Fracture toughness characterization through notched small punch test specimens

Abstract

In this work a novel methodology for fracture toughness characterization by means of the small punch test (SPT) is presented. Notched specimens are employed and fracture resistance is assessed through a critical value of the notch mouth displacement δ SPT . Finite element simulations and interrupted experiments are used to track the evolution of δ SPT as a function of the punch displacement. The onset of crack propagation is identified by means of a ductile damage model and the outcome compared to the crack tip opening displacement estimated from conventional tests at crack initiation.The proposed numerical-experimental scheme is examined with two different grades of CrMoV steel and the differences in material toughness captured.Limitations and uncertainties arising from the different damage phenomena observed in the lowest toughness material examined are thoroughly discussed.

Damage modeling in Small Punch Test specimens

“…Attending to the specimen geometry and test setup, quasi-static conditions are assumed and a 3-D approach is adopted, taking advantage of symmetry when possible. As described elsewhere [18,26], 8-node linear brick elements are employed, with the mesh gradually being refined towards the notch, where the characteristic element length is determined from a sensitivity study. The lower matrix, the fixer and the punch are modeled as rigid bodies and their degrees of freedom are restricted except for the vertical displacement of the punch.…”

“…Different methodologies have been proposed to fit model parameters from a variety of experimental tests (see, e.g., [8,11,18]). A common procedure in the literature is to assume constant values of the parameters q 1 and q 2 (with…”

“…19 one can easily observe that the onset of damage and subsequent propagation is accurately captured by the numerical model. This is particularly useful for the development of new methodologies for fracture toughness assessment within the SPT, as it allows to identify crack propagation patterns and measure the crack tip opening displacement [18]. …”

“…The aforementioned testing methodology, commonly known as Small Punch Test (SPT), employs very small specimens (generally, 8 mm diameter and 0.5 mm thickness) and may be considered as a non-destructive experiment. The SPT has consistently proven to be a reliable tool for estimating the mechanical [2,3] and creep [4,5] properties of metallic materials and its promising capabilities in fracture and damage characterization have attracted great interest in recent years (see, e.g., [6][7][8][9][10][11][12][14][15][16][17][18]). …”

“…This is not the case of the SPT, hindering a direct comparison and leading to predictions that may significantly differ from the plane strain fracture toughness. Hence, current research efforts are mainly devoted to the development of notched or cracked SPT samples with the aim of increasing the attained triaxiality level [7,18]. In this work, the influence of the shape of the notch on the SPT response is extensively investigated, considering both the constraint conditions and the fabrication process.…”