Service loading test as a source for the fatigue curve parameters' estimation

Very important question of fatigue curve parameter estimation might be explored from the point of view of service loading. Comparative testing at varied stress levels with subsequent computation might reveal some peculiarities of fatigue curves and substantial changes of their parameters due to service loading factors. __________ Translated from Problemy Prochnosti, No. 1, pp. 76–82, January–February, 2009.     

Strain–life curves of steels and methods for determining the curve parameters. Part 2. Methods based on the use of artificial neural networks

The authors look into the possibility of using artificial neural networks for predicting the deformation characteristics of steels (the parameters of the Basquin–Manson–Coffin strain–life curve equation) based on static strength and plasticity characteristics, by constructing four independent neural networks with different configurations of input and output data. The prediction of parameters of the Basquin–Manson–Coffin equation and the fatigue life calculations by means of artificial neural networks are demonstrated to provide a better accuracy in comparison to the available conventional methods.     

Mechanical behavior of NiTi shape memory alloy fiber reinforced Sn matrix “smart” composites

The detrimental effects of Pb on the environment and human health have provided the driving force for replacement of Pb–Sn solders with Pb-free alternatives. Sn-rich Pb-free solder alloys with silver and copper alloying additions have higher strength but lower elongation-to-failure than Pb–Sn solders. Thus, these alloys are more susceptible to failure under mechanical shock, drop, and thermal fatigue conditions. In this article, mechanical tensile testing of NiTi–Sn3.5Ag single fiber composites demonstrates superelastic behavior of the composite with 85% strain recovery. Fatigue experiments show an evolution in damage over cycles, and an S–N curve shows sharp transition between a nearly vertical low-cycle fatigue behavior and the high-cycle fatigue regime. The solder composite exhibits constant fatigue strength over the superelastic range of the NiTi fiber.     

Strain-controlled tests for determining the changes in the fatigue parameters of materials

We present the results of fatigue testing of cyclically hardened 2017(A)-T4 aluminum alloy (AlCu₄ Mg Si) for five levels of cyclic strains. The cyclic instability of this material hardened under variable cyclic loads leads to the variations of the coefficients of fatigue characteristics of the Manson-Coffin-Basquin curve. The analysis of these changes in the process of evaluation of the fatigue strength of materials enables us to improve the values of this quantity for cyclically unstable materials. We propose a model for the evaluation of the fatigue strength of materials under random loading and, in particular, for the variable characteristics and coefficients of fatigue strength and plasticity. Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 4, pp. 42–46, July–August, 2007.     

Fatigue lifetime of ADI from ultimate tensile strength to permanent fatigue limit

S-N curve of austempered ductile iron was obtained in the range of lifetime including low cycle fatigue domain and high cycle fatigue domain up to 10⁸ cycles. Ultimate tensile strength is used as a limiting value of the curve. Symmetric push-pull fatigue and tensile tests were performed at room temperature on isothermally treated nodular cast iron alloyed with copper and nickel having positive impact on mechanical, technological and fatigue properties of austempered ductile iron. Suitable functions for the fit of experimentally determined points were tested and their parameters were calculated. The best results were obtained using the Palmgren function and the function introduced by Kohout and Věchet. Since the loading frequency in high-cycle region is two orders higher than in low-cycle region, the effect of loading cycle frequency on fatigue behavior of the studied material is also studied. A possibility of discontinuity of experimental data between low-cycle and high-cycle regions is discussed. __________ Translated from Problemy Prochnosti, No. 1, pp. 40–43, January–February, 2008.     

Low-Cycle Fatigue of the Equipment of Nuclear Power Plants Made of 15Kh2MFA Steel

We summarize experimental data on the low-cycle fatigue resistance of 15Kh2MFA steel, which is widely used in VVER-440 reactors, and its welds. These results were obtained for a fairly great number of melts of the base metal (21 melts) and welds (16 specimens). We compare these experimental data with the reference curve for low-alloy heat-resistant steels according to the Standards PNAE G-7-002-86 currently in force. We show that the values of safety factor n _σ = 2 and n _N = 10 are provided in the assessment of fatigue strength for equipment made of 15Kh2MFA steel, which is very important in connection with the extension of the service life of VVER reactor vessels for 10 – 15 year. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 41, No. 3, pp. 112 – 118, May – June, 2005.     

