What is Tensile Testing?
Initial Linear Response and Elastic Modulus
The tensile behaviour of polyethylene is routinely measured on a mechanical tensile testing machine (e.g. Instron machine), by stretching a dumbbell of material at a fixed test speed).
The initial response is linear, i.e. stress and strain increase in proportion. This is known as the initial elastic response. The gradient of this linear portion (indicated by the red line in the picture above) is known as the elastic modulus and is usually denoted by the symbol, E.
E = stress/strain
E can also be referred to as the Young’s modulus or tangent modulus. For accurate measurement of the modulus during tensile testing it is necessary to use an extensometer, which is a device that accurately measures the strain in the parallel central region of the specimen.
Following the initial linearity, there is a gradual softening of the material as the crystal structure reorganizes by intra-crystalline shear. This process takes place in the 0 – 5% strain region. It is not unusual for values of modulus to be quoted for specific strain values, (e.g. the 1% secant modulus is defined as the stress at 1% strain divided by 0.01 or E1%)
Tensile testing Yield and Drawing…
Further extension of polyethylene beyond strains of 5% leads to inter-crystalline shear as the crystal blocks themselves shear and lead to a dramatic softening of the material. This is known as the tensile yield point. The stress at which this occurs is the tensile yield stress or tensile yield strength.
The strain at which yielding occurs is typically 8 – 25% depending on the density and crystal structure of the polyethylene. Further extension of the material leads to a reduction in the measured stress – the yield process can be regarded as an energy barrier. In conjunction with this stress reduction, the cross-sectional area of the yielded region decreases and a “neck” is formed in the material where subsequent yielding is localised.
A region of extension at constant stress then follows as the “neck” propagates. This is referred to as the “drawing” process.
Ultimate Extension and Failure…
Finally, after complete propagation of the “neck” into the shoulders of the tensile specimen, the material hardens due to tightening of the extended molecular network and the stress rises. The rate of rise of the stress in this region depends on the detailed molecular structure and is related to the stress crack resistance of the material, (e.g. copolymers).
The material eventually breaks during tensile testing when the fully stretched molecules are no longer capable of sustaining the tensile stress. It is important to have uniformly prepared specimens with good surface finish to make the best evaluation of ultimate properties. Ultimate tensile strength is the tensile strength at failure. Ultimate elongation is the strain at failure.
Tensile Testing in action…