Effect of load excursions and specimen thickness on crack closure measurements
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-amplitude 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 constant-amplitude crack opening stresses.
- Research Organization:
- Southwest Research Inst., San Antonio, TX (US)
- OSTI ID:
- 20005707
- Resource Relation:
- Conference: Second Symposium on Advances in Fatigue Crack Closure Measurement and Analysis, San Diego, CA (US), 11/12/1997--11/13/1997; Other Information: PBD: 1999; Related Information: In: Advances in fatigue crack closure measurement and analysis: Second volume. ASTM special technical publication 1343, by McClung, R.C.; Newman, J.C. Jr. [eds.], 490 pages.
- Country of Publication:
- United States
- Language:
- English
Similar Records
Crack growth under variable amplitude and spectrum loading in 2024-T3 aluminum alloys
An experimental investigation of fatigue crack growth of stainless steel 304L