An improved analytical method for life prediction of bolting

A research project was conducted to develop and validate an improved, analytical life prediction method for high temperature turbine and valve studs/bolts. The life prediction method used the two-parameter creep equation, an incremental calculation procedure, and a strain hardening flow rule. The failure criterion was an accumulated inelastic or creep strain limit of 1%. The life prediction procedure recommends the use of the service history of operating temperature, number/stress level of tightenings, cycle time, etc., to calculate the stress relaxation behavior. Life assessment uses the measured bolt length to calculate the accumulated creep strain. The link between the current condition, i.e., accumulated creep strain, and the remaining creep life, i.e., time to accumulate 1% strain, is obtained by a prediction of the future creep strain accumulation under the intended loading cycle(s) imposed during future operation. In order to validate the approach, the calculated results were compared to the results of uniaxial stress relaxation testing, bolt model testing, and service experience. The analytical procedure coupled with other industry wide NDE and measurement procedures is expected to provide broad guidelines to utilities for bolting life assessment.