Energy-based Low-cycle Fatigue Research Articles

An energy-based low-cycle fatigue life evaluation method considering anisotropy of single crystal superalloys

The crystal orientation significantly affects the low-cycle fatigue (LCF) properties of single crystal (SC) superalloys. However, the orientation-dependent LCF life model with precise mechanisms and strong applicability is still lacking. This investigation aims at establishing an energy-based LCF life evaluation method that could consider the orientation effect. First, the influencing factors of anisotropy were identified through the literature review. Secondly, the multiaxial formula of the Ramberg-Osgood (R–O) equation was established to describe the anisotropic cyclic deformation characteristics. Furthermore, the strain energy density of SC superalloys was determined based on this equation, and the effective strain energy density was introduced to account for the effect of orientation. Finally, the energy-based method was validated by its application to several SC superalloys. Results showed that the crystallographic orientation with a lower Young's modulus usually exhibits better LCF resistance. This phenomenon could be attributed to the different values of strain energy density dissipated in one cycle. The multiaxial R–O relationship could capture the anisotropic cyclic deformation response of DD6. Compared with the classical methods, the energy-based model is favored by its precise mechanism and strong applicability. And it also exhibited better prediction accuracy. Most data points of different crystallographic orientations lay within the ±3 error band.

An Energy-Based Unified Approach to Predict the Low-Cycle Fatigue Life of Type 316L Stainless Steel under Various Temperatures and Strain-Rates.

An energy-based low-cycle fatigue model was proposed for applications at a range of temperatures. An existing model was extended to the integrated approach, incorporating the simultaneous effects of strain rate and temperature. A favored material at high temperature, type 316L stainless steel, was selected in this study and its material characteristics were investigated. Tensile tests and low-cycle fatigue tests were performed using several strain rates at a temperature ranging from room temperature to 650 °C. Material properties were obtained in terms of temperature using the displacement-controlled tensile tests and further material response were investigated using strain-controlled tensile tests. Consequently, no pronounced reduction in strengths occurred at temperatures between 300 and 550 °C, and a negative strain rate response was observed in the temperature range. Based on the low-cycle fatigue tests by varying strain rates and temperature, it was found that a normalized plastic strain energy density and a strain-rate modified cycle were successfully correlated. The accuracy of the model was discussed by comparing between predicted and experimental lives.

A Global Approach for the Fatigue of Shape Memory Alloys

This review paper, related to previous works of the authors, documents a global approach for structural fatigue of shape memory alloys. It includes two steps. First, a cyclic finite-strain thermomechanically coupled constitutive model is developed in order to compute the asymptotic thermomechanical state. Second, fatigue behavior of SMAs is comprehensively investigated: (i) for low cycle fatigue, strain, and stress-controlled fatigue tests are carried out, the effect of the frequency on low cycle fatigue of SMAs is studied and a strain energy-based low-cycle fatigue criterion is proposed; (ii) for high-cycle fatigue, a shakedown-based high-cycle fatigue criterion is developed to predict whether an SMA structure submitted to high cycle loading would undergo fatigue or not.

Energy-based low cycle fatigue analysis of low yield point steels

As examples of the new advanced high-performance structural steels, low yield point steels have attracted increasing interest owing to their excellent properties in the technology of energy dissipation and seismic design. Because dynamic cyclic loading is inevitable during service life in engineering applications, it is critical to develop in-depth understanding of the fatigue behavior of this material. Here, the low cycle fatigue behavior of low yield point steels produced in China, namely LY100, LY160, and LY225, is investigated using an energy-based approach. Axial steel coupons are tested by fully reversed and push-pull cyclic loading with a nominal strain ratio R = −1 at a constant strain rate of 0.1% S−1. The strain amplitudes range from 0.5% to 6.0% in 0.5% increments. First, experimental details and results of fatigue life are introduced. Subsequently, using an energy-based approach, the cyclic plastic strain energy, cyclic hysteresis loop properties, and fatigue life prediction are thoroughly analyzed. Finally, a simplified method for fatigue life prediction is proposed. The results show that plastic strain energy density is an important parameter for predicting the low cycle fatigue life of low yield point steels with an acceptable degree of accuracy. The proposed simplified method can provide an effective and reliable alternative for low cycle fatigue life prediction of low yield point steels.

Energy-based low cycle fatigue indicator prediction of non-load-carrying cruciform welded joints

A new uniform analytical formulation is proposed to predict the low cycle fatigue assessment indicator of non-loading cruciform welded joints under tensile cyclic loading considering the effects of plasticity and mechanical heterogeneity of the welded materials and geometry configurations. Particularly, weld toe of the joints is dealt with a notch rounding in order to avoid the notch tip singularity in accordance with generalized Neuber concept of Fictitious Notch Rounding (FNR). The relationship between elastic and plastic energy concentration factor of weld toe is firstly determined by the combination of notch stress and strain distribution with the increases of nominal loading, then the main parameters affecting the energy concentration factor are investigated by separating elastic and plastic stages. Stress and strain field intensities are finally expressed in terms of total energy values, which are linked to the nominal energy ones. The energy prediction function which takes into account the material and geometry factors is capable of providing a closed form analytical expression about the state of energy both under small and large-scale yielding for industrial facilities by the comparison against finite element analysis results.

Different Applications of the Gleeble® Thermal–Mechanical Simulator in Material Testing, Technology Optimization, and Process Modeling

Abstract In this study, our aim is to present a compilation about different applications of the Gleeble® thermal–mechanical system concerning material testing, characterization of advanced thermomechanically controlled processes and modeling the multiaxial forging technique. The first topic aims to provide an accurate input dataset for developing an energy-based fatigue model for 15Ch2MFA reactor steel. The process characterization intends to quantify the effect of deformation-stored energy on the start temperature and kinetic of the austenite–ferrite transformation. The modeling topic purposes to predict the evolution of the flow stress and the hardening behavior during processing a mild steel by multiaxial forging. All of these experiments were carried out on a Gleeble® 3800 simulator using different mobile conversion units. The fatigue tests were conducted under uniaxial tension–compression loading with in-phase thermal cycles. To develop a new energy-based low-cycle fatigue (LCF) model, the energy balance of the plastic deformation was also investigated. Moreover, the evolution of the microstructure was observed by transmission electron microscopy (TEM) studies at the different stages of the nominal failure lifetime. The deformation-induced ferrite transformation (DIFT) effect was characterized by a high-resolution dilatometer under uniaxial hot compression tests. An advanced simulation program and experimental setup have been developed for the calculation of reliable flow curve from the data, sampled during the multiaxial forging simulation. Concerning the thermomechanical fatigue tests, the experimental results correspond to the Coffin–Manson curve. Furthermore, temperature evolution measurements were performed to provide input data to the calculation of the plastic strain energy partition converted to heat. Finally, the effect of the deformation stored energy on the start and finish temperature of austenite transformation was obtained and quantified.

New energy-based low cycle fatigue model for reactor steels

Abstract A new low cycle fatigue criterion is presented based on the stored energy, which accumulates in the material’s microstructure during fatigue loading. The new damage parameters are based on the assumption that only the stored part of the introduced energy causes the changes in the microstructure, while the other part does not contribute to the fatigue process. Moreover the dissipated part may depends on the fatigue conditions, especially on the strain rate, which could affect the lifetime prediction accuracy of the applied models. To demonstrate the prediction capability of the proposed model a comprehensive experimental work were conducted on two types of reactor pressure vessel structural material. Investigation on the fraction of plastic work dissipated to heat was carried out to provide information on thermo-mechanical behavior on the tested materials, which can be used as input parameters of the new model. Furthermore isothermal and thermo-mechanical low cycle fatigue tests were performed with in-service loading conditions. The result is higher prediction accuracy than by the classical strain amplitude and strain energy based approaches. The developed model may provide a useful analytical tool for the low cycle fatigue evaluation of reactor components.

Cyclic behavior and energy approach of the fatigue of shape memory alloys

This paper presents an energy-based low-cycle fatigue criterion that can be used in analyzing and designing structures made from shape memory alloys (SMAs) subjected to cyclic loading. Experimentally, a response similar to plastic shakedown is observed: during the first cycles the stress-strain curve shows a hysteresis loop which evolves during the first few cycles before stabilizing. By adopting an analogy with plastic fatigue, it is shown that the dissipated energy of the stabilized cycle is a relevant parameter for estimating the number of cycles to failure of such materials. Following these observations, we provide an application of the cyclic model, previously developed by the authors within the framework of generalized standard materials with internal constraints [16], in order to evaluate such parameter. Numerical simulations are presented along with a validation against experimental data in case of cyclic superelasticity.

