ABSTRACT:The cure kinetics for a commercial epoxy have been established and the influence of the degree of cure on the glass transition determined. Time-temperature and time-conversion superposition principles have been built into a model that successfully predicts the development of the viscoelastic properties of the epoxy during isothermal cure from gelation to after vitrification.
Elastic and dissipative properties of granular assemblies under uniaxial compression are studied both experimentally and by numerical simulations. Following a novel compaction procedure at varying oscillatory pressures, the stress response to a step-strain reveals an exponential relaxation followed by a slow logarithmic decay. Simulations indicate that the latter arises from the coupling between damping and collective grain motion predominantly through sliding. We characterize an analogous "glass transition" for packed grains, below which the system shows aging in time-dependent sliding correlation functions.( Dated: Phys. Rev. Lett. 95, 128001 (2005) ) Mechanically agitated granular materials are characterized by slow relaxation dynamics, arising from the rearrangement of the constituent grains within the volume in which they are confined [1]. This leads to a slow compaction of the system volume, which follows a logarithmic decay in time, as seen in several experiments and predicted by theoretical models of granular compaction [2,3]. This property has prompted analogies between the physics of athermal granular materials and thermal glasses, the theory of which is better understood at the fundamental level [1]. The field of granular matter therefore benefits from such parallels, as new ways of investigating the system's complex properties are discovered. An advantage of using granular materials over glasses is that it facilitates a much easier exploration of the microstructure through grain-grain interactions.Logarithmic relaxation and rate-dependent strengthening have also been observed in compressed granular matter [4], although the underlying mechanism is still under much debate. The collective rearrangement of the grains in the bulk could be responsible for the slow relaxation, as suggested by experiments on slowly sheared granular materials by Hartley and Behringer [4]. On the other hand, it is also known that aging occurs at the contacts between the particles [5], which manifests itself as a logarithmic increase of the friction coefficient between grains as a function of time. It could also be responsible for the observed slow dynamics, as has been suggested in experiments by Ovarlez et al. and Nasuno et al. [6,7], which show rate dependence and slow strengthening characteristics of aging at the interparticle contacts, respectively.The goal of this Letter is to demonstrate the existence of slow relaxation in the response of dense granular matter to infinitesimal strain perturbations and to elaborate on the origin of the dynamics. The experiments reveal a very slow stress relaxation under a constant applied differential strain. This behavior is well characterized by a two-step relaxation dynamics, analogous to the slow relaxation in "glassy" systems [1].We investigate this dynamics via computer simulations, which employ various dissipative processes into the system in order to compare their relative effects.The results show that the main process responsible for the logarithmic stress relaxation is the co...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.