Keywords: fracture mechanics, concrete, wedge splitting test, T-stress, numerical simulation.

For the determination of certain fracture characteristics of the material, for example specific fracture energy or tension softening properties, it is necessary to perform fracture tests with displacement control beyond the maximum load to record also the softening part of the specimen's fracture behaviour, which can be rather difficult to obtain e.g. in cases of tensile test geometries. The wedge splitting test (WST), introduced by Linsbauer and Tschegg [2] and later developed by Brühwiler and Wittmann [1], is an interesting experimental method since it does not require sophisticated testing equipment to achieve a quasi-static fracture process. Testing specimens are usually prepared from standard cube specimens, but the experiment can also be performed on adapted core-drilled specimens. The present paper continues and elaborates the previous study of the authors [3], where the cylinder shaped WST specimens and classical and round CT specimens are studied in the framework of two-parameter fracture mechanics.

The purpose of this paper is to investigate the influence of the vertical compressive force applied to the specimen as a reaction of the bottom support, on the fracture parameters via tools of the two-parameter fracture mechanics.

A high level of crack-tip constraint can reduce fracture toughness and, in the case of fatigue cracks, can reduce the fatigue crack growth rate [4]. Knowledge of the constraint enables a transferability of fracture toughness values obtained from laboratory specimens to engineering structures. In general, the two-parameter fracture mechanics approaches imply that the laboratory specimens must match the constraint of the structure. The two geometries must have the same values of the T-stress expressed by B₂, values in order to transfer fracture toughness from laboratory specimens to engineering structures. The approach of two-parameter fracture mechanics is descriptive but not predictive. From this point of view the approximation of B₂ values calculated for homogenous specimens is sufficient and can be used to characterise constraint for non-homogenous cases as well. To use the results the constraint for the considered wedge splitting test has to be calculated.

1

E. Brühwiler, F.H. Wittmann, "The wedge splitting test, a new method of performing stable fracture mechanics test", Engineering Fracture Mechanics 35, 117-125, 1990. doi:10.1016/0013-7944(90)90189-N

2

H.N. Linsbauer, E.K. Tschegg, "Fracture energy determination of concrete with cube-shaped specimens", Zement und Beton 31, 38-40, 1986.

3

S. Seitl, L. Routil, V. Veselý, "Numerical analysis of stress field for wedge splitting geometry", Proceedings of Applied Mechanics 2009, Smolenice 6.-9.4.2009, 270-278, 2009.

4

Z. Knésl, K. Bednár, J.C. Radon, "Influence of T-stress on the rate of propagation of fatigue crack", Physical Mesomechanics, 5-9, 2000.

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