Papers by Author: Guo Zheng Kang

Abstract: The thermal ratcheting boundary of pressure pipeline is a popular topic in nuclear power engineering. The existed thermal ratcheting boundary based on the Bree diagram is conservative for structures subjected to the thermo-mechanically coupled loadings since it was obtained only from an elastic-perfectly plastic model. Therefore, it is necessary to improve the existed thermal ratcheting boundary based on a reasonable constitutive model. The Bree diagram was validated firstly by the linear relationship between the plastic strain increment and mechanical stress by finite element method. And then the influences of different constitutive models, such as elastic-perfectly plastic, multi-linear kinematic hardening, Chaboche and Abdel Karim-Ohno models, on the thermal ratcheting boundary of pressure pipeline were investigated numerically. It is found that the elastic-perfectly plastic and multi-linear kinematic hardening models provide the lower and upper bounds for the thermal ratcheting boundary, respectively. Finally, an improved thermal ratcheting boundary by introducing the dimensionless axial tensile stress was proposed based on the Bree diagram, the improved thermal ratcheting boundary covered the present cases with different ratios of mechanical stress over thermal stress.

Abstract: A plastic strain correction factor is used in a simplified elastic-plastic fatigue analysis of nuclear power plant components. Numerical investigation on the plastic strain correction factor is presented for the case of the primary and secondary stress range exceeding three times the design stress intensity value under thermal-mechanical loadings. The plastic strain correction factor was computed separately by following the RCC-M code and applying the elastic-plastic finite element analysis. The influence of loading ratio, loading controlled mode and ambient temperature on the plastic strain correction factor was discussed. It was shown that the plastic strain correction factor computed from the RCC-M code is not as conservative as that from the complete elastic-plastic finite element analysis when the primary plus secondary stress range is close to three times the design stress intensity value. However, it is too conservative when the primary plus secondary stress range is more than three times the design stress intensity value multiplying parameter m (use in RCC-M code). Additionally, a new formula of plastic strain correction factor was proposed to provide a complete envelope curve to the entire primary plus secondary stress range.

Abstract: Macroscopic cyclic tension-unloading experiments are conducted to investigate the cyclic deformation behaviors of CB filled vulcanized hydrogenated nitrile butadiene rubber at room temperature. In the load-controlled cyclic tension-unloading tests, remarkable ratchetting occurs, and the effects of the level and rate of cyclic loading on the ratchetting are also investigated. The ratcheting strain increases with the increasing mean stress and stress amplitude, and more obvious ratchetting is observed in the cyclic test with load-hold or at lower loading rate. In the displacement-controlled cyclic tension-unloading tests, the responding peak stress of the H-NBR decreases continuously, but the residual strain increases with the increasing number of cycles. Furthermore, the zero-stress hold at the end of cyclic test demonstrates that the residual strain will be recovered partially, which implies that the residual strain of the H-NBR after cyclic test consists of the reversible and irreversible parts.

Abstract: Experiments on U75V rail steel were carried out to investigate the cyclic feature, ratcheting behavior and low-cycle fatigue under both strain- and stress-controlled loadings at room temperature. It was found that U75V rail steel shows strain amplitude dependent cyclic softening feature, i.e., the responded stress amplitude under strain-controlled decreases with the increasing number of cycles and reaches a stable value after about 20th cycle. Ratcheting strain increases with an increasing stress amplitude and mean stress, except for stress ratio, and the ratcheting strain in failure also increases with an increasing stress amplitude, mean stress and stress ratio. The low-cycle fatigue lives under cyclic straining decrease linearly with an increasing strain amplitude, the fatigue lives under cyclic stressing decrease with an increasing mean stress except for zero mean stress, and decrease with an increasing stress amplitude. Ratcheting behavior with a high mean stress reduces fatigue life of rail steel by comparing fatigue lives under stress cycling with those under strain cycling. Research findings are helpful to evaluate fatigue life of U75V rail steel in the railways with passenger and freight traffic.

