PSI - Issue 33

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

1134

4

The average size of the cleavage facets in the steel (75  m), represents the basic fracture unit (Park and Bernstein, 1979; Lewandowski and Thompson, 1986) when it is typically brittle. This fracture unit can be considered as the weakest link in the fracture phenomenon, so that when a critical equivalent stress is achieved over this weakest link the unstable fracture takes place.

Fig. 2. Distribution of  eff at the fracture instant for the different notched specimens A, B, C and D exhibiting very distinct triaxiality levels. 4. Micro-fracture maps (MFMs) Scanning electron microscope (SEM) fractographic analysis showed that the fracture always initiates by micro void coalescence (MVC) in a fibrous or dimpled region and propagates in unstable manner by cleavage-like (C) in geometries A, B and C, or in stable manner by MVC followed by shear lip (L) in the case of geometry D. The macroscopic fracture mode is peripheral and plane in the first case and central with cup and cone shape in the second. Cleavage is oriented in A and C (single initiation point) and randomly oriented in B (multiple initiation locations). The depth of the MVC region (process zone) in each geometry is the following: Geometry A: x MVC = 25  m Geometry B: x MVC = 75  m Geometry C: x MVC = 300  m Geometry D: net section Micro-fracture maps (MFMs) were assembled covering the whole fracture surface of all geometries and containing information on the micromechanisms of fracture in the material (Fig. 3). In a previous paper (Toribio and Vasseur; 1996) it was shown that the MFMs can be interpreted by numerical analysis of the stress-strain distribution at the failure situation, and the triaxiality turns out to be the variable governing the extension of the MVC region where fracture initiates. Although the void growth rate increases as the local triaxiality does, from the global point of view the MVC zone is a decreasing function of the triaxiality factor (maximum value of triaxiality in the sample), since the critical void growth rate decreases as the triaxiality increases. A characteristic value of triaxiality seems to exist below which fracture is MVC and above which it is cleavage like. This is consistent with the existence of a critical size of micro-void which is a decreasing function of the triaxiality (Pineau, 1981; Beremin 1981).