Roles of the Interphase Stiffness and Percolation on the Behavior of Solid Propellants

Atomic force microscopy has provided access to local moduli for propellants prepared with bonding agents, which create a stiffness gradient in the matrix producing a stiffer interphase surrounding the fillers. The reinforcing impact of the bonding agent appears up to some distance and interphase percolation is observed. In order to better understand the impact of bonding agents on the stress and strain at break of propellants, finite element simulations are performed. Two-dimensional periodic cells containing randomly dispersed particles are considered, including both a cohesive zone model at the filler/matrix interface to account for possible debonding and an interphase that percolates or not. The influence of the interphase stiffness and of its percolation, on the stress and strain at break of the model propellants are evaluated through the use of a microstructure-based failure criterion

Thermal oxidation of aromatic epoxy-diamine networks

The thermal oxidation of DGEBA-DDS (bisphenol A diglycidyl ether + 4,4′-diaminodiphenyl sulfone) and TGMDA-DDS (4,4′-methylenebis(N,N-diglycidylaniline) + 4,4′-diaminodiphenyl sulfone) was performed at 80, 120, and 200 °C and was monitored by FTIR. Oxidation was shown to generate amides and carbonyls. Comparisons were done with model systems displaying some common reactive groups, which highlighted the predominating role of methylene in α position of ether in DGEBA-DDS and methylene in α position of nitrogen hold by TGMDA in TGMDA-DDS. The participation of CH2 in α position of DDS hardener group seems to depend on the temperature and decrease when lowering it. The oxidation of such complex systems must hence be described by a co-oxidation model where each kind of reactive sites is described by its own set of kinetic constants

A numerical study of the influence of polydispersity on the behaviour until break of a reinforced hyperelastic material with a cohesive interface

Solid propellants manufacturers commonly monitor the granulometries of the explosive fllers they introduce in the material to pack high fller volume fraction and thus obtain satisfactory energetic performance. However, to our knowledge, the effect of a mix of small and large particles in the micrometric size range in flled elastomers has not yet been fully understood. This work aims at producing a better understanding of the underlying mechanisms that take place in a bidisperse flled elastomer composite under uniaxial loading by using finite element simulations. An original process for creating bidisperse microstructures is proposed and analyzed. The key role of the fller/matrix interface is emphasized through the use of a cohesive zone model. Plane- strain simulations in uniaxial tension of such cells with different fractions of large and small particles are performed

On the account of a cohesive interface for modeling the behavior until break of highly filled elastomers

The nonlinear behavior and failure of highly filled elastomers are significantly impacted by the volume fraction, the size and nature of fillers and the matrix stiffness. Original experimental data obtained on glass beads reinforced acrylates and on propellants allow illustrating and discussing the main effects generally observed. In order to better understand the effects of the microstructure and constitutive parameters on the behavior and failure of highly filled elastomers, a composite model, represented by a 2D periodic cell with randomly dispersed particles, with an account of a cohesive zone at the filler/matrix interface is used. Finite element simulations with finite strain provide insight on the stress-strain responses dependence to the model parameters and allow defining a failure criterion perceived by the appearance of a critical fibrillar microstructure

Propellant cohesive fracture during the peel test of a propellant/liner structure

The integrity of propellant/liner structures in rocket motors is critical to ensure controlled combustion of the engine. In an effort to improve the bonding between the liner and the propellant, it is necessary to characterize it well. Therefore, a propellant–liner structure, bounded thanks to co-curing, has been submitted to a peel test while recording the macroscopic fracture energy and the local displacement field on the propellant-free surface. The experimental setup includes two cameras in order to record the displacement field on the propellant-free surface. Upon loading, the peel force stabilizes quickly due to a cohesive fracture in the propellant, providing access to the fracture energy. While the crack propagates through the propellant, it is observed that only a small localized area is submitted to strain, and most of the structure remains unstrained

Effects of small particles on the mechanical behavior and on the local damage of highly filled elastomers

