Deformation Under Load Creep

This property is an important consideration in the design of parts from fluoroplastics because they deform substantially over time when subjected to load. Metals similarly deform at elevated temperatures. Creep (also called cold flow) is defined as the total deformation under stress after a period of time, beyond the instantaneous deformation upon the application of load. Significant variables that affect creep are chemical structure of resin, load, time under load, and temperature. Creep is measured under various conditions tensile, compressive, and torsional. 

Perfluorinated fluoroplastics have higher creep tendency than partially fluorinated fluoropol5miers. Polytetrafluoroethylene, comprised of 100% tet-rafluoroethylene monomer, has the highest tendency to exhibit cold flow. Copolymers of tetrafluoroeth-ylene containing small amounts (0.1% by weight) of certain other fluorinated monomers, also referred to as modifiers, have significantly lower creep. These modifiers form pendent groups in the polymer chain. 

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Deformation Under Load (Creep) and Cold Flow... 

Creep, Stress Relaxation, Elastic Recovery. Olefin fibers exhibit creep, or time-dependent deformation under load, and undergo stress relaxation, or the spontaneous relief of internal stress. High molecular weight and high orientation reduce creep. 

In the last decade, many new oxide fibers with improved high-temperature performance have been commercialized. The keys to these improvements has been (1) the design of fiber microstructures to reduce the volume of amorphous phases and (2) the development of multiphase polycrystalline fibers. Eliminating amorphous phases prevents rapid, viscous deformation under load at high temperatures. Multiphase polycrystalline microstructures appear to inhibit creep, particularly at elevated temperatures. Examples of developmental fibers with improved high-temperature properties include polycrystalline AI2O3, YAG, and mullite filjers. 

Fluoropolymers have outstanding chemical resistance, low coefficient of friction, low dielectric constant, high purity, and broad use temperatures. Most of these properties are enhanced with an increase in the fluorine content of the polymers. For example, polytetrafluoroethylene, which contains four fluorine atoms per repeat unit, has superior properties compared to polyvinylidene fluoride, which has two fluorine atoms for each repeat unit. Generally, these plastics are mechanically weaker than engineering polymers. Their relatively low values of tensile strength, deformation under load or creep, and wear rate require the use of fillers and special design strategies. 

When a plastic material is subjected to a constant load, it deforms quickly to a strain roughly predicted by its stress-strain modulus, and then continues to deform slowly with time indefinitely or until rupture or yielding causes failure (16). This phenomenon of deformation under load with time is called creep. All plastics creep to a certain extent. The degree of creep depends upon several factors, such as type of plastic, amount of load, temperature, and time. 

Figure 15-26. (a, b) Creep (deformation under load over time)-related failure due to overtightening. (Courtesy of The Madison Group.)... 

Another aspect of plasticity is the time dependent progressive deformation under constant load, known as creep. This process occurs when a fiber is loaded above the yield value and continues over several logarithmic decades of time. The extension under fixed load, or creep, is analogous to the relaxation of stress under fixed extension. Stress relaxation is the process whereby the stress that is generated as a result of a deformation is dissipated as a function of time. Both of these time dependent processes are reflections of plastic flow resulting from various molecular motions in the fiber. As a direct consequence of creep and stress relaxation, the shape of a stress—strain curve is in many cases strongly dependent on the rate of deformation, as is illustrated in Figure 6. 

Creep the dimensional change of a plastic under load with time followed by the instantaneous elastic or rapid deformation at room temperature permanent deformation caused by prolonged application of stress below the elastic limit. 

If the same stress level prevails for 200 hours, the total strain will be the sum of the initial strain plus the strain due to time. This total strain can be obtained from a creep-data curve. If, for example, the total deformation under a tension load for 200 hours is 0.02 in. 

Figure 8.12 illustrates the general effect of creep, plotted as a function of log strain versus log time. Under applied load a sample gradually deforms until a critical time (tc) after which it deforms rapidly. If creep is allowed to go unchecked, the sample may break abruptly. 

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