Lecture 12.Mechanical Properties

Engineering Stress < True Stress

True Stress True Strain

The mechanical properties of a polymer involve its behavior under stress. These properties tell a polymer scientist or engineer many of the things he or she needs to know when considering how a polymer can be used.

How strong is the polymer? How much can you stretch it before it breaks?

How stiff is it? How much does it bend when you push on it?

Is it brittle? Does it break easily if you hit it hard?

Is it hard or soft?

Does it hold up well under repeated stress?

The mechanical properties of polymers are one of the features that distinguishes them from small molecules.

Poisson's ratio n is the ratio of transverse contraction strain to longitudinal extension strain in the direction of stretching force. Tensile deformation is considered positive and compressive deformation is considered negative. The definition of Poisson's ratio contains a minus sign so that normal materials have a positive ratio.

n = - etrans / elongitudinal

e = DL/L.

                                     Poisson's ratio: why usually positiveVirtually all common materials become narrower in cross section when they are stretched. The reason why, in the continuum view, is that most materials resist a change in volume as determined by the bulk modulus K more than they resist a change in shape, as determined by the shear modulus G.

Change in volume duringDeformation

Change in volume duringDeformation

Poisson’s Ratio

A stress-strain curve is a graph derived from measuring load (stress - σ) versus extension (strain - ε) for a sample of a material. The nature of the curve varies from material to material. The following diagrams illustrate the stress-strain behavior of typical materials in terms of the engineering stress and engineering strain where the stress and strain are calculated based on the original dimensions of the sample and not the instantaneous values. In each case the samples are loaded in tension although in many cases similar behaviour is observed in compression.

isotropic material:  A material which has the same mechanical properties in all directions.

Increasing temperature

In class quiz