Masatsugu Yaguchi

Creep-fatigue damage assessment by subsequent fatigue straining

Abstract A series of creep-fatigue tests has been conducted with modified 9Cr-1Mo steel at 873 K in a high vacuum environment of 0.1 mPa. In order to investigate the accumulation of creep-fatigue damage, the creep-fatigue test programme includes changes in strain waveform during the test: from creep-fatigue type to fatigue type and from fatigue type to creep-fatigue type. The conventional linear cumulative damage rule for fatigue and/or creep-fatigue damage fails in evaluating the creep-fatigue life under the present complicated strain wave history. The linear summation of the life fraction is smaller than unity when the prior loading is creep-fatigue type and larger than unity when the prior loading is fatigue type. Scanning electron microscope (SEM) observation of the fracture surface was also conducted. In the case where the strain waveform changes from prior creep-fatigue type to subsequent fatigue type, the crack mode changes from transgranular to intergranular with an increase in the prior creep-fatigue loading history. In the case where the strain waveform changes from prior fatigue type to subsequent creep-fatigue type, the primary crack mode is generally intergranular regardless of the prior fatigue loading history.

Mechanistic approach for creep-fatigue evaluation of 9Cr-1Mo-V-Nb steel

A mechanistic model was developed to describe the creep-fatigue interaction of 9Cr-1Mo-V-Nb steel based on a damage mechanics approach. The model assumes that each of the fatigue and creep damage consists of basic damages with a size and density. The mechanism of the creep-fatigue interaction is considered to be an early growth of a size of fatigue basic damage from a creep basic damage formed prior to fatigue loading. The model interprets well the experimental creep-fatigue life under complex strain histories.

Damage Evolution and Life Prediction of a P91 Longitudinal Welded Tube Under Internal Pressure Creep

The clarification of creep damage mechanism and the establishment of remaining life prediction methods of longitudinal welded piping of P91 steel are important subjects to maintain a reliable operation of boilers in thermal power plants. Internal pressure creep tests were conducted on P91 steel longitudinal welded tubes to characterize the evolution of creep damage with time and to evaluate a life prediction method. Interrupted creep tests were performed for damage observation in addition to rupture tests. Three dimensional finite element creep analyses of the longitudinal welded tube specimens were conducted to identify the stress and creep strain distributions within the specimen during creep. Failure occurred at a heat affected zone (HAZ) without a significant macroscopic deformation. It was found that the initiation of creep voids had concentrated at the midthickness region in the HAZ rather than in the surface. The creep analysis results indicated that the triaxial tensile stress yielded at the midthickness region in the HAZ due to difference of creep deformation property among the base metal, the HAZ, and the weld metal. It was suggested that the triaxial stress state caused acceleration of the creep metal. It was suggested that the triaxial stress state caused acceleration of the creep damage evolution in the HAZ, resulting in internal failure of the tube specimens. A rupture time prediction method of the welded tube is proposed based on the maximum principal stress and the triaxial stress factor in the HAZ. The void growth behavior in the HAZ was well predicted by the previously proposed void growth simulation method by introducing a void initiation function to the method.

Creep-Fatigue Damage Evaluation of Modified 9Cr-1Mo Steel Based on the Overstress Concept

A series of creep-fatigue tests was conducted with Modified 9Cr-1Mo steel in a very high vacuum environment. The test results reflect the creep-fatigue behavior of this material which is completely free from the environmental effect of the air. The frequency effect is not observed in the vacuum environment but the strain wave effect is still found to remain. The overstress and the internal backstress were experimentally investigated and a creep-fatigue damage model is developed based on the overstress. Finally, SEM observations were conducted and the micro-mechanical implementation of the damage model is discussed.

High temperature Strength. Inelastic Constitutive Equations of Ni-Based Superalloy IN738LC under Anisothermal Cyclic Deformation Conditions.

A series of deformation tests was conducted on Ni-based superalloy IN738LC under isothermal and anisothermal conditions between 450°C and 950°C. Under the isothermal conditions, the material showed no rate/time-dependency below 700°C, while there was distinct rate/time-dependency above 800°C. Anomalous inelastic behavior was observed under the anisothermal conditions; with an increase of cycles, stress at higher temperatures became smaller in absolute value, and stress at lower temperatures became larger. Based on the experimental results, the previously proposed viscoplastic constitutive model for IN738LC at 850°C was extended to the anisothermal conditions. In the constitutive model, evolution of formally incorporated variable Y was assumed to be active under the higher temperatures and negligible under the lower temperatures. The extended constitutive model was applied to the anisothermal cyclic loading as well as monotonic tension, stress relaxation, creep and cyclic loading under the isothermal conditions. It was demonstrated that the present constitutive model was successful in describing the inelastic behavior of the material adequately, including the “anomalous inelastic behavior” observed under the anisothermal conditions.