm thickness and a surface density of 9.89 kg/m2) presented back faces with only micro-exfoliated spots of the matrix for tests with both impactors. These results encourage the continuation of research on actual components for car and naval industries subjected to low velocity impacts.

Failure Investigation of Layered LFT SB1plus Package after Ballistic Tests for Level IIA.

The main objective of this study focuses on designing and testing body protection systems using advanced materials based on aramid fibers, for high impact speeds of up to 420 ± 10 m/s. Ballistic applications of aramid fiber-based composites mostly include soft body armors. The investigation of the failure mechanisms identifies issues of protective fabrics, major challenges and technological problems for efficient development of these systems. The authors present an investigation on the failure processes and destructive stages of a ballistic package made of successive layers of LFT SB1plus, a trade name for a multiaxial fabric by Twaron Laminated Fabric Technology (LFT), taking into account the particular test conditions from NIJ Standard-0101.06 Ballistic Resistance of Body Armor. The main parameter of interest was the backface signature (BFS), but also details of projectile arrest and SEM investigation could offer arguments for using this material for individual protection. For the reported tests, the maximum and minimum values for BFS were 12 mm and 24 mm, the mean value being 18.66 mm and the standard deviation being 3.8 mm.

Tribology of Polymer Blends PBT + PTFE.

This paper presents results on tribological characteristics for polymer blends made of polybutylene terephthalate (PBT) and polytetrafluoroethylene (PTFE). This blend is relatively new in research as PBT has restricted processability because of its processing temperature near the degradation one. Tests were done block-on-ring tribotester, in dry regime, the variables being the PTFE concentration (0%, 5%, 10% and 15% wt) and the sliding regime parameters (load: 1, 2.5 and 5 N, the sliding speed: 0.25, 0.5 and 0.75 m/s, and the sliding distance: 2500, 5000 and 7500 m). Results are encouraging as PBT as neat polymer has very good tribological characteristics in terms of friction coefficient and wear rate. SEM investigation reveals a quite uniform dispersion of PTFE drops in the PBT matrix. Either considered a composite or a blend, the mixture PBT + 15% PTFE exhibits a very good tribological behavior, the resulting material gathering both stable and low friction coefficient and a linear wear rate lower than each component when tested under the same conditions.

Numerical and Experimental Results on Charpy Tests for Blends Polypropylene + Polyamide + Ethylene Propylene Diene Monomer (PP + PA + EPDM).

This paper presents results from numerical and experimental investigation on Charpy tests in order to point out failure mechanisms and to evaluate new polymeric blends PP + PA6 + EPDM. Charpy tests were done for initial velocity of the impactor of 0.96 m/s and its mass of 3.219 kg and these data were also introduced in the finite element model. The proposed model takes into account the system of four balls, including support and the ring of fixing the three balls and it has a finer discretization of the impact area to highlight the mechanisms of failure and their development in time. The constitutive models for four materials (polypropylene with 1% Kritilen, two blends PP + PA6 + EPDM and a blend PA6 + EPDM) were derived from tensile tests. Running simulations for each constitutive model of material makes possible to differentiate the destruction mechanisms according to the material introduced in the simulation, including the initiation and the development of the crack(s), based on equivalent plastic strain at break (EPS) for each material. The validation of the model and the simulation results were done qualitatively, analyzing the shape of broken surfaces and comparing them to SEM images and quantitatively by comparing the impact duration, energy absorbed by the sample, the value of maximum force during impact. The duration of the destruction of the specimen is longer than the actual one, explainable by the fact that the material model does not take into account the influence of the material deformation speed in Charpy test, the model being designed with the help of tests done at 0.016 m/s (1000 mm/min) (maximum strain rate for the tensile tests). Experimental results are encouraging for recommending the blends 20% PP + 42% PA6 + 28% EPDM and 60% PA6 + 40% EPDM as materials for impact protection at low velocity (1 m/s). Simulation results are closer to the experimental ones for the more brittle tested materials (with less content of PA6 and EPDM) and more distanced for the more ductile materials (with higher content of PA6 and EPDM).