stress-strain behavior of polymers

Stress-strain behavior of polymer

The mechanical properties of polymer are specified by,

1. Value of elasticity
2. Yield strength

• Tensile strength

The most important mechanical properties of polymer highly sensitive to

• Rate of deformation
• Temperature of polymer
• Chemical nature of environment such as presence of water, oxygen, or organic solvents etc.

For many polymeric materials simple stress-strain test is employed for the characterization of polymer. Three different types of stress-strain behavior of polymer related to nature of polymer is shown in figure.

The three different types of polymer depending on mechanical properties are

1. Brittle polymer
2. Polymer which have plastic deformation
3. Elastomers (totally elastic)

‘Curve A’ illustrate stress-strain characteristics curve for brittle polymer. It indicates that, with increase in stress, strain linearly increases and the fracture strength is high ‘curve B’ represents stress-strain behavior of polymers which have plastic deformation. It is almost similar that of metal in which initial deformation is elastic and the material reach to yield strength. After that strength the behavior of polymer is plastic. ‘Curve C’ represents deformation which is totally elastic in nature. Larger amount recoverable strain produced at low stress level. This type of behavior is found in a class of polymer known as elstomers.

In polymer, the value of tensile modulus and ductility, percentage elongation as same manner as for metals. The maximum stress at which the linear portion of curve ends is known yields strength (\(\sigma_y\)). The tensile strength corresponds to stress at which fracture occur.

For brittle polymer tensile strength (\(T_s\)) is almost equal to yield strength (\(\sigma_y\)) of the material. But for plastic deformation polymer tensile strength is slightly below yield strength.

For elastomer, \(T_s\) is greater than yield strength.

From the above interpretation we can say that some of the polymers are similar like that of metals. But, the tensile strength for polymer is about 100 Mpa whereas for metal alloy it is about 4100 Mpa to 4GPa. Due to the structure of polymer the stress-strain curve is more sensitive to change in temperature. With increases in temperature there is

1. Decreases in elastic modulus due to vi8bration of bonds
2. Reduction in tensile strength due to wreaking of bonds
3. Enhancement of ductility

The influence of rate of strain is also important. In general, decreasing the rate of deformation has the same influence on stress-strain characteristics as increasing the temperature the material become softer and more ductile .

Viscoelastic deformation of polymer

Viscoelasticity

An amorphous polymer, which behaves like a glass at low temperature (brittle), rubbery solid at intermediate temperature and viscous liquid at high temperature. For small deformation, the mechanical behavior at low temperature may be elastic i.e. it obeyed hook’s law (stress\(\propto\) strain). For intermediate temperature polymer is rubbery solid which exhibits the extreme behavior and this condition is known viscoelascticity. It is the property of polymer in intermediate temperature. In the diagram when a polymeric material in the intermediate temperature is made is subjected to stress then there is linearly increase in stress similar that of metal. After yield strength without appreciable increase in strain. Which is the form of viscous behavior of elastic polmer.

When time-dependent stress is applied on polymer the strain produced in the material polymer the strain produced in the material is also time dependent.

Figure 1 shows a constant load is applied in time \(t_a\) and released instantaneously on time\(t_r\).

Figure 2 shows that the strain time graph for perfectly elastic material.

Figure 4 represents deformation is delayed or dependent in time and this process is not reversible, which is viscous behavior of the material. For the intermediate viscoelastic behavior the imposition of stress results and instantaneous elastic strain and then strain increases progressively. It is time-dependent as shown in figure 3 and this behavior is viscoelastic behavior of polymer.

References:

Callister, W.D and D.G Rethwisch. Material Science and Engineering. 2nd. New Delhi: Wiley India, 2014.

Lindsay, S.M. Introduction of Nanoscience . New York : Oxford University Press, 2010.

Patton, W.J. Materials in industry . New Delhi : Prentice hall of India, 1975.

Poole, C.P. and F.J. Owens. Introduction To Nanotechnology. New Delhi: Wiley India , 2006.

Raghavan, V. Material Science and Engineering. 4th . New Delhi: Pretence-Hall of India, 2003.

Tiley, R.J.D. Understanding solids: The science of Materials. Engalnd : John wiley & Sons , 2004.