"破坏时伸长量" can be translated as "elongation at failure" or "elongation at rupture" in English.
"Elongation at failure" emphasizes the elongation or stretching that occurs up to the point of material failure.
"Elongation at rupture" specifically refers to the elongation when the material ruptures or breaks.
Both terms are commonly used in materials science and engineering to describe the behavior of materials under tension until they break.