Determination of the characteristics of distribution of the fatigue limits of materials on the basis of the energy approach

We propose a method for the determination of the mean value of the fatigue limit of a material and its mean-square deviation according to the results of standard fatigue tests. The interpretation of these results from the viewpoint of energy enables us to establish the relationship between the individual durability of specimens and their potential ability to withstand fatigue fracture in the form of individual fatigue limits. Institute for Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 4, pp. 28–33, July–August, 1998.     

Material characterization of SS 316 in low-cycle fatigue loading

This study deals with simulation of low-cycle fatigue (LCF), followed by evaluation of fatigue parameters, which would be suitable for estimating fatigue lives under uniaxial loading. The cyclic elastic–plastic stress–strain responses were analyzed using the incremental plasticity procedures. Finite-element (FE) simulation in elastic–plastic regime was carried out in FE package ABAQUS. Emphasis has been laid on calibration of SS 316 stainless steel for LCF behavior. For experimental verifications, a series of low-cycle fatigue tests were conducted using smooth, cylindrical specimens under strain-controlled, fully reversed condition in INSTRON UTM (Universal Testing Machine) with 8,800 controller at room temperature. The comparisons between numerical simulations and experimental observations reveal the matching to be satisfactory in engineering sense. Based on the cyclic elastic–plastic stress–strain response, both from experiments and simulation, loop areas, computed for various strain amplitude, have been identified as fatigue damage parameter. Fatigue strain life curves are generated for fatigue life prediction using Coffin–Manson relation, Smith–Watson–Topper model, and plastic energy dissipated per cycle (loop area). Life prediction for LCF has been found out to be almost identical for all these three criteria and correlations between predicted and experimental results are shown. It is concluded that the improvement of fatigue life prediction depends not only on the fatigue damage models, but also on the accurate evaluations of the cyclic elastic–plastic stress/strain responses.     

Prediction of fracture characteristics of metals from high-frequency test data

A method is proposed for the prediction of cyclic crack resistance characteristics of metallic materials under low-frequency loading from high-frequency test data, which is based on a model of development of local plastic deformation regions during the accumulation of fatigue damages and fatigue crack growth with allowance for cyclic loading rate. We performed a comparative analysis of the results of prediction of fatigue fracture diagrams with test data for VT22, VT18U, VNS-25, and AMg6N alloys in a frequency range of 20 Hz–10 kHz. Report on International Conference “Dynamics, Strength, and Life of Machines and Structures” (1–4 November 2005, Kiev, Ukraine). __________ Translated from Problemy Prochnosti, No. 2, pp. 121–128, March–April, 2007.     

New representation of the diagrams of ultimate stresses for the fatigue analysis of aluminum structures

We propose a new analytic representation of the diagrams of ultimate stresses for constant fatigue life. The diagrams are constructed according to limited amount of experimental data obtained as a result of fatigue testing of smooth specimens of the metal of welded joints of aluminum alloys. In determining the “hot-spot” stressed state, these diagrams enable us to perform the fatigue analysis of welded aluminum structures with regard for the stress concentration, the level and character of distribution of residual stresses, misalignment in the zone of the joint, and other factors. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 42, No. 2, pp. 94–98, March–April, 2006.     

Influence of nonmetallic inclusions on the fatigue strength of metals

A model of growth of short fatigue cracks in the vicinity of nonmetallic inclusions in crystalline metallic materials is proposed. We establish the influence of inclusions on the fatigue strength of materials of this type. A conclusion is made that the inclusions whose typical size is smaller than the grain size exert practically no influence on the fatigue strength of the material. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 2, pp. 5–9, March–April, 2007.     

Scatter in the fatigue characteristics of steels and its analysis with allowance for cyclic inelastic strains

The regularities of the scatter in the characteristics of fatigue and inelasticity of steels 45 and 1Kh13 have been studied under steady-state, step, and block loading. It is shown that the use of the inelastic strain per cycle as a measure of the fatigue damage accumulation rate makes it possible to take into account the scatter in the number of cycles to fracture and in the fatigue limits and to substantiate the cumulative fatigue damage rule. __________ Translated from Problemy Prochnosti, No. 3, pp. 5–22, May–June, 2007.     

On the prediction of the fatigue life of thin-walled structures based on a statistical model

We present experimental results of fatigue tests of AMg6 aluminum alloy model specimens-plates with a single notch, subjected to fully reversed and asymmetric cyclic axial loading. The experimentally determined lives of model specimens are used as the results of the base experiment for predicting the fatigue strength characteristics of large-scale cylindrical shells with circumferential ring-shaped stress raisers loaded by axial forces of a cyclic nature. A numerical study has been performed of the statistical model adopted for predicting the fatigue life of such shells. __________ Translated from Problemy Prochnosti, No. 3, pp. 112–122, May–June, 2006.     

