Device for carrying out environmental very high cycle fatigue tests with ultrasonic excitation in asymmetric push-pull mode

摘要

The increasing lifetime of many engineering components leads to a growing demand for accelerated testing methods. Fatigue failure of components submitted to cyclic loading at stress levels below the endurance limit occurs even beyond 10~7 cycles which has been the traditional limit for fatigue testing in most laboratories. Test programs covering this range of cycles on servo-hydraulic or resonance machines are very time consuming. Therefore methods for very high cycle fatigue (VHCF) testing at ultrasonic frequencies have been developed and are now used routinely. These methods rely on the formation of a longitudinal standing ultrasonic wave inside a test specimen. The wave exerts an alternating tensile and compressive stress on the specimen. Because of their origin in a standing wave, the tensile and compres sive stresses usually have the same magnitude, i.e. the test is carried out under fully reversed conditions. Several test rigs have been proposed and built to overcome this drawback by coupling an ultrasonic loading device with a classical uniaxial test bench and superposing the ultrasonic stress to a constant or slowly varying stress. We present a different approach for overcoming that limitation where the constant stress is generated by a pressure difference. This approach is especially useful for testing in hazardous environments since all movable parts like pull rods passing through the walls of the test chamber are avoided. We describe the design and the performance of such a VHCF device and present first test results demonstrating the deterioration of the lifetime of Inconel 718 specimens in high pressure gaseous hydrogen compared to argon.