Low Cycle Fatigue Conditions Research Articles

Enhanced Cyclically Stable Plasticity Model for Multiaxial Behaviour of Magnesium Alloy AZ31 under Low-Cycle Fatigue Conditions.

Magnesium alloys, particularly AZ31, are promising materials for the modern automotive industry, offering significant weight savings and environmental benefits. This research focuses on the challenges associated with accurate modelling of multiaxial cyclic plasticity at small strains of AZ31 under low-cycle fatigue conditions. Current modelling approaches, including crystal plasticity and phenomenological plasticity, have been extensively explored. However, the existing models reach their limits when it comes to capturing the complexity of cyclic plasticity in magnesium alloys, especially under multiaxial loading conditions. To address this gap, a cyclically stable elastoplastic model is proposed that integrates elements from existing models with an enhanced algorithm for updating stresses and hardening parameters, using the hyperbolic tangent function to describe hardening and ensure a stabilised response with closed hysteresis loops for both uniaxial and multiaxial loading. The model is based on a von Mises yield surface and includes a kinematic hardening rule that promises a stable simulation of the response of AZ31 sheets under cyclic loading. Using experimental data from previous studies on AZ31 sheets, the proposed model is optimised and validated. The model shows promising capabilities in simulating the response of AZ31 sheet metal under different loading conditions. It has significant potential to improve the accuracy of fatigue simulations, especially in the context of automotive applications.

Tensile and low-cycle fatigue failure of carbon fiber reinforced aluminium laminates

Fiber metal laminates (FMLs) have emerged as advanced materials for engineering applications due to their high strength-to-weight ratio. This study aims to understand the mechanical response and failure mechanisms of carbon-fiber reinforced aluminium laminates (CARALL) under monotonic and cyclic loading, specifically uniaxial tensile and low-cycle fatigue (LCF) conditions. CARALL, fabricated with alternating (3/2) layers of AA2024-T3 and unidirectional carbon fiber composite, exhibits superior ultimate tensile strength (UTS) compared to AA2024-T3 sheets, with a distinctive three-zone stress-strain curve. The first zone shows elastic strain in both the aluminium alloy and laminate. The second zone features elastic strain in the laminate and plastic strain in the aluminium alloy. In the third zone, surface modifications enhance adhesion, leading to a sudden load drop due to fiber fracture or delamination. Fractographic analysis indicates no significant necking in the laminate, contrasting with the ductile failure of AA2024-T3. The carbon fiber layer exhibits a pure brittle fracture with visible delamination at the interphase of carbon fiber layer and aluminium alloy sheet. Under LCF conditions, the laminate’s cyclic stress response remains stable, with fatigue cracks initiating in the aluminium layers but mitigated by the carbon fibers, leading to cyclic softening. High-magnification images reveal striation marks and transverse micro-cracks in the aluminium, highlighting the complex interaction between aluminium and carbon fiber layers during fatigue.

Experimental Research on the Low-Cycle Fatigue Crack Growth Rate for a Stiffened Plate of EH36 Steel for Use in Ship Structures

This paper presents a straightforward approach for determining the low-cycle fatigue (LCF) crack propagation rate in stiffened plate structures containing cracks. The method relies on both the crack tip opening displacement (CTOD) and the accumulative plastic strain, offering valuable insights for ship structure design and assessing LCF strength. Meanwhile, the LCF crack growth tests for the EH36 steel were conducted on stiffened plates with single-side cracks and central cracks under different loading conditions. The effects of stress amplitude, stress ratio, and stiffener position on the crack growth behavior were examined. Fitting and verifying analyses of the test data were employed to investigate the relationship between CTOD and the crack growth rate of EH36 steel under LCF conditions. The results showed that the proposed CTOD-based prediction method can accurately characterize the LCF crack growth behavior for stiffened plate of EH36 steel for use in ship structures.

Investigating the relationship between fracture entropy and stress amplitude in CFRP laminates under low-cycle fatigue loading

Fracture fatigue entropy (FFE) is considered to be independent of the loading amplitude in fatigue experiments, and this conclusion has been applied to fatigue life prediction of composites and metals with satisfactory prediction accuracy. However, the existing research work mainly focuses on high-cycle fatigue, and it is not clear whether the FFE is independent of the loading amplitude in low-cycle fatigue, which requires more in-depth research. Fatigue experiments were conducted on three CFRP laminates with different ply orientations, and the FFE values corresponding to low-cycle fatigue were obtained by thermographic analysis. The results show that the FFE value is no longer independent of the loading amplitude under low-cycle fatigue conditions. On the contrary, it decreases with the increase of fatigue loading amplitude and also varies with different ply orientations. The relationship between low-cycle fracture fatigue entropy and loading amplitude of composites with different ply orientations was analyzed and modeled, and a novel low-cycle fatigue life prediction method based on FFE was developed.

Effect of Testing Conditions on Low-Cycle Fatigue Durability of Pre-Strained S420M Steel Specimens.

S420M steel subjected to strain-controlled low-cycle fatigue does not exhibit a period of cyclic properties stabilization. The maximum stress on a cycle continuously drops until fracture. For this reason, it is difficult to plan experimental research for different types of control in such a way that their results can be considered comparable. The aim of this paper is to present and discuss the results of tests conducted in various conditions of low-cycle fatigue of S420M steel specimens, both undeformed and pre-strained. In both loading conditions, after initial deformation, a significant change in the cyclic properties of steel described by the parameters of the hysteresis loop was observed. Also, the fatigue life of the pre-strained specimens appeared to be different from unstrained specimens and was affected by the test loading conditions. The reduction in life under controlled stress conditions was attributed to the increase in the extent of plastic deformation and stress and the occurrence of creep.

Experimental Study on Low-Cycle Fatigue Characteristics of Marine Structural Steel

This research focuses on the experimental investigation of the low-cycle fatigue characteristics of marine structural steel. The study aimed to explore the behavior of marine-grade steel under low-cycle fatigue conditions. The experimental parameters include the fatigue life of the material, crack propagation behavior, and a comprehensive analysis of mechanical properties associated with various loading conditions. Based on the experimental results, a low-cycle fatigue crack propagation rate model for marine structural steel plates was established using CTOD as a characterization parameter. The primary objective of this research is likely to enhance the durability and safety of maritime structures, providing valuable technical insights for the field of naval engineering.

Constitutive model of an additively manufactured combustor material at high-temperature load conditions

ABSTRACT In this paper, the high-temperature constitutive behaviour of an additively manufactured ductile nickel-based superalloy is investigated and modelled, with application to thermomechanical fatigue, low-cycle fatigue and creep conditions at temperatures up to 800 ∘ C. Thermomechanical fatigue tests have been performed on smooth specimens in both in-phase and out-of-phase conditions at a temperature range of 100 − 800 ∘ C, and creep tests at 625 ∘ C, 700 ∘ C, 750 ∘ C and 800 ∘ C. Additionally, low-cycle fatigue tests at different strain ranges and load ratios have been performed at 700 ∘ C, and tensile tests have been performed at 600 ∘ C, 700 ∘ C and 800 ∘ C. A clear anisotropic mechanical response is obtained in the experiments, where the anisotropic effects are larger at high stress levels in creep loadings. To capture this behaviour, a rate-dependent strain based on a double-Norton model has been adopted in the model, by which the creep and mid-life response of the thermomechanical fatigue tests can be simulated with good accuracy.

Microstructure analysis and life prediction of DD6 superalloy under 760 °C combined high and low cycle fatigue conditions

The impact of high and low cycle combined loads on the fatigue performance of DD6 alloy was investigated in this work. Specifically, the combined high- and low-cycle fatigue response of DD6 alloy was examined at a temperature of 760 °C under different low cycle stress amplitudes (960, 1000, and 1040 MPa). The findings revealed that higher low cycle stress amplitudes result in shorter combined high and low cycle fatigue life and less dispersion in life. Correlations between the microstructural features (crystal plane distribution, fracture elevation difference, and the area proportion of crack propagation zone) and fatigue performance were accurately established, demonstrating a linear relationship between these microstructural features and the logarithm of fatigue life. The study also uncovered the evolution rule of microstructural features in high- and low-cycle combined fatigue. Furthermore, the fracture mechanism was analyzed and the distribution of high and low cycle combined fatigue life of DD6 alloy at 760 °C was statistically validated using the dual-parameter Weibull distribution theory. Building upon the crystal plasticity theory, a fatigue life prediction model was developed, and the combined cycle fatigue lifespan under various low cycle stress amplitudes was compared. The comparison between the predicted life and actual fatigue life indicated that the predicted lifespan falls in three times the scatter band.

Numerical Determination of the Life of Steel Exposed To Low Cycle Fatigue

In the paper, based on the results of the experimental research of high-strength low-alloy steel (HSLA), Nionicral 70 (NN-70), under conditions of low-cycle fatigue (LCF), a numerical analysis of stress and determination of the life of steel samples was performed. Experimental testing of the behavior of the samples were performed with controlled and fully reversible strain (/2 = const, R = min/max = -1), according to the standard ISO 12106:2003 (E). For computational analyses, the following were used: the method of least squares and the method of finite elements (FEM). On the basis of the analysis of the results of the stress-deformation state and the determination of the life span through the isolines of the life span and comparison with the results of experimental tests, a graphic representation is given. The analysis justified the effort to numerically solve the estimation of the lifespan of steel under low cycle fatigue (LCF).

Damage-coupled unified constitutive modeling of 316LN stainless steel including dynamic strain aging under various tension dwell time: A macroscopic phenomenological study

A damage-coupled unified constitutive model is developed for 316LN stainless steel based on the framework of the Abdel-Karim and Ohno model. In the modified model, a kinematic hardening coefficient related to accumulated plastic strain is introduced in the linear hardening term, and a damage coefficient is incorporated in the static recovery term. Meanwhile, the parameters of isotropic hardening and kinematic hardening are associated with the maximum plastic strain rate and plastic strain memory to describe the effects of dynamic strain aging and plastic strain memory. Additionally, the kinematic hardening coefficients and static recovery coefficients correlate with dwell time to simulate stress relaxation throughout the whole-life time. The comparison between the simulation and experimental results indicates the validity of the modified model under the conditions of low-cycle fatigue and creep-fatigue interaction. After identifying the material parameters using a combination of classic parameter determination methods and optimization algorithms, the cyclic stress response and hysteresis loops can be accurately simulated throughout the whole-life time.

Discrepancy in low cycle fatigue and creep-fatigue life of 720Li alloy tested at 720 °C: Role of crystallographic texture evolution

Low cycle fatigue (LCF) and creep-fatigue (CF) interaction behavior of 720Li alloy were studied for the first time at 720 °C with different values of total strain amplitudes (Δεt/2) and hold time (HT). At Δεt/2 = 0.6%, CF life is less than LCF life. However, at Δεt/2 = 0.44, 0.48 and 0.55% CF life increased with increase in HT. Texture analysis of specimen tested at Δεt/2 = 0.44%, revealed that initial loading direction ǁ [11¯1] found to be rotated: (i) [21¯1] in LCF condition; and (ii) [11¯0] and [01¯1] in the CF condition predominantly. Notably, texture components developed in the CF specimens tested at Δεt/2 = 0.6% is quite similar to that in the LCF specimen tested at Δεt/2 = 0.44%. Development of [21¯1] texture component is due to multi planner slip (i.e. octahedral and cubic plane slip) under the influence of compressive load which is also confirmed by viscoplastic-simulation. Evolution of [21¯1] texture component leads to greater extent of inter crystalline cracks due to latent strain hardening and strain incompatibility between adjacent grains. Further, high static interfacial energy of annealing twin boundaries, confirmed through five-parameter stereological analyses, in LCF specimen tested at Δεt/2 = 0.44% and CF specimen tested at Δεt/2 = 0.6% resulted annealing twin boundary cracking. In contrast, evolution of low modulus texture components (i.e. [11¯0] & [01¯1]) has been inferred to be responsible for higher cyclic life in CF specimen tested at Δεt/2 = 0.44% and LCF specimen tested at Δεt/2 = 0.6%.

Effect of loading conditions on corrosion fatigue process of FSW AA6061-T6 joint in 3.5% NaCl studied by electrochemical noise

Electrochemical noise (EN) signals generated during corrosion fatigue (CF) process of friction stir welded (FSW) AA6061-T6 joints in 3.5 wt% NaCl solution under different loading conditions were monitored. Wavelet analysis and cumulative probability calculations showed that the increase in peak stress, stress ratio and stress frequency all lead to a decrease in Rn and an increase in the incubation rate of local events. Under current experimental conditions, the influence of peak stress is the most significant, while the influence of frequency is the least. The combination of EN signal analysis and transmission electron microscopy (TEM) observations indicates that the coarsening of β′ phase and the extensive dissolution of β'' phase are the main reasons for preferential failure in the weakest heat affected zone. The main contribution of this study is preliminary investigating the CF behavior of FSW AA6061 joints under different low cycle fatigue conditions. Moreover, EN is an effective strategy for monitoring CF process and establishes a connection between an alternative EN technique and in-situ CF behavior research.

Identifying the effect of coherent precipitates on the deformation mechanisms by in situ neutron diffraction in an extruded magnesium alloy under low-cycle fatigue conditions

As a new class of heat-treatable magnesium alloys, the low-alloyed Mg-Al-Ca-Mn alloy has great engineering potential because of its excellent extrudability and high strength by the dispersion of Guinier-Preston (G.P.) zones. The complex deformation mechanisms associated with hexagonal crystals, the interactions between defects with such G.P. zones, as well as their impacts on fatigue lifetime, are all critical issues before the actual deployment of such materials. In this study, this type of Mg alloy with and without G.P. zone dispersion during cyclic deformation was investigated by in situ neutron diffraction measurements. The relationship between the macroscopic cyclic deformation behavior and the microscopic response (particularly twinning and detwinning) at the grain level was established. The general deformation mechanism evolution in samples under the solution-treated (S.T.) state was generally similar to that in samples under the peak-aged (P.A.) state. Both samples plastically deformed by extension twinning during compression, and by a sequential process of detwinning and dislocation motion under reverse tension. Results suggest that the precipitates provided the greatest strengthening against the prismatic slip (∼ 45% increase), the moderate to the {101¯2} extension twinning (∼ 27 %), and the least to the basal slip (∼ 11% increase). The P.A. samples have shorter fatigue lifetime primarily due to the excessive dislocation pileups and the higher backstress that promote crack initiation.

Cycle life prediction under low cycle fatigue using nonlinear Marco - Starkey model

The process of deformation and fracture of structural alloys under low-cycle fatigue in conditions of uniaxial loading with axial strain control under complex cycle shape and block loading has been studied. The obtained results of experimental studies of structural alloys were used to assess the possibility of using the nonlinear Marco - Starkey damage accumulation model. The processing of the nickel alloy cyclic tests results with a simple and complex form of the cycle has been carried out. A combination of exponents included in the non-linear Marco - Starkey model was selected which for the case of low-cycle fatigue with a complex M-shaped cycle made it possible to predict durability which is in good agreement with experimental test data. The main problem of using the nonlinear Marco - Starkey model for predicting cyclic durability with an M-shaped cycle is the presence of many combinations of exponents that allow predicting cyclic durability with the same accuracy. To determine the uniqueness of the solution it is proposed to carry out a set of tests under simple and block loading. Experimental results have been obtained on the processes of deformation, fracture of D16T aluminum alloy under low-cycle fatigue conditions under simple cycle forms with constant parameters, and block loading with variable cycle parameters in tests for uniaxial loading with axial strain control. Based on the obtained experimental results according to the new method combinations of the m degrees coefficients were selected, which was carried out in the range of exponents from 0.2 to 10 with a step of 0.2. Comparison of the forecasting results for blocks consisting of three groups made it possible to find several combinations of general exponents m for two groups that are present in all blocks. The final choice of a pair of coefficients was carried out on the condition that the predicted damage is close to unity in the first (test) block. The selected values of the coefficients made it possible to predict the durability under block low-cycle loading using the nonlinear Marco - Starkey model.

Avoiding inelastic strains in solder joint interconnections of space electronics

AbstractA simple and physically meaningful analytical (“mathematical”) predictive model is developed for the evaluation of the sizes of the zones of inelastic deformations, if any, at the ends of a soldered electronics assembly in a packaged IC device. The emphasis is on the IC devices intended for space applications and on the incentive for using column‐grid‐array (CGA) rather than a ball‐grid‐array (BGA) technology for lower thermal interfacial stresses. Such devices, when employed outside spacecraft, experience extraordinarily low temperatures, and their solder joint interconnections, when used interchangeably inside and outside the craft, are subjected to significant and variable inelastic strains, possibly leading to low cycle fatigue conditions. Because of that, the useful lifetime of the device might be shorter than required for particular applications. There is an obvious incentive to avoid, if possible, the inelastic deformations in the solder material or, at least, to predict the sizes of the inelastic zones at the peripheral portions of the assembly. The developed model shows how this could be done. Both shearing and peeling interfacial stresses are addressed. The numerical examples are carried out for an IC‐package mounted on a ceramic (Al203) substrate using 3%–4%Ag0.51%Cu lead‐free solder alloy. In addition, an extreme response of an assembly of the type in question to the random temperature cycling is briefly addressed. It is concluded that several possible and independent ways to avoid or, at least, to minimize the inelastic deformations in the solder material could be employed: the use of high yield stress solder and/or a solder with a low melting temperature and/or a low expansion substrate and/or compliant interfaces, such as, for example, low‐modulus‐and‐compliant CGA design instead of high‐modulus‐and‐stiff BGA, and/or inhomogeneous attachment designs, such as, for example, those, when high‐modulus and/or high‐soldering‐temperature solder is used in the assembly's major mid‐portion, where heat‐transfer considerations play the major role, and low‐modulus and/or low‐melting‐temperature solder – at its peripheral portions, where mechanical strength is critical. It is concluded also that because, based on the calculated data, the peeling stresses are typically considerably lower than the shearing ones and are proportional to the shearing stresses, only the latter could be considered and possibly minimized. As to the random loading, whose effect was assessed by considering the extreme elastic shearing stress in an assembly with an extraordinarily high yield stress (so that inelastic deformations in it are impossible), it has been concluded that a probabilistic‐design‐for‐reliability of the solder joint interconnections in IC packages intended for space application should be considered in addition to the deterministic approach taken in this paper, but this analysis is beyond the scope of the present study and will be done as future work. Finally, it is concluded that the analytical ("mathematical") modeling should always be considered, in addition to computer simulations, in every important physical design related undertaking, such as the one addressed in this analysis: these two major modeling tools are based on different assumptions and employ different calculation techniques, and if the calculated data obtained using these tools are in agreement, then there is a good reason to believe that these data are accurate and trustworthy.

