Fracture propagation involves the coupling of many length scales ranging from the sample loading geometry to the molecular level. In brittle materials, the length scales of the damage process zone are reduced to a submicrometric scale and the coupling with the macroscopic scale is expected to be the domain of linear elastic fracture mechanics (LEFM). However, although 2D elastic analyses are generally adequate to describe the sample deformation at macroscopic scales, local investigations of failure mechanisms at the sample free surface require the use of 3D mechanical tools due to the crack front local curvature and to the corner point singularities at the intersection between the crack front and the external surfaces of the sample. We present here a thorough multiscale investigation of-2-the slow crack growth of a sharp crack in oxide glasses in the stress-corrosion regime, combining experimental and numerical analyses of the displacement fields from the millimeter scale to the nanoscale range. The principal aim of the study is identifying the length and time scales of the mechanisms of damage and interaction between water and glass, which have been the subject of an extensive debate in the last decades.

The Atomic Force Microscope tip was used to progressively abrade the surface of non-cutted starch granules embedded in the endosperm protein matrix in grain sections from wheat near-isogenic lines differing in the puroindoline b gene and thus hardness. In the hard near-isogenic wheat lines, starch granules exhibited two distinct profiles corresponding either to abrasion in the surrounding protein layer or the starch granule. An additional profile, only identified in soft lines, revealed a marked stop in the abrasion at the protein-starch transition similar to a lipid interface playing a lubricant role. It was related to the presence of both wild-type puroindolines, already suggested to act at the starch-protein interface through their association with polar lipids.This study revealed, for the first time, in situ differences in the nano-mechanical properties at the starch-protein interface in the endosperm of wheat grains depending on the puroindoline allelic status.

Wheat endosperm is a cemented granular material composed of a binary-sized mixture ofalmost spherical starch granules bounded to one another with an amorphous proteinmatrix. The milling properties of wheat grains depend on this typical microstructure, largelycontrolled by the genetic background and the growing conditions. An original Atomic ForceMicroscopy nano-scratch methodology has been developed [1] to assess the localrheological properties of the protein matrix, the starch granules and their interface. Thedetermined relative stiffness and failure strength, together with information on the phasedistribution, were then used to construct 2D numerical samples of wheat endosperm. Thegranular structure of the sample with different granular packing was computed using aMolecular Dynamics approach. The protein matrix was added in the form of bridgesconnecting neighboring particles. The samples were then meshed using a triangular latticeof one-dimensional spring elements that were characterized by stiffness and yield force.The rheological properties of each element were set according to the location of its twonodes leading to five different elements: starch, matrix, starch-matrix, starch-starch andvoids. The samples were then subjected to an increasing uniaxial tensile stress until failureusing an iterative procedure based on conjugate gradient minimization. The LatticeElement Method, developed by Topin [2,3], was used for the simulations and a parametricstudy was performed where the protein content, the starch granular packing and thestarch-protein adhesion, suggested to be responsible of the wheat fragmentation, werevaried. The results showed that, depending on the sample porosity, the bulk elasticproperties do not follow the mixing law of diluted composites, highlighting the granularbackbone effect of percolating particles. A non-linear evolution of the bulk elastic modulusas a function of the sample porosity was also noted, with little effect of the granule solidfraction. Concerning the failure properties, the results showed that the particle volumefraction has a greater influence on the yield stress at high starch-protein adhesion and thatfor the same porosity, increasing particles volume fraction leads to higher yield stress.Finally, we noted that the yield stress strongly depends on the sample porosity at highstarch-protein adhesion and is weakly affected by the porosity at low adhesion, whateverthe particle volume fractions.