An Evaluation of the Double Torsion Technique

Abstract

Double Torsion (DT) is a powerful testing technique for fracture mechanics characterisation of brittle materials as, in principle, it provides a crack length independent test configuration. However, several corrections have been proposed to address variations of experimental results reported from various laboratories. These correction factors address the validity of the DT configuration and its crack length independent stress intensity. Never the less, there seems to be no consensus in literature on the various corrections and the reason of reported variations. This paper presents firstly a critical review of the DT technique, followed by proposed corrections through an experimental analysis using the proposed corrections, a Finite Element model of the geometry and the use of Digital Image Correlation to measure out-of-plane surface deformations. It focuses on the validity of the constant stress intensity regime and the independence of crack length in a critical evaluation using Polymethylmethacrylate test specimens. Assessment of three un-grooved specimen geometry configurations demonstrated the apparent regime of approximately constant stress intensity, although a small but clear dependence of the stress intensity on crack length was observed in all specimen configurations. This dependence is attributable to significant load-point deflections and out-of plane deformations that are not accounted for in the DT analysis. Revisions of the proposed analysis methodologies show that a crack length independent specimen geometry can be achieved, however at the cost of less accurate data. Reliable and accurate data can be achieved with a DT testing configuration using an optimum specimen configuration.

Three-dimensional crack observation, quantification and simulation in a quasi-brittle material

“…compact tension test) and the critical stress r c may be assumed to be equivalent to the ultimate stress in a tensile test. For Gilsocarbon graphite, the typical uniaxial flexural strength has been measured to be 28 MPa [21] and the critical energy release rate to be 260 J m À2 [22]. These studies used the same Gilsocarbon grade of nuclear graphite, with a nominal void volume fraction of $20% (the flexural strength and critical energy release rate are microstructure-dependent, being affected by the porosity of graphite [68]).…”

“…In-plane constraint effects have been studied in polygranular graphites through the sensitivity of their R-curve to stress state [21] and geometry [22]. Although indications of out-of-plane constraint effects, such as crack tunnelling [23], have been also observed, it has not been investigated fully.…”

Damage, crack growth and fracture characteristics of nuclear grade graphite using the Double Torsion technique

“…The specimen has dimensions: length L, width W and thickness d. Because the crack profile in DT is curvilinear a crack front ratio is defined as c = Da/d (Da is the difference between the upper (tensile) and lower (compressive) surface crack lengths), where typically c = 5 [23]. This property is especially useful in crack tip observation since the highest stress intensity factor (SIF) is along the upper (tensile surface) [13].…”

“…up to 75 mm for a specimen of 150 mm length) contrasted with sandwiched bend specimens, for example, where crack extension is seldom longer than a few mm [12]. In addition the configuration allows for easy crack tip observations, due to the way the specimen is loaded [13]. Since graphite is said to undergo considerable non-linear deformations during fracture [5,6] a modification to the DT technique is proposed, which allows for elastic-plastic fracture mechanics (EPFM) conditions.…”

Measurement of dynamic fracture toughness by double torsion testing

“…Several testing methods for measuring fracture toughness have been suggested, improved, and standardized over the years, such as compact tension, double-edge notched tension, single-edge notched bending. The double torsion test (Trantina, 1977), (Fuller, 1979), (Pletka, et al, 1979), (Tait, et al, 1987), (Shyam and Tadaharu ADACHI*, Zoltan MAJOR**, Kenji FUJII***, Kohei MIKUMA****, Markus Karamoy UMBOH*****, Kyohei TAKEO****** and Yosuke ISHII******* Lara-Curzio, 2006), (Becker, et al, 2011) is one of the most useful testing methods for studying rate dependencies of the fracture toughness of brittle materials. Because the double torsion test is based on a four-point bending test of a precracked specimen, fracture behavior can be measured independently of length of crack in the specimen as it propagates in a uniform tensile stress field.…”