Multiaxial creep-fatigue rules

Within the UK, a comprehensive procedure, called R5, is used to assess the high temperature response of structures. One part of R5 deals with creep-fatigue initiation, and in this paper we describe developments in this part of R5 to cover multiaxial stress states.To assess creep-fatigue, damage is written as the linear sum of fatigue and creep components. Fatigue is assessed using Miner's law with the total endurance split into initiation and growth cycles. Initiation is assessed by entering the curve of initiation cycles vs. strain range using a Tresca equivalent strain range. Growth is assessed by entering the curve of growth cycles vs. strain range using a Rankine equivalent strain range. The number of allowable cycles is obtained by summing the initiation and growth cycles. In this way the problem of defining an equivalent strain range applicable over a range of endurance is avoided.Creep damage is calculated using ductility exhaustion methods. In this paper we address two aspects: first, the effect of multiaxial stress on creep ductility; secondly, the nature of stress relaxation and, hence, accumulated creep strain in multiaxial stress fields. ((orig.))