I'm reading the book The New Science of Strong Materials by J.E. Gordon. He writes
when we plotted (...) breaking strain, against thickness, we found it did not matter what the whiskers were made of, for they all plotted on the same curve. (p87, Penguin)
He proves this point with a graph where the y-axis is "Fracture stress $\%E$". However, he previously stated that stress is very different to strain—stress is the load per area, while strain is the extension of the material under said load. Thus, the graph doesn't seem to correlate with the argument. I'm thinking that the $\%E$ is therefore relevant. Can someone clarify this for me?
Here is the graph: