A Comparison of Fatigue Crack Growth Rates as Determined by Striation Measurements and by Observations of Crack Length on the Specimen Surface during the Test.

摘要

Fatigue striation spacings have been measured on DTD 5050 test pieces that had been tested at stress ratios of 0.05 and 0.65. Crack growth rate curves were obtained from these measurements and compared with those produced by the direct measurement of crack length as seen on the specimen surface. The aim was to establish what errors in the general growth rate might be expected from the fractographic method. The use of cellulose-acetate-carbon two stage replication gave consistently low growth rates whereas the scanning electron microscope gave good agreement with the direct method up to rates of 0.6 micrometers/cycle. Above this rate striation measurements underestimate because of the contribution of local tensile tearing.