According to our experimental investigations of varying load amplitude , it was found that the phenomena of fatigue crack growth retardation occurred for pmma material following a high overload cycle exerted among the regular constant amplitude load cycles , the ratio of peak value of overload to that of the regular cycles should be over at least 1 . 35 to make the retardation taking effect . the experiment scheme for measuring the shutting - off ratio of overload ( defined as the overload ratio when da / dn < 10 - 7 / cyc . ) 本文試驗分析了變幅載荷對裂紋擴展的影響,證實了有機玻璃也存在超載遲滯現(xiàn)象,通過金相觀察分析,提出超載遲滯的過程實際上是裂紋前緣萌生新的裂紋的過程的觀點,同時提出超載遲滯的合理的解析,并且給出超載遲滯比,為隨機載荷下疲勞擴展模型的建立提供了一個重要的參數(shù)。