Cut Growth and Flex Testing of Natural Rubbers and Synthetic Elastomers

Abstract

Abstract In the last decades, numerous studies have appeared concerning the behavior of tears in natural rubber and synthetic elastomers. In the United States' and England there are even standardized test methods. It can be seen from all the investigations and test instructions that the results are very poorly reproducible and only rarely agree with practical experience. In investigations on the behavior of tears, the two stages—the beginning of the tear and the growth of the tear (tear propagation)—must be differentiated. The following factors must be given primary consideration in starting the tear: external damage, the action of ozone and oxygen, frequently changing tensile or compression stresses. Here oxygen plays a role similar to that in the corrosion of metals. Fatiguing mechanical stress undoubtedly leads to the appearance of tears. But it can be assumed that the beginning of a tear observed in the area of maximum stress takes place at a preferred point on the surface. Such preferred points are always present. As is well known, the surface of a material— even on a high polymer—must not be regarded as completely “smooth”. In place of this supposed smoothness, there appears under microscopic or electron-microscopic observation a kind of “mountain system”. Observed under sufficient magnification, the “valleys” or cuts in the mountains are comparable to tears. Under mechanical stress, such cuts will then continue to grow particularly rapidly (nick effect). Now it is largely dependent on the kind of observation and the observer as to whether a small irregularity in the surface is called a tear. The beginning of the tear cannot, therefore, be defined clearly. The case is quite different with tear growth. The initial conditions—the tear present at the beginning of measurement—are given and only the growth of the tear has to be followed by measurement. The growth of the tear therefore seems to us to be well suited for detailed investigation.