各种平均应力下高周疲劳极限间的定量关系

A QUANTITATIVE RELATIONSHIP OF HIGH CYCLE FATIGUE LIMIT WITH MEAN STRESSES

疲劳极限是材料工程应用中的一个重要特性参数,但它并不是材料常数。在不同循环载荷作用下,材料会有不同的疲劳极限值。疲劳极限值很大程度上依赖于循环载荷中的平均应力或最大应力幅值,两者之间的关系是材料自身固有的特性关系。因为在接近疲劳极限的低循环应力幅下,S-N曲线受疲劳极限控制,所以在工程应用中对其定量关系有迫切的需求,但现有的经典经验关系只能对其进行非常粗略的估算,不能满足工程应用中有关寿命设计的需要。现状是只能通过大量的实验,得出其实验关系。该文提出了一种函数形式的关系式,该关系式可以足够精确地描述各种材料的疲劳极限和平均应力之间的关系,且只包含三个材料常数。只要这三个材料疲劳特性常数被事先确定,则任意疲劳载荷下的疲劳极限值,都可由该关系式估计得到,因此可以作为材料本身固有的疲劳强度的状态关系式使用。

Fatigue limit is a very important property of engineering materials in applications, but it is not a constant of material in fact. It is well known that a fatigue limit is dependent on fatigue cyclic properties, especially on either mean stress or maximum stress amplitude. Since the S-N curve corresponding to low stress amplitudes is strongly influenced by a fatigue limit, the quantitative and intrinsic relationship of a fatigue limit with mean stress is necessary for the life design in engineering applications. However, the classical experience relationships generally can only give a very rough estimation, thusly special experiments are usually necessary to obtain experimental results to meet engineering needs. This paper proposes a functional relationship to describe the variation of a fatigue limit with mean stresses. It is found that this relationship can well express the fatigue limits under different mean stresses for various engineering materials, and contains only three material constants. When these three material constants have been pre-determined, the fatigue limit for any fatigue cyclic loadings can be determined quantitatively. This functional relationship is referred to as the state relationship of fatigue strength, which is an intrinsic relationship of material property.