Comment on the role of the size of the circles placed on the surfaces of sheet metals in determining their formability. Are square grid patterns, as shown in Fig. 7.65, useful? Explain. We note in Fig. 7.65 on p. 400 that, obviously, the smaller the inscribed circles, the more accurately we can determine the magnitude and location of strains on the surface of the sheet

being formed. These are important considerations. Note in the figure, for example, how

large the circles are as compared with the size of the crack that has developed. As for square

grid patters, their distortion will not give a clear and obvious indication of the major and minor

strains. Although they can be determined from geometric relationships, it is tedious work to do

so.