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{{Language_sel|LANG=ger|ARTIKEL=Langsames Risswachstum}}
{{PSM_Infobox}}
<span style="font-size:1.2em;font-weight:bold;">Slow crack growth</span>
__FORCETOC__

==Causes of slow crack growth==

Slow crack growth (SCG) occurs in [[Plastics|plastics]], e.g. in polyethylene ([[Plastics – Symbols and Abbreviated Terms|abbreviation]]: PE) pipes, as a result of inhomogeneities such as surface scratches and/or [[Notch|notches]] (see also: [[Notch Sensitivity|notch sensitivity]]) in the presence of a relatively low stress concentration over a long period of time [1]. The [[Stress|mechanical stress]] present in a pipe system results from a combination of internal pressure, manufacturing-related [[Tensile Test Residual Stresses Orientations|residual stresses]] and external load stresses (e.g. due to installation, earth masses and car traffic). Since slow crack growth is influenced by a variety of [[Environmental Stress Cracking Resistance|factors]], there are a large number of studies and investigations in the literature that describe the SCG phenomenon and aim to increase the [[Crack Toughness|resistance]] to slow crack growth in polyethylene [2‒12].

==The mechanism of slow crack growth==

The SCG mechanism is characterised by [[Crack Initiation|crack initiation]] and [[Crack Propagation|crack propagation]], followed by brittle fracture (see: [[Fracture Types|types of fracture]]). Crack initiation occurs at the [[Crack|crack]] tip with the formation of a deformation zone (see: [[Fracture Process Zone|fracture process zone]]) and the formation of [[Micropores|micropores]], [[Craze-Types|fibrils]] and [[Micromechanics & Nanomechanics|crazes]]. The crack propagation stage is characterised by stretching and the associated weakening of the fibrils and the propagation of [[Crazing|crazes]]. According to Lustiger [4] and Huang and Brown [5, 6], tie molecules are important for this process. This tie molecule model cannot explain the outstanding combination of properties in high-density PE materials with a pronounced bimodal molecular weight distribution due to their low concentration [8]. Within the highly oriented ‘craze’ fibrils, the microfibrils can slide past each other [8]. The entanglement density [10, 11] and co-crystallisation effects between low-molecular-weight homo- and high-molecular-weight copolymer species [12] play a decisive role in this process. The formation of the microfibril network is controlled by the Tie molecules.

In addition to molecular and structural conditions, SCG is also influenced by external factors such as ambient temperature, loading speed (see: [[Test Speed|test speed]]), media and/or geometric conditions (see: [[Brittle Fracture Promoting Factors|factors promoting brittle fracture]]). This means that the experimental conditions of the detection methods are particularly important when characterising resistance to SCG.

==Significance of slow crack growth==

Current estimates of the service life of highly developed PE 100 materials, based on detection methods for slow crack growth and developed over a period of 50 years, are approximately 100 years [2].

The use of polyethylene (PE) as a pipe material requires a high degree of safety, reliability and quality and has been established since the 1950s. The slow propagation of [[Crack|cracks]], e.g. caused by scratches on the [[Surface|surface]] or notches, is the most common cause of failure (see: [[Fracture|fracture]]) of pipe systems made of PE materials. It is a decisive factor in assessing the service life and economic efficiency of these [[Plastic Component|plastic components]]. The further development of more economical and environmentally friendly installation techniques, such as sandbed-free or trenchless laying of drinking water pipes, required the development of increasingly resilient and tougher materials (see also: [[Ductility Plastics|ductility plastics]]) with reliably verifiable resistance to slow crack growth.

The PE pipe materials used today are third-generation materials that are classified as PE 100. The difference to the second generation (PE 80) is the bimodal molar mass distribution, which has resulted in optimised physical and mechanical properties and thus also increased resistance to slow crack growth.

==The Full-notched Creep Test (FNCT)==

A recognised method for testing slow crack growth is the [[Full Notch Creep Test (FNCT)]] [13]. However, a disadvantage of this test method is that testing a PE 100 material takes an immense amount of time, up to 8,000 hours or 333 days. For this reason, attempts are being made to characterise the [[Toughness|toughness]] of various PE materials through short-term tests and to find possible correlations with the FNCT or the results of the accelerated (acc.) FNCT. Due to the significantly increased information content compared to conventional toughness testing methods, the application of [[Fracture Mechanical Testing|fracture mechanics material diagnostics methods]] is particularly suitable for this purpose in order to evaluate the resistance to [[Crack Initiation|crack initiation]] and [[Crack Propagation|crack propagation]] [14].

Von Langer et al. present experimental fracture mechanics results for three selected pipe materials in [15].

