金属在交变应力作用下导致的失效称为“疲劳”。疲劳的外观现象是,在应力远低于材料的静强度极限,甚至在低于屈服极限和比例极限的情况下,材料经过许多周次应力反复后即造成失效。疲劳过程中材料内部变化分为:疲劳裂纹的发生、发展和最后断裂三个阶段。如果某一因素促使这三个阶段的加速,则材料的疲劳抗力将降低。在水下工作 Metal failure caused by alternating stress is called “fatigue”. The appearance of fatigue is that, at stresses well below the static strength limit of the material, the material fails after repeated stress cycles, even below the yield and proportional limits. The internal changes in the process of fatigue are divided into three parts: the occurrence of fatigue cracks, development and the final fracture. If a certain factor to promote the acceleration of these three stages, the fatigue resistance of the material will be reduced. Work under water