A Theoretical And Experimental Investigation Of The Influence Of Crack Tip Plasticity On Fatigue Crack Closure PDF Download

A theoretical and experimental basis regarding the fatigue crack closure model, proposed by measuring the externally applied loads versus crack opening displacement, was established. The obstructions of crack closure were simulated using an artificial asperity. The crack closure model was defined with three distinct stages of the unloading curve: (i) the asperity does not contact the upper crack face, (ii) a process where both the asperity and specimen deform elastically, and (iii) the plastic deformation in the specimen around the asperity occurs. An equation was proposed to estimate the crack opening displacement values, where the effects of deformations in the asperity and specimen were considered. Fundamentals of crack surface contact and crack closure behaviors were examined via two special experiments: the collection of the fracture debris and the direct observation of the contact zones on the fracture surface through an ink dyeing method. To obtain different severity of crack surface contact, the fatigue tests were conducted under different loading conditions. The results of this study showed that the stress amplitude and R-ratio were the major factors that determine the contacting status between the mating fracture surfaces, where the higher stress amplitude and the lower R-ratio make stronger crack surface contact. The contact severity was also attributed to the location of fracture surface, e.g., the severity of fracture surface contact in the area near crack tip was relatively stronger than that in the other area. A crack closure characteristic during fatigue was examined using an artificial asperity. With asperity wedging, the fatigue crack growth rate was obviously reduced. The crack growth rate was then correlated with the effective crack driving force, defined as DeltaPeff = Pmax--Pmin, real, which itself was also correlated to the plastic deformation level in the asperity and specimen. Moreover, the DeltaPeff value was found to change with increasing the crack length. In a short crack range, the value of Delta Peff decreased due to the artificial asperity- and plasticity-induced crack closing behaviors. As the crack length increased, the Delta Peff value increased as well until just prior to the fracture because of the reduction in the closing severity.