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Torsion fatigue tests have been conducted at 20kHz ultrasonic fatigue testing system, andcompared to the torsion fatigue data generated on 35Hz conventional fatigue test machine todetermine if there are any frequency effects. Failure modes and fracture surfaces in long liferegime were examined and discussed.. The difference in fatigue behavior and fracturemechanism under pure ultrasonic torsion loading for three kinds of alloys including castaluminum 2-AS5U3G-Y35, D38MSV5S steel and 100C6 steel are discussed. Results indicatedthat the S-N curves exhibit decrease in fatigue strength beyond 107 cycles.