On damage distribution in the upsetting process of sintered porous materials

The common information about the average porosity of a given porous material (like sintered powder materials) may not be sufficient for design purposes. It is evident that an occasional localization of pores (‘damage’) may trigger premature failure at a low global average porosity. In order to avoid this limitation we suggest here two measuring techniques to assess the spatial distribution of the damage in one and two dimensions. (1) Macro-measurement: The idea is based on removing (in consecutive steps) thin layers from the outer boundary of the workpiece and measuring the density and weight of the residual workpiece between each step. The repeated cycles of ‘cutting and measuring’ give the distribution of the pores in the cross section of the workpiece (averaged across the height of the section). (2) Micro-measurement: Magnified pictures are taken from selected metallographic sections of the workpiece. An area scanner is employed to measure the ‘local porosity’ by providing the ratio between the dark areas to the total area occupied by each picture. This procedure renders directly the 2D distribution of the pores in a given cross section. Porous specimens made by powder metallurgy (Fe and 304 stainless steel) are compressed unidirectionally. The evolution of the porosity due to the compacting process is measured by these two techniques. The measurements show how the initial pore distribution evolves in space and time during the compression process. The results are compared to a semi-analytical simulation of the densification process using the limit analysis formulations.