Materials for engineering, 3rd Edition - (Malestrom)

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Organic polymeric materials 175 Polystyrene (PS) and polymethylmethacrylate (PMMA) are both glassy polymers that can readily form crazes, and both materials exhibit a welldefined region I. At the higher ∆σ end of region II, slow growth of crazes and their transformation into cracks are dominant mechanisms of failure. In contrast, the fatigue life in region III is controlled by the incubation time for the nucleation of microscopic flaws. Fatigue crack growth The fatigue crack growth (FCG) behaviour of a wide variety of amorphous and semicrystalline polymers can be characterized in terms of the stress intensity factor, ∆K, Fig. 5.10(a) for PMMA, epoxy, polycarbonate (PC), Nylon 66, polyacetal (PA), and poly(vinylidene fluoride) PVDF (after Hertzberg, Nordberg and Manson, J. Mater. Sci., 1970, 5, 521–526, and Hertzberg, Skibo and Manson, Spec. Tech. Publ., 1979, 675, 471–500). Since these materials have very different elastic moduli, for comparison purposes it is instructive to plot da/dN against ∆K/E, where E is the relevant Young’s modulus. An example is shown in Fig. 5.10(b), (after Hertzberg, Skibo and Manson, 1979, and Manson, Hertzberg and Bretz, 1981, in: Advances in Fracture Research (ed. D. François), 1, 443–448). In both diagrams, data for two engineering alloys are included for comparison, namely a 2 1 / 4 Cr– 1Mo steel and a 7075 aluminium alloy, and it is evident that plastics will exhibit superior or inferior FCG resistance compared with metals depending on whether cycling is conducted under strain-control or stress-control, respectively. Among the polymers, it appears that in general semicrystalline polymers exhibit a superior FCG resistance to amorphous ones. 5.5 Joining of polymers The question of joining similar or dissimilar polymers often arises, as it is frequently necessary to assemble two sub-components when a product is manufactured. Again, items are commonly fabricated from sheet or semifinished sections, which requires the employment of fastening or welding techniques. We will consider three modes of joining polymers: welding, adhesives and mechanical fasteners. 5.5.1 Welding of polymers Since welding involves melting and flow of the surfaces to be joined, the technique will only be applicable to thermoplastic polymers. The workpiece may have its temperature raised either by performing local mechanical work of friction or by the application of thermal energy.
176 Materials for engineering 10 –1 PMMA PC PVC 10 –2 Epoxy PVDF da/dN (mm/cycle) 10 –3 10 –4 Nylon 66 7075–T651 aluminum 2 1 / 4 Cr–1 Mo steel 10 –5 10 –6 PA 0.2 1 10 20 40 60 80 ∆K (MPa √m) (a) da/dN (mm/cycle) 10 –1 10 –2 PMMA Epoxy PVC PC PVDF 2 1 / 4 Cr–1 Mo 10 –3 10 –4 7075 PA Nylon 66 10 –5 10 –6 10 –4 2 4 6 810 –3 2 4 6 ∆K/E (√m) (b) 5.10 (a) FCG characteristics of some amorphous (dashed lines) and semicrystalline (solid lines) polymers. Comparative data for a steel and an aluminium alloy are shown. (b) FCG rates shown in Fig. 5.10a replotted as a function of ∆K/E.
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Materials for engineering Third edi
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Woodhead Publishing Limited and Man
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vi Contents 3.4 Degradation of meta
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Preface to the third edition The cr
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xvi Introduction Young’s modulus,
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1 Structure of engineering material
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Structure of engineering materials
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1.2 Microstructure Structure of eng
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Page 234: Part III Problems
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Appendix I Useful constants Symbol
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234 Appendix II Fracture toughness
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Appendix IV Sources of material pro
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Sources of material property data 2
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Index acrylics 160 adhesives 121 ad
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Index 245 smart fibre 189 stiffness
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Index 247 GPC (gel permeation chrom
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Index 251 nitriding 83-4 normalisin
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Materials for engineering, 3rd Edition - (Malestrom)

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