Short-term mechanical tests

Brown R P (Ed) Handbook of Polymer Testing Short Term Mechanical Tests, Rapra Technology, Shawbury, 2002... 

In Fig. 2.2 an overview about some fundamental experimental methods in short-term mechanical testing of polymers and the related typical loading speeds are given which form the basis to determine basic material properties such as ductility, strength, stiffness, toughness and hardness to be explained more detailed in the following. 

Strength and Stiffness. Thermoplastic materials are viscoelastic which means that their mechanical properties reflect the characteristics of both viscous liquids and elastic solids. Thus when a thermoplastic is stressed it responds by exhibiting viscous flow (which dissipates energy) and by elastic displacement (which stores energy). The properties of viscoelastic materials are time, temperature and strain rate dependent. Nevertheless the conventional stress-strain test is frequently used to describe the (short-term) mechanical properties of plastics. It must be remembered, however, that as described in detail in Chapter 2 the information obtained from such tests may only be used for an initial sorting of materials. It is not suitable, or intended, to provide design data which must usually be obtained from long term tests. 

The creep compliance of rocks reflects their deformability whereas the cohesion reflects their strength. In Section 1, the expressions are derived for the time-temperature equivalence for rocks and the testing results obtained in Sections 2 and 3 have shown clearly that not only the deformability of the TGP granite rock but also its strength follow the time-temperature equivalent principle. The establishment of shift factor eq. (25) and the determination of its parameters make it possible to predict correctly the long-term mechanical response of rock at lower temperatures according to its short-term mechanical behaviour at higher temperatures. 

With the practical approach, most plastics are required to withstand only short-term static mechanical loads—that is, no dynamic loads. Thus, conventional short-term static tests generally suffice. The engineering approach recognizes that many plastic products have been used since their inception to take long-term dynamic or static loads. Thus, they consider fatigue, torsion, creep, and other data that include plastic s viscoelastic properties. See kiss plastic processing. 

One of the most characteristic features of polymer materials is the time dependence of the mechanical properties. For the experimental determination of time dependent mechanical properties in short-term, dynamic test methods (widely used with oscillating loading) are used. 

With the practical approach most plastic products are required only to withstand static mechanical loads (that is, no dynamic loads). Thus, conventional short-term static tests generally suffice. 

This chapter is concerned with the short-term mechanical properties — moduli and strengths — of glass, aramid and carbon fibres in a thermosetting resin matrix. A little information on reinforced thermoplastic matrix systems is also included. The data mainly refer to the room temperature properties of 55-65 v/o fibre, unidirectional, systems. The effects of the variation in fibre volume loading, method of test and instantaneous and long term exposure to temperature are briefly mentioned. Longitudinal properties tend to be fibre dominated, and so are compressive properties to some extent for glass and carbon fibres. The anisotropy of unidirectional materials is noticeable. 

Behavior under conditions of long-term exposure to temperature cannot be ascertained in short-term mechanical or thermo-mechanical tests, because time-dependent aging and relaxation processes are involved. 

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