Fatigue Crack Growth Databases and Analyses for Threshold Behavior in Rotorcraft Materials

摘要

Accurate representation of fatigue crack thresholds is extremely important for many rotorcraft applications. If the measured threshold is unconservatively high, then a structural component designed with these data may fail long before fatigue analyses predict. Currently, in North America, the threshold crack growth regime is experimentally defined by using ASTM Standard E 647, which has been shown to exhibit anomalies due to the load-reduction test procedure. The load-reduction test procedure has been shown to induce remote crack surface closure, which prematurely slows crack growth and produces an abnormally high threshold for some materials. The scope of this research project was to (1) further develop the compression precracking (CP) threshold test methods, (2) determine fatigue crack growth rates in the near-threshold regime and over a wide range in rates (from threshold to near fracture) for three materials (7075-T651, Ti-6Al-4V a-STOA, and 4340 steel), and (3) analyze fatigue crack growth data on three rotorcraft materials (7050-T7451, Ti-6Al-4V a-STOA, and Mg AZ91E) tested at the National Aeronautics and Space Administration (NASA) Langley Research Center (LaRC). In addition, a few tests on the 7050 alloy were conducted at Mississippi State University (MSU) using the new threshold test methods. At MSU, three types of tests were conducted on each material: (1) ASTM E 647 load-reduction procedure, (2) CP constant-amplitude test method, and (3) CP load-reduction test method. An additional test method, CP constant-AK, was also used on two materials. All tests were conducted under a laboratory air environment at room temperature. Alternative methods to determine thresholds and near-threshold fatigue crack growth rate behavior for metallic materials were developed. The titanium (a-STOA) alloy showed large differences between the ASTM LR and CP tests but the 7075 and 4340 steel showed small differences.