Impact of microstructure, temperature and strain ratio on energy-based low-cycle fatigue life prediction models for TiAl alloys

In this paper, two fatigue lifetime prediction models are tested on TiAl intermetallic using results from uniaxial low-cycle fatigue tests. Both assessments are based on dissipated energy but one of them considers a hydrostatic pressure correction. This work allows to confirm, on this kind of material, the linear nature, already noticed on silicon molybdenum cast iron, TiNi shape memory alloy and 304 L stainless steel, of dissipated energy, corrected or not with hydrostatic pressure, according to the number of cycles to failure. This study also highlights that, firstly, the dissipated energy model is here more adequate to estimate low-cycle fatigue life and that, secondly, intrinsic parameters like microstructure as well as extrinsic parameters like temperature or strain ratio have an impact on prediction results.

Cyclic behavior and energy approach to the fatigue of shape memory alloys

This paper presents an energy-based low-cycle fatigue criterion that can be used in analyzing and designing structures made from shape memory alloys (SMAs) subjected to cyclic loading. Experimentally, a response similar to plastic shakedown is observed: during the first cycles the stress-strain curve shows a hysteresis loop which evolves during the first few cycles before stabilizing. By adopting an analogy with plastic fatigue, it is shown that the dissipated energy of the stabilized cycle is a relevant pa- rameter for estimating the number of cycles to failure of such materials. Following these observations, we provide an application of the cyclic model, previously developed by the authors within the framework of generalized standard materials with internal constraints (16), in order to evaluate such parameter. Numerical simulations are presented along with a validation against experimental data in case of cyclic superelasticity.

A modified energy-based low cycle fatigue model for eutectic solder alloy

Gintic Institute of Manufacturing Technology, Nanyang Drive, Singapore 638075*School of Mechanical and Production Engineering, Nanyang Technological University,Nanyang Avenue, Singapore 639798(Received April 21, 1999)(Accepted May 7, 1999)1. IntroductionSurface mount technology (SMT) is increasingly used in microelectronics to mount components bysoldering onto the printed circuit board (PCB). The solder alloys are used as the electrical andmechanical connections between the component and the board. Fatigue failure of solder joints isrecognized as a major cause of failure in electronic devices. An approach to this problem is to determinethe fatigue behaviors of solder alloy by accelerated fatigue testing at different temperatures. In thepresent research, smooth specimens made entirely of a 63Sn/37Pb solder alloy were tested over a widetemperature range at various low frequencies to study its low cycle fatigue properties.Strain-based models, notably the Coffin-Manson model (1–2), have been widely used to characterizelow cycle fatigue behaviors of engineering materials including solder alloys (3–5). However, it ispractically very difficult to obtain a single plastic strain value in a solder joint because of the complexstress state (6). In contrast, it is much easier to calculate energy density from the low cycle hysteresisloops for any types of solder joints under test (7). Therefore, in recent years, energy-based low cyclefatigue models have been increasingly used for solder alloys (7–9). These models are not so wellestablished as the strain-based ones, and some researchers (10) even expressed doubt on whether energydensity is a true parameter governing fatigue life of solder alloy. Therefore, in the study special effortwas made to examine the existing energy-based models. In the end, a flow stress modified energy-basedmodel is proposed based upon the examination and the experimental results. It is demonstrated in thepaper that the new fatigue model can be used to predict the fatigue life of solder alloy at differentfrequencies or temperatures.2. Experimental ProceduresThe material used in the study was a eutectic alloy 63Sn/37Pb. This material is widely used as a solderin SMT. The chemical composition of the solder is as follows (in wt.%): 63.2 Sn, 0.006 Sb, 0.002 Cu,0.004 Bi, 0.001 Zn, 0.002 Fe, 0.001 Al, 0.01 As, 0.001 Cd, and remainder Pb. Rods of the solder wereproduced by casting and then machined into cylindrical fatigue specimens of 90 mm long. The fatiguespecimens have a diameter of 12 mm at the two ends and a central diameter of 6 mm with a radius ofcurvature of 105 mm in the gauge section to prevent any stress concentration due to sharp corners. After