Crack Resistance Curve – Examples

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Crack resistance curve – Examples

General information

The toughness of plastics is characterised on the basis of the crack resistance (R) curve concept; where the material exhibits elastic-plastic behaviour, the stages of the entire fracture process—crack blunting, stable crack initiation, stable crack propagation, and, following this, often unstable crack propagation and fracture — can be described.

R-curve for isotactic polypropylene (PP)

Using isotactic polypropylene (abbreviation: PP) as an example, this paper demonstrates the possibility of characterising structural modifications (see also: microscopic structure) caused by nucleation in PP materials [1].

To experimentally determine the crack resistance curves as a basis for establishing the fracture mechanics parameters of crack initiation and crack propagation, the quasistatic tensile test was employed using SENT (Single-Edge-Notched Tension) specimens [2–6].

The addition of nucleating agents influences the structure of PP. Semi-crystalline PP is polymorphic, i.e. it can crystallise into the α, β or γ modification depending on the cooling conditions and the nucleation process. The α-modification is characterised by a helical structure in a monoclinic unit cell, which is the thermodynamically most stable form and thus the most common modification. In comparison, the β modification is less ordered and hexagonal, featuring non-parallel, crossed lamellae. The orthorhombic γ modification can be induced under high pressure or by the use of a nucleating agent [7–9]. The crystal modification influences the fundamental mechanical properties due to its different physical and mechanical properties [8, 9]. PP with a predominant β-modification has a lower modulus of elasticity and yield stress at a comparable strain rate, but increased impact strength, tensile strain at break [7] and higher crack toughness [10] compared with PP with an α-modification. The crystal structure can be influenced, amongst other things, by the addition of nucleating agents or by the cooling rate.

The isotactic PP was blended with an α-nucleating agent (αPP) or a β-nucleating agent (βPP). The plastics were cooled at a rate of 1 K/min (abbreviated as -1) and moulded into sheets (see also [11, 12]). The crack resistance curves (R-curves) of the various plastics were evaluated in accordance with the procedure set out in ESIS TC4 [13].

Fig. 1:

(a) R-curves for the plastics PP-1 and αPP-1, and (b) a typical fracture surface of a test specimen showing a metal-blade notch (1), stable crack propagation (2), the damaged zone (3) and the brittle fracture surface (4)

Effect of nucleation on polypropylene (PP)

The R-curves for the three PP materials with different nucleation types are shown in Fig. 1a. At small stable crack extensions, the differences are relatively minor. At higher stable crack extensions, clear differences emerge between the non-nucleated PP-1 and the two nucleated types. For a comparable stable crack extension Δa, the J-values of αPP-1 and βPP-1 are significantly higher, although there is no significant difference between the two nucleated PP types.

This shows that, at a cooling rate of 1 K/min, the resistance of nucleated PP materials to stable crack propagation is higher than that of non-nucleated PP. A typical fracture surface of a PP material is shown in Fig. 1b. The metal blade notch, the zone of stable crack growth—characterised by a very smooth surface and a clear boundary with the damage zone—and the residual fracture surface can be seen.

See also

• Fracture mechanics
• Crack resistance (R) curve
• Crack resistance curve – Experimental methods
• Crack resistance curve – Elastomers quasistatic
• Tearing modulus

