Announcing the new guidelines, Mr. Trump repeated stress that they were voluntary.

在宣布新的指导方针时 特朗普再次强调 这些指导方针是自愿的。

The surprise came when he discovered how his new material responded to repeated stress.

当他注意到新材料受到连续作用力而产生反应时，令他惊讶的现象出现了。

A new material that gets stiffer when it is stressed ONE of the valuable properties of bone is that when it endures repeated stress it responds by becoming denser and stronger.

一种受外力时硬度提高的新材料骨头有一种很珍贵的属性，那就是当受到反复的击打时，其质地反而会变得更加紧密，坚硬度变得更高。