Brittleness of materials: implications for composites and a relation to impact strength

Brittleness of materials—whether it occurs naturally or with aging—affects significantly performance and manifests itself in various properties. In the past, brittleness was defined qualitatively, but now a definition of brittleness for viscoelastic materials exists, enabling analysis of all types of polymer-based materials. The quantity brittleness, B, has been evaluated for neat thermoplastics, but here composites and metal alloys are also assessed. The physical significance of brittleness is connected to the dimensional stability of materials. The connections of brittleness to tensile elongation and to fatigue are explored while its relationship to surface properties— specifically wear by repetitive scratching—is examined more closely. The economic impact of wear results in monetary loss associated with failure and reduced service life of plastic parts—thus its connection to brittleness finds use across a broad spectrum of industrial applications which utilize plastics for manufacturing, processing, etc. We also demonstrate a correspondence between impact strength (Charpy or Izod) and brittleness of polymers. It is shown that the assumption hardness is equivalent to brittleness is inaccurate; this fact has important implications for interpreting the results of mechanical testing of viscoelastic materials. Introduction and scope Brittle materials are frequently encountered, whether expected or not. In the case of polymers, some are by nature brittle while others become brittle due to environmental conditions or aging. Embrittlement of polymers with aging is commonly observed and is reported in the literature [1–4]. Similarly, metals and ceramics are also discussed in terms of brittleness. Clearly, the notion of brittleness is not new; rather it is a significant concept in all of materials science and engineering. The creation of composite materials is one way to avoid the problem of brittleness; a variety of studies report on the changes in mechanical properties of polymers reinforced with fillers including fibers, nanotubes, and others [5–7]. Other options for modifying polymers include fluorination [8], fluoropolymer additives to fluorless polymers [9, 10], intentional synthesis of multiphase systems [11], nanocomposite formation [12], and/or special processing such as in supercritical carbon dioxide [13]. Manuscripts from a 1974 symposium on toughness and brittleness of plastics [14] provide a collection of knowledge regarding brittle behavior of polymer-based materials (PBMs) and different factors which contribute to its manifestation. Despite this, the quantity brittleness was defined largely by the visual assessment of fractures and related properties. For example, Yee et al. [15] report the ductileto-brittle transition by changes in the strain behavior and by electron micrographs of the fracture surfaces. The authors provide no quantitative measure of brittleness allowing direct comparison of one material to another. This work was presented at POLYCHAR 17 World Forum on Advanced Materials, April 20–24, 2009, in Rouen, France. W. Brostow H. E. Hagg Lobland (&) Laboratory of Advanced Polymers & Optimized Materials (LAPOM), Department of Materials Science & Engineering, University of North Texas (UNT), 1155 Union Circle #305310, Denton, TX 76203-5017, USA e-mail: [email protected] URL: Website: http://www.unt.edu/LAPOM/ W. Brostow e-mail: [email protected] 123 J Mater Sci (2010) 45:242–250 DOI 10.1007/s10853-009-3926-5