一试验的原理和方法升降法是在指定循环基数N_0(如10~7)下测定疲劳极限,或在某一指定寿命下测定疲劳强度的方法。它适用于长寿命区,即疲劳寿命在10~6或10~6循环以上的情况。采用单点试验法测定疲劳极限时,在逐渐降低应力水平的过程中,如某一试件在第i级应力σ_i的作用下未达到循环基数N_0就发生破坏,而依次试验的另一试件在较低的第i+1级应力σ_(i+1)作用下“越出”(达到指定寿命未破坏),其寿命大于循环基数N_0,那么可以设想,对应循环基数N_0的应力(疲劳极限)必发生在σ_i和σ_(i+1)之间。若两个应力差值(σ_i-σ_(i+1))很小,小于σ_(i+1)的5%,则可取σ_i和σ_(i+1)的平均值近似作为疲劳极限: A test principle and method of Ascending and descending method is the determination of the fatigue limit under the specified cycle base number N_0 (such as 10 to 7), or fatigue strength measured at a specified life. It applies to long-life zone, the fatigue life of 10 to 6 or 10 to 6 cycles or more. When using the single point test to determine the fatigue limit, during the process of gradually decreasing the stress level, if one test piece fails to reach the cyclic base number N_0 under the i-th stress σ_i, another test piece In the case of lower “i + 1” stress σ_ (i + 1) “beyond” (to reach the specified life without damage), the life is greater than the cycle base number N_0, then it can be assumed that the corresponding cyclic base number N_0 stress Fatigue limit) must occur between σ_i and σ_ (i + 1). If the two stress differences (σ_i-σ_ (i + 1)) are small and less than 5% of σ_ (i + 1), then the average of σ_i and σ_ (i + 1) can be approximated as the fatigue limit: