Creep behaviour

This competition between mechanisms is conveniently summarised on Deformation Mechanism Diagrams (Figs. 19.5 and 19.6). They show the range of stress and temperature (Fig. 19.5) or of strain-rate and stress (Fig. 19.6) in which we expect to find each sort of creep (they also show where plastic yielding occurs, and where deformation is simply elastic). Diagrams like these are available for many metals and ceramics, and are a useful summary of creep behaviour, helpful in selecting a material for high-temperature applications. 

From the technical point of view the outstanding property of polybut-1-ene is its creep behaviour. Possibly because of its very high molecular weight the polymer has a very high resistance to creep for an aliphatic polyolefin. One... 

Occasionally creep curves are plotted as log (strain) against log (time). This is convenient because quite often this results in straight line plots suggesting that the creep behaviour can be described by an equation of the type... 

For some plastics, particularly nylon, the moisture content can have a significant effect on the creep behaviour. For such plastics, creep curves are normally available in the wet and dry states and a knowledge of the service conditions enables the appropriate data to be used. 

Example 2.12 An acrylic moulding material is to have its creep behaviour simulated by a four element model of the type shown in Fig. 2.38. If the creep curve for the acrylic at 14 MN/m is as shown in Fig. 2.40, determine the values of the four constants in the model. 

Example 2.13 A plastic which can have its creep behaviour described by a Maxwell model is to be subjected to the stress history shown in Fig. 2.43(a). If the spring and dashpot constants for this model are 20 GN/m and 1000 GNs/m respectively then predict the strains in the material after 150 seconds, 250 seconds, 350 seconds and 450 seconds. 

Maxwell and Kelvin-Voigt models are to be set up to simulate the creep behaviour of a plastic. The elastic and viscous constants for the Kelvin-Voigt models are 2 GN/m and 100 GNs/m respectively and the viscous constant for the Maxwell model is 200 GNs/m. Estimate a suitable value for the elastic constant for the Maxwell model if both models are to predict the same creep strain after 50 seconds. 

Mechanical properties per se concerns with the qualities which determine the behaviour of a material towards applied forces. The ability to support weight without rupture or permanent deformation, to withstand impact without breaking, to be mechanically formed into different shapes - all these depend upon a combination of mechanical properties characteristic of metals. Four types of behaviour of a material under stress are very important linear or elastic behaviour, plastic behaviour, creep behaviour and fatigue behaviour. 

ISO 899-1, Plastics - Determination of creep behaviour - Part 1 Tensile creep, 1993. 

Creep behaviour is a commonly used and very important measure of the effect of mechanical stress on plastics, but it is less used as a means of monitoring degradation due to environmental agents. At shorter times, the measured creep is predominantly due to physical effects and it is only at longer times that environmental effects will be apparent. It can be noted that creep tests use the same test piece at successive time intervals which is advantageous from the point of view of reproducibility. 

Takahashi, M. (1981). Bending creep behaviour of plywoods under long term exposure to fungal attack. International Research Group on Wood Preservation, Doc. No. IRGAVP 81-2163. 

Better general creep behaviour, the links between the chains restricting the relative displacements of the macromolecules, one against the other. 

ISO 899-1 2003 Plastics - Determination of creep behaviour - Part 1 Tensile creep ISO 899-2 2003 Plastics - Determination of creep behaviour - Part 2 Flexural creep by three-point loading... 

HDPE same properties as LDPE but more rigid better thermal and creep behaviour lower coefficient of friction and higher pressure strength, allowing antifriction applications with higher PV (pressure velocity) factor more transparent. 

Linear PE same properties as the equivalent branched PE with an improvement in the mechanical properties, thermal and creep behaviour, and resistance to stress cracking. 

To illustrate the differences in creep behaviour between the main polyolefins, after 1000 h under 8 MPa at room temperature, the elongations are, for example ... 

Polycarbonates are used in optics, transparent technical parts, aesthetic applications and protection devices for their transparency, mechanical properties, impact resistance, rigidity, good creep behaviour, fatigue resistance, insulating properties, dimensional stability, low moisture uptake, broad range of service temperatures (-100°C up to -i-135°C),food contact and sterilization possibilities for suitable grades. 

PPEs are used in technical parts because of the good price/performance ratios for mechanical and electrical properties creep behaviour, low moisture uptake, heat and low-temperature behaviour, resistance to moisture and hot water good dimensional stability. 

Reinforcement with glass fibres improves moduli and creep moduli, leading to fair creep behaviour, as we can see in Figure 4.91, which shows creep moduli at room temperature and 100°C under an unknown loading. 

Reinforcement with carbon fibres improves moduli and creep moduli, leading to fair creep behaviour. 

Creep behaviour is good even when the temperature rises. Neat polysulfones have medium moduli that involve medium strains for moderate loading. Consequently, creep moduli are also in a medium range at room temperature but they decrease more slowly than the majority of other thermoplastics when the temperature rises. Reinforcement with fibres leads to high moduli and consequently high creep moduli. 

Polyamide-imides are appreciated for good mechanical and electrical properties high service temperatures (up to 220°C with possible long service times at 260°C) rigidity good creep behaviour fatigue endurance low shrinkage and moisture uptake inherent flame retardancy chemical resistance usability down to -196°C. 

Creep behaviour is good even when the temperature rises. 

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