Fatigue resistance of high-temperature alloys under cyclic temperature variation. Part 2. Method of strength and service life prediction under two-frequency nonisothermal loading

We propose a criterion of the limiting state of a metal under two-frequency nonisothermal loading. The fatigue curves that are calculated using the linear damage summation hypothesis for the studied materials and thermocycling regimes lie above the experimental curves, while the curves calculated using the proposed technique are in good agreement with experiment. Analysis shows that the described technique for calculating the service life and the fatigue limits of materials under cyclic nonisothermal loading is universal, effective, and fully adequate for engineering calculations.Translated from Problemy Prochnosti, No. 4, pp. 16–22, April, 1994.     

Fatigue strength and cyclic crack resistance of titanium alloy VT3-1 in different structural states. Communication 1. Study procedure and experimental results

Procedures are described for studying the fatigue strength and cyclic crack resistance characteristics of alloy VT3-1 in different structural states achieved by varying thermomechanical treatment (TMT) parameters. TMT schedules, alloy structure and texture characteristics, and also the size of structural elements typical for an alloy structural state are given. Results are given for static tensile, fatigue, and crack resistance tests on alloy VT32-1 in different structural and textural states, and they are analyzed.Translated from Problemy Prochnosti, Nos. 5–6, pp. 3–11, May–June, 1995.     

Fatigue resistance of aligned carbon nanotube arrays under cyclic compression

Structural components subject to cyclic stress can succumb to fatigue, causing them to fail at stress levels much lower than if they were under static mechanical loading. However, despite extensive research into the mechanical properties of carbon nanotube structures for more than a decade, data on the fatigue behaviour of such devices have never been reported. We show that under repeated high compressive strains, long, vertically aligned multiwalled nanotubes exhibit viscoelastic behaviour similar to that observed in soft-tissue membranes. Under compressive cyclic loading, the mechanical response of the nanotube arrays shows preconditioning, characteristic viscoelasticity-induced hysteresis, nonlinear elasticity and stress relaxation, and large deformations. Furthermore, no fatigue failure is observed at high strain amplitudes up to half a million cycles. This combination of soft-tissue-like behaviour and outstanding fatigue resistance suggests that properly engineered nanotube structures could mimic artificial tissues, and that their good electrical conductivity could lead to their use as compliant electrical contacts in a variety of applications.     

Investigation into threshold stress intensity factors of materials under cyclic loading. Report 1. Methods and experimental results

On the basis of the results of studying threshold stress intensity factors of stainless and heat-resistant steels and titanium and nickel alloys, the author considers the correlation between numerical values of the threshold stress intensity factors and the yield stress, ultimate strength, and fatigue limit, as well as the influence of temperature, environment, cycle asymmetry, and overloads on the threshold stress intensity factor. Institute for Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 4, pp. 5–15, July–August, 1998.     

A constitutive model for cyclic actuation of high-temperature shape memory alloys

In this work, a three dimensional constitutive model for High Temperature Shape Memory Alloys (HTSMAs) is presented. To describe the evolution of the cyclic actuation behavior of such alloys, viscoplastic mechanisms and transformation induced plasticity are introduced in addition to the classical transformation behavior of shape memory alloys. Based on continuum thermodynamics, the evolution of phase transformation, plasticity induced transformation, retained martensite and viscoplasticity are described. Deformation mechanisms that occur over the operational range of such HTSMAs have been identified from the thermomechanical behavior of a NiTiPd alloy. The proposed model has therefore been calibrated and validated based on the thermomechanical response of the studied NiTiPd HTSMA alloy during thermal cycles under compression. Careful attention is devoted to the calibration procedure to identify the contribution of the different mechanisms independently. Finite Element Analysis (FEA) is performed to demonstrate the capabilities of the model to describe the cyclic behavior of HTSMA devices.     

