Fatigue Failure Criterion: Comparing Maximum Phase Angle with Minimum Curvature of Stiffness Curve

This study compares two fatigue failure criteria using uniaxial constant crosshead cyclic fatigue testing results of 14 specimens. To guarantee that the failure occurred in the middle of the specimen, where the strain measurements were carried out, the tests were performed with an asphalt mixture performance tester (AMPT) on thin, right rectangular prismatic specimens trimmed into a dog-bone shape. Furthermore, the tests were conducted at 40°C to address the potential of fatigue cracking in a high temperature environment such as that of Hawaii. The first criterion used was the number of repetitions where the phase angle of the mix becomes a maximum. The second criterion consists of estimating the number of repetitions where the stiffness vs. loading cycles curve reaches a minimum (most negative) curvature. Both criteria provide estimates of the number of cycles where a rapid loss in stiffness due to macrocracking starts. For 3 of the 14 specimens evaluated in this study, the phase angle vs. number of cycles curve was very flat. The flatness of the curve combined with some noise in the data resulted in a substantial underestimation of the location of the failure point using the maximum phase angle criterion. For the other 11 specimens, the values determined with both criteria were extremely consistent. Considering only these specimens, the R² of the linear relationship between the two criteria was 0.997. Hence, the minimum curvature of the stiffness curve appears to provide very consistent results with the accepted existing phase angle criterion when that criterion resulted in reasonable results and provided more realistic failure estimates when the phase angle criterion failed. Hence, the minimum curvature criterion is recommended as a potential alternative for robust estimation of the fatigue failure point.