a pP 
1 200 um} 200 pm 
Fig. 1: Regular surtace structure, A: Undeformed, B: Deformed to the 
limit of uniform elongation 
It is well known, that the surface roughness of sheets changes during forming process (7, 8). Thus, 
the local strain can be derived by establishing empirical relationships between strain and roughness 
parameters. For this purpose 12 of the 3D parameters varying systematically with strain (method 1) 
were selected. 
Etched measuring grids typically have a line distance of 1 mm. A finer grid is available using the mern 
RSSE (method 2). An even finer grid, though irregular, is made up by the microsructure inherent in yp: 
essentially any engineering material. For the sheet steel used here, the grain structure is revealed by ter har 
etching and the grain size can be measured (method 3). Comparison of images of initial and final me 
state at the same location is used for both the RSSE (method 4, see Fig. 1) and the grain structure som * 
(method 5, see Fig. 2). In contrast to method 1, engineering or true strain can be directly calculated  ,..... 
from the length change of the RSSE or the grains. erie 
In order to compare the five methods on an equal basis, experimental errors were determined pe N 
graphically as described in the following for method 1. Significant changes of surface morphology fr 4A 
occur during deformation (Fig. 1). The originally circular lubricant pockets (Fig. 1A) take an van el 
elongated shape, and, due to the increasing roughness, become faint (Fig. 1B). va. 
The 12 deformation sensitive 3D roughness parameters are plotted logarithmically as a function of Forma 
the true plastic strain of the sheet tested in tension in Fig. 3. All parameters increase by a factor of ~~ 
about 2 for a true plastic strain of 0.5. Re 
, > Fe TB} 
k \ 
Tim, Som | 
Fig. 2: Comparison of grain morphology at same location: (A) 
undeformed, (B) deformed to the limit of uniform elongation 
(Features marked a and b are easily discerned as corresponding 
locations. Dark zones at the left and the bottom are marks of overhead 
markers to facilitate relocation of the image position). 
66