SEDL / STP / STP1343-EB / STP15765S

Measurement of Fatigue Crack Closure for Negative Stress Ratio

Romeiro, FFJ
Assistant professor, Polytechnic Institute of Leiria, Lisbon,

Domingos, CA
Assistant professor, Polytechnic Institute of Setubal, Lisbon,

de Freitas, MJM
Associate Professor, Instituto Superior Tecnico, Technical University of Lisbon, Lisbon,

Pages: 16    Published: Jan 1999

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Abstract

The concept of fatigue crack closure, introduced in the 60's by Elber, has been used to explain a wide range of positive stress ratio crack propagation results (R ⩾ 0). Less attention has been given to fatigue loading for negative stress ratios (R < 0). In this work the results of crack propagation tests of middle-crack tension M(T) specimens of a normalized medium carbon steel DIN Ck 45 for a wide range of stress ratios from 0.7 ⩾ R ⩾ -3 are presented. Crack closure loads were measured with the compliance technique at test frequency, using a data acquisition system. Negative crack closure loads were found for negative stress ratios R ⩽ -1, which can explain higher crack propagation rates and accelerations in crack growth during variable-amplitude tests where compressive loads of different stress ratios are present.