Abstract—Crack tip closure measurements were performed in a 2124 T3 51 aluminum alloy by electronfractography. The technique makes use of high resolution fractography to correlate a closure in striation spacing with a sudden change of the stress intensity factor range following steady state crack growth. The crack growth tests under programmed loads were performed with a servohydraulic machine interfaced to a digital computer. An average value ofKop/Kmaxequal to 0·15 was found in the range of 10 MPa√m<ΔK<35 MPa√m. The higher closure developing after overloads was found a suitable explanation for crack growth reta