Abstract: A computer controlled procedure is developed for automatic measurements of the crack opening stress Sop during fatigue tests A crack opening displacement gauge(CODmeter)is used to obtain digital data on load COD curves. Three methods for deriving Sop from the da- ta sets are compared:(1) slope method,(2)tangent lines intersection method,and(3)tangent point method. The effect of the position of the COD meter with respect to the crack tip is studied in tests on 2024T3 specimens. Results on crack growth and Sop are presented for CA loading with an overload and with an overload followed bv an underload.

Key words: fatigue, crack closure, overload