Project "SGP-GAPS"

Experimental and numerical investigation of elastic gaps in strain gradient plasticity theories

Pioneered by Aifantis (1984), strain gradient plasticity (SGP) has received a strong scientific interest in recent years and numerous SGP theories have been developed. These theories offer a very promising way to model the size-dependent behavior of materials at small scales. Including internal length scale(s), such theories are capable of accounting for the gradient of plastic deformations which correlate with size effects, as experimentally observed and numerically predicted using dislocation mechanics.

Despite their interesting features, application of SGP theories to real engineering problems is up to now very limited due to lack of maturity. Some major size effects numerically predicted by these theories have never been observed neither experimentally nor numerically using accurate small-scale approaches. In this context, almost all existing SGP theories including thermodynamically-consistent higher-order dissipation lead to elastic gaps under certain loading conditions (i.e., delay in plastic flow under infinitesimal loading change). More specifically, an infinitesimal change in the loading conditions may lead to a finite change in the generalized stresses, causing the interruption of the plastic flow until the mechanical state (evolving elastically) reaches a new yield point.

The phenomenon of elastic gaps represents a major source of confusion and uncertainty, preventing the development of robust SGP models capable of correctly predicting size-dependent behaviors of materials. In the absence of works studying these gaps from a physical point of view, the scientific investment on SGP theories has reached a bifurcation point making the community divided into those who consider elastic gaps as “unknown” size effects (then possibly physical) and those who see no physical reasons for their occurrence in reality (as they reflect an instantaneous finite change in the generalized stresses). Therefore, investigation of such gaps and their existence in reality is highly needed to clear up this ambiguity and to allow for uniting the scientific efforts on SGP theories in the right direction.

This is the goal of the present project which represents a logical continuation of the strong scientific effort performed up to now on the development of robust SGP theories. Particularly, this project aims at providing the compelling answer to the question "are elastic gaps physical?". For this purpose, a multi-disciplinary synergy will be created between original small-scale experiments, implying, for the first time, complex non-proportional loading conditions, and extensive discrete-dislocation-based simulations. To achieve its goals the project is divided into three tasks as detailed below.