Multiple Crazing

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Multiple Crazing or Multiple craze formation (Author: Prof. Dr. G. H. Michler)

Introduction

For many years, the toughness of brittle polymer materials has been technically improved by modifying them with rubber particles. Extensively investigated model examples are impact-resistant and high-impact modified polystyrene materials ( abbreviation: PS-HI – High Impact PS or HIPS). The incorporation of rubber particles (with content of 10–25 vol.-%) reduces strength and stiffness, but greatly increases elongation at break and toughness. The main effect of the rubber particles is the initiation of local flow processes in the material, the so-called crazes [1–3], which can occur in different craze-types. This effect is the basis for increased energy absorption in the material, whereby the main proportion comes from the formation of crazes (around 90 %), while the other mechanisms only provide small contributions. The toughness gain of the high impact polymers is often more than ten times greater than the toughness of the unmodified matrix [4].

The three-stage mechanism of multiple crazing

The mechanism of multiple craze formation is illustrated as a three-stage mechanism of multiple crazing in Fig. 1 [1, 5].

The three main steps are:

(a) Craze initiation: the rubber particles initiate stress concentrationens σ_K with the largest values in the equatorial zones. Crazes form here and propagate perpendicular to the acting stress;

b) Overlapping effect: The stress fields around the rubber particles overlap intensively if the distance is smaller than the particle radius, i.e. with a particle content of approx. 15 vol.-%;

c) Crack stop and crack propagation: If cracks have formed in the crazes, they can only propagate the short lengths between the crazes and are then stopped at the next particles (by crack tip rounding – crack tip blunting).

Fig. 1:

Three-stage mechanism for increasing toughness (multiple crazing) a) Stress concentration on individual particles b) Superposition of stress concentrations, higher stress concentrations leading to thicker crazes and wide crack bands c) Crack length limitation and crack arrest on and in the particles

This effect is shown in Fig. 2 in a deformation test of HIPS in a 1,000 kV transmission electron microscope (HVEM) [6, 7]. The rubber particles and the crazes initiated on them appear brighter than the matrix. Fig. 2a shows an overview of the material after stressing with two cracks on the left and right. Starting from these cracks, light-colored crack bands spread into the specimen perpendicular to the direction of stress. At higher magnification, Fig. 2b clearly shows the rubber particles and the crazes formed on them.

Fig. 2:

Deformation structures in an impact-resistant polymer (HIPS) in the HVEM (see arrow for direction of strain); a) Overview of the deformation zone between the two cracks on the left and right b) in higher magnification area in front of a crack tip with rubber particles (gray) in the matrix (black) with crazes ( bright)

The crack stop on the rubber particles is an essential step in achieving high toughness, as this prevents premature crack propagation and further energy-absorbing cracks can form. Fig. 3 shows a crack stop caused by a crack running into a soft rubber particle in a deformation test in the 1,000 kV TEM.

Fig. 3:

HIPS under stress with a cracked craze (crack from top left), the crack is stopped in the rubber particle (deformed thin sample in the HVEM)

After the material has broken, the plastic deformations are visible in strongly stretched fibrils of the matrix material – as shown in Fig. 4 in images taken with a scanning electron microscope (SEM).

Fig. 4:

Fracture surface of an impact-resistant polymer with plastically strongly stretched fibrils of the matrix polymer at lower (a) and higher magnification (b), SEM images

See also

• Craze-types
• Micromechanics & Nanomechanics
• Polymers & structure
• Microscopic structure
• Scanning electron microscopy

References

[1]	Michler, G. H.: Kunststoff-Mikromechanik: Morphologie, Deformations- und Bruchmechanismen. Carl Hanser, Munich (1992); (ISBN 3-446-17068-5, see AMK-Library under F 4)
[2]	Bucknall, C. B.: Toughened Plastics. Applied Science Publ., London (1977); https://doi.org/10.1002/pol.1978.130160714
[3]	Bucknall, C. B.: British Plastics 40, 1181–1122 (Nov.), 84–86 (Dec.) (1967)
[4]	Michler, G. H.: Plaste und Kautschuk, 26, 680–684 (1979)
[5]	Michler, G. H., Balta-Calleja, F. J.: Nano- and Micromechanics of Polymers: Structure Modification and Improvements of Properties, Carl Hanser, Munich (2012); (ISBN 978-3-446-42767-9; e-Book 978-3-446-42844-7; see AMK-Library under F 13)
[6]	Michler, G. H.: Werkstoffwissenschaft und Kunststoffe. Schriften der Sudetendeutschen Akademie der Wissenschaften und Künste. Band 43, Forschungsbeiträge der Naturwissenschaftlichen Klasse, Munich (2024) 27–58; see AMK-Library under F 33
[7]	Michler, G. H.: Mechanik–Mikromechanik–Nanomechanik. Vom Eigenschaftsverstehen zur Eigenschaftsverbesserung. SpringerSpektrum (2024), ISBN 978-3-662-66965-5; e-book: ISBN978-3-66966-2; https://doi.org/10.1007/978-3-662-66966-2; see AMK-Library under F 34

Weblinks

• Wikipedia – Die freie Enzyklopädie: Crazes: https://de.wikipedia.org/wiki/Crazes
• Michler, G. H.: Modellierung des Einflusses des Kautschukgehaltes auf die Craze-Bildung in schlagzähen Polymeren. Acta Polymerica Vol. 36, Issue 6 (1985)325-330; https://doi.org/10.1002/actp.1985.010360607