Mitsuo Miyahara

Fatigue-creep life prediction for a notched specimen of 214Cr1Mo steel at 600°C

Abstract This paper presents the life prediction of 2 1 4 Cr ue5f81 Mo notched specimens subjected to fast-fast, slow-slow and hold-time loadingsat 600°C. The crack initiation lives of notched specimens were estimated based on the local stress-strain calculated by inelastic finite element analyses. For the life prediction, combinations of seven different constitutive models and five fatigue-creep damage laws were used. The applicability of the constitutive model and damage law is discussed. The constitutive models predict similar stress-strain relations at the notch root, leading to similar predicted lives. The damage model, however, has a much larger influence on the life prediction.

Evaluation of Hydrogen Environment Embrittlement and Fatigue Properties of Stainless Steels in High Pressure Gaseous Hydrogen: Investigation of Materials Properties in High Pressure Gaseous Hydrogen—2

Hydrogen environment embrittlement (HEE) susceptibility in high pressure gaseous hydrogen was investigated on 300 series austenitic stainless steels and A6061-T6 aluminum alloy. Tensile properties of these materials were evaluated by Slow Strain Rate Testing (SSRT) in gaseous hydrogen pressurized up to 90MPa (13053 psig) in the temperature range from −40 to 85 degrees C (−40 to 185 degrees F). HEE susceptibilities of austenitic stainless steels strongly depended upon the chemical compositions and testing temperatures. A6061-T6 aluminum alloy showed no degradation by hydrogen. Fatigue properties in high pressure gaseous hydrogen were evaluated by the external cyclic pressurization test using tubular specimens. The tubular specimen was filled with high pressure hydrogen gas, and the outside of the specimen was cyclically pressurized with water. Type 304 showed a decrease in the fatigue life in hydrogen gas, while as for type 316L and A6061-T6 the difference of the fatigue life between hydrogen and argon environments was small. HEE susceptibility of investigated materials was discussed based on the stability of an austenitic structure.Copyright

Fatigue Strength Prediction of Spot-Welded Joints Using Small Specimen Testing

It is well known that fatigue strength of spot weld of high strength steel sheet is not improved, compared with that of mild steel sheet. In this study, the governing factors and the effects of steel grade on fatigue strength of spot weld is investigated. Firstly, small specimens with total length of less than 3mm are taken from the spot weld of mild steel sheet (270MPa-grade) and high strength steel sheet (590MPa-grade). And then, tensile and high cycle fatigue properties are individually evaluated by newly-developed testing technique. Secondly, finite element analyses of tensile-shear specimen of spot-welded joints under cyclic loading are carried out and fatigue limit of the joints are predicted, using the above-mentioned local material strength properties and considering welding residual stresses around spot weld. Predicted results are nearly equal in both steels, which coincides with experimental results. It is found that fatigue strength of HAZ, which is the crack initiation site in joint, of 590MPa-grade steel is higher than that of 270MPa-grade steel. However, residual stress in 590MPa-grade steel is also higher and this may be one of the reasons why 590MPa-grade steel exhibits little improvement in fatigue strength of the joint over 270MPa-grade steel.

A new high-temperature creep-fatigue damage rule based on the strain range partitioning concept

Abstract A new life prediction model based on the strain range partitioned crack growth rate equations is presented which can evaluate the material damage and the residual life under variable creep-fatigue strainings. Two-step sequential creep-fatigue straining tests such as High-Low and Low-High tests were conducted on Mod. 9Cr-1Mo steel and type 316LC stainless steel in order to examine the validity of the proposed model. It was found that the proposed model can be more successfully applied to the creep-fatigue life prediction under variable strainings than the conventional life prediction methods.

Creep-Fatigue Properties of Fe-Ni Base 0.08C-23Cr-45Ni-7W Alloy for Piping in 700°C A-USC Power Plants

Creep-fatigue properties of Fe-Ni base 0.08C-23Cr-45Ni-7W alloy (HR6W) are investigated at 700°C and compared with Ni base superalloy Alloy617 in order to examine applicability of these materials to piping in 700°C A-USC power plants. It is clarified that the fatigue lives of HR6W and Alloy617 are almost the same under PP (fast-fast) type strain waveform, but the life of HR6W is longer than that of Alloy617 under CP (slow-fast) type strain waveform. The crack initiation lives at the ligament parts of superheater outlet headers, which are made of HR6W and Alloy617, are predicted from the results of three-dimensional FE-analyses. The transient heat transfer analysis and the elastic-plastic-creep thermal stress analysis are conducted on several kinds of typical operating conditions to evaluate the inelastic strain caused by the thermal cycles during start-up and shut-down of power plants. The maximum steam temperature and pressure are selected as 700°C and 34.3MPa, respectively, and the maximum cycle time of operation is 1 year. On the basis of the analytical results, creep-fatigue and creep rupture properties of the materials, the crack initiation lives of the headers are predicted with the strain range partitioning method. Inelastic strain in the header of HR6W is larger than that of Alloy617, because HR6W has lower creep deformation resistance and a slightly higher thermal expansion coefficient. However the predicted life of the header of HR6W is longer than that of Alloy617, because HR6W is much superior to Alloy617 in creep ductility and creep-fatigue properties.Copyright

High Temperature Fatigue Properties and Life Prediction of SUS304 Stainless Steel under Variable Straining

ABSTRACT In order to examine the applicability of the proposed fatigue life prediction model under variable straining, PP type High-Low and Low-High tests were conducted for the as solution-treated SUS304 at 700, 800°C and for the thermally aged SUS304 at 700°C in air. It was found that the experimental results for the as solution-treated material at 800°C and for the aged material at 700°C were well explained by the proposed model. On the other hand, in the case of the as solution-treated material at 700°C the deviation from the proposed model was observed in some testing condition. By comparing the cyclic hardening behavior under each testing condition, it was supposed that the discrepancy from the proposed model prediction was caused by the influence of the cyclic hardening during the primary straining on the inelastic strain range of the secondary straining.