PSI - Issue 2_B

Chretien Gaëlle et al. / Procedia Structural Integrity 2 (2016) 950–957 Gaëlle Chr ti n et al. / Structural Integrity Proced a 00 ) 000–000

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Fig. 1. Material microstructure after etching with Kroll’s reagent.

Fig. 2. A schematic plot of the machining process to create a short crack from a long crack on a CT specimen.

2.2. Creation of artificial 2D short through cracks in CT specimens The obtainment of artificial short crack consists in three steps: pre-cracking, threshold and wake machining. During the pre cracking step, the crack is propagated at an initial applied nominal SIF range of 12 MPa√m at ambient air regardless of the temperature uses for the threshold. This SIF is maintained constant from the crack initiation up to a crack length of about 4 to 5 mm by decreasing the applied loading range during crack advance. The crack length measurements are performed by mean of a travelling binocular microscope on both sides of specimen, the reported crack length being the mean value of both measurements. Then a threshold test is performed in order to obtain a crack with a weak plasticized zone at the crack tip. At 20°C, crack length is measured with the same binocular microscope, but at 400°C, crack length is watched by potential method. This threshold test lies in the use of a load shedding procedure with load steps of about 5%. The shedding procedure is programmed in order to reach the near-threshold domain for a crack length ratio a/W around 0.5. Indeed, this value makes easier crack closure detection by the technique of compliance variation. Last but not least, the machining step consists in removing the plastic wake of the initial long crack in order to obtain a 2D physically short crack. This machining, which is realized by using an electro discharge machining (EDM), removes gradually the plastic wake with a systematic measure of crack closure after each. Figure 2 illustrates this procedure. The EDM wire is chosen with the smallest available diameter (0.25 mm) to minimize the change in the specimen geometry and to reduce at minimum the size of the heat affected zone near the crack tip. Thus the loading history attached to the crack wake of the long crack at threshold is mainly removed by the machining. Before machining, it is necessary to ensure crack is the most symmetric to facilitate this step and the obtaining of a really short crack. It is challenging to create a remaining 2D short crack with a length of about 0.1 mm on both sides by EDM machining, given the possible tilt and deflection of the crack plane and crack front misalignment. 2.3. Testing conditions Tests in air at 20°C and 400°C are carried out on a standard 100 kN hydraulic testing machine INSTRON at 30 Hz under load control and at a load ratio R = 0.1. The objective is to get several successive threshold evaluations at increasing short crack lengths as illustrated on figure 3. As effective threshold stress intensity factor range for long cracks at 20°C and 400°C are respectively 2.49 MPa√m and 1.9 MPa√m, the short crack threshold approach is started respectively at low SIF range : ΔK nom = 4 MPa√m and 2.8 MPa√m respectively. ΔK nom is then gradually decreased until the crack growth rate is below 10 -10 m/cycle. The short crack length grown at ambient temperature is measured with the travelling optical binocular microscope as described above for long cracks. For short cracks grown at 400°C, crack length is approximately known with the potential method and when threshold is reached, that is to say when no significant evolution of the potential is observed after one million cycles, specimen is removed from oven and optical crack length is measured. 2.4. Crack closure measurement The opening SIF K op is identified using the compliance method as initially proposed by Elber (1970), complemented by the differential technique developed by Kikukawa et al. (1982) and the numerical differentiation method suggested by Kim (2006) as illustrated in figure 4. First tension load P is recorded at a low frequency of 0.2 Hz as a function of the crack edge displacement