==See also==

* [[Pennsylvania Edge Notch Tensile (PENT) Test]]
* [[Full Notch Creep Test (FNCT)]]
* [[Strain Hardening Test (SHT)]]

==References==

{|
|-valign="top"
|[1]
|Langer, B., Schmidt, A., Enderle, H.-F., [[Grellmann,_Wolfgang|Grellmann, W.]]: Risswachstum in PE-Rohrwerkstoffen – Deformations- und Schädigungsmechanismen. In: Grellmann, W. (Eds.): Neue Entwicklungen in der Werkstoffprüfung – Herausforderungen an die Kennwertermittlung. Tagung „Werkstoffprüfung 2011“, 1. und 2. Dezember 2011, Berlin, Proceedings pp. 179‒184 (ISBN 978-3-9814516-1-0; see [[AMK-Library]] under A 13)
|-valign="top"
|[2]
|Hessel, J.: 100 Jahre Nutzungsdauer von Rohren aus Polyethylen, Rückblick und Perspektiven. 3R international (46) Heft 4, (2007) 242−246
|-valign="top"
|[3]
|[[Michler,_Goerg_Hannes|Michler, G. H.]]: Kunststoff-Mikromechanik, Morphologie, Deformations- und Bruchmechanismen. Carl Hanser, Munich Vienna (1992) (ISBN 3-446-17068-5: see [[AMK-Library]] under F 4)
|-valign="top"
|[4]
|Lustiger, A., Markham, R. L.: Importance of Tie-molecules in Preventing Polyethylene Fracture under Long-term Loading Conditions. Polymer 24 (1983) 1647−1654; [https://doi.org/10.1016/0032-3861(83)90187-8 https://doi.org/10.1016/0032-3861(83)90187-8]
|-valign="top"
|[5]
|Huang, Y. L., Brown, N.: The Effect of Molecular Weight on Slow Crack Growth in Linear Polyethylene Homopolymers. Journal Material Science 23 (1988) 3648–3655; [https://doi.org/10.1007/BF00540508 https://doi.org/10.1007/BF00540508]
|-valign="top"
|[6]
|Huang, Y. L., Brown, N.: Dependence of Slow Crack Growth in Polyethylene on Butyl branch Density: Morphology and Theory. Journal Polymer Science B 29 (1991) 129–137; [https://doi.org/10.1002/polb.1991.090290116 https://doi.org/10.1002/polb.1991.090290116]
|-valign="top"
|[7]
|Lilge, D., Enderle, H. F.: Lifetime Determining Processes in HDPE: From the Molecular Level to Macroscopic Properties. International Conference on Deformation, Yield and Fracture of Polymers (2009) Rolduc Abbey, Kerkrade, Proceedings pp. 75‒78
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|[8]
|Men, Y., Rieger, J., Lindner, P., Enderle, H. F., Lilge, D., Kristen, M. O., Mihan, S., Jiang, S.: Structural Changes and Chain Radius of Gyration in Cold-Drawn Polyethylene after Annealing: Small- and Wide-Angle X-ray Scattering and Small-Angle Neutron Scattering Studies“. J. Phys. Chem. B 109, (2005) 16650 – 16657; [https://doi.org/10.1021/jp052723g https://doi.org/10.1021/jp052723g]
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|[9]
|Lindner, P., Zemb, T. (Eds.): Neutrons, X-rays and Light: Scattering Methods Applied to Soft Condensed Matter, North-Holland (2002): J. Rieger: Use of Scattering Methods in Chemical Industry SAXS and SANS from Fibers and Films. Hardback ISBN: 978044451122-5; Paperback ISBN: 9781493302260; E-Book ISBN: 9780080930138
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|[10]
|Seguela, R.: Critical Review of Molecular Topology of Semi-Crystalline Polymers: The Origin and Assessment of Intercrystalline Tie Molecules and Chain Entanglements. Journal Polymer Science Part B: Polymer Physics 43 (2005) 1729–1748; [https://doi.org/10.1002/polb.20414 https://doi.org/10.1002/polb.20414]
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|[11]
|Men, Y., Rieger, J., Strobl, G.: Role of the Entangled Amorphous Network in Tensile Deformation of Semicrystalline Polymers. Physical Review Letters 91 (2003) 095502; DOI: [https://doi.org/10.1103/PhysRevLett.91.095502 https://doi.org/10.1103/PhysRevLett.91.095502]
|-valign="top"
|[12]
|Krishnaswamy, R. K., Yang, Q., Fernandez-Bellester, L., Kornfield J.: Effect of the Distribution of Short-Chain Branches on Crystallization Kinetics and Mechanical Properties of High-Density Polyethylene. Macromolecules 41 (2008) 1693–1704; [https://doi.org/10.1021/ma070454h https://doi.org/10.1021/ma070454h]
|-valign="top"
|[13]
|ISO 16770 (2019-09): Plastics − Determination of Environmental Stress Cracking (ESC) of Polyethylene. Full-Notch Creep Test (FNCT)
|-valign="top"
|[14]
|[https://www.researchgate.net/profile/Wolfgang-Grellmann Grellmann, W.], [[Seidler,_Sabine|Seidler, S.]]: Polymer Testing. Carl Hanser, Munich Vienna (2022); (ISBN 978-1-56990-806-8; E-Book: 978-1-56990-807-5; see [[AMK-Library]] under A 22)
|-valign="top"
|[15]
|Langer, B., Berthold, A., [https://de.wikipedia.org/wiki/Wolfgang_Grellmann Grellmann, W.], Enderle, H. F.: Mechanische Kurzzeitprüfung zur Bewertung des Verhaltens von PE-Rohrwerkstoffen beim langsamen Risswachstum. Materialprüfung 54 (2012) 9, pp. 580‒585; https://doi.org/10.3139/120.110364
|}

[[Category:Fracture Mechanics]]

Slow Crack Growth - Revision history