Determination of Strength and Fracture
Toughness from Indentation Tests
in the Framework of Finite Fracture Mechanics
Jonathan Hahn and Wilfried Becker (B)
TU Darmstadt, 64287 Darmstadt, Germany
becker@fsm.tu-darmstadt.de
Abstract. Many technically relevant materials show brittle failure behaviour. For
a safe and reliable use often a fracture mechanical assessment is important for
which the knowledge of the fracture mechanical characteristics is essential. The
classical identification of these characteristics often requires much effort so that
indentation tests seem to be an advantageous alternative. The common use of
sharp indenters, however, is connected with some uncertainty how to describe
the fracture process properly and the results cannot be reproduced in a reliable
manner. On this background we suggest the use of blunt spherical indenters and a
physically motivated modelling of the crack initiation in the framework of finite
fracture mechanics. This allows a qualitatively good explanation of experimental
findings.
Keywords: Indentation cracks · Finite fracture mechanics · Parameter
identification
1 Introduction
As brittle materials are often prone to fracture failure the knowledge of their fracture
mechanical properties is essential. The identification of these properties often requires
a big experimental effort, in particular when well-defined fracture specimens have to be
manufactured with defined initial cracks. The effort can be reduced by the identification
of the fracture toughness through indentation tests as suggested for instance by Evans
[1]. In most cases a sharp indenter (Vickers, Berkovich) is used to generate cracks at
the indentation location. The generated cracks can be detected reasonably well and their
dimensions are identified microscopically. Then simple relations give the fracture toughness, see e.g. Lawn et al. [2]. The relations, however, contain an empirical correction
factor that has to be identified experimentally and that depends on the underlying material. This gives raise to several points of criticism as it is summarized by Quinn/Bradt [3].
A proper fracture mechanical derivation is difficult due to the complex stress state, the
complex crack patterns and localized plastic yielding at the indenter tip and round-robin
studies reveal serious problems with the reproducibility of results.
On this background we suggest the use of blunt axisymmetric indenters in the framework of finite fracture mechanics. This is to enable a physically motivated modelling
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
S. N. Atluri and I. Vušanovi´ c (Eds.): ICCES 2020, MMS 97, pp. 44–51, 2021.
https://doi.org/10.1007/978-3-030-64690-5_5
Toughness from Indentation Tests
in the Framework of Finite Fracture Mechanics
Jonathan Hahn and Wilfried Becker (B)
TU Darmstadt, 64287 Darmstadt, Germany
becker@fsm.tu-darmstadt.de
Abstract. Many technically relevant materials show brittle failure behaviour. For
a safe and reliable use often a fracture mechanical assessment is important for
which the knowledge of the fracture mechanical characteristics is essential. The
classical identification of these characteristics often requires much effort so that
indentation tests seem to be an advantageous alternative. The common use of
sharp indenters, however, is connected with some uncertainty how to describe
the fracture process properly and the results cannot be reproduced in a reliable
manner. On this background we suggest the use of blunt spherical indenters and a
physically motivated modelling of the crack initiation in the framework of finite
fracture mechanics. This allows a qualitatively good explanation of experimental
findings.
Keywords: Indentation cracks · Finite fracture mechanics · Parameter
identification
1 Introduction
As brittle materials are often prone to fracture failure the knowledge of their fracture
mechanical properties is essential. The identification of these properties often requires
a big experimental effort, in particular when well-defined fracture specimens have to be
manufactured with defined initial cracks. The effort can be reduced by the identification
of the fracture toughness through indentation tests as suggested for instance by Evans
[1]. In most cases a sharp indenter (Vickers, Berkovich) is used to generate cracks at
the indentation location. The generated cracks can be detected reasonably well and their
dimensions are identified microscopically. Then simple relations give the fracture toughness, see e.g. Lawn et al. [2]. The relations, however, contain an empirical correction
factor that has to be identified experimentally and that depends on the underlying material. This gives raise to several points of criticism as it is summarized by Quinn/Bradt [3].
A proper fracture mechanical derivation is difficult due to the complex stress state, the
complex crack patterns and localized plastic yielding at the indenter tip and round-robin
studies reveal serious problems with the reproducibility of results.
On this background we suggest the use of blunt axisymmetric indenters in the framework of finite fracture mechanics. This is to enable a physically motivated modelling
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
S. N. Atluri and I. Vušanovi´ c (Eds.): ICCES 2020, MMS 97, pp. 44–51, 2021.
https://doi.org/10.1007/978-3-030-64690-5_5