Fracture assessment of U-notches under mixed mode loading: two procedures based on the ‘equivalent local mode I’ concept

Abstract

Two fracture criteria are proposed and applied to blunt-notched components made of brittle materials loaded under mixed mode; the former is based on the averaged strain energy density over a given control volume, the latter on the cohesive crack zone model. In both instances use of the equivalent local mode I hypothesis is made. Only two material properties are needed: the ultimate tensile strength and the fracture toughness. Numerical predictions of rupture loads from the two criteria are compared with experimental measurements from more than 160 static tests with notched beams. The samples are made of PMMA and tested at −60 • C to assure a bulk behaviour almost linear elastic up to rupture. Notch root radii range from 0.2 to 4.0 mm and load mixicity varies from pure mode I to a prevailing mode II. The good agreement between theory and experimental results adds further confidence to the proposed fracture criteria.

Fracture behaviour of notched round bars made of PMMA subjected to torsion at −60°C

“…In previous works by the authors [28,[37][38][39] tensile tests with PMMA specimens at low temperatures had shown an almost linear elastic mechanical behaviour up to failure (see Fig. 8a).…”

“…The problem becomes more involved if the loading symmetry is lost, i.e. when the notched structural component is subjected to mixed mode loading as widely discussed in previous works by other researchers [31][32][33][34][35][36] and also by the present authors [37][38][39][40][41][42].…”

“…The averaged value of the strain energy density (SED) over a well-defined control volume surrounding the notch tip has been used profitably to summarise the static strength data of notched specimens under static loads [19,[37][38][39][40][41][42] as well as notched components and welded joints under fatigue loading. Dealing with static loads, the SED approach has successfully been applied to assess the critical loads to failures of brittle or quasi-brittle materials weakened by U-and Vnotches.…”

“…While systematic experimental data on fracture of blunted notched specimens (with notches of different root radii) loaded under mode I and mixed mode loading (1+ II) have been recently provided [33,34,[37][38][39][40][41], the results from blunted notched specimens under torsion loading are relatively scarce since the research activity has mainly considered cracked bars under torsion [43][44][45][46][47][48].…”

“…The specimens have been tested at -60 °C to have a behaviour closer to the linear elastic one, as previously made under mode I and mode II loadings [37][38][39][40][41].…”