References

[1]	Monami, A., Langer, B., Grellmann, W.: Moderne Methoden der Kunststoffprüfung zur Werkstoffentwicklung und Bauteilprüfung. Werkstoffprüfung. Fortschritte in der Werkstoffprüfung für Forschung und Praxis (2016), December 1 and 2, 2016, Neu-Ulm, Proceedings pp. 233–238 (ISBN 978-3-514-00830-4)
[2]	Grellmann, W., Reincke, K., Monami, A., Kretzschmar, B.: Bruchmechanische Zähigkeitscharakterisierung von schichtsilikatverstärktem Polypropylen. In: Pohl, M. (Eds.): Konstruktion, Qualitätssicherung und Schadensanalyse. Stahleisen, Düsseldorf (2007), pp. 115−120 (ISBN 978-3-514-00753-6)
[3]	Grellmann, W., Langer, B., Bierögel, C., Schoßig, M., Mecklenburg, T.: Bruchmechanische Zähigkeitsbewertung nukleierter glasfaserverstärkter Polyolefinwerkstoffe. In: Pohl, M. (Eds.): Konstruktion, Qualitätssicherung und Schadensanalyse, Publishing House Werkstoff-Informationsgesellschaft mbH, Frankfurt (2004) pp. 321−326 (ISBN 3-88355-337-9)
[4]	Kroll, M., Langer, B., Grellmann, W.: Toughness optimization of elastomer-modified glass-fiber reinforced PA6 materials. Journal of Applied Polymer Science 127 (2013) 57−66 DOI: https://doi.org/10.1002/app.36853
[5]	Kroll, M., Langer, B., Schumacher, W., Grellmann, W.: The influence of carbon black batches on the fracture behavior of glass fiber reinforced PA6/PA66 blends. Journal of Applied Polymer Science 116 (2010) 610−618 DOI: https://doi.org/10.1002/app.31611
[6]	Langer, B., Bierögel, C., Grellmann, W., Fiebig, J., Aumayr, G.: Material optimization of PP-short glass fibre compounds. In: Grellmann, W., Seidler, S. (Eds.): Deformation and Fracture Behaviour of Polymers. Springer, Berlin, Heidelberg (2001) (ISBN 978-3540412472; see AMK-Library under A 7)
[7]	Maier, C., Calafut, T.: Polypropylene – The Definitive User’s Guide and Databook, William Andrew Publishing/Plastics Design Library (1998) (ISBN 978-0-0809-5041-9)
[8]	Marigo, A., Causin, V., Marega, C., Ferrari, P.: Crystallization of the gamma form in random propylene-ethylene copolymers. Polymer International 53 (2004) 2001−2008; DOI: https://doi.org/10.1002/pi.1613
[9]	Marigo, A., Marega, C., Causin, V., Ferrari, P.: Influence of thermal treatments, molecular weight, and molecular weight distribution on the crystallization of beta-isotactic polypropylene. Journal of Applied Polymer Science 91 (2004) 1008−1012; DOI: https://doi.org/10.1002/app.13260
[10]	Raab, M., Kotek, J., Baldrian, J., Grellmann, W.: Übermolekulare Struktur und mechanische Eigenschaften von isotaktischem Polypropylen. In: Grellmann, W., Seidler, S. (Eds.): Deformation und Bruchverhalten von Kunststoffen. Springer Berlin Heidelberg (1998) (ISBN 3-540-63671-4; e-Book: ISBN 978-3-642-58766-5; see AMK-Library under A 6)
[11]	Monami, A., Langer, B., Sadlik, J., Kucera, J., Grellmann, W.: Fracture mechanics properties of polymorphic molypropylene. Procedia Materials Science 3 (2014) 276−281; DOI: https://doi.org/10.1016/j.mspro.2014.06.048
[12]	Androsch, R., Monami, A., Kucera, J.: Effect of an alpha-phase nucleating agent on the crystallization kinetics of a propylene/ethylene random copolymer at largely different supercooling. Journal of Crystal Growth 408 (2014) 91−96; DOI: https://doi.org/10.1016/j.jcrysgro.2014.09.028
[13]	Hale, G. E., Ramsteiner, F.: A Testing protocol for conducting J-crack growth resistance curve on plastics. In: Moore, D. R., Pavan, A., Williams, J. G. (Eds.): Fracture Mechanics Testing Methods for Polymers Adhesives and Composites, Elsevier, Amsterdam, London, New York, Oxford, Paris, Shannon, Tokyo (2001) 123–157 (ISBN 0-0804-3689-7)