An increment of natural strain ei = dl / l' , where l' is length at beginning of increment, but not the original length, since the previous strain increment changed l'. The natural strain is the sum of all the increments. If you integrate from an initial length to a final length, then by definition you end up with the natural strain = ln ( final length/ original length). the graph of incremental strain against the angle by which the fibre chord departs are plotted to get the maximum and mean incremental strain