铝在疲劳载荷作用下所发生的基本过程

【摘　要】进行了99.6％工业纯铝和99.99％高纯铝(退火和冷加工)的扭转疲劳试验,测定了经过各种应力循环数N以后的滞后迴线的形状和面积,从而算出了在每次循环中的能量消耗ΔE和最大抗扭矩T_m。对于所得到的ΔE-N曲线和T_m-N曲线进行了分析,并与在疲劳试验的各个阶段对于试样表面所作的金相观测结果作了比较,认为在试样中未出现局部滑移区以前,引起ΔE和T_m的基本过程是空位对于位错的钉紥作用,这是一种体积效应,但是在局部滑移区出现以后,这种滑移区引起额外的能量消耗,而对于T_m的影响并不显著,因而在这个阶段里T_m和ΔE的变化便不再彼此相对应。这种分析可以澄清过去文献中关于ΔE和T_m(代表硬度变化)在疲劳过程中的变化的许多互不一致的结果。假定实验所观测到的曲线是这两种过程(整体过程和局部过程)所引起的效应的迭加,并且假定这两种过程对于ΔE和T_m的贡献由于疲劳振幅的大小、试样的处理状态(退火或冷加工)以及所合杂质之不同而异,可以满意地解释实验所观测到的各种曲线的形状和位置。这个观点的正确与否还有待于进一步的实验证实。

BASIC PROCESSES TAKING PLACE IN ALUMINUM DURING FATIGUE LOADING

Torsional fatigue experiments were carried out on 99.6% commercial aluminum and 99.99% high-purity aluminum (annealed and cold-worked). The shape and the area of the hysteresis loop (stress versus strain) after various stress cycles N were determined, from which the energy loss△E in each cycle and the maximum torque T_m of the specimen were calculated. The △E-N curves and T_m-N curves were analyzed and different stages of the curves were correlated with the results of metallographic observations on the specimen surface during the process of fatigue. It is then concluded that, in the initial stage of fatigue loading, the basic process that gives rise to the change of △E and T_m is the pinning of dislocations by the vacancies generated during fatigue and this occurs as a bulk effect. Later on, the localized coarse slip regions appearing in the specimen give rise to an extra energy loss △E, but have only an insignificant effect on T_m. Consequently, the changes of T_m and △E in this stage no longer correspond to each other. Such an analysis may clarify many of the dis-crepencies reported in the literature concerning the results of observation on the change of △E and T_m (representing a change in hardness) during fatigue.Assuming the curves experimentally observed to be a summation of the effects given rise by these two processes (a bulk process and a localized process), and taking into account that the contributions from these two processes on △E and T_m vary with the fatigue amplitude, the condition of specimen treatment (annealed or cold-worked), and the impurity content of the specimen, the shape and the position of the numerous experimental curves of △E-N and T_m-N can be interpreted satisfactorily. However, the validity of this viewpoint needs further experimental verification.