Polymer Engineering
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  1 Topic 2 Part 1 Polymer Properties Characterization Mechanical Properties ãIn order to facilitate comparisons with the behaviourof other classes of materials the approach taken was to refer to standard methods of data presentation such as stress-strain graphs.ãIt is important to note that when one becomes involved in engineering design with plastics, such graphs are of limited value.ãThe reason is that they are the results of relatively short-term tests and so their use is restricted to quality control and, perhaps, the initial sorting of materials fin terms of stiffness, strength etc.  2ãThe modulus obtained from a short-term test would not predict accurately the long-term behaviourof plastics because they are viscoelasticmaterials.ãThe viscoelasticitymeans that quantities such as modulus, strength, ductility and coefficient of friction are sensitive to straining rate, elapsed time, loading history, temperature, etc.ãThe manufacturing method used for the plastic product can create changes in the structure of the material which have a pronoucedeffect on properties. ãTherefore, the behaviourof the moulded product is different from the behaviourof a moulded test-piece of the same material.ãThe time dependent change in the dimensions of a plastic article when subjected to a constant stress is called creep.  3 ãFor most traditional materials, the objective of the design method is to determine stress values which will not cause fracture.ãHowever, for plastics it is more likely that excessive deformation will be the limiting factor in the selection of working stresses. Tensile Properties ãIn typical tensile test, a polymer sample, in the form of a dogbone, is clamped at one end and pulled at a constant rate of elongation at the other clamped end.ãThe thinner portion of the tensile specimen encourages the sample to fail at the center of the bar, where the stress is the highest, and not at the grip sites, where stress concentration may otherwise result in premature failure.  4 ãThe initial length of a central section contained within the narrow region of the tensile specimen is called the initial gage length, Lo. ãDuring the deformation, force, F, is measured as a function of elongation at the fixed end by means of a transducer. Usually, the tensile response is plotted as engineering stress, σ versus engineering strain, ε , o  L L ∆= ε   o  AF  = σ  
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