Mechanical Behavior of Plastics

The applicability of Nutting's equation of the mechanical behavior of some soft plasticized polyvinyl chloride preparations noted by Dyson in creep experiments is further demonstrated by force‐length measurements at constant rate of strain. This suggests new criteria for the characterization of soft plasticized PVC. For example, in a stretching experiment in which the sample is subjected to a strain (L−1) which increases linearly with time (t) at the rate (R), definition of ``modulus'' as the limit at small strains of the ratio of the force (F) to the strain is clearly inappropriate. That is, when $$\begin{array}{c}{\text{L−1=GF}}^m{t}^n,\\ \lim {\displaystyle {lim}_{\text{L−1→0}}}\left(\frac{F}{\text{L−1}}\right){\text{=(L−1)}}^{\text{(1−n−m)/m}}{R}^{\mathrm{n/m}}{G}^{\text{−1/m}},\end{array}$$ which may be zero or infinite and also depends on the strain rate. Some even more general force‐length‐time relationships for plastic are discussed which have been found useful for predicting the conditions of tensile failure. More complicated relationships of particular interest provide for the possibility of force‐time superposition of creep curves. Published creep data are considered from this point of view.