TAILIEUCHUNG - Fundamentals of Polymer Engineering Part 13

Tham khảo tài liệu 'fundamentals of polymer engineering part 13', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 12 Mechanical Properties INTRODUCTION It is usually material properties in addition to cost and availability that determine which class of materials-polymers metals or ceramics and which particular member within that class are used for a given application. Many commodity thermoplastics for example begin to soften around 100 C and this essentially limits their use to temperatures that are a few tens of degrees Celsius below this value. A major factor in favor of polymers though is their low density by a factor of 4 or 5 relative to metals the possibility of a large weight savings coupled with high strength makes plastics very attractive for automotive marine and aerospace applications. In terms of choosing a specific polymer however it is necessary to consider whether the application of interest is structural or nonstructural. In the former case mechanical properties such as tensile strength stiffness impact strength and chemical resistance might be relevant whereas important considerations in the latter case might include surface finish ease of painting and the influence of humidity and ultraviolet radiation on the tendency of the material to crack. In this chapter we will consider mechanical properties of polymers at small strains as well as large strains. In general the mode of deformation could be tension compression shear flexure torsion or a combination of these. To keep the discussion manageable we will restrict ourselves to tension and shear. Note however that we can use viscoelasticity theory 1 487 Copyright 2003 Marcel Dekker Inc. 488 Chapter 12 especially at small strains to predict the behavior in one mode of deformation from measurements made in another mode of deformation. As with metals we expect that the measured properties depend on the chemical nature of the polymer and the temperature of measurement. However what makes data analysis and interpretation both fascinating and challenging are the facts that results also depend on time of loading .

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