On the connection between microstructure and surface roughness of brass sheets and their formability

Abstract

The aim of this work was the analysis of the relationships between material properties and forming limits of a sheet, caused by the strain localization in the groove. In particular, we aim at the replacement of a nonmeasurable inhomogeneity coefficient of the material, the value of which has to be taken a priori, by measurable coefficients. In the proposed model, it is assumed that the material heterogeneity is a result of surface roughness and presence of internal defects (voids). The value of inhomogeneity coefficients changes with increasing strain. The experiments were carried out for M85 brass sheets, annealed to produce different microstructures. The measured values of some material parameters: strain-hardening constants n and C, normal anisotropy factors, inhomogeneity coefficients (surface roughness and volume fraction of voids) as well as forming limit curve demonstrated their dependence on the grain size. As a result of the experimental work, original equations were proposed, describing the relationship between inhomogeneity coefficients and effective strain and grain size. These equations were used in the theoretical calculation of the forming limit diagram. The theoretical calculations of the strain limits of the tested sheets were based on the associated flow rules, assuming strain-hardening and strain-softening process (Shima-Oyane equation and equations based on the Gurson theory). The analysis of the influence of different material parameters on the forming limit diagram has shown that in the case of the material tested, the value of limit strains depends decisively on the values of the inhomogeneity coefficients and grain size. The postulates showing proportional relationship between the value of a strain-hardening exponent and limit strains were not confirmed.

Analysis of sheet surface roughness change under contact with flat and spherical indenters

“…5). This dependence is nearly linear (R 2 > 0.91), which supports the work of Stachowicz [24]. In the case of Ra parameter measured along the rolling direction in the range of strain between 0 and 20 %, a fast increasing of its value is observed.…”

Analysis of Contact of a Rigid Sphere Against a Deformable Flat

“…As we found, the determination coefficient R 2 between roughness parameters values and the strain value is higher than 0.9104. The linear character of this relation is confirmed by Stachowicz [26]. The increase in surface roughness of the sheet is the result of reorientation and fragmentation of the individual grains, mainly in the subsurface.…”

Effects of loading conditions on free surface roughening of AISI 420 martensitic stainless steel

“…Moreover, most studies showed that roughness linearly increases with the applied strain. For example, Stachowicz (2016) explored the surface roughness of different brass sheets in uniaxial and biaxial tensile tests, and found linearity between the surface roughness increment and the effective strain. Most studies, however, reported that the roughness increment is not a linear function.…”