Abstract: Glassy shape memory polymer materials are applied successfully in biomedical fields due to their large recovery deformation, excellent biocompatibility and unique biodegradability. To predict the thermo-mechanical behavior of glassy shape memory polymers in biomedical devices accurately, a reasonably three-dimensional thermo-mechanical constitutive model must be established firstly. A one-dimensional linear-elastic constitutive model proposed by Tobushi et. al. (1997) was extended to capture the loading level dependent degradation of shape memory effect by introducing new nonlinear evolution equations with threshold values. Comparisons between experiments and simulations were carried to validate the extended model. Simulation results agree with experiments well, especially for the high loading levels.

Abstract: Fatigue tests were carried out at frequent of 20 kHz for 5083 aluminum alloy. The loading way is uniaxial and bending loading. The S-N curve of uniaxial loading presents a duplex curve corresponding to surface fracture and interior fracture. However the S-N curve of the bending fatigue shows the continuous curve. This demonstrates that different loading ways lead to different S-N curve characteristics. For uniaxial loading, almost all crack initiated interior of specimen in the very high cycle regime. The crack source zone appears wear away because of the constant pressure and grinding of this area in the process of cyclic loading. For the symmetric bending loading, the crack of corner in the specimen expands at different rates and direction.

Abstract: The uniaxial time-dependent cyclic deformation of POM/PTFE blends was studied experimentally under the stress-controlled and strain-controlled loading. The volume fraction of PTFE in the POM/PTFE blends was 20%. It was shown that the creep behavior of POM/PTFE blends is better than that of POM. In the cyclic strain-controlled experiments, the responded stress amplitude has a close relationship with the applied strain amplitude and strain rate. As a result of stress relaxation, the responded stress amplitude decreases as the hold time at peak strain increases. Ratcheting occurs in the blends subjected to asymmetric stress-controlled cyclic loading, and the ratcheting strain depends on the applied stress amplitude and stress rate. Hold time at peak stress and low stress rate lead to an increased ratcheting strain. The POM/PTFE blends will fail as the hold time at peak stress is longer than a critical value.

Abstract: Uniaxial ratchetting-fatigue interaction of tempered 42CrMo alloy steel was observed by various cyclic stressing tests at room temperature. The ratchetting deformation and low cycle fatigue (LCF) property of the material as well as their interaction occurred in cyclic stressing were discussed. It is shown that progressive ratchetting deformation causes the decrease of fatigue life, and the fatigue life of the material depends greatly upon the applied mean stress, stress amplitude, maximum stress and stress ratio. Since tempered 42CrMo steel presents significant cyclic softening feature, a tertiary ratchetting is observed. Based on the experimental results, a simple and reasonable failure model convenient to engineering application was constructed to predict the fatigue life of the material in uniaxial cyclic stressing. The basic variables of the model are maximum stress and stress ratio, and the effect of cyclic softening feature on the ratcheting-fatigue interaction is also included in the model by introducing a new variable. It is shown that the predicted lives are in fairly good agreement with the experimental ones.

Abstract: Based on three dimensional cubic unit cell models containing several particulates with certain particulate arrangements, the monotonic tensile and uniaxial ratcheting behaviors of particulate reinforced metal matrix composites (i.e., T6-treated SiCP/6061Al composites) were numerically simulated by using elastic-plastic finite element code ABAQUS with help of newly developed user material subroutine (UMAT). In the simulations, the effects of different particulate arrangements inside the unit cell models on the monotonic tensile and ratcheting behaviors of the composites were discussed. It is shown that the effect of particulate arrangement on the ratcheting of the composite depends on the arranged modes and the number of particulates contained in the model, and the interaction between particulates can be represented reasonably by the cubic unit cell model with a suitable distribution of multi-particulates.

Abstract: The uniaxial/multiaxial cyclic deformation behaviors of SiCp/6061Al alloy composites with various particulate volume fractions were studied by uniaxial and multiaxial cyclic straining or stressing tests at room temperature. The cyclic softening/hardening features and ratcheting behaviors of T6-treated composites and un-reinforced matrix were discussed in different loading conditions. It is shown that the ratcheting also occurs in the composites under uniaxial and multiaxial asymmetrical cyclic stressing, and the ratcheting strain increases with stress amplitude and mean stress; however, the addition of SiC particulates into the matrix increases the resistance of the composite to ratcheting. The ratcheting depends greatly on the shapes of loading paths and mainly occurs in the direction of non-zero mean stress.