Abstract
This paper is an attempt to present an analized picuture of the variation in the mechanical property of tyre cord in relation to the growing of fatigue given by th repeated flexing fatigue test under various load.
In the diagram of the tensile strength of cord and numbers of flexing cycle (in logarithms) no change is shown at a certain critical point of fatigue cycle, but over this point the values of tensile strength decrease rapidly and almost linearly with numbers of flexing cycle. The larger the weight of the original static stress is, the lower the limit of flexing cycle is, and the rate of decrease of the tensile strength is smaller with the larger original static stress. The rate of decrease of the tensile strength depends on the types of fibers, e.g., it becomes very slow in High Tenacity Rayon Cord and sharp in cotton and BX Cords.
It is found that the limit of decreased tensile strength by flexing cycle (the ultimate strength) is much more than the weight of the original static stress, of which the reason cannot be explained for the present. It is, however, very intesting to know that the above result coincides with following fact, that is, in the case of the practical use the ultimate strength is far greater than the breaking strength of the cord.
Variation on impact breaking energy corresponding to the growing fatigue is similar to that in the case of the tensile strength, but the rate that of decrease of impact breaking energy is more greater and the limit of the decrease is lower compared with the rate of decrease of the tensile strength. This suggests that the breaking of the cord is brought from the impact rather than quasi static.
The increasing fatigue leads to the breaking of monofilament independently of the tensile strength of the cord, so that there is no change of the tensile strength until the half number of filaments of the cord are broken down. The breaking of monofilament, of course, is most ready to take place at the part where the most frequent flexings are given. It is often observed that at many points single filament is broken down in the flexing parts of the cord. Therefore it comes to the conclusion that the tensile strength depends upon both the strength of monofilament and the frictional force between monofilaments.
The decrease of the tensile strength is not so remarkable in both broken monofilament and unbroken one. But the decrease of elongation is distinctly recognized, and the ultimate elongation is a half of the original one. Then it is supposed that the cause for the breaking of monofilament is not due to the decrease of the tensile strength, but is due to the decrease of elongation.
