When diameter-thickness ratio is not less than 5, an ideal HPT region on center of copper specimens exists, and no ideal HPT region size is less than its thickness on samples margin.

• 当径厚比不小于5时，试样中心存在一个可用纯扭转变形描述的区域，非理想HPT区域大小不超过试样厚度尺寸；