The mechanical behavior and damage of highly filled elastomers such as propellants is studied experimentally. A model material made of a polyacrylate matrix filled with glass beads and energetic binders filled with ammonium perchlorate and HMX have been formulated. The focus is on materials containing micrometric size particles. The size of fillers was varied from a few microns to hundreds of microns in order to study the impact of the size of particles. The materials stress-strain responses and the volume changes during uniaxial tensile tests have been recorded. Microtomographic slices of strained samples have been obtained in order to look at the type of damage sustained by the acrylate/glass bead materials. It appears that in the presence of large particles, composites showing early prominent crack benefits from the addition of small particles, whereas composites showing well dispersed matrix/particle decohesion without large cracks show no change of behavior when small particles are added.financement DG

Roles of the Interphase Stiffness and Percolation on the Behavior of Solid Propellants

Atomic force microscopy has provided access to local moduli for propellants prepared with bonding agents, which create a stiffness gradient in the matrix producing a stiffer interphase surrounding the fillers. The reinforcing impact of the bonding agent appears up to some distance and interphase percolation is observed. In order to better understand the impact of bonding agents on the stress and strain at break of propellants, finite element simulations are performed. Two-dimensional periodic cells containing randomly dispersed particles are considered, including both a cohesive zone model at the filler/matrix interface to account for possible debonding and an interphase that percolates or not. The influence of the interphase stiffness and of its percolation, on the stress and strain at break of the model propellants are evaluated through the use of a microstructure-based failure criterion

Platicizer effect on network structure and hydrolytic degradation

The hydrolytic degradation of fully cured polyester-urethane networks polymerized in the presence of several weight ratios of triacetin was monitored by the residual concentration in elastically active chains obtained from modulus and equilibrium solvent swelling measurements. The presence of triacetin does not change the water uptake but induces a lower rate of degradation. Comparisons were performed with networks in which triacetin was removed before ageing, and with networks in which polyester-urethane was first polymerized and then impregnated by triacetin. Data suggest that the presence of triacetin during polymerization induces the presence of elastically inactive chains such as dangling chains, loops… the hydrolysis of which does not change the elastic properties of the network. This explanation was checked from relaxation measurements by n.m.r and d.m.a, and by the analysis of the soluble fraction generated by hydrolysis

Network Topology of the Interphase between Cross-Linked Polyurethane/Ethylene Propylene Diene Terpolymer Elastomers for Adhesion Applications

Understanding the interfacial phenomena involved in the adhesion between elastomer layers on a molecular basis is an important topic from both fundamental and applied aspects. Nevertheless, this topic has been poorly addressed experimentally. This report aims at rationalizing differences in the adhesion behavior of polyurethane (PU) elastomers cured on an ethylene–propylene–diene terpolymer (EPDM) substrate, based on a detailed description of their local network-like topology, determined thanks to 1H solid-state nuclear magnetic resonance (NMR) spectroscopy. The polyurethanes, composed of the same fraction of hydroxy-terminated poly(butadiene) and isophorone diisocyanate, were cured under different reaction conditions: nature and concentration of the catalyst as well as the cross-linking temperature. The rigid domains formed by the hard segments, the proportion of elastically active chains, and the distribution of the topological constraints in the soft domains were investigated by 1H solid-state NMR, taking advantage of the magic sandwich echoes and double quantum-based experiments. The PU network topology within 20 μm thick slices collected near the interface with the EPDM layer was systematically compared to the one observed for 60 μm thick slices, located 500 μm from the interface, corresponding to the bulk regions. Curing at a low temperature (30 °C) with a low amount of catalyst (0.02 wt %) leads to elastically active poly(butadiene) chains close to the interface with, on average, higher molecular weights between topological constraints than the ones in the bulk. Such differences between interfacial and bulk regions are not observed any longer as the catalyst concentration is increased to 0.2 wt %. These variations of the local PU network topology, occurring over several tens of micrometers, allow one to account for the adhesion testing results

Stress-strain response and volume change of a highly filled rubbery composite: Experimental measurements and numerical simulations

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