Classical constitutive models of cyclic plasticity are very poor in predicting the progressive deformation of ratchetting, though ratchetting is an important factor in the design of structural components. Recent works done in the last decade, however, have enabled us to simulate the strain accumulation due to ratchetting with reasonable accuracy. In the present paper, first, the state of the art in constitutive modeling for ratchetting is described by criticizing the classical models and by reviewing the recent modifications introduced for ratchetting. Then, the application of recent and calssical models to ratcheting problems such as the thermal ratchetting induced by moving temperature distribution is discussed to show the effectiveness of recent models in simulating ratchetting.
Long-term stress relaxation behavior was investigated on NCF 800H alloy. Complicated stress relaxation behaviors such as rapid decrease of residual stress and increase of residual stress were observed. Relationship between stress relaxation behavior and microstructural changes was examined. The decrease of residual stress was caused by the reduction of creep deformation resistance due to coarsening of carbides. Minus relaxation behavior which is equivalent to the increase of residual stress was related to shrinking of testing material due to precipitation of carbides. Therefore, it is difficult to extrapolate the long-term stress relaxation curve from the result of short-term tests, and the long-term residual stress value should be predicted using aged material.
The paper deals with estimation of thermal ratchetting strain in cylinders subjected to short axial travelling of temperature distribution. The estimation is made by considering partly pressurized cylinders as a model. It is assumed that the pressurized region, which is taken to occupy the temperature travel region, is ruled by the deformation theory with a constant secant modulus so as to estimate the thermal ratchetting strain conservatively. This assumption enables us to utilize a linear elastic solution for thin-walled cylinders. The secant modulus is specified using the analytical solution derived in the long travel case by assuming Masing's rule and no cyclic hardening. It is shown that the model gives good estimates to finite element analysis of the thermal ratchetting strain as well as to experiments on 316FR steel cylinders done by Kitade et al. An extension of the model, in which plastic yielding outside the travel region is taken into account, is also discussed.
The thermomechanical hysteretic behavior in an Fe-9%Cr-5%Ni-14%Mn-6%Si polycrystalline shape memory alloy is experimentally investigated under tensile and compressive cyclic thermomechanical loading. Similar to the other shape memory alloys, such as Ti-Ni and Cu-based alloys, the Fe-based shape memory alloy also exhibits the cyclic effect which appears macroscopically as the change in the hysteresis loop with cycling. The convergence to a limit loop is, however, not observed in the present alloy. The cyclic effect on the residual strain, the transformation strain and the martensite start stress is explained by the two metallurgical processes; the accumulation of the perfect dislocations and the shape irreversibility in the reverse transformation.
The prediction of remaining creep life by the Iso-stress method was investigated for 153500h service-exposed boiler tubes of 1.25Cr-0.5Mo-Si, 2.25Cr-1Mo and 9Cr-1Mo steels. At first, the internal pressure creep rupture strength of the service-exposed tube specimens of the steels was shown to be the same as the tensile creep rupture strength of the sheet specimens taken from the tubes. Then the temperature-accelerated creep rupture tests under iso-stress were carried out by the tensile creep rupture tests, because of simplicity of the tests. The Iso-stress method was verified for the virgin specimens of the steels by comparing the predicted long-term rupture times as long as 105h with the data in the NRIM Creep Data Sheets. The predicted rupture times agreed with the observed data within a factor of 2 or 3. The temperature-accelerated tests of the sheet specimens taken from the service-exposed tubes showed that the linear extrapolation of the rupture times to the service temperature gave the remaining creep rupture life of about 8×105h for the 1.25Cr-0.5Mo-Si steel and about 1.6×106h for the 2.25Cr-1Mo steel. However, the Iso-stress method could not be applied for the 9Cr-1Mo steel, because the logtr versus T curve was not linear, where tr is the creep rupture time and T is the testing temperature. It is concluded that the Iso-stress method is useful when the significant microstructural change does not take place during accelerated creep rupture tests at high temperatures.
In order to elucidate the fracture process in high-temperature low-cycle fatigue of a cast Ti-34wt.%Al with lamellar structure and to derive the life law of smooth specimens, the behavior of initiation and growth of microcracks is observed in detail. The results obtained are summarized as follows. (1) Microcracks, which originate from casting defects at and/or beneath the specimen surface, begin to initiate at less than 15% of the failure life. (2) The transgranular microcracks grow perpendicularly to the stress axis and transversely to the lamellar boundaries. They are neither blocked nor decelerated by the grain boundary. (3) Although the growth rate of microcracks has a large scatter, the average rate coincides with the fracture mechanics law which is characterized by the relationship between the crack growth rate, dc/dN, and the fatigue J-integral range, ΔJf. (4) The crack initiating at the earliest stage of many distributed cracks tends to grow up to be a main crack for the failure. (5) The fatigue life of smooth specimens, Nf, is successfully correlated with ΔWf-parameter which is derived by integrating fracture mechanics law of dc/dN vs. ΔJf relation.
On the basis of the concept of ductility exhaustion, a method of predicting the creep-fatigue lives with good precision has been developed for the material in the components operated at elevated temperatures, such as FBR and power boiler, in particular. Comparison of predicted lifetimes with experimental results for the FBR grade Type 316 stainless steel and for the power boiler grade Mod. 9Cr-1Mo steel has proved that the proposed method assesses the creep-fatigue life more accurately and always on the safer side than the currently popular method based on the time exhaustion concept.
The outline of the creep-fatigue life prediction model proposed by the authors and its applicability to the life prediction under variable strainings were summarized. In the model, crack growth curves in smooth specimens were predicted by the strain range partitioned crack growth rate equations, and these crack growth curves were used as damage accumulation curves. The sequential loading effect on creep-fatigue properties was evaluated considering the strain history dependency of cumulative damage curves. Results of two-step sequential straining tests such as High-Low and Low-High tests on Mod. 9Cr-1Mo, SUS304 and 316LC steel were presented and it was shown that the proposed model can be more successfully applied to the creep-fatigue life prediction under variable strainings than the conventional linear damage rule. Furthermore, in order to certify the validity of the model, the correspondence between predicted crack growth behaviors and experimentally obtained behaviors was examined. Surface crack growth behaviors in smooth bar specimens were observed for Mod. 9Cr-1Mo and 316LC steel and compared with those predicted. It was found that the predicted crack growth curves corresponded well with the relationships between the maximum crack length and the life ratio. Consequently, it was concluded that the proposed model can evaluate the material damage and the residual life under variable creep-fatigue strainings.
The microscopic aspect of creep-fatigue failure process in a 6061 aluminum alloy composite reinforced with SiC whiskers, SiCw/6061, was studied at high temperature, with specific attention being paid to the interaction between whiskers and the small crack growth. It was shown that the creep-fatigue damage progressed according to the following procedures: (i) the nucleation and the growth of small cracks, or microcracks in the matrix near the interface, (ii) the growth of the small cracks constrained by whiskers, and the increase of the crack density, (iii) the crack face bridging by whiskers, accompanying with the inhibition of the small crack growth by whiskers, (iv) the coalescence of the small cracks, accompanying with the pull-out of whiskers, followed by the final rupture. The crack propagation rates of the SiCw/6061 were also studied, comparing with those of the matrix alloy, on the basis of non-linear fracture mechanics parameter; creep J-integral. An interesting correlation of the creep-fatigue crack growth rates between the SiCw/6061 and the matrix alloy and other kinds of metallic materials was found, by taking account of the creep ductility.