Fatigue resistance at variation of temperature-time factors

The equations of an elasto-plastic static and cyclic deformation at high temperature, force and deformation criteria of quasistatic and fatigue failures, criterion of initiation and propagation of cracks, features of the equations for linear and nonlinear fracture mechanics are described. The mechanics of a nonlinear deformation and fracture is used for calculations of strength, life-time and crack resistance of constructions at high temperature.     

Fatigue behavior of aluminum alloys under biaxial loading

The biaxiality effect, especially the effect of non-singular stress cycling, on the fatigue behavior was studied, employing cruciform specimens of aluminum alloys 1100-H14 and 7075-T651. The specimens, containing a transverse or a 45^o inclined center notch, were subjected to in-phase (IP) or 100% out-of-phase (hereinafter referred to as “out-of-phase or OP”) loading of stress ratio 0.1 in air. The biaxiality ratio λ ranged from 0 to 1.5, and 3 levels of stress were applied. It was observed that: (1) at a given λ, a lower longitudinal stress induced a longer fatigue life under IP and OP loading, and the fatigue life was longer under IP loading, (2) the fatigue crack path profile was influenced by λ, phase angle (0^o or 180^o), and initial center notch (transverse or 45^o inclined); (3) the fatigue crack path profiles, predicted analytically and determined experimentally, had similar features for the specimens with a transverse center notch under IP loading; and (4) the fatigue crack growth rate was lower and the fatigue life longer for a greater λ under IP loading, whereas it changed little with change in λ under OP loading. These results demonstrate that non-singular stress cycling affects the biaxial fatigue behavior of aluminum alloys 1100-H14 and 7065-T651under IP and OP loading.     

Fracture toughness of structural alloys under cyclic loading. Communication 2

Strength of Materials -     

Fatigue crack initiation and propagation under cyclic contact loading

A computational model for contact fatigue damage analysis of gear teeth flanks is presented in this paper. The model considers the conditions required for the surface fatigue crack initiation and then allows for proper simulation of the fatigue crack propagation that leads to the appearance of small pits on the contact surface. The fatigue process leading to pitting is divided into crack initiation and a crack propagation period.The model for prediction of identification of critical material areas and the number of loading cycles, required for the initial fatigue crack to appear, is based on Coffin-Manson relations between deformations and loading cycles, and comprises characteristic material fatigue parameters. The computational approach is based on continuum mechanics, where a homogenous and elastic material model is assumed and results of cyclic loading conditions are obtained using the finite element method analysis.The short crack theory together with the finite element method is then used for simulation of the fatigue crack growth. The virtual crack extension (VCE) method, implemented in the finite element method, is used for simulating the fatigue crack growth from the initial crack up to the formation of the surface pit. The relationship between the stress intensity factor K and crack length a, which is needed for determination of the required number of loading cycles N_p for a crack propagation from the initial to the critical length, is shown.     

On the cyclic behavior of cast and extruded aluminum alloys. Part A: Fatigue crack propagation

The fatigue crack propagation (FCP) response of a cast and extruded aluminum alloy was examined as a function of mean stress and specimen orientation. The extruded alloy was tested in both the longitudinal and transverse orientation and no difference in FCP response was noted. FCP tests were conducted at R ratios of 0.1, 0.5, 0.65 and 0.8. In the threshold regime, it was seen that as R ratio increased, $\text{Δ}\text{K}{}_{\mathrm{TH}}$ decreased. In addition, ΔK_TH values determined for the cast alloy were superior to those determined for the extruded alloy at all R ratios examined. The threshold regime was also shown to be K_MAX rather than $\text{Δ}\text{K}$ dependent. At intermediate ΔK levels, a mean stress effect was seen for both alloys at R ratios less than 0.5. Crack closure was monitored during testing so that ΔK_EFF values could be determined. $\text{Δ}\text{K}{}_{\mathrm{EFF}}$ was seen to explain mean stress effects at intermediate $\text{Δ}\text{K}$ levels.