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Back-Face Strain for Monitoring Stable Crack Extension in Precracked Flexure Specimens

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Calibrations relating back-face strain (BFS) to crack length in precracked flexure specimens were developed for different strain gage sizes. The functions were verified via experimental compliance measurements of notched and precracked ceramic beams. Good agreement between the functions and experiments occurred, and fracture toughness was calculated using several operational methods: maximum test load and optically measured precrack length; load at 2% crack extension and optical precrack length; and maximum load and BFS crack length. All the methods gave comparable results. The initiation toughness, KIi, was also estimated from the initial compliance and load. The results demonstrate that the stability of precracked ceramic specimens tested in four-point flexure is a common occurrence, and that methods such as remotely monitored load-point displacement are only adequate for detecting stable extension of relatively deep cracks.

Document Type: Research Article


Affiliations: NASA Glenn Research Center, Cleveland, Ohio

Publication date: September 1, 2010

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