This kind of had me puzzled for the last week. If you apply a tensile stress on a body, it will elongate, and get a little bit thinner. The amount of stretch will be a function of the stresses. But then I started wondering, "Well, if that’s the case, then it will stretch and get a little thinner. But then, I can say there is a stress still (after it got thinner), and I could repeat my logic using the strain equation based on the stress, over and over again, until this thing becomes thin as a wire." That’s clearly not what’s happening. I think, to be EXTRA precise, the book should say that the strain is a result of a SUDDENLY APPLIED load. You apply a load, it will have a strain. But after that strain, the load is no longer suddenly applied, but steadily applied. So now your strain will be zero, because there is no NEW applied load. In short, strain is a result of a load being suddenly applied or an already applied load changing in magnitude will result in a change in the strain. Because to not say that, would infer that you could continue to use the strain equation to as many times as you like to (because hey, there will always be stresses present right?) , but that’s obviously not correct. (All this assuming the elastic region, of course).