Measurement of Fatigue Crack Growth in Compact Tension Specimen Using an Optical Extensometer

ABSTRACT In this investigation, fatigue crack growth (FCG) tests are performed on transformation induced plasticity steels to characterize the relevance of stress intensity and crack growth rates with standard compact tension (CT) specimens. An optical extensometer that can achieve real-time displacement measurement is developed based on digital image correlation. Instead of a displacement gauge, the optical extensometer is adopted in FCG tests to measure CT specimen crack opening displacement (COD). Compared with stress intensity, it is difficult to calculate crack growth rates with COD. Therefore, the compliance equation and incremental polynomial method are programmed with C++ languages to obtain the steady crack growth curve directly. The proposed method is verified with experimental results obtained by the displacement extensometer. In addition, the precision of the displacement measurement is assessed, and the error induced by displacement output rates is discussed.