Question

1. Define tensile strength. Explain how polymers are characterized vis-à-vis differences in tensile stress-strain behavior.

Answer 1

The mechanical properties of plastic materials depend on both the strain (rate) and temperature. At low strain, the deformation of most solids is elastic, that is, the deformation is homogenous and after removal of the deforming load the plastic returns to its original size and shape. In this regime, the stress (σ) is proportional to the strain (ε):

Stress = Constant x Strain

σ = E ε

POLYMER PHYSICS

RELATED TOPICS:

• Mechanical Moduli of Viscoelastic Materials
• Tensile Properties of Solid Polymers
• Shear Properties of Solid Polymers
• Strain Hardening of Plastics
• Crack Propagation and Griffith Criteria
• Stress Relaxation and Creep
• Dynamic Mechanical Analysis (DMA)

TENSILE TESTING MACHINES

• Instron
• MTS
• Zwick
• Shimadzu

BACK TO INDEX

STRESS-STRAIN BEHAVIOR OF POLYMERS

One of the most important characteristics of polymers is their inherent toughness and resistance to fracture (crack propagation). It is not coincidence that the name plastic, which describes any kind of polymeric material, is similar to the word plasticity which is the propensity of a solid to undergo permanent deformation under stress.

The mechanical properties of plastic materials depend on both the strain (rate) and temperature. At low strain, the deformation of most solids is elastic, that is, the deformation is homogenous and after removal of the deforming load the plastic returns to its original size and shape. In this regime, the stress (σ) is proportional to the strain (ε):

Stress = Constant x Strain

or

σ = E ε

where E is the tensile (or Young's) modulus of the plastic which is a measure of the stiffness of the material. This relationship is known has Hooke's law. It means, when a plastic specimen is pulled at a (constant) strain rate the applied stress (or load) is directly proportional to the observed strain (or elongation). The maximum stress up to which the stress and strain remain proportional is called the proportional limit. If a plastic material is loaded beyond its elastic limit, it does not return to its original shape and size, i.e. a permanent deformation occurs. With increasing load a point is eventually reached at which the material starts yielding. This point is known as the yield point. A further increase in strain occurs without an increase in stress.

The stress-strain behavior of a polymer greatly depends on the temperature. At very low temperature well below the glass transition temperature, brittle failure is observed as a break at low strain rate at the stress maximum. If the temperature is increased, a polymeric material changes from brittle (crazing) to ductile (yielding) behavior in deformation and fracture. This temperature is called brittle ductile transition temperature.