Fatigue Damage Mitigation for Welded Beam-to-Column Connections in Steel High-Rise Buildings Using Passive Vibration Control

To investigate the use of vibration control systems in fatigue damage mitigation for welded beam-to-column connections in steel high-rise buildings, cases of both a single connection under constant amplitude cyclic loading and multi-connections in a high-rise building under the stochastic wind, with and without the fluid viscous damper (VFD) and the tuned mass damper (TMD), are discussed respectively. The finite element analysis and the fatigue assessment are conducted so that the mitigation effect, the effect of technical parameters and the conditions of both high-cycle fatigue and low-cycle fatigue are all discussed. The results show that the VFD and the TMD systems are both effective in the mitigation of local fatigue damage along with the structural displacement for both cases. The VFD generally has a better mitigation effect than the TMD and it starts to take effect instantly with the external loading, but it causes a phase difference in structural responses, while the situation of the TMD is quite the opposite. The displacement and the local stress show similar and synchronous mitigation trends so that the damping systems can be designed based on either of them. The VFD should be designed with a smaller damping exponent and a larger damping coefficient in a braced installation form, while the TMD can be designed using the optimal parameters. The optimized VFD layout plan is that VFDs are placed between the two connections with large relative displacement and relative velocity on higher floors and these two connections with VFDs should be near to the targeted connection. The negative fatigue damage mitigation mainly stems from insufficient lateral support force so that the direct installation of VFDs may result in a negative fatigue damage mitigation effect in the connections with limited lateral support.

Finite element analysis on prosthetic leg under different loads and flexion angles for medical applications

Prosthetic legs (PL) are mainly used to perform leg amputations more easily. The design and performance of these legs mainly depend on the material selection and manufacturing methods. In the present study, alternative materials are considered for PL such as Al alloy, Ti alloy, Uni directional Carbon fiber epoxy (UDCFE), and combined composite material (CF, UDCFE and Ti alloy) to perform static and fatigue analysis . Static analysis was performed under four different load conditions 60 kg,70 kg, 80 Kg and 90 kg. Fatigue analysis was done under low cycle fatigue conditions. Theoretical calculations are evaluated at various inclinations of the foot 10°, 20°, and 30° with the ground. The results obtained theoretically are compared with Ansys results. The best material which gives the lesser value of deformation and lower damage factor is selected for the design. Further, experimental studies were suggested based on the results obtained from this work.

Predicted Low-Cycle-Fatigue Lifetime of Solder Joint Interconnections: Application of Hall's Approach and Boltzmann-Arrhenius-Zhurkov (Baz) Model

A predictive model is suggested for the evaluation of the low-cycle-fatigue lifetime of a solder joint interconnection in an avionic IC package subjected to temperature cycling and experiencing low cycle fatigue conditions. The model is based on two significant findings in the field of electronic materials reliability. The first one is Hall's plastic strain energy approach suggested about forty years ago.

Constitutive model for thermomechanical fatigue conditions of an additively manufactured combustor alloy

In this study, the mechanical response of an additively manufactured nickel-based combustor alloy, subjected to thermomechanical fatigue (TMF) loadings has been investigated and modelled. TMF tests were performed in both in-phase and out-of-phase conditions with different strain ranges and temperature ranges of 100 °C–450 °C and 100 °C–600 °C, respectively. The smooth specimens were manufactured in two different orientations to study the influence of anisotropy, and the specimens were machined to final dimensions with conventional techniques. A constitutive model with focus on describing the mid-life behaviour was developed where the total inelastic strain was divided into one plastic (rate-independent) and one creep (rate-dependent) part, to be able to describe both the rate-dependent effects from TMF conditions as well as rate-independent responses. A cycle jumping procedure was used, which enables to simulate the mid-life response of the material for TMF as well as low-cycle fatigue conditions within three simulated loading cycles.

Мезомеханика фрикционного контакта в условиях малоцикловой усталости

Мезомеханика фрикционного контакта в условиях малоцикловой усталости