Extension of the Theta Projection Concept in Multi-Axial and Stress-Change Creep Behavior

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The purpose of this project is to estimate creep behavior at the stress-change state or under multi-axial stresses with a help of both the theta projection concept and the strain hardening law.(1)Initially a series of constant-load creep tests were conducted on austenitic stainless steel AISI type 304 at 923K.Four parameters in the theta projection concept were computed for each creep curve using the non-linear least square method. Since these parameters were well expressed as an exponential function of the applied stress, creep curves even under unknown stresses could be easily drawn.(2)Stress-change creep tests were performed between the stresses of 90MPa and 140MPa. After the specimens were crept at the initial stress of 90MPa for 50, 100, 200, 300 and 400 hours, the applied stress was increased to 140MPa. These hours when the creep tests were interrupted corresponded to the primary, secondary, and (former, middle, and latter part of) tertiary creep respectively.(3)Creep behavior after the change of the applied stress was calculated using the proposed method. The estimation curves were compared with the experiments. When the interrupted points were places within the former part of the tertiary stage of creep, they well agreed each other. However the calculation were underestimated over the range of the middle part of the tertiary stage. Then we concluded that the application of the theta projection concept should be limited within the initial part of the tertiary creep if the strain hardening law as employed together.(4)The experiments reported here were the results when the stress was increased after some creep deformation. However we should investigate whether this method can also be applied to the stress-change creep test after the applied stress was decreased.