PSI - Issue 33

Jesús Toribio et al. / Procedia Structural Integrity 33 (2021) 1131–1138 Jesús Toribio / Porcedia Structural Integrity 00 (2021) 000–000

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MVC

MVC

MVC

MVC

MVC

L

MVC

C

C

C

MVC L Fig. 3. Micro-fracture maps (MFMs) at the fracture for the different notched specimens A, B, C and D exhibiting very distinct triaxiality levels; MVC: micro-void coalescence ; C: cleavage-like topography; L: shear-lip . 5. Process zone fracture criterion The next step to develop the fracture criterion consists of using the process zone concept, so that instead of employing a constant critical distance (a microstructural characteristic unit of the material), the critical domain or fracture region to formulate the fracture criterion is the microstructurally damaged area, i.e., the zone where the fracture process initiates (process zone) which in the case of the notched samples of pearlitic steel under consideration is the MVC region in the MFMs. This statement is physically sound since it combines both the macro- (continuum mechanics and computational methods) and micro- (scanning electron microscopy and fractographic analysis) approaches. As a matter of fact, although the final failure process develops by cleavage in a typically brittle manner, the fracture process initiates by MVC in the vicinity of the notch tip. To apply the criterion in the form described in the previous paragraph, the average value of the distortional part of the strain energy density (or, accordingly, the effective or equivalent stress in the von Mises sense) has to be computed over the process zone, i.e., over the MVC region. To this end, the stress-strain state at the fracture instant is used as calculated by elastic-plastic finite element analysis. C