Crack Toughness

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Crack toughness

Explanation of terms

In fracture mechanics, crack toughness or fracture toughness describes the resistance of a component or material to crack propagation of any kind. Cracks are undesirable material discontinuities that arise as a result of manufacturing and/or post-treatment and processing, but often also due to external (forces and moments) or internal (residual stresses and orientations) stresses [1, 2].

In the case of unstable crack propagation, the characteristic parameter of fracture toughness is the critical stress intensity factor K_Ic. Crack toughness is calculated from the load stress or force acting perpendicular to the crack flanks (see: crack opening modes). Fracture is the most dangerous cause of failure on the material side, as it occurs without warning, i.e. without prior plastic deformation, in the case of unstable crack propagation. Once a critical stress is reached, brittle fracture is triggered. In plastics, fracture is accompanied by the tearing of molecular chains, the pulling out of molecular chains (see also: fibre-reinforced plastics fracture model and fracture behaviour) and the tearing of phase boundaries [3].

Practical Application examples

A number of measures are used in the design and dimensioning of plastic components to increase their crack toughness and improve their fracture behaviour [4].

• Radial ribbing on bucket bottoms,
• Ribs and stringers on structural elements in aircraft construction
• Radial and tangential ribs in washing machine lye tubs (Fig. 1),
• Special ribs with a defined degree of freedom of deformation (Fig. 2a),
• Avoiding wall thickness variations in injection-moulded ropes (Fig. 2b),
• Setting defined orientations in injection-moulded parts through the design of the gate and flow path obstacles,
• Avoiding geometric heterogeneities such as edges and corners or cavities (Fig. 2c).

Fig. 1:

Radial and tangential ribbing of a lye tub in washing machines

Fig. 2:

(a) Ribs with and without degrees of freedom of deformation, (b) wall thickness variation, and (c) external and internal inhomogeneities in injection-moulded parts

See also

• Toughness
• Toughness temperature dependence
• Brittle-tough transition
• Levels of knowledge in fracture mechanics
• Crack tip opening displacement concept (CTOD)
• Crack propagation energy

References

[1]	Blumenauer, H., Pusch, G.: Technische Bruchmechanik. Deutscher Verlag für Grundstoffindustrie, Leipzig (1993) 3rd Edition p. 15, (ISBN 3-342-00659-5; see AMK-Library under E 29-3)
[2]	Schwalbe, K.-H.: Bruchmechanik metallischer Werkstoffe. Carl Hanser, Munich Vienna (1980), (ISBN 3-446-12983-9; see AMK-Library under E 15)
[3]	Grellmann, W.: Fracture Toughness Measurements in Engineering Plastics. In: Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser, Munich (2025) 3rd Edition, pp. 229–281 (ISBN 978-1-56990-806-8; E-Book: ISBN 978-1-56990-807-5; see AMK-Library under A 22)
[4]	Bierögel, C., Langer, B., Müller, H., Grellmann, W.: Schadensfallanalyse an Kunststoffbauteilen. In: Pohl, M. (Ed.): Konstruktion, Qualitätssicherung und Schadensanalyse. Tagung Werkstoffprüfung 2004, Neu-Ulm, 25.–26. November 2004, Werkstoff-Verlag Informationsgesellschaft mbH Frankfurt (ISBN 3-88355-337-9; see AMK-Library under M 12), Proceedings pp. 231–236 Download as pdf-file