During technical operation, high performance materials are very often exposed to cyclic loading conditions leading to damage accumulation until the final fatal fracture. Concurrently, quite high requirements with regard to high number of cycles without any damage are claimed for the majority of parts in components.
In the past, the fatigue behavior of Al-matrix composites (Al-MMCs) reinforced by alumina particles or short fibers was intensively studied in the LCF and HCF range. Nowadays, also the long life behavior of this material class up to the very high cycle fatigue regime (VHCF) is of great interest.
The present investigations are focused on Al-MMCs with 15 Vol.% of alumina particles in comparison to 20 Vol.% of alumina short fibers tested by ultrasonic fatigue under symmetric loading conditions (R=-1) and stress amplitudes ? 140 MPa up to 109 cycles. Furthermore, the aging conditions of the aluminum matrix (peak aged, overaged) is relevant for the fatigue behavior. Different specimen geometries and investigation methods were used for a better characterization of the underlying fatigue mechanisms. The fatigue tests were complemented by in situ full surface view thermography and digital image correlation, respectively. Moreover, the resonance frequency as well as the damage parameter were evaluated in order to determine the beginning of damage in the Al-MMCs. It is possible to determine critical changes in temperature, resonance frequency and damage parameter leading to the final fatale fracture. Furthermore, local heating as well as strain localizations developing in dependence on the number of cycles in the specimens allow conclusions on the damage behavior. The results of fatigue tests were correlated with subsequent microstructural observations, micro hardness measurements and X-ray diffraction analysis.
The combination of all methods contributes to a better understanding of damage mechanisms and damage accumulation in metal matrix composites in dependence on the different reinforcements, stress amplitudes and aging conditions in comparison to their unreinforced counterparts.
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