Appendix: Measuring Strains and The FLC
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To determine strains they have to be measured. This is done by marking the surface in some way, and to compare the situation after forming to that before forming. These markings can easily be made by painting, but there is a possibility that the paint wears off when the surface slides over the tool. Another way is to etch the markings chemically into the surface. This supplies very robust markings, but the procedure is tedious. Also, a deep etching may act as a stress concentrator initiating early failure.