Initial Crack Length

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Initial crack length

Manual notch insertion

In the fracture mechanics testing of plastics with the aim of specifying geometry-independent plastic-specific characteristic values (see also: geometry criterion), the mechanically introduced notch plays a decisive role (see: notching). In contrast to metallic materials, where fatigue cracks are preferably generated, mechanical notches or cracks, e.g. with metal or razor blades, are applicable for plastics. These initial cracks can be introduced directly with a metal blade, either pneumatically or manually, for small a/W-ratios (Fig. 1).

Fig. 1:

Notching equipment from Polymer Service GmbH Merseburg

Motorised notch insertion

In testing practice, particularly for larger test specimens, motorised notch insertion (Fig. 2) has proven effective, where, for example, a notch can be planed in using a special device. For larger a/W-ratios, a V-notch is first milled in and then the required sharp notch is created using a metal blade.

Fig. 2:

Motorised Notchvis notch device from Instron/Ceast, Italy

Determination of the initial crack length

The notch produced with a metal blade is preferably measured under a light microscope, and the notch length is referred to as the initial crack length or true crack length (see: effective crack length). If plastic phenomena, e.g. in the form of plastic zones, are detected on the fracture surface using light or scanning electron microscopy (SEM) methods as an expression of stable crack propagation, the initial crack length a must be extended by the microscopically measured length of the stable crack growth r_pl.

See also

• Effective crack length
• Notching
• Fracture surface
• Plastic zone
• Notch

References

[1]	Blumenauer, H., Pusch, G.: Technische Bruchmechanik, Deutscher Verlag für Grundstoffindustrie, Leipzig Stuttgart (1993), (ISBN 3-342-00659-5; see AMK-Library under E 29-3)
[2]	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 22)