Fatigue and Crack-Growth Behavior in a Titanium Alloy Under Constant-Amplitude and Spectrum Loadin

A titanium alloy plate material was provided by University of Dayton Research Institute. Fatigue-crack-growth tests on compact specimens have been previously performed at Mississippi State University. These tests used newly-developed compression pre-cracking methods to generate fatigue-crack-growth-rate data in the near-threshold regime. A crack-closure model was then used to determine an effective stress-intensity-factor-range relation over a wide range in rates and load ratios (R = Pmin/Pmax). Some engineering estimates were made for extremely slow rates, below commonly defined threshold rate. Single-edge-notch-bend fatigue specimens were machined from the titanium alloy plates and were tested at two load ratios (R = 0.1 and 0.5) and a modified Cold-Turbistan engine spectrum. Calculated fatigue lives from FASTRAN using small-crack theory with an equivalent-initial-flaw-size of 9 microm in radius at the center of the semi-circular edge notch fit the constant-amplitude test data fairly well, but underpredicted the spectrum loading results by a factor of 2 to 3.