rate of crack propagation

• 裂纹传播速率,裂缝扩展速度

rate of crack propagation的用法和样例：

例句

1. Today, the rate of desert expansion in Niger is dropping and the amount of harvested crops is up.
   现在，尼日尔沙漠的扩展速度在下降，而农作物的总数则在上升。