is an open access repository that collects the work of Arts et Métiers ParisTech researchers and makes it freely available over the web where possible. In passive safety structures the use of composite materials has increased significantly recently due to their low specific mass and high energy absorption capacities. The purpose of this experimental study is to describe the macroscopic behaviors of different Kevlar woven composite materials with different kinds of matrix (pure and with acrylate based block copolymer additives: Nanostrength Ò ) under lowvelocity impact. Tests were performed with a drop weight tower on square plates (100 Â 100 mm 2 )clamped by means of a circular fixture. Images were recorded during impact by a high-speed video camera fixed underneath the plate. It was found that Kevlar epoxy composite material with Nanostrength M52N has the best resistance to perforation. The second purpose is to study the influence of physicochemical parameters (fibers ratio, percentage of M52N, micro-porosity) on the behavior of the selected composite material. Based on correlation between pictures, displacement, and loading histories, two criteria are defined to quantify the energy absorption capability of the composite material just before the fibers' failure and after perforation of the plate. A high-fiber weight improves performance regarding criteria and also improves the efficiency of the block copolymer present in the epoxy matrix.
Rheological models based on molecular dynamics (as opposite to empirical relationships) are now preferred to link the molecular weight distribution (MWD) of linear polymers to their rheological properties. These models incorporate the double reptation concept, which represents the relaxation modulus as an integral over the molecular weight distribution. We propose a method that incorporates a detailed modeling of all the relevant relaxation processes, including Rouse fast and longitudinal modes and glassy relaxation. In addition, we take into account the effect of polydispersity on the relaxation times for reptation, i.e., “tube renewal.” In order to demonstrate the importance of these features of our technique, we compare it with one involving the direct inversion of the double reptation integral without accounting for tube renewal and additional relaxation processes. To invert the relaxation modulus in terms of the molecular weight distribution, one must either solve the ill-posed problem using an efficient numerical algorithm or postulate a function to describe the MWD. The second approach is more robust and less sensitive to noisy data, but one must assume the form of the MWD a priori. We present a procedure for selecting this function and use it to compare the two approaches to inverting a G(t) model. Data for several binary blends and commercial polymers are analyzed using both approaches. We conclude that the more detailed technique is necessary when the MWD is broad or when there is a significant amount of low-molecular weight material present.
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