PSI - Issue 17

Trevor Sabiston et al. / Procedia Structural Integrity 17 (2019) 666–673 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 1 fatigue specimen geometry (modified Waterloo geometry) with 6.344 mm gauge center width

Trial fatigue testing was conducted using the four geometries to evaluate the spread in the data and other issues that would present themselves under fatigue loading such as buckling. The trial fatigue testing was conducted at room temperature under fully reversed loading. The fatigue tests were conducted using load control on MTS servo hydraulic frames the data acquired using load cells and extensometers were recorded for each test. Further fatigue testing was conducted on the chosen specimen geometry under fully reversed loading at room temperature and 130 ºC at 2 Hz frequency until failure or 1 million cycles. Environmental chambers were used around the specimens at the 130 ºC test temperature. During the fatigue tests, infrared imaging was also used to ensure that the temperature rise during the testing was less than 10 ºC. The fatigue results at room temperature and 130 ºC are fit to a Basquin equation of the form = (2 ) (1) where is the safe stress amplitude for a given number of cycles, and are fitting parameters, and is the number of cycles. Analysis of the failure modes of the specimens post fatigue test were conducted at a macroscopic level to determine the primary macroscopic failure modes. Scanning Electron Microscopy (SEM) of the fracture surfaces was used to determine the micro level failure mechanisms of the NCF material subjected to fatigue loading.

3. Results and Discussion

3.1. Strain Fields and Uniaxial Test Results

The strain fields developed in the four specimen geometries during a uniaxial tensile test is shown in Fig. 2. From the 3D DIC analysis the gauge section of all four of the geometries considered undergo uniform strain fields along their in plane surfaces. The DIC was conducted using an equivalent 0.24 mm gauge length resolution for strain measurement. For the modified Waterloo geometry specimens the strain distribution along the through thickness direction is not uniform as it is on the in plane surface due to the curvature.

Fig. 2. Axial strain field in the loading direction developed under 275 MPa tensile load for a) 10 mm strip, b) 6.344 mm Waterloo geometry, c) 10 mm Waterloo geometry, d) 15 mm Waterloo geometry