Fatigue Curves Research Articles

Microstructure-Based Model for Sharp Stress Raiser-Related Fatigue Stage Length Assessment

A model for evaluating the fatigue life of specimens/structure elements with sharp stress raisers/defects is presented. The model permits of computing the number of cycles to fatigue crack initiation and its growth from a sharp stress raiser to failure at a constant stress span with the only application of characteristics of static strength and microstructure of the initial material. The model can be used to assess the fatigue life of components that contain structural stress raisers and defects stemming from their manufacturing technique (surface roughness, surface cuts, scratches, and microcracks). The model reliability was verified with experimental results taken from the literature, calculations appeared to be in good agreement with experimental data. Fatigue curves to a grain-size crack initiation and to fracture of smooth specimens and those with a chemically-notched blunt raiser that simulates the casting defects in aircraft components were calculated. The two sets of specimens from a Ti–6Al–4V titanium alloy differing in the cross-section (rectangular and cylindrical) and in microstructure (different grain sizes). Smooth specimens exhibited the test surface roughness Rv = 10 and 19 μm (average dent depth), which was assumed to be a sharp raiser for calculations. The model need not long-term and labor-consuming high-cycle fatigue tests to construct the fatigue curve.

Estimation of temporary decommissioning impact on reliability and durability of oil pipelines

Purpose: Due to changes in the volume of oil transportation, part of the pipelines must be temporarily decommissioned. The purpose of the work is theoretical and experimental study of pressure fluctuations during operation of oil pipelines and their impact on the durability of the pipeline material in work state and after temporary decommissioning (conservation). Design/methodology/approach: The results of oil pressure fluctuation research have given a chance to choose the terms of experimental research providing of the pipe metal mechanical features changes research during oil pipeline exploitation. Findings: Fatigue test modes are selected based on the calculations of the mathematical model developed. Experimental studies of the dependence of the fatigue strength of the pipe material on the conditions of operation have been carried out, which made it possible to determine the parameters of the fatigue curve of the samples. Has been defined that fatigue strength for the new metal pipe samples is more for 20-25% than for the metal samples which had the contacts with lime milk and for 30-40% more than for metal samples which were under exploitation. Research limitations/implications: In the future, more combination of "pipe material – preservation medium" should be explored to establish pain of general regularities. Practical implications: The probabilistic curves of the pipeline non-destruction are constructed, which will be used for practical calculations of the reliability and durability of the 13G1S-U steel pipelines. Originality/value: Mathematical model is made that describes non-stationary oil moves in oil pipeline, that has been caused by jump like changes of oil supply in oil pipeline, on this basis was defined, that oil pressure in oil pipeline provides within non stationary process in a range of 0.4-0.6 Hz frequency and amplitude fluctuation of pressure is 0.1-0.5 MPa.

Cumulative Damage Model for Glass-Fiber Reinforced Composites under Two Blocks Loading

A fatigue damage accumulation model for composite materials is proposed. There is unanimity on the complex nature of the mechanisms of damage inherent to these types of materials. This led to proposals for nonlinear models, but relatively expensive in experimental requirements and determination of parameters. Thus, the proposed model is simple and does not require parameters apart the ultimate strength and the fatigue curve. A program is developed to allow the follow-up of the damage evolution during the cycles of loading. The proposed model is applied to E-Glass/Epoxy [0°/90°] under two blocks loading. Prediction results are relatively good and fit well with the experimental results compared to similar models.

Assess the impacts of different autonomous trucks’ lateral control modes on asphalt pavement performance

With ongoing massive investment in the field of autonomous vehicles, fully autonomous self-driving vehicles are set to become a reality soon. Despite the benefits in traffic safety and economy, the potential effects of autonomous vehicles, especially autonomous trucks, on transportation infrastructure remain to be determined. In this study, a finite element model was developed based on a typical flexible pavement structure. The effects of autonomous trucks’ lateral distribution within the lane with respect to rutting depth and fatigue damage were estimated by finite element analysis under certain environmental condition. Characteristics of the pavement structure and environmental condition have been collected and presented in this paper. Four possible lateral control modes on managing autonomous trucks’ lateral distribution were proposed, listed as zero-wander mode, uniform mode, double peak Gaussian mode, two-section uniform mode. Considering the difference between autonomous trucks and human-driven trucks, the ratio of autonomous trucks was also evaluated. Based on the simulation results, it has been confirmed that, if controlled appropriately, autonomous trucks could be highly beneficial to asphalt pavements for wider using of pavement. Conversely, if without appropriate lateral control, the effect of autonomous trucks on the typical flexible pavement life could be negative, owing to repeated single point load caused by lane centering and keeping. For example, the uniform mode, double peak Gaussian mode and two-section uniform mode all throw positive effect on the pavement. On the contrary, under the zero wander mode, the time when the rutting depth of the proposed pavement reaches 15 mm (defined as maintenance year) may advance by 1.56 years and fatigue damage at the bottom of asphalt layer increases by 146%. Compared with the normal situation nowadays, the two-section uniform distribution of autonomous trucks, the best performing control mode, delays the maintenance year by 2.3 years. From the aspect of fatigue, a much uniform distribution curve of fatigue within the lane can also be found under this mode. When all trucks’ lateral positions were in control, the two-section uniform mode could reduce the fatigue damage by up to 35% under repeated standard axle load on the flexible pavement. Finally, a framework of dealing with the lateral control was proposed as a reference for different conditions, such as asphalt pavement structures and materials.

Fatigue equation for asphalt mixture under low temperature and low loading frequency conditions

Abstract In order to reveal the thermal fatigue behavior of asphalt mixtures under low temperature conditions, the four-point bending beam tests of strength and fatigue were carried out. The asphalt mixtures are typical viscoelastic materials whose fatigue properties are significantly influenced by low loading frequency. Thus, the fatigue tests under low loading frequency were used to simulate the thermal fatigue of asphalt mixtures. Based on the nominal stress ratio (not considering the effect of loading rate on the strength value) and the real stress ratio (the strength value corresponding to the loading rate of fatigue test), the fatigue equations of asphalt mixtures under low temperature and low frequency conditions were established. The flexural strengths of the four-point bending beam tests at different loading rates at 0 °C, −10 °C and −20 °C were studied. The results show that the flexural strength of asphalt mixtures expresses a power function on the pattern of variation with the loading rates. The intersection of fatigue curve characterized by the nominal stress ratio and the abscissa axis is much larger than 1. The fatigue curve based on the nominal stress ratio cannot be extended to fatigue failure point (1,1). But the fatigue curve characterized by the real stress ratio can be extended to the strength failure point, whose fatigue life is 1. So, the behavior of strength failure and fatigue failure was integrated. The uniform characterization model of fatigue equations for asphalt mixture under low temperature and low loading frequency conditions was realized, which means that the fatigue equations and curves under different loading frequencies could be characterized by one equation and one curve. