Abstract
It is well known that the elastic-plastic fracture toughness for a cyclic load is smaller than that for a monotonic load. This study was aimed at investigating the root cause for the reduction in the fracture toughness of carbon steel due to application of a cyclic load. First, fatigue crack growth tests were conducted using plate specimens for a fully-reversed load of various stress amplitudes. The crack growth equation, in which the effective strain intensity factor range was quoted as the crack driving force, was derived to predict the crack growth up to 0.5 mm/cycle. Then, the same material was used to conduct the fracture toughness tests for monotonic and cyclic loading conditions. Plate specimens were used to enable the nominal strain to be measured during the tests. It was shown that the J-R curve and J0.2 fracture toughness were reduced due to application of the cyclic load. To investigate why the fracture toughness was reduced, it was assumed that the ductile crack extension was enhanced by the cyclic load. It was concluded that the reduction in the fracture toughness due to application of the cyclic load could be attributed to the fatigue crack growth. The fatigue crack growth was brought about not only by the unloading cycles but also by monotonic loading. The growth of the ductile crack extension was well predicted by considering the fatigue crack growth and the apparent crack growth caused by crack tip blunting.
Published Version
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