A demonstration of problems associated with crack closure measurement techniques

✍ Scribed by Randy Bowman; Stephen D. Antolovich; R.C. Brown

• Publisher: Elsevier Science
• Year: 1988
• Tongue: English
• Weight: 764 KB
• Volume: 31
• Category: Article
• ISSN: 0013-7944
• DOI: 10.1016/0013-7944(88)90111-7

✦ Synopsis

Closure load measurements were performed on an experimental Ni base superalloy compact tension (CT) specimen which exhibited a significant degree of surface roughness. To calibrate the test procedure, a back face strain gage, front face clip gage and a laser extensometer were used to measure the load-displacement response for an untracked specimen. Closure measurements were then performed as a function of AK during a fatigue crack propagation test. It was found that a single back face strain gage could not be used to obtain accurate closure measurements due in part to the apparent strains generated as a result of temperature fluctuations. The clip gage was somewhat more accurate but was subject to rotation and slippage on the knife edges which affected the output. The laser extensometer gave the most reliable results and required no additional specimen preparation. Machine alignment was critical and measured closure loads were affected by seemingly unimportant alterations in the test set-up. Limitations and sources of potential errors associated with these measurement techniques are discussed.