PSI - Issue 2_B
Chretien Gaëlle et al. / Procedia Structural Integrity 2 (2016) 950–957 Gaëlle Chretien et al. / Structural Integrity Procedia 00 (2016) 000–000
954
5
3. Experimental results 3.1. Threshold of long cracks
The variations of da/dN for long cracks grown following the procedure described above for the threshold test at 20°C and 400°C in air are plotted in figures 6 a and b on a bi-logarithmic diagram. Crack propagation curves of long crack realized in same conditions of temperature and load ratio R are well repeatable, that is of a fundamental importance for the study of short cracks. Increasing temperature leads to lower thresholds but the effect of temperature is specially marked at growth rate of 10 -8 m/cycle due to the presence of a plateau, which has been related to an environmental effect more pronounced with temperature (Figure 7). The crack closure correction highlights the contribution of closure more important at threshold for each temperature. The effective propagation curves at 20°C and 400°C at near-threshold are closed (Figure 7). Temperature effect in the low growth rate range seems to be limited to the domain of the plateau, where an intrinsic effect of temperature occurs. The values of the nominal and effective SIF ranges under which crack growth rate is lower than 10 -10 m/cycle are considered representative of the threshold ranges and are recorded in table 1. The nominal and effective thresholds decrease with temperature. a b
Fig. 6. Fatigue crack propagation da/dN vs ΔK nom and ΔK eff at : (a) 20°C ; (b) 400°C.
Fig. 7. Comparison of effective crack propagation curves at 20°C and 400°C.
ο ܭ ௧ 2.49
ο ܭ ௧ 5.22
Table 1. Effective and nominal thresholds for long cracks Temperature (°C)
20
400
3.73
1.9
3.2. K op of short cracks As shown in figure 3, short crack growth rates are faster than long crack growth rates due to the less crack closure level. The crack opening ratio U = ΔK eff /ΔK of the successive thresholds are measured for crack lengths ranging from 0.21 mm to 6.26 mm and enables to determine the evolution of the opening SIF K op . The evolution of K op with crack length at 20°C and 400°C are reported on figures 8 a and b respectively and are compared to K op measured during machining of the plastic wake. The black symbols correspond to K op measured during machining and the open symbols correspond to K op measured during the short crack propagation. In both cases, the relation between K op and Δa is similar, with a substantial increase in K op with increasing Δa up to reach the K op for long crack. A critical length Δa cr considered to define the boundary between the short crack domain and the long crack domain appears to ranging near 3mm at 20°C, and shorter than 2mm at 400°C. The general trend of the observed evolution of K op with Δa is consistent with previous result provided by the literature for physically short cracks grown in conditions where LEFM concepts can be used (Petit (1999)).
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