A note on the physical meaning of gradient constitutive relations

Gradient plasticity theory proposed initially by Aifantis and co-workers has proven very useful in problems dealing with material heterogeneity and material instabilities. Although it has been used successfully in many applications by many authors, still some open questions remain unanswered. The modest goal of the present short note is to provide some thoughts on these open questions, providing a new interpretation of the gradient terms leading to a successful interpretation of both extrinsic and intrinsic size effect phenomena.