Invited
THE DISTRIBUTION OF PLASTIC STRAIN IN POLYCRYSTALLINE MATERIALS: MICROSCOPIC VS> MACROSCOPIC APPROACHES

According to the continuum mechanics approach, plastic deformation takes place in locations were the local stress reaches the yield surface. Afterwards, plastic strain proceeds in the direction normal to the yield surface. This view is followed by modern computational methods and yields good conformance with the experiment.

Microscopically, each crystal in a polycrystalline material can slip only along a discrete set of slip planes and slip occurs on a certain slip system when the resolved shear stress on this system exceeds the yield stress. This gives rise to a very inhomogeneous plastic deformation in microscopic scale, since the slip in neighboring crystals greatly differ in magnitude, direction and number of active slip systems. Our work aims to investigate the distribution of local strain and to reveal whether the microscopic strains average to the strains calculated by continuum mechanics approaches.

We have tensile tested 304 stainless steel specimen with a circular hole to a 3% plastic strain. The slip bands generated on the free surface were observed by SEM, AFM and EBSD. The local shear strains were analyzed and the average strains were calculated and compared with the strain calculated by the finite element method.

We found that the slip magnitudes were distributed in a nearly normal distribution in each area. The average microscopic strains were proportional to the strains calculated by the continuum mechanics approach. However, the local microscopic strains greatly exceeded the macroscopic strains. It is concluded that the plastic strains in polycrystalline materials are inefficient in complying with the external constraints, but concurrently, the results illustrate that the ductility of metals may be much larger than that observed macroscopically.