Monitoring of glass-ceramic composites under static and dynamic loading by combined NDE methods

Abstract

Current work deals with the nondestructive evaluation (NDE) of the fracture behavior of ceramic matrix composite (CMCs) materials using combined infrared (IR) thermographic and acoustic emission (AE) characterization. IR thermography as a non-destructive, real-time and non-contact technique, allows the detection of heat waves generated by the thermo-mechanical coupling and the intrinsic energy dissipated during mechanical cyclic loading of the sample. Two different thermographic methodologies, based on the measurement of the surface temperature and on the intrinsically dissipated energy respectively, were applied in order to monitor the crack initiation and propagation and to rapidly assess the fatigue limit of cross-ply SiC/BMAS composites. Simultaneously, AE monitoring was employed to record a wide spectrum of cracking events ranging from matrix cracking to fiber fracture and pull-out. AE event rate, as well as qualitative indices like the rise time and peak frequency reveal crucial information allowing the characterization of the severity of fracture in relation to the applied load. Additionally, rapid assessment of the fatigue limit of CMCs composites was also attempted by AE. Testing a specimen at different load levels for predetermined blocks of cycles shows that the AE acquisition rate remains low for loads below the fatigue limit, while it increases abruptly for higher levels. The thermographic assesment of fatigue limit is in total agreement with the AE results enabling the reliable evaluation of the fatigue limit of the material by testing just one specimen. The application of combined NDE techniques proved very valuable for benchmarking purposes while the sensitivities of the methods act complementarily to each other providing a very detailed assessment of the damage status of the material in real time.