In mechanics of materials, stress is a measure of the effect of loads on an object; more specifically, it expresses the internal forces that neighboring particles of a material exert on each other. When an object becomes stressed, it may change shape i.e. deform or break, depending on the magnitude of stress on the object and the material the object is made from. The measure of this deformation is called strain.
Most of used materials are generally polycrystalline. They consist of millions of single crystals called grains. Grains have different orientations of the atom lattice and they are separated from the neighboring grains by interfaces called Grain Boundaries (GB). It is well established that the irreversible (or plastic) deformation of a sample originates mainly from the nucleation and the propagation of more than hundreds of billions per cm³ of micrometric (even nanometric) linear defects of the regular crystal lattice called dislocations. Dislocations move through the grain and interact with each other or with GB. GB may act in several ways: sinks, traps and sources of dislocations.
Nowadays, one knows almost how one dislocation interacts with one model GB, but understanding the response of several real GB (contained in a real bulk polycrystalline sample) after receiving numerous dislocations is still a major scientific challenge. The mystery becomes inextricable when one considers that there are more than hundreds of billions of dislocations per cm³ of sample interacting together and with billions of GB… Even more inextricable if one wants to take into account the influence of the distribution of GB, other types of interfaces, grain shape, grain orientation and defects in the bulk sample i.e. its microstructure. Due to this inherent complexity, we have to link two extreme scales: sample (or macro-) scale and dislocation (or micro-) scale. It is obvious that these two worlds interact each other but their connections remain extremely difficult to understand because of the need of extrapolations.
The response of one grain of interest in a polycrystalline sample cannot be understood individually without taking into account its neighborhood. Surprisingly, literature suffers a lack on this crucial point. For instance, one generally supposes that the solicitation experienced by one grain is the same than that one experienced by the all specimen, i.e. the stress field inside one grain (σ) is supposed to be the same that the macroscopic stress field applied (Σ) on the sample during the deformation test. Therefore, the influence of the surrounding microstructure and the crystalline anisotropy are considered as negligible. Despite intense research, this question remains partially unexplored to date essentially because of numerous technological issues.
By nature, solving this mystery requires a breakthrough in the existing approaches. The role of micromechanical modeling coupled with experiments brings important insight. Such computational schemes need constitutive equations that have to be “fed” with experimental criteria and parameters, capturing the important operative mechanisms. However, such valuable experiments are still marginal and all suffer numerous constraints making interpretations not reliable.
Ongoing projects on the deformation of materials
2018–2021: Thesis of Frederic HABIYAREMYE
Experimental and mesoscopic approach for understanding the fundamental deformation mechanisms of materials.
2020–now: Postdoc of Dr. Kaustubh VENKATRAMAN
Influence of the microstructure on the internal stress field within grains.