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–6Al–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–6Al–4V specimens. Some questions are raised regarding the validity of the step method when a large number of steps are used.