SEDL / STP / STP982-EB / STP27209S

Comparison of Methods for Measuring Fatigue Crack Closure in a Thick Specimen

Ray, SK
Graduate assistant and professor and head, School of Aeronautics and Astronautics, Purdue UniversityBoeing Aircraft Company, West LafayetteRenton, INWA

Grandt, AF
Graduate assistant and professor and head, School of Aeronautics and Astronautics, Purdue UniversityBoeing Aircraft Company, West LafayetteRenton, INWA

Pages: 17    Published: Jan 1988

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Abstract

The objective of this paper is to compare fatigue crack closure loads obtained by various crack opening displacement methods. Crack closure loads are obtained by three different techniques: optical interferometry, backface strain measurements, and crack mouth opening displacement (CMOD) gage measurements. Optical interferometry gives complete through-the-thickness variation of crack closure, while backface strain and CMOD measurements provide an average (bulk) crack opening load for the entire specimen thickness. The present paper focuses on comparing the bulk specimen measurements of crack opening load with point measurements obtained at various points along the crack front from the interferometry method. It was observed that the closure loads obtained from backface strain measurements agree well with the CMOD closure values, although the interferometric crack opening loads are higher than either compliance techniques.