Ref HAL: hal-01904561_v1
DOI: 10.1007/978-3-319-50257-1_125-1
Exporter : BibTex | endNote
Résumé:

Fracture propagation is inherently a multiscale problem, involving the coupling of many length scales from sample dimension to molecular level. Fracture mechanics provides a valuable link between the macroscopic scale of the structural loading of the samples and the scale of the process zone for brittle materials. Modeling the toughness of materials requires yet an investigation at scales smaller than this process zone, which is nanometric in oxide glasses and micrometric in polymer glasses.We present here the important insights that have been obtained through an in situ experimental investigation of the strain fields in the micrometric neighborhood of a propagating crack. We show the richness of atomic force microscopy combined with digital image correlation although it limits the observations to the external surface of the sample and to very slow crack propagation (below nm/s). For oxide glasses, this novel technique provided enlightening information on the nanoscale mechanisms of stress corrosion during subcritical crack propagation (Ciccotti, J Phys D Appl Phys 42:214006, 2009; Pallares et al., Corros Rev 33(6):501–514, 2015), including the relevance of crack tip plasticity (Han et al., EPL 89:66003, 2010), stress-induced ion exchange processes (Célarié et al., J Non-Cryst Solids 353:51–68, 2007), and capillary condensation in the crack tip cavity (Grimaldi et al., Phys Rev Lett 100:165505, 2008; Pallares et al., J Am Ceram Soc 94:2613–2618, 2011). An extension of this technique has recently been developed for glassy polymers (George et al., J Mech Phys Solids 112:109–125, 2018), leading to novel insights on the transition between crazing and shear yielding mechanisms and to promising new ways to link the toughness properties to the time-dependent large strain material properties of these nominally brittle materials.

Ref HAL: hal-01904339_v1
DOI: 10.1016/j.jcs.2018.07.009
WoS: 000448494900008
Exporter : BibTex | endNote
3 Citations
Résumé:

Light transmission through wheat (T. aestivum L.) grain longitudinal cross sections of different thickness was used to study the endosperm microstructure and was shown to strictly follow a Beer-Lambert law allowing a non ambiguous quantification of the endosperm vitreousness. Therefore similar samples obtained from near-isogenic lines differing by hardness and grown in two distinct environments affecting their vitreousness were analyzed and confirmed the relationship between light transmission and the endosperm microstructure. In each sample, moreover analysis of light transmission within the different grain parts highlighted the greater compactness of the central endosperm cheeks in comparison with the distal and the proximal regions. These results helps a better understanding of the endosperm microstructure.

Ref HAL: hal-01904079_v1
Ref Arxiv: 1804.01810
DOI: 10.1073/pnas.1717403115
WoS: 000429012500051
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
19 Citations
Résumé:

Material failure is ubiquitous, with implications from geology to everyday life and material science. It often involves sudden, unpredictable events, with little or no macroscopically detectable precursors. A deeper understanding of the microscopic mechanisms eventually leading to failure is clearly required, but experiments remain scarce. Here, we show that the microscopic dynamics of a colloidal gel, a model network-forming system, exhibit dramatic changes that precede its macroscopic failure by thousands of seconds. Using an original setup coupling light scattering and rheology, we simultaneously measure the macroscopic deformation and the microscopic dynamics of the gel, while applying a constant shear stress. We show that the network failure is preceded by qualitative and quantitative changes of the dynamics, from reversible particle displacements to a burst of irreversible plastic rearrangements.

