PSI - Issue 42

Jean-Baptiste Delattre et al. / Procedia Structural Integrity 42 (2022) 886–894 Jean-Baptiste Delattre / Structural Integrity Procedia 00 (2019) 000–000

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cooling rate, the general yield load, Pgy and the maximum load, Pm,determined from the load vs. hammer displace ment curves, were plotted as a function of testing temperature. The value of the load, P*, for which both curves intersected was used to evaluate the fracture stress in the absence of significant ductile crack advance. This point should match the case where fracture occurred right after the onset of plastic yield (as requested in the physical cleav age fracture mechanism). Following Chaouadi and Fabry (Chaouadi and Fabry), the fracture stress, σ * was estimated by: σ ∗ = K sp . K b . P ∗ with K sp = 2 . 35 and K b = 45 . 8 MPa . kN − 1 (3) These estimates of the fracture stress appeared to significantly depend on the quenching rate and the tempering conditions (figure 7). The fracture stress increased with the quenching rate and there was a clear di ff erence between proeutectoid ferrite bearing microstructures (150°C / h) and fully bainitic ones (10000°C / h). The stress values were in good agreement with values found in other studies (between 1250 - 2200 MPa approximately) (Bowen et al.; Chen et al.).

Fig. 6. Experimental load vs. hammer displacement curves for tests with an energy between 40J and 60J. The curves have been o ff set by the displacement at Pgy to make them overlap (along the horizontal axis) at the onset of plastic yield. Each color corresponds to a di ff erent tempering condition. The dotted lines correspond to the 150°C / h cooling rate and the solid lines to the 10000°C / h cooling rate. The testing temperatures are also indicated.

Fig. 7. Critical stress estimates for each heat treatment condition.