Study of Pd–Ag dental alloys: examination of effect of casting porosity on fatigue behavior and microstructural analysis

The goals of this study were to investigate the fatigue limits of two Pd–Ag alloys (Ivoclar Vivadent) with differing mechanical properties and varying proportions of secondary alloying elements, examine the effect of casting porosity on fatigue behavior, and determine the effect of casting size on microstructures and Vickers hardness. The alloys selected were: IPS d.SIGN 59 (59.2Pd–27.9Ag–8.2Sn–2.7In–1.3Zn); and IS 64 (59.9Pd–26.0Ag–7.0Sn–2.8Au–1.8 Ga–1.5In–1.0Pt). Tension test bars, heat-treated to simulate dental porcelain application, were subjected to cyclic loading at 10 Hz, with R-ratio of −1 for amplitudes of compressive and tensile stress. Two replicate specimens were tested at each stress amplitude. Fracture surfaces were examined with a scanning electron microscope (SEM). Sectioned fatigue specimens and additional cast specimens simulating copings for a maxillary central incisor restoration were also examined with the SEM, and Vickers hardness was measured using 1 kg load. Casting porosity was evaluated in sectioned fatigue fracture specimens, using an image analysis program. The fatigue limit (2 × 10⁶ loading cycles) of IS 64 was approximately 0.20 of its 0.2% yield strength, while the fatigue limit of d.SIGN 59 was approximately 0.25 of its 0.2% yield strength. These relatively low ratios of fatigue limit to 0.2% yield strength are similar to those found previously for high-palladium dental alloys, and are attributed to their complex microstructures and casting porosity. Complex fatigue fracture surfaces with striations were observed for both alloys. Substantial further decrease in the number of cycles for fatigue failure only occurred when the pore size and volume percentage became excessive. While the heat-treated alloys had equiaxed grains with precipitates, the microstructural homogenization resulting from simulated porcelain firing differed considerably for the coping and fatigue test specimens; the latter specimens had significantly higher values of Vickers hardness.     

Creep and fatigue behaviors of the lead-free Sn–Ag–Cu–Bi and Sn60Pb40 solder interconnections at elevated temperatures

Creep and fatigue behaviors of the interconnections soldered by the lead-free Sn–Ag–Cu–Bi solder were investigated at different elevated temperatures (with the homologue temperature in the range of 0.71– 0.82), with a comparison to that of a traditional Sn60Pb40 solder. The results show that the lead-free Sn–Ag–Cu–Bi solder shows a superior anti-creep performance over the Sn60Pb40 solder, in terms of a much lower creep strain rate and a vastly elongated creep fracture lifetime; in the secondary creep regime, the calculated creep-activation energy for two solders is reasonably close to other published data. In addition, it has also been shown that the joints soldered by the lead-free Sn–Ag–Cu–Bi solder exhibits a superb fatigue property.     

On a reliable equation for the second portion of the fatigue crack growth diagram

The fatigue crack growth kinetics on the second portion of the fatigue crack growth diagram is described using an equation that takes into account both the current crack tip opening displacement and the instant when the crack closure in a cycle vanishes and does not appear until the specimen failure. This instant corresponds to the critical effective SIF range, ΔK_fcl. Institute of Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 3, pp. 83–87, May–June, 1999.     

A study of the influence of a lubricant composition on fatigue and wear processes jointly or severally occurring in steel 45

Analysis of experimental data on fatigue and wear of tribosystem parts with lubricants of various compositions has demonstrated that the purposeful doping of the lubricants enables one to control the processes of surface passivation in friction, which govern the materials’ resistance to fatigue and wear. Fatigue life of tribosystem parts can be greatly varied by changing the position and size of the region of existence of surface structures. Translated from Problemy Prochnosti, No. 4, pp. 48–61, July–August, 2009.     

Prediction of the Durability of Cyclically Loaded Structural Elements

A procedure of determination of the durability of cyclically loaded notched specimens is proposed and experimentally verified. The procedure is based on the concepts of the unified model of fatigue fracture treating the processes of initiation and propagation of a fatigue macrocrack from the common point of view. For specimens with structural stress concentrators of two types, we compute the periods of initiation and growth of a fatigue macrocrack and the number of loading cycles to failure on the basis of the diagrams of fatigue crack growth rates. The numerical results agree with the experimental data with an error of at most 38% depending on the method of calculations and durability. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 41, No. 4, pp. 39–44, July–August, 2005.