PSI - Issue 33

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

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1. Introduction The analysis of fracture processes in notched geometries is a topic of major importance in fracture mechanics of engineering materials for both scientific and engineering reasons. From the scientific point of view, notch-like defects (i.e., with root radius different from zero) generate a triaxial stress distribution near the notch (cf. cracked ones), which allows a detailed analysis of the influence of stress state and triaxiality on ductile failure (Hancock and Brown, 1983) and microscopic mechanisms of fracture (Beremin, 1980). In engineering, notches can be present in structural components due to previous defects or to particular working conditions (e.g. anchorages for prestressed concrete). In a previous paper (Toribio, 1996) a fracture criterion was proposed for high-strength pearlitic steel bars subjected to multiaxial stress states produced by notches of very different geometries. In another scientific contribution (Toribio and Vasseur, 1996), the analysis was focused on the development of micro-fracture maps (MFMs) from testing results and ulterior interpretation by adequate computational methods for stress-strain analysis at the failure situation of the notched specimens. This paper goes further in the analysis and presents a combined micro- and macro-approach to the fracture of high-strength steel notched bars. Considering the slow-fracture region as a process zone, fracture domain or microstructurally damaged area, the fracture criterion developed by Toribio (1996) over a constant characteristic microstructural length of the material —and appropriate for engineering design on the basis of the weakest link concept— is extended towards a more physically sound criterion on the basis of the process zone concept, i.e., the critical domain or fracture region may be considered as not constant, but depending on the stress state (constraint) in

the vicinity of the notch tip. 2. Experimental programme

A high strength pearlitic steel was used, whose chemical composition and mechanical properties are respectively given in Tables 1 and 2. It presents a coarse pearlitic microstructure, with a pearlite interlamellar spacing of 0.3  m, an average size of the cleavage facet of 75  m, and an average pearlite colony size of about 15  m.

Table 1. Chemical composition (wt %) of the steel.

C

Mn

Si

P

S

Cr

Ni

Mo

0.85

0.60

0.26

0.010

0.030

0.02

0.02

0.001

Table 2. Mechanical properties of the steel _____________________________________________________________________________________________ Young's Yield UTS Elong. Ramberg-Osgood Modulus Strength at UTS parameters (GPa) (MPa) (MPa) (%) P (MPa) n _____________________________________________________________________________________________ 199 600 1151 6.1 2100 4.9 _____________________________________________________________________________________________ P,n: Ramberg-Osgood Parameters  =�  /E +(  /P) n

Fracture tests under tension loading were performed on axisymmetric notched specimens with a circumferentially-shaped notch (Fig. 1). Four notch geometries A, B, C and D were used with different depths and radii, in order to achieve very distinct stress states in the vicinity of the notch tip (cf. Fig. 1).