This International Standard specifies a method for calculating the maximum allowable hoop stress applicable to pipes exposed to varying internal pressures and/or temperatures during their expected lifetime. This method is generally known as Miner's rule. It is necessary to apply Miner's rule to each failure mechanism separately. Thus, for mechanical failure due to internal pressure, other failure mechanisms, such as oxidative or dehydrochlorinative degradative failure mechanisms, are to be neglected (assuming, of course, no interaction). A material may be used only when it is proven to conform to all failure mechanism criteria. NOTE — Miner's rule is an empirically based procedure, and is only a first approximation to reality.