A physical mechanism based constitutive model for temperature-dependent transformation ratchetting of NiTi shape memory alloy: One-dimensional model

In this paper, the effect of test temperature on the transformation ratchetting of super-elastic NiTi shape memory alloy was first investigated in the cyclic tension-unloading tests. It is shown that all the residual strain, dissipation energy, the start stress of martensite transformation and their evolutions during the cyclic loading depend greatly upon the test temperature. Based on the experimental observations, a new one-dimensional constitutive model is constructed by considering two different inelastic deformation mechanisms (i.e., martensite transformation and transformation-induced plasticity). The proposed model employs a new evolution rule of transformation-induced plasticity which considers the physical mechanism of the plastic deformation, i.e., the dislocation slipping in the austenite phase near the austenite–martensite interfaces. Furthermore, the interaction between dislocation and martensite transformation is also taken into account in the proposed model. The capability of the proposed model to predict the uniaxial temperature-dependent transformation ratchetting of NiTi shape memory alloy is verified by comparing the predictions with the experimental data.     

Deformation analysis of notched components and assessment of approximate methods

Finite element modelling was conducted on notched members subjected to proportional and non-proportional loading. A recently developed cyclic plasticity model capable of accurately describing cyclic material behaviour was implemented into a finite element code. A plate with a central hole and a shaft with a circumference groove were studied. Approximate methods for the notched problems were critically evaluated using the finite element results.     

Fretting fatigue crack nucleation in Ti–6Al–4V

Fretting fatigue crack nucleation in Ti?6Al?4V when fretted against itself is investigated to determine the influence of contact pressure, stress amplitude, stress ratio, and contact geometry on the degradation process. For the test parameters considered in this investigation, a partial slip condition generally prevails. The resulting fatigue modifying factors are 0.53 or less. Cycles to crack nucleation, frictional force evolution, crack orientations and their relationship to the microstructure are reported. The crack nucleation process volume is of the same order as the microstructural length scales with several non‐dominant cracks penetrating 50 μm or less. The effective coefficient of friction increases during early part of fretting. Observations suggest that cyclic plastic deformation is extensive in the surface layers and that cyclic ratchetting of plastic strain may play a key role in nucleation of the fretting cracks. A Kitagawa–Takahashi diagram is used to relate the depth of fretting damage to the modifying factor on fatigue life.     

Investigation of the multiaxial fatigue behaviour of 316 stainless steel based on critical plane method

In this work, the multiaxial behaviour of 316 stainless steel is studied under the lens of critical plane approach. A series of experiments were developed on dog bone–shaped hollow cylindrical specimens made of type 316 stainless steel. Five different loading conditions were assessed with (a) only tensile axial stress, (b) only hoop stress, (c) combination of axial and hoop stresses with square shape, (d) combination of tensile axial and hoop stresses with L shape, and (e) combination of compressive axial and hoop stresses with L shape. The fatigue analysis is performed with four different critical plane theories, namely, Wang‐Brown, Fatemi‐Socie, Liu I, and Liu II. The efficiency of all four theories is studied in terms of the accuracy of their life predictions and crack failure plane angle. The best fatigue life predictions were obtained with Liu II model, and the best predictions of the failure plane were obtained with Liu I model.     

SIMULATION OF NONPROPORTIONAL CYCLICEXPERIMENTS OF IN738LC AT 850℃

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Investigation of rolling contact crack initiation in bainitic and pearlitic rail steels

The stress–strain history and the crack initiation lives of bainitic and head‐hardened pearlitic rail steels were determined under rolling contact loading by implementing the semi‐analytical Jiang–Sehitoglu rolling contact model that incorporates both ratchetting and multiaxial fatigue damage. The calculations revealed that the bainitic steel withstands higher loads than the pearlitic steel at low shear tractions, however; both materials behave in an increasingly similar manner as the shear tractions increase. Furthermore, maximum damage occurs in both steels when ratchetting and fatigue damage coincide on the surface. In addition to shedding light on the rolling contact fatigue (RCF) performance of bainitic and pearlitic rail steels, the current work also establishes a methodology for the realistic prediction of crack initiation under RCF.     

Geometric effects on shakedown and ratchetting of axisymmetric cylindrical shells subjected to variable thermal loading

This paper presents results obtained from numerical simulations of the responses of an elastic-plastic thin cylindrical shell to fluctuating axisymmetric temperature in the presence of uniform axial stresses. The engineering situation considered has practical importance in nuclear reactors and has been the subject of a number of earlier studies. The main purpose is to assess quantitatively the influence of geometry changes, primarily due to plastic yielding, on shakedown and ratchetting (incremental collapse) phenomena. In particular, these phenomena are investigated with respect to both the stabilizing effects of tensile primary stresses on them, and their strong interference with elastoplastic buckling. The systematic evolutive analyses presented herein are also intended to critically assess the validity of earlier results (mainly condensed in the so-called Brussels diagrams) which have been established by simplified methods of shakedown based on the small deformation (no geometric effects) hypothesis.     

An experimental study of creep‐ratchetting behavior of rolled AZ31B magnesium alloy at room temperature

To study the interaction effect of creep and ratchetting for rolled AZ31B magnesium alloy at room temperature, a series of stress‐controlled tests were designed. In the tests, four loading types with different mean stresses were considered, and dwell loading was applied to explore the creep effect on the ratchetting. The test results indicated that the sequence of ratchetting and creep loading is crucial for the strain evolution. The amount of twinning/detwinning increased as the mean stress decreased, leading to an exhaustion of nonbasal slip during ratchetting, and then suppressed the creep ductility. However, the creep sequence exerted little influence on the strain shift of ratchetting while large amount of twinning/detwinning was involved.     

Temperature‐dependent uniaxial ratchetting of ultra‐high molecular weight polyethylene

Uniaxial stress‐controlled cyclic tests were performed on the ultra‐high molecular weight polyethylene (UHMWPE) polymer at different temperatures. The effects of stress level, stress rate, peak/valley stress hold and loading history on the ratchetting of the UHMWPE were discussed at different temperatures, and the temperature‐dependence of ratchetting was addressed. It is concluded from the experimental observations that the ratchetting of the UHMWPE depends greatly on the test temperature, and the ratchetting strain increases with the increasing temperature from ?20 to 37 °C, but decreases when the temperature is equal to or higher than 60 °C in some cases. The ratchetting is also time‐dependent and increases with the increase of peak stress hold time and the decrease of stress rate. The ratchetting presents apparent loading history dependence, and previous cyclic history with higher stress level remarkably restrains the occurrence of ratchetting in the subsequent cyclic loading cases with lower stress levels, but previous cyclic history with lower stress level hardly influences the ratchetting in the subsequent cyclic loading cases with higher stress levels.     

Strain Accumulation in Ti-6Al-4V during Fatigue at High Mean Stress

The effect of mean stress and frequency on the high cycle fatigue behavior of Ti-6Al-4V has been investigated. It has been shown that a transition in the fatigue behavior occurs at a stress ratio of approximately 0.7. Above this value, the material exhibits measurable strain accumulation and necking. Since Ti-6Al-4V is susceptible to room temperature creep, an empirical model was developed using static creep data in an attempt to predict the cyclic behavior of the material. The model was unable to account for the large amounts of strain seen experimentally. In addition, closer examination of the data revealed that the deformation was more closely related to the number of cycles than to time.