Brittle-Tough Transition

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Brittle-tough transition

Description of the brittle-tough transition

In a physical dependence of material toughness (as a measure of energy dissipation or resistance to crack propagation), the brittle-tough transition characterizes a characteristic point at which a fundamental change in material behaviour occurs, e.g. a transition from dominantly unstable to predominantly stable crack propagation. This is associated with a strong increase in the toughness parameters, e.g. the critical J values or crack tip opening displacements, from a low level, the low toughness, to a high level, the high toughness. Due to the change in material behaviour, it is also possible to detect changes in deformation and fracture behaviour on the fracture surface that have resulted from pronounced plastic deformation processes. Brittle-tough transitions in plastics can be caused by a variation of loading parameters (such as temperature, loading rate or aging time in media), structural parameters (such as phase fractions e.g. rubber content, particle size and particle distance, molar mass distribution, degree of crystallinity, etc.) and/or specimen geometry (such as specimen thickness).

Causes can be assumed to be, for example, the increasing molecular mobility of the macromolecules with an increase in temperature (e.g. for semi-crystalline and rubber-modified thermoplastics at the glass transition temperature of the toughness-determining phase), stress field superpositions with a reduction in particle distance, or the transition from a predominantly plane strain state to a plane stress state with a decrease in specimen thickness.

Example of a PC/ABS/PMMA blend

The present example of a ternary polymer blend, a PC/ABS/PMMA blend, reveals a structurally induced brittle-tough transition at a PMMA mass fraction of 10 %. Since the transition is associated with a bimodal force–deformation and fracture behaviour, particularly high standard deviations in the toughness characteristics are induced (Fig. 1). At PMMA contents higher than 10 %, unstable crack growth (brittle fracture) and linear-elastic material behaviour occur dominantly. At PMMA contents lower than 10 %, stable crack growth (ductile fracture) and elastic-plastic material behaviour are observed exclusively.

Fig. 1:

Struktural caused Brittle-tough transition (BTT) in PC/ABS/PMMA-Blends at variation of PMMA content for ratio PC/ABS of 75:25

See also

• Fracture mechanics
• Impact test

References

• Lach, R.: Correlations between Fracture Mechanical Material Properties and Molecular Relaxation Processes of Amorphous Polymers. VDI-Fortschritt-Berichte, Reihe 18: Mechanics/Fracture Mechanics, Nr. 223, VDI-Verlag, Düsseldorf (1998), (ISBN 3-18-322318-X; see AMK-Library under B1-7)
• Rybnicek, J., Lach, R., Lapcikova, M., Steidl, J., Krulis, Z., Grellmann, W., Slouf, M.: Increasing Recyclability of PC, ABS and PMMA: Morphology and Fracture Behavior of Binary and Ternary Blends. Journal of Applied Polymer Science 109 (2008) 3210–3223, DOI: https://doi.org/10.1002/app.28376