Commentaires: . Réf Journal: PNAS 115, 3587 (2018)

Ref HAL: hal-01896690_v1
DOI: 10.1039/c8sm00911b
WoS: WOS:000442269000021
Exporter : BibTex | endNote
1 Citation
Résumé:

We use space-resolved dynamic light scattering in the highly multiple scattering regime (Photon Correlation Imaging Diffusing Wave Spectroscopy, PCI-DWS) to investigate temperature-induced phase transitions in polymorphic materials. We study paraffin wax as a simple model system and chocolate, a prototypical example of fat-based products exhibiting complex, history-dependent phase transitions. We find that microscopic dynamics measured using PCI-DWS show remarkable, non-monotonic behavior upon heating: they transiently accelerate when crossing phase transition and slow down above the transition temperature. Sub-micron resolution measurements of the local drift of the sample surface reveal that the speed-up of the dynamics is due to the strain field induced by the change in density at transition temperature. The transition temperatures obtained from PCI-DWS are found to be in excellent agreement with those inferred from complementary differential scanning calorimetry and X-ray scattering experiments, thereby validating PCI-DWS as a new, powerful tool for the characterization of phase transitions in complex soft matter. Finally, we demonstrate the unique possibilities afforded by space-resolved DWS by investigating the spatially heterogeneous response of poorly manufactured or composite chocolate samples.

PMID 29758608
DOI: 10.1103/PhysRevE.97.040601
WoS: WOS:000429636700001
30 Citations
Résumé:

We explore the glassy dynamics of soft colloids using microgels and charged particles interacting by steric and screened Coulomb interactions, respectively. In the supercooled regime, the structural relaxation time τα of both systems grows steeply with volume fraction, reminiscent of the behavior of colloidal hard spheres. Computer simulations confirm that the growth of τα on approaching the glass transition is independent of particle softness. By contrast, softness becomes relevant at very large packing fractions when the system falls out of equilibrium. In this nonequilibrium regime, τα depends surprisingly weakly on packing fraction, and time correlation functions exhibit a compressed exponential decay consistent with stress-driven relaxation. The transition to this novel regime coincides with the onset of an anomalous decrease in local order with increasing density typical of ultrasoft systems. We propose that these peculiar dynamics results from the combination of the nonequilibrium aging dynamics expected in the glassy state and the tendency of colloids interacting through soft potentials to refluidize at high packing fractions.

Ref HAL: hal-01889803_v1
Ref Arxiv: 1802.03820
DOI: 10.1122/1.5025622
WoS: 000449684700010
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
10 Citations
Résumé:

Power law rheology is of widespread occurrence in complex materials that are characterized by the presence of a very broad range of microstructural length and time scales. Although phenomenological models able to reproduce the observed rheological features exist, in general a well-established connection with the microscopic origin of this mechanical behavior is still missing. As a model system, this work focuses on a fractal colloidal gel. We thoroughly characterize the linear power law rheology of the sample and its age dependence. We show that at all sample ages and for a variety of rheological tests the gel linear viscoelasticity is very accurately described by a Fractional Maxwell (FM) model, characterized by a power law behavior. Thanks to a unique set-up that couples small-angle static and dynamic light scattering to rheological measurements, we demonstrate that the power law rheology observed in the linear regime originates from reversible non-affine rearrangements and discuss the possible relationship between the FM model and the microscopic structure of the gel.

Commentaires: . Réf Journal: Journal of Rheology, 62, 1429-1441 (2018)

Ref HAL: hal-01881804_v1
DOI: 10.1016/j.tca.2018.07.019
WoS: 000444663600024
Exporter : BibTex | endNote
9 Citations
Résumé:

Despite an abundant literature on epoxy-amine systems, a complete description of the curing kinetics in epoxy-phenol composites was still lacking. In this study, the curing kinetics of an epoxy-phenol composite relevant to microelectronics industry is probed by isothermal and non-isothermal differential scanning calorimetry. An isoconversional analysis of the data reveals a significant contribution of the diffusion of molecular species to the activation energy, at low temperature and high degrees of curing. A model-fitting of the kinetics is performed in two successive steps: high-temperatures data are fitted with Arrhenius law and nth order autocatalytic model (where the diffusion contribution is neglected), whereas low-temperature data are fitted using Rabinowitch and modified-Williams-Landel-Ferry models (considering a diffusion contribution related to the glass transition). The chemical and diffusion contributions to the rate constants are calculated at various temperatures, clarifying the kinetics regimes with precision. Finally, the kinetics regimes are summarized in an improved time-temperature-transformation diagram.