Fracture Surface

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Fracture surface

Fracture Surface

A fracture surface, i.e. a free surface area, is created by the destruction of atomic or molecular bonds and is associated with the loss of load-bearing capacity of a structural component. In plastics, this material separation occurs through the fracture of molecular chains, the pulling out of molecular chains and the tearing open of phase boundaries.

Local plastic deformations, such as crazes or shear bands, become microscopically visible on the fracture surface, providing information about the causes of failure on the material side (see: failure analysis ‒ basics). Such micromechanical deformation mechanisms are described in detail in the literature [1‒5].

Determination of the effective crack length

In fracture mechanics polymer testing [6], the focus is on determining material values, which requires measuring the effective crack length on the fracture surface. The effective or fracture-mechanically effective crack length is composed of the length of the initial crack (mechanical notch; true crack length) and the length of stable crack growth (fracture mirror; radius of the plastic zone).

Two examples of selected fracture surfaces are shown in Figs. 1 and 2.

Fig. 1:

Fracture surface of a heterophasic copolymer of propylene with ethylene (HeCo)

Fig. 2:

Scanning electron microscope (SEM) images of the fracture surface morphology of a heterophasic PP/EPR/PE blend with 90 wt.-% RAHECO® [7] from an R-curve test (see: crack resistance curve – experimental methods)

The scanning electron microscope (SEM) examinations of the fracture surfaces are described in detail under the explanation of the term ‘stretch zone’. At the end of the crack propagation area (stable crack growth; fracture mirror; see: effective crack length), the formation of a stretch zone is observed using a scanning electron microscope.

Determination of the stretch zone height SZH and stretch zone width SZW

The measured stretch zone height SZH in the example of the PP/EPR/PE blend (see: stretch zone) varied slightly around the value SZH = 30 μm, but showed no systematic dependence on the RAHECO content, which can be explained by the low influence of morphology on crack initiation.

Assuming proportionality between the stretch zone height SZH and the stretch zone width SZW, and taking into account the general relationship between the stretch zone height and crack opening displacement (see: extended CTOD concept)

δ = 2 SZH

it can be assumed that, with constant SZH, the crack toughness at the actual physical crack initiation δ_i is not significantly influenced by the morphological changes (see: micromechanics & nanomechanics) in the material [8].

See also

• Effective crack length
• Initial crack length
• Notching
• Fracture mirror
• Plastic zone
• Fracture process zone

References

[1]	Michler, G. H.: Kunststoff-Mikromechanik. Morphologie, Deformations- und Bruchmechanismen. Carl Hanser, Munich Vienna (1992) (ISBN 3-446-17068-5; see AMK-Library under F 4)
[2]	Michler, G. H.: Electron Microscopy of Polymers. Springer Verlag, Berlin (2008) (ISBN 978-3-54036350-7; see AMK-Library under F 1)
[3]	Michler, G. H., Balta-Calleja, F. J.: Nano- and Micromechanics of Polymers: Structure Modification and Improvement of Properties. Carl Hanser, Munich (2012) (ISBN 978-3446427679; see AMK-Library under F 13)
[4]	Woodward, A. E.: Understanding Polymer Morphology. Carl Hanser, Munich (1994) (ISBN 3-446-17431-1; https://doi.org/10.1002/pi.1995.210370411)
[5]	Michler, G. H.: Atlas of Polymer Structures. Morphology, Deformation and Fracture Structures. Carl Hanser, Munich (2016) (ISBN 978-1-56990-557-9; E-Book ISBN 978-1-56990-558-6; see AMK-Library under F 14)
[6]	Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser, Munich (2022) 3rd Edition, pp. 235–236, (ISBN 978-1-56990-806-8; E-Book: ISBN 978-1-56990-807-5; see AMK-Library under A 23)
[7]	Cäsar, T., Seidler, S.,Grellmann, W.: Bruchmechanische Zähigkeitsbewertung des Rißinitiierungs- und Rißausbreitungsverhaltens von Ethylen-Propylen-Random-Copolymerisaten. In: Grellmann, W., Seidler, S. (Eds.): Deformation und Bruchverhalten von Kunststoffen. Springer, Berlin, Heidelberg (1998) pp. 271–284, (ISBN 3-540-63671-4; e-Book (2014): ISBN 978-3-642-58766-5; see AMK-Library under A 6)
[8]	Seidler, S.: Anwendung des Risswiderstandskonzeptes zur Ermittlung strukturbezogener bruchmechanischer Werkstoffkenngrößen bei dynamischer Beanspruchung. Habilitation (1997), Martin-Luther-Universität Halle-Wittenberg, VDI-Reihe 18: Mechanik/Bruchmechanik Nr. 231, VDI-Publishing (1998), Düsseldorf (ISBN 978-3-1832-3118-8; see AMK-Library under B 2-1)