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题目材料：
Composites are immensely popular engineering materials because they are significantly lighter than aluminum yet 20 percent stronger. However, great care must be taken when using them because composites fatigue and fail differently from metals. In a metal, although a fatigue crack can appear at a very low stress level--about 10 percent of the metal's breaking stress--such cracks normally grow only slowly, allowing ample time to spot them before catastrophic failure. To start a crack in a composite, on the other hand, requires somewhere between 50 to 70 percent of its breaking stress. But while it is much more difficult to create the crack in the first place. it will then reach a critical size much more quickly.
以上解析由 考满分老师提供。
