AbstractThe relationship is shown between the flex life and long‐termtensile strength of flexible composite material. The long‐term strength is interpreted in terms of a kinetic theory of failure. However, it is not posible to predict the flex life from tensile strength or even long‐term strength data unless the stiffness or modulus is considered along with the thickness of the sample. With these factors it becomes possible to predict the flex life of a material and to compare various flex‐testing instruments utilizing a new concept, which material and to compare various flex‐testing instruments utilizing a new concept, which we call “effective force.” The usefulness of the concept of effective force
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