Ultrasonic fully reversed axial tests for exploring the very high cycle fatigue of composite materials

In the present work, the feasibility of axial ultrasonic tests for exploring the fully reversed fatigue response of composite materials even in the Very High Cycle Fatigue (VHCF) regime is proved. VHCF tests are run on hourglass specimens made of twill 2x2 carbon woven fabric impregnated with epoxy resin with stacking sequences [0]8 and [0/90/+45/-45]s and designed through Finite Element (FE) modal analysis. The stress distribution within the specimen and the absence of buckling are first determined through an extensive strain gage campaign, which has validated the FE model. As the temperature is a main concern in ultrasonic tests, the temperature increment within the composite specimen is investigated by means of an embedded fiber optic sensor and controlled during the tests with an infrared sensor. With the proposed experimental setup, fully reversed ultrasonic tests have been carried out up to 109 cycles and the failure of the two investigated specimen types has been analyzed by comparing the failure origin location in relation to the stress distributions.