PSI - Issue 7

Benoît Bracquart et al. / Procedia Structural Integrity 7 (2017) 242–247 B. Bracquart et al. / Structural Integrity Procedia 00 (2017) 000–000

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2.3. Experimental set-up

Fully reversed stress-controlled push-pull fatigue tests ( i.e. R σ = − 1) have been carried out on an Instron 8802 servohydraulic testing machine, using a specific anti-buckling fixture to prevent buckling of the specimen. The fatigue tests have been conducted with a frequency of 12 Hz and stopped after 10 6 loading cycles if failure was not observed. During cyclic tests, two di ff erent experimental configurations have been used to investigate the influence of grain size and / or defect size. The first configuration uses specimens for which a single hemispherical defect has been introduced at the center of the gauge area. Using an optical camera to record a picture of the defect surroundings every 1000 loading cycles, the length of cracks initiating from the defect can be determined. The detection of cracks is limited to surface ones with a minimum length of about 100 µ m. This configuration has been used only for specimens containing a single large defect. The second configuration uses specimens with multiple defects, and allows determining, for a given stress amplitude, the number of cracked defects from a posteriori SEM observations.

3. Results

3.1. Specimens with a single defect

Stress-controlled fatigue tests have been carried out on specimens with a single defect to evaluate the impact of grain size on fatigue crack initiation. For these tests, the defect size is held constant ( d l = 1000 µ m). As illustrated by Figure 1, the observation of the defect surrounding area with the optical camera allows monitoring early crack propagation from the defect, on the specimen surface. Based on fatigue test results, the evolution of the stress amplitude as a function of the number of cycles to crack initiation N i is displayed in a S-N diagram, Figure 2. In the present work, the crack initiation criterion is arbitrarily set to a total surface crack length of 100 µ m, which is the shortest detectable crack length. One should note that, as a result, especially for the small grain microstructure, the number of cycles to initiation may include a portion of early crack propagation.

3.2. Specimens with multiple defects

To investigate the influence of defect size on fatigue crack initiation, some specimens with multiple defects have been submitted to stress-controlled fatigue tests. In this case, both the grain size φ and the defect size d have been

Fig. 1: Example of crack tracking from the defect during fatigue test. Number of cycles, from left to right, and top to bottom: 0; 26,000; 69,000; 84,000; 102,000; 289,000. Loading axis is vertical.