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The effects of simple load excursions on fatigue crack growth and crack closure measurements in aluminum alloy 2024-T351 are presented. The crack closure loads were measured local to the crack tip in 6-mm and 14-mm-thick specimens, using an Elbert-type gage for measuring crack-tip opening displacement (CTOD). All specimens were manufactured from a single lot of 15.7-mm-thick plate. Preliminary tests to establish the level of crack closure for a given level of remote constant-am- plitude loading were also undertaken, with consistently different levels of crack opening and closing loads observed. Crack opening and closing stresses obtained from the Elber-type (CTOD) gage, when compared with those from a (global) clip gage, showed identical results for opening stresses, but crack closing stresses were approximately 15% to 20% higher in the CTOD measurements. The results apparently contradict many analytical closure models, which have crack closing stresses lower than crack opening stresses.
Simple overload, underload and over/underload cycles were performed. Differences in the number of post-overload retardation cycles for the two thicknesses, in the single overload test, were obtained. It was found that the 6-mm-thick specimen's fatigue life was two times larger than for the 14-mm-thick specimen. The measured opening stresses were found to be in general agreement with the trends obtained for the fatigue crack growth results. Greater fatigue lives were obtained in the overload tests, with subsequent higher crack opening stresses measured. After a certain number of cycles following an overload, constant-amplitude crack growth rates were restored. However, the crack opening stresses did not return to the preoverload constant-amplitude values, but increased relative to the preoverload constantamplitude crack opening stresses.
crack closure, fatigue, crack growth, aluminum alloy, overloads, thickness
Engineer, Mechanical and Materials Engineering, Southwest Research Institute, San Antonio, TX
Professor, University of Bristol, Bristol,