13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- Fig. 1. Microstructures of the materials used for the fatigue tests. (a). Alloy 690, (b). Ti6Al4V, (c). Martensitic ferritic steel (MFS), (d). Martensitic austenitic steel (MAS). 2.2 Experimental Two types of stress controlled fatigue tests have been carried out. One was performed using an Amsler machine with a frequency of about 140 Hz up to 5x108 cycles. Another test was performed using an ultrasonic (Piezo) fatigue testing machine with a frequency of 20 kHz up to 5x109 cycles. A round sample with a diameter up to 6mm was used. Pulsating tensile stresses with a stress ratio R=0.1 were applied for both tests. The origins of fatigue crack initiation and fatigue pre-initiation damage (dislocation slip bands) were investigated using a JEOL 840 scanning electron microscope (SEM). In order to investigate the fatigue crack initiation mechanism and material damage process after the fatigue, the microstructures or damage in the samples after the VHCF testing were investigated using two scanning electron microscopy techniques: electron back scatter diffraction (EBSD) and electron channeling contrast image (ECCI). The EBSD technique was used to analyze the strain or stress localization or the influence of VHCF on the strain localization. Orientation maps were performed in a 6500 F JEOL field emission gun-scanning electron microscope (FEG-SEM) equipped with a TSL OIM EBSD system. EBSD maps were measured at 15 kV acceleration voltage and a working distance of 15mm. The ECCI technique has been recently proven as a powerful technique to image deformation damage and even dislocation structures steels by using a SEM [23]. ECCI observations were carried out in a Zeiss Crossbeam instrument (XB 1540, Carl Zeiss SMT AG, Germany) consisting of a Geminitype field emission gun (FEG) electron column and an focused ion beam (FIB) device (Orsay Physics). ECCI was performed at 10 kV acceleration voltage and a working distance of 5 mm, using a solid state 4-quadrant BSD detector. The microscope was run in the “high current” mode and an objective lens aperture of 120_m was used. 3. Results and discussion 3.1Fatigue life and crack initiation behavior (d) (c)
RkJQdWJsaXNoZXIy MjM0NDE=