Notch effect on structural strength of components at elevated temperature under creep, fatigue and creep-fatigue loading conditions : phenomenon and mechanism

Structural discontinuities (e.g., nozzle, hole, and groove) widely occur in many high temperature components of nuclear and fossil power plants. In general, the notched component is used for simplified tests and analyses due to the complexity of the introduction of a practical component. In the previous work, the effects of the notch on failure life of the components have been reported experimentally, including the strengthening and weakening effects; however, the internal mechanisms have not been clearly demonstrated. This work reviews the notch effects on the structural strength of the notched components at elevated temperatures under creep, fatigue, and creep-fatigue loading conditions. Experimental phenomena (i.e., strengthening or weakening effects) for typical notched specimens subjected to the above three loading conditions are summarized, and the related factors for notch effects on creep rupture life or cycle to failure of the components are discussed. The mechanisms for the strengthening or weakening effects induced by a notch are described. Evaluation procedures for notch effect analysis under complex loading conditions are also included, and the primary challenges concerning the notch effect are provided for further investigations

Comparison Between Single Loading–Unloading Indentation and Continuous Stiffness Indentation

Experiments are performed on fused silica, Si, and duplex stainless steel to examine whether the CSM (continuous stiffness indentation) method will provide approximately the “same” results of contact modulus and indentation hardness as those measured from the quasi-static single loading–unloading indentation. The experimental results show that the elastic modulus measured by the CSM method is compatible with that by the quasi-static loading–unloading method for hard materials, while there exists a percentage difference of ∼21.3% between the smallest value and the largest vale of the measured indentation hardnesses from the CSM method for fused silica and a percentage difference of ∼15.3% between the hardnesses measured by the CSM method and the single indentation for duplex stainless steel. The large percentage difference suggests that the indentation hardness measured by the CSM method may not be compatible with that measured by the quasi-static loading–unloading method for hard materials. The finite element results reveal the percentage difference between the indentation hardness at the wave peak and that at the wave valley for the CSM method increases with the increase of the ratio of elastic modulus to yield stress

A direct approach to the evaluation of structural shakedown limit considering limited kinematic hardening and non-isothermal effect

This paper presents a novel direct method for the structural shakedown analysis considering limited kinematic hardening and non-isothermal effect. The Melan’s static shakedown theorem is extended to consider limited kinematic hardening material and implemented into the Linear Matching Method (LMM) shakedown module. Instead of using a specific kinematic hardening rule and an explicit back stress field, the general nonlinear hardening laws are considered by using a two-surface hardening model. A two-stage procedure is developed in the extended LMM algorithm, which can generate the limited hardening shakedown envelope and the unlimited hardening curve efficiently and accurately. Also, the material non-isothermal effect is considered during the computation process of the shakedown limit by proposing a temperature-dependent hardening factor, in place of a constant and fictitious one. To validate the extended LMM method, a numerical test on a thin cylinder pipe with temperature-independent material properties is performed, and the results match well with ones from literature. Then, a numerical study on a typical aero-engine turbine disk is conducted to investigate the influence of temperature-dependent material properties and operating conditions. Several shakedown curves considering kinematic hardening effect are derived and adequately discussed. As a result, the extended LMM shakedown module is proven to be a robust, efficient and versatile tool for practical industrial problems

Crack tip strain evolution and crack closure during overload of a growing fatigue crack

It is generally accepted that fatigue crack growth is retarded after an overload, which has been explained either by plasticity-induced crack closure or near-tip residual stress. However, any interpretation of overload effect is insufficient if strain evolution in front of crack tip is not properly considered. The current understanding of overload-induced retardation lacks the clarification of the relationship between crack closure at crack wake and strain evolution at crack tip. In this work, a material with low work hardening coefficient was used to study the effect of overload on crack tip strain evolution and crack closure by in-situ SEM observation and digital image correlation technique. Crack opening displacement (COD) and crack tip strain were measured before and after the overload. It was observed that the evolution of crack tip strain follows the crack opening behaviour behind the crack tip, indicating a smaller influence of overload on micro-mechanical behaviour of fatigue crack growth. After the overload, plastic strain accumulation was responsible for crack growth. The strain at a certain distance to crack tip was mapped, and it was found that the crack tip plastic zone size correlated well with crack growth rate during post-overload fatigue crack propagation

Enhanced fatigue damage under cyclic thermo-mechanical loading at high temperature by structural creep recovery mechanism

Creep-cyclic plasticity of a benchmarked holed plate subjected to thermo-mechanical loading is investigated by means of nonlinear finite element analysis. From the analyses, a structural creep recovery response is found within a dwell period, which has serious repercussions on structural integrity. The structural creep recovery can take place by reversing the creep stress in sign during the stress relaxation due to the creep stress redistribution, consequently enhancing unloading plasticity which causes a substantial increase of total strain range within a cycle. Based on this critical observation, further analyses and discussions are provided to investigate the root cause of this precautious structural response. Various cyclic loadings with a dwell at the peak thermal load are analysed to define factors influencing the structural creep recovery mechanism, and to investigate how the mechanism affects the lifetime of the structure. To show the effectiveness of the structural creep recovery mechanism under cyclic loading, Chaboche nonlinear kinematic hardening model is adopted. Limitations of applying elastic follow-up in predicting creep strains and appropriate creep fatigue damage calculation methods are discussed in the presence of this structural creep recovery mechanism. This research work confirms that when a structure experiences the structural creep recovery it can reduce creep damage, nevertheless the structure may experience significant fatigue damage due to creep enhanced plasticity

A comparative study on the cyclic plasticity and fatigue failure behavior of different subzones in CrNiMoV steel welded joint

The cyclic plasticity and the low cycle fatigue failure behavior of the weld metal (WM) and base metal (BM) of the CrNiMoV steel welded joint under the strain and stress-control modes were investigated respectively. Significant cyclic softening was observed for both the WM and BM under the low cycle fatigue tests with the two control modes. Besides, obvious ratcheting happened in the WM and BM under the stress-controlled cyclic loading conditions. It is shown that both the WM and BM exhibited lower fatigue strength at the stress control mode than that at the strain control mode due to the influence of tension-compression asymmetry. Meanwhile, the WM showed larger cyclic softening rate, lower ratchetting deformation and fatigue strength than the BM under the same loading levels. The failure location of the WM specimens shifted from BM region (nearby the heat affected zone) to the center of WM with the increasing of strain amplitude under the strain-controlled tests, which can be explained with the similar maximum equivalent plastic strain amplitude location shifting behavior observed from the corresponding finite element simulations