Crack Opening Modes
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Crack opening modes
General information
The terms ‘crack opening modes’ and ‘crack opening types’ are used interchangeably in the literature on fracture mechanics. Fracture mechanics assumes that the fracture of a component and thus of the material occurs as a result of the propagation of initial cracks. It examines the conditions for crack propagation and allows quantitative relationships to be established between the external stress, i.e. the nominal stress acting on the component or test specimen, the size and shape of the cracks, and the resistance of the material to crack propagation [1].
Types of crack opening
In practice, a crack in a workpiece is usually subject to quite complex stress fields, which can be represented by the superposition of three simple characteristic types of stress (modes) (see also: fracture modes).
Depending on the possible relative movement of the crack surfaces, a distinction is made between the following modes, depending on the external stress
- Mode I: simple crack opening; symmetrical lifting of the crack edges,
- Mode II: longitudinal shear; sliding of the crack surfaces in the crack plane
and
- Mode III: longitudinal shear; sliding of the crack surfaces in the crack plane
whereby these types of crack opening can be represented schematically using the example of an edge crack in a pane:
| Figure: | Schematic illustration of the three types of crack opening that are possible in principle |
Practical significances
Crack opening mode I is the most important in practice [2]. It is effective, among other things, in components with internal or surface cracks that are subjected to tensile or bending stresses, as well as in cracks in components under internal pressure (see: component failure). Modes II and III occur, for example, under shear or torsional stress. Mode II and mixed stress (mixed mode) have become more important in practice for fibre-reinforced composites (FRCs). Mode III tests are of lesser importance in practice.
Based on the description of the fracture modes shown in the figure, special fracture mechanics test specimens have been developed which are used to determine geometry-independent characteristic values (see: geometry criterion).
The application of fracture mechanics to the evaluation of fibre-reinforced composite components requires the availability of these characteristic values, which can be determined using the procedure described by Altstädt in [3] (see: testing of composite materials).
See also
References
| [1] | Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser, Munich (2022) 3rd Edition, pp. 231/232 (ISBN 978-1-56990-806-8; E-Book: ISBN 978-1-56990-807-5; see AMK-Library under A 22) |
| [2] | Blumenauer, H., Pusch, G.: Technische Bruchmechanik. Deutscher Verlag für Grundstoffindustrie, Leipzig Stuttgart (1993) 3rd Edition, (ISBN 3-342-00659-5; see AMK-Library under E 29-3) |
| [3] | Altstädt, V.: Testing of Composite Materials. In: Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser, Munich (2022) 3rd Edition, pp. 515–568 (ISBN 978-1-56990-806-8; E-Book: ISBN 978-1-56990-807-5; see AMK-Library under A 22) |