Issue 35

E. Fessler et alii, Frattura ed Integrità Strutturale, 35 (2016) 223-231; DOI: 10.3221/IGF-ESIS.35.26 225 The first part of the test consisted in a fatigue crack growth test under load control conditions. The tests were performed using 10-X-10 trapezoidal load cycles, where 10 is the time in seconds used to load and to unload the sample, and X denotes the hold time (in seconds) at maximum load. Hold times of 300 s and 1200 s have been investigated. All the tests were performed at a constant load ratio of R=0.05. During tests, the crack growth was monitored using a direct current potential drop technique (DCPD) previously calibrated by finite element calculations (see for e.g. [10]). These testing conditions were applied until the crack propagates over 200 µm in order to collect data in the Paris regime. The second part of the test focused on the low ΔK regime and consisted in a K-decreasing procedure performed by shedding the applied force step wisely while applying the investigated 10-X-10 cycle. ASTM E-647 recommends applying this method until a crack growth rate of 10 -10 m/cycle or less is reached to determine the crack growth threshold. Doing so with 10-X-10 cycles would be time consuming and for this reason, tests were performed until crack growth rates between 10 -8 m/cycle and 10 -9 m/cycle were reached. Another fatigue crack growth test can be subsequently performed with hold time under load control conditions to acquire more data. On the other side, several tests were stopped directly after the K-decreasing procedure to obtain information at the crack tip in the low ΔK regime. Scanning electron microscopy was used in order to determine the transgranular or intergranular nature of the fracture surfaces. Crack path observations were also performed. The samples (broken at the end of the test) were axially cut, perpendicularly to the fracture surface, with a diamond tip cutter. The crack path appears along the edge of the cut plane. In order to protect the crack path during polishing and to avoid blunting it, a nickel coating was applied on the fracture surface. This way, the blunting will affect the nickel coating edge and allow keeping the crack path undamaged. The crack path will then be observed using electron backscatter diffraction and back-scattered electron (BSE) imaging. R ESULTS Fracture surfaces observations canning electron microscopy was used to observe the fracture surfaces of samples tested under hold time conditions in the Paris regime first. For all the investigated temperatures (from 550 °C up to 650 °C) and hold time durations (300 s and 1200 s), the fracture surfaces exhibited an intergranular fracture (see Fig. 1-b). For the same temperature, under 2 Hz pure fatigue loading, transgranular fracture was observed (see Fig. 1-a). A large amount of δ phase particles were observed on both transgranular and intergranular fracture surfaces (indicated by white arrows on Fig. 1-a and 1-b). Figure 1 : a) Transgranular fracture under sinusoidal 2Hz load cycle, b) Intergranular fracture under hold time conditions. The black arrows indicate the crack propagation direction. The fracture surfaces in the low ΔK regime were also investigated. A progressive transition from a fully intergranular fracture (Fig. 2-b) to transgranular fracture (Fig. 2-c) was observed as K decreases. No fatigue striations were observed on the transgranular domain. Such a transition has also been observed by Li [9] in alloy 720Li, using a load increasing procedure. This procedure consists in applying the 10-X-10 cycle at a load level corresponding to a K slightly higher than the fatigue crack growth threshold ΔK th . The maximum load level is then progressively increased until a significant crack growth is detected using the DCPD method. This method was also used on some samples. The load level was increased by 10 % every 24 h if no significant crack growth was detected as monitored with the DCPD method. Using this testing S