Mechanisms of toughening brittle polymers

doi:10.1016/0921-5093(94)90961-X

Materials Science and Engineering: A

Volume 176, Issues 1–2, 31 March 1994, Pages 79-90

https://doi.org/10.1016/0921-5093(94)90961-X Get rights and content

Abstract

Heavily cross-linked thermosetting polymers such as epoxies and certain flexible chain thermoplastic polymers which undergo crazing show brittle behavior in tension but can undergo quite significant plastic deformation in compression. The cause of the tensile brittleness of the former is a relatively low fracture toughness, while that of the latter results from premature fracture, precipitated in craze matter, due to acquisition of ill-adhering inorganic particulate inclusions of larger than micron size by the crazes during their growth. Both types of polymers can be effectively toughened. A particularly attractive mechanism of toughening the epoxies is by a crack-trapping process through the use of a well-adhering set of tough fibers which force the brittle crack to bow around the fibers. A very effective technique to toughen the brittle crazable polymers is to decrease the craze flow stress below the critical threshold strength defined by the size of the inclusions. This is accomplished by a novel mechanism of prepackaged, stress-activated diluents that plasticize the polymer on demand during crazing. Examples of these two mechanisms are presented.

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A comprehensive analysis on the failure mechanism of epoxy resin was provided by Kinloch et al. [13], who proposed a quantitative model accounting for the blunting at the notch tip prior to the onset of crack initiation. Several potential toughening mechanisms for polymer structures weakened by cracks and sharp notches were discussed by Argon et al. [15,16] whereas the effect of different pre-notching methods on the fracturing behavior of polymers was studied in [18–21]. In these contributions it was proposed that residual plastic stresses induced by the manufacturing of the notch may lead to very high values of apparent fracture toughness of the polymer.
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Polymers are often modified with rubber inclusions to improve their toughness [1–3]. The mechanisms of toughening have been a subject of intense interest and many studies have identified a range of mechanisms that include cavitation [4–11,20,21], crazing [17,24,25], crack bridging [6,7], micro cracking [6,7] and shear banding [4–11]. These mechanisms are operative in the process zone ahead of a crack tip and their sequence together with their relative contribution towards toughness has been a subject of debate.
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Mechanisms of toughening brittle polymers

Abstract

J. Polym. Sci. (Phys.)

Philos. Mag.

Polymer

Philos. Mag.

J. Chem. Phys.

Toughened Plastics

Macromolecules

Macromolecules

Macromolecules

Macromolecules

Deformation, Yield and Fracture of Polymers

J. Mater. Sci.

J. Mater. Sci.

Philos. Mag.