Procedures for Precision Measurement of Fatigue Crack Growth Rate Using Crack-Opening Displacement Techniques

Abstract

This paper describes experimental and analytical procedures whereby the conventional commercial fracture mechanics clip-gage can be used for precision measurement of fatigue crack growth rate in compact-type specimens. Potential sources of error in measuring crack length via crack-opening displacement (COD) techniques are delineated. Comparisons are made between crack-length data obtained via specimen surface observations and COD techniques. Comparisons ate also made between data analyzed by the secant and 7-point incremental polynomial methods. It is emphasized that COD techniques can enhance the accuracy of the secant method of data reduction. Step-loading procedures using COD techniques are described. Proposed amendments to ASTM Tentative Test Method for Constant-Load-Amplitude Fatigue Crack Growth Rates Above 10~* m/Cycle (E 647-78 T) regarding incrementing of crack-length measurements via COD techniques are discussed.