Identification of the intrinsic material length in gradient plasticity theory from micro-indentation tests

Recent experimental and theoretical studies show that constitutive models without internal material length cannot predict effects of the specimen size. To assess the size effects it is necessary to incorporate an intrinsic material length into the constitutive equation. In the present paper we are going to discuss a gradient plasticity model which possesses an intrinsic micro-material length and may catch strength variations with the specimen size. Our analysis confirms that the gradient plasticity model is suitable to simulate micro-indentation hardness tests and provides the depth-dependent hardness which has been observed in micro- and nano-indentations. The quadratic dependence of the micro-hardness on the inverse of the indentation depth is computationally verified and the material length scale can be identified from such hardness measurements. The paper presents a method for identifying the intrinsic material length scale parameter. Comparison with published experiments displays that computational results can catch size effects in micro-indentation tests accurately. (C) 2001 Elsevier Science Ltd. All rights reserved.