Fracture Mirror

Sprachauswahl/Language selection

Dieser Artikel ist auch auf Deutsch verfügbar Bruchspiegel

A service provided by
Polymer Service GmbH Merseburg
Tel.: +49 3461 30889-50 E-Mail: info@psm-merseburg.de Web: https://www.psm-merseburg.de
Our further education offers: https://www.psm-merseburg.de/weiterbildung
PSM on Wikipedia: https://de.wikipedia.org/wiki/Polymer Service Merseburg

Fracture mirror

General information

Information about the cause of the fracture (see: fracture formation), the type of stress (static, dynamic or tensile, bending, torsion, etc.) and design and material (see: errors) (inhomogeneities, foreign inclusions) can be obtained from the fracture surface.

Consideration of the fracture mirror when specifying characteristic values

The diagnosis of the fracture is referred to as fractography or microfractography. The fracture surfaces of standardised test specimens for the fracture mechanical determination of parameters contain information on the appearance and length of the initial crack and on the formation of the plastic zone in front of the crack tip. The length of the plastic zone is used in LEFM with small-scale yielding to determine characteristic values. Provided that this plastically deformed area is small in relation to the component dimensions and the ligament length (W–a), it can be approximated by assuming an effective crack length

$a_{eff}\,=\,a\,+\,r_{pl}$

with

rpl		radius of the plastic zone
a		length of the initial crack, notch depth (often also referred to as a0)

in the form of the effective stress intensity factor

$K_{I}(a_{eff})\,=\,\sigma \left(\pi a_{eff}\right)^{\frac {1}{2}}\,f\left({\frac {a_{eff}}{W}}\right)$

Statements on the theoretical basis of this extension of linear elastic fracture mechanics (LEFM) to small-scale yielding and the size of the circular plastic zone are presented in the fracture mechanics literature [1–3].

In fracture mechanics value determination, the radius of the plastic zone is observed as a fracture surface phenomenon in the form of the fracture mirror, whereby the initial crack length a is extended by the length of stable crack growth, which is often measured microscopically, and with

$a_{eff}\,=\,a\,+\,a_{BS}$

with

aBS

fracture surface, length of stable crack growth

the transition from linear elastic fracture mechanics (LEFM) to LEFM with small-scale yielding is formally completed [4]. In very brittle structures, at high test speed or low temperatures, the fracture surface is very small or negligible.

Formation of the fracture mirror on broken glass surfaces

The existence of a fracture mirror was first proven for glass and is described by Spauszus in [5]. Spauszus describes the fracture pattern and the appearance of the fracture surface in a glass rod under tensile stress (Figure 1).

Fig. 1:

Fracture surface after tensile stress on a round glass rod (according to Leeuverik and Burgers) [5] a) Light microscopic; b) Schematic)

Sp		Mirror
Rf		Surface with fine roughness
Rg		Surface with coarse roughness; G must reach or exceed the critical value Gc required for fracture initiation.

The fracture origin is followed by a circular, completely smooth area on the fracture surface, which is referred to as a fracture mirror (S_p) due to its appearance.

Both mirror surfaces of the rod half can be joined with optical precision. An area of fine roughness (R_f) is followed by a zone of coarse roughness (R_g). While the mirror surfaces are oriented perpendicular to the main stress direction, the rough zones form angled surfaces as a result of a fracture branch to the mirror, whereby a wedge-shaped piece of glass is often ejected during the fracture process (Figure 2).

Fig. 2:

Fracture pattern in a glass rod under tensile stress [5] 1 Mirror surface 2 Fracture branching 3 Rough fracture surface 4 Wedge-shaped piece of glass

See also

• Fracture surface
• Fracture process zone
• Fracture formation
• Effective crack length

References

[1]	Blumenauer, H., Pusch, G.: Technische Bruchmechanik. Verlag für Grundstoffindustrie, Leipzig (1987) 2nd Edition, p. 66 (see AMK-Library under E 29-2)
[2]	Anderson, T. L.: Fracture Mechanics, Fundamentals and Application. 3rd Ed., CRC Press Boca Raton (2005) (ISBN 978-0849342608; siehe AMK-Library under E 8-2) DOI: https://doi.org/10.1201/9781420058215
[3]	Schwalbe, K.-H.: Bruchmechanik metallischer Werkstoffe. Carl Hanser, Munich Vienna (1980) (ISBN 3-446-12983-9; see AMK-Library under E 15)
[4]	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 AMK-Library under A 23)
[5]	Spauszus, S.: Werkstoffkunde Glas. Deutscher Verlag für Grundstoffindustrie, Leipzig (1974) (see AMK-Library under Q 2)