Fracture energetics, polymer

Fracture Energetics and Surface Energetics of Polymer Wear... 

Since the first review on the effect of surface energetics on polymer friction and wear published in 197, (0 many new works have appeared. Some of these papers(2- ) are on fracture mechanics. In this paper we shall review our current knowledge about both fracture energetics and surface energetics of polymer wear. First, we discuss wear mechanisms and then emphasize these two aspects related to each wear mechanism. 

Abrasive Wear. Abrasive wear(18) is common for brittle, ductile and elastomeric polymers. Abrasion is the wear by displacement of materials from surfaces in relative motion caused by the presence of hard protruberanees or by the presence of hard particles either between the surfaces or embedded in one of them. As a result, microploughing, microshearing or microcutting can occur. Thus, fracture energetics and contact mechanics are involved in analyzing the wear results. We shall discuss briefly the wear rate with respect to different types of polymers. 

In comparison with metals, most conventional polymers are low in wear resistance. For wear control, we need to understand various wear mechanisms for each polymer system (V). As discussed in a previous paper, for adhesive wear, surface energetics can determine the extent of surface wear. Thus, a low surface energy is preferred to minimize the surface attrition. In addition, a harder polymer is desired to lower the wear rate. For abrasive wear, fracture energetics become important a harder and tougher material should be more wear resistant. 

Because of the importance of failure phenomena in any consideration of multicomponent polymer systems, a brief review of the energetics and mechanics of fracture follows (Andrews, 1968, 1972 Berry, 1961 Broutman and Kobayashi, 1972 Eirich, 1965 Griffith, 1921 Halpin and Polley, 1967 Hertzberg et al, 1973 Johnson and Radon, 1972 Kambour and Robertson, 1972 Krauss, 1963 Lannon, 1967 Manson and Hertzberg, 1973u Mark, H., 1943,1971 Orowan, 1948 Prevorsek, 1971 Prevorsek and Lyons, 1964 Radon, 1972 Riddell et ai, 1966 Rivlin and Thomas, 1953 Rosen, 1964 Tobolsky and Mark, 1971 Williams, M. L., and DeVries, 1970 Zhurkhov and Tomashevskii, 1966). Specific cases are discussed in Sections 3.2 and 12.1.2. 

In dispersions stabilized on the basis of phospholipids, an additional fraction of very unstable particles was detected in the cryo-preparation. These particles possibly have a spherical shape as particles with an onion-like structure were detected in the freeze-fracture preparations. Due to the layered structure of the smectic phase, a cylindrical particle shape should be energetically more favorable than a spherical one. This may also be the reason for the higher sensitivity of spherical particles towards the electron beam. In formulations with polymeric stabilizers (e.g. poloxamer, polyvinyl alcohol and Tween 80), such highly unstable particles have not been detected yet. Polymer-stabilized smectic nanoparticles possess a more round, paving-stone-like particle shape. In dependence on the stabilizer system, additional colloidal structures (e.g. vesicles and micelles) formed by the excess of emulsifiers could be detected as well (Fig. 10.11). 

Depending on plasticity degree a polymers behavior in impact tests is described by either the Eq. (10.21) or the Eq. (10.22). Let us note, that the Eq. (10.21) was derived from the conditions of elastic energy in sample accumulation and dissipation and therefore, fracture process itself in the obvious form (i.e., new surfaces formation) is not taken into consideration at the derivation [23]. The Eq. (10.22) was obtained from the modified energetic Griffith criterion, in which a new surfaces formation at crack If ont advancement is directly taken into consideration [22]. At such treatment it is supposed, that crack edges are absolutely flat, though experimental observations... 

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