Biaxial compression fatigue and discontinuity of concrete

A biaxial compression fatigue machine was built to test plain concrete plates of 6 x 6 x 1.5 in. Brush-type loading platens were used to eliminate the edge friction. The test program covered four principal biaxial-compression stress ratios and a range of load cycles from 1 to 2,000,000. Within this range of cycles, the S-N (fatigue stress versus number of cycles) curve was found not to be a straight line as commonly assumed. This S-N curve can be closely represented by a modified expression developed by Richard and Abbott. The observed S-N curves of concrete can each be idealized by two straight lines that have significantly different slopes. The slope for low-cycle fatigue is compared with that for high-cycle fatigue. The S-N curves obtained in the literature are compared with the experimental results.