Volume 11, Issue 2 (March 1983)
Accuracy and Precision of Crack Length Measurements Using a Compliance Technique
A compliance crack measurement technique has been implemented in a computer-controlled fatigue crack growth rate test written to conform to the ASTM Test for Constant-Load-Amplitude Fatigue Crack Growth Rates Above 10−8 m/cycle (E 647). The use of a “correction” factor in the compliance expression is discussed and relevant details of the computer implementation are provided. Possible sources of error in the technique have been investigated and the relative precision and absolute accuracy determined using 12.6-mm-thick compact tension specimens of 2024 T351 (Unified Numbering System [UNS] A92024) aluminum having widths of 51 mm. The relative precision of individual measurements using standard fixtures was found to be about 0.05 mm, but this can be reduced to as little as 0.01 mm by averaging over 20 measurements. For most crack lengths, the calculated lengths were within 0.25 mm of the average physical crack lengths measured from profiles on fractured specimens.