Search

Description, Tough elastomers that resist fracture at high loads are not optimum for sustaining many cycles at low loads., Although elastomers often experience 10 to 100 million cycles before failure, there is now a limited understanding of their resistance to fatigue crack propagation. We tagged soft and tough double-network elastomers with mechanofluorescent probes and quantified damage by sacrificial bond scission after crack propagation under cyclic and monotonic loading. Damage along fracture surfaces and its spatial localization depend on the elastomer design, as well as on the applied load (i.e., cyclic or monotonic). The key result is that reversible elasticity and strain hardening at low and intermediate strains dictates fatigue resistance, whereas energy dissipation at high strains controls toughness. This information serves to engineer fatigue-resistant elastomers, understand fracture mechanisms, and reduce the environmental footprint of the polymer industry.

International audience; Designing innovative multi-functional thermoset polymers represents a major challenge in materials science. In this work, two novel polyhedral imidazolium ionic liquid supported on oligomeric silsesquioxane nanocages having chloride (Cl-) and bistrifluoromethanesulfonimidate (NTf2-) counter anions have been synthesized. These ionic nano-objects were used to nanostructure a model epoxy network. Hence, networks displaying a homogeneous nanoscale morphology were obtained by introducing 5wt % of such organic-inorganic (O/I) silicon-oxo clusters functionalized with ionic liquids. The obtained ionic hybrid O/I epoxy-amine networks display an excellent thermal stability (> 400 °C) compared to their neat counterparts, as well as a pronounced hydrophobic character (23 mJ m-2) and good mechanical performances. Moreover, these O/I nanostructured networks demonstrate an improved fire resistance according to pyrolysis-combustion flow calorimetry (PCFC) and cone calorimetry analyses. In fact, the use of low amounts of imidazolium ionic liquid-modified polyhedral oligomeric silsesquioxanes induce a significant decrease of heat release rate (about 55 %), as well as a retardation of the ignition time (close to 29 %) compared to the neat epoxy-amine network.

Tremendous progress in nanomechanical testing and modelling has been made during the last two decades. This progress emerged from different areas of materials science dealing with the mechanical behaviour of thin films and coatings, polymer blends, nanomaterials or microstructure constituents as well as from the rapidly growing field of MEMS. Nanomechanical test methods include, among others, nanoindentation, in-situ testing in a scanning or transmission electron microscope coupled with digital image correlation, atomic force microscopy with new advanced dynamic modes, micropillar compression or splitting, on-chip testing, or notched microbeam bending. These methods, when combined, reveal the elastic, plastic, creep, and fracture properties at the micro- and even the nanoscale. Modelling techniques including atomistic simulations and several coarse graining methods have been enriched to a level that allows treating complex size, interface or surface effects in a realistic way. Interestingly, the transfer of this paradigm to advanced long fibre-reinforced polymer composites has not been as intense compared to other fields. Here, we show that these methods put together can offer new perspectives for an improved characterisation of the response at the elementary fibre-matrix level, involving the interfaces and interphases. Yet, there are still many open issues left to resolve. In addition, this is the length scale, typically below 10 micrometres, at which the current multiscale modelling paradigm still requires enhancements to increase its predictive potential, in particular with respect to non-linear plasticity and fracture phenomena.

© 2018 Elsevier Ltd The deformation behavior of polymer materials below their glass transition temperature involves a complex intermingling of elasticity, of viscoplastic yielding with softening and large strain hardening, and of significant rate, temperature and pressure dependency. During unloading, the response exhibits large non-linearity as well, combining forward and backward creep. Even though the nature of the atomistic underlying mechanisms is relatively well understood, a full picture of the interactions between the elementary distortion mechanisms and their collective organization capturing the complexity of the macroscopic behavior is still lacking. Here, we propose a mesoscale approach based on the heterogeneous activation of molecular level shear transformation zones and we show that it is rich enough to unravel the physical origin of most aspects of the macroscopic viscoplastic response. The study is based on a wide micro- and macro-mechanical test campaign on a thermoset material. In addition to providing a full characterization of all macroscopic properties, evidence of a complex rate reversal mechanism upon unloading is given. Other experiments provide insights about the characteristic length scales at play. Compared to existing continuum models involving typically more than 25 parameters, the present formalism is based on only 7 physical parameters while allowing quantitative predictions of all the experimental data. The present approach opens new avenues for the prediction of the mechanical response of a variety of polymer applications in confined state such as in composites or in adhesive joints. It also offers a new line of thought to address the mechanics of polymers at small and intermediate length scales.

A new fracture criterion for semi brittle epoxy materials is developed, expressed in terms of the attainment of a local critical value of the maximum principal stress at the tip of small internal defects. The criterion is assessed on the highly cross-linked structural epoxy resin RTM6 used as matrix in fiber reinforced composites. Based on finite element simulations of unit cells, the local stress field at the tip of ellipsoidal defects is related to the macroscopic loading. The overall fracture stress levels measured on uniaxial tension and compression specimens are used to identify the two parameters of the failure criterion which involves the characteristic aspect ratio of the microdefects and the critical maximum principal tensile stress. An upper bound to the size of the microdefect is determined based on fracture mechanics arguments. The fracture criterion accurately predicts the fracture for a wide range of stress triaxialities from overall compression conditions to tensile with additional hydrostatic component related to different notched configurations.