Brittle Fracture Promoting Factors

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Brittle fracture promoting factors

General information

The failure of components made of plastics and composite materials (see: fracture behaviour of plastics components) is promoted by a number of factors known as brittle fracture promoting factors. In addition to an increased strain rate (see: strain rate – basics), low temperatures and/or multiaxial stress states, including residual stresses, are of crucial importance. The formation of brittle fracture is particularly promoted by stress concentrations at corners, notches or cracks, so that the deformation behaviour under impact stress on notched test specimens with varying temperatures must be considered as the critical stress.

Influence of internal condition

When investigating the deformation behaviour under impact loading, it is not possible to derive generally valid statements about the behaviour of plastics under this stress, even as a funkctino of temperature, using just a few characteristic values. Particularly in the case of thermoplastics, test specimens, moulded parts and semi-finished products manufactured from a material differ in their processing-related internal states:

• Orientations
• Residual stresses
• Morphology
• Degree of crystallinity and
• Spherulite size.

In general, it can be said that higher orientation in amorphous plastics leads to higher impact strength (see: toughness). In semi-crystalline plastics, high orientations can reduce the deformation reserves and thus lead to a reduced characteristic value level.

Furthermore, as the temperature decreases, the molecular mobility freezes, resulting in increasing ambrittlement.

Selected testing method

The following methods, which are also listed in this encyclopaedia, are hoghly relevant in practice for investigating the effects of factors that promote brittle fracture:

• [Impact Test|Impact tests]] according to Charpy, Izod and Dynstat
• Instrumented Charpy impact test (ICIT)
• Conventional Tensile impact test
• Conventional and Instrumented tensile impact test (ITIT)
• Instrumented puncture impact test

There are also different technical variants and test specimen shapes for these tests.

Factors influencing fracture behaviour

Fig. 1 below shows the factors influencing the fracture behaviour of plastics components, categorised according to stress, geometry, material and environment.

Fig. 1:

Factors influencing the fracture behaviour of plastic components

When analysing the mechanical properties and evaluating the deformation and fracture behaviour of plastic components, it is necessary to combine microscopic and macroscopic aspects in order to contribute decisively to the understanding of deformation and fracture mechanisms.

See also

• Failure analysis
• Failure analysis plastics products, VDI Guideline 3822
• Fibre-reinforced plastics fracture model
• Crack toughness
• Slow crack growth
• Vibration fracture

References

• Blumenauer, H., Pusch, G.: Technische Bruchmechanik. Deutscher Verlag für Grundstoffindustrie, Leipzig Stuttgart (2003), 3rd Edition (ISBN 3-342-00659-5; see AMK-Library under E 29-3)
• Anderson, T. L.: Fracture Mechanics; Fundamental and Applications. CRC Press, Boca Raton (2005) (ISBN 978-0849342608; see AMK-Library under E 8-2); https://doi.org/10.1201/9781315370293
• Grellmann, W., Seidler, S.: Mechanical and Thermomechanical Properties of Polymers. Landolt-Börnstein. Volume Band VIII/6A3, Springer, Berlin (2014), (ISBN 978-3-642-55165-9; see AMK-Library under A 16)
• Krüger, L., Trubitz, P., Hentschel, S.: Bruchmechanisches Verhalten unter quasistatischer und dynamischer Beanspruchung. In: Biermann, H., Krüger, L.: Moderne Methoden der Werkstoffprüfung. Wiley-VCH Publishing, Weinheim (2014) pp. 1–52; ISBN 978-3-527-33413-1 (see AMK-Library under M 35)