Validation of the step test method for generating Haigh diagrams for Ti-6Al-4V

Step testing provides a method for generating an endurance stress for a particular stress ratio with a single specimen as well as a method of generating endurance limits for one-of-a-kind specimens, e.g, specimens that may have been subjected to pre-test damage that cannot be accurately reproduced. The objective of this paper was to determine the statistical validity of the step test method for specimens excised from Ti-6A1-4V forgings. Room temperature endurance limits and constant-life Haigh (modified-Goodman) diagrams for smooth Ti-6Al-4V specimens generated by both the step method and conventional method (using S-N curves) are analyzed and statistically compared. Using lognormal-based statistical methods, the endurance stress at prescribed reliability limits and a confidence level (50%) is determined for the 10(7) cycle life. It appears that step testing yields results that are within the statistical limits of conventional S-N curve results and therefore is a valid method for generating endurance limits and therefore Haigh diagrams for Ti-6A1-4V specimens. Some questions are raised regarding the validity of the step method when a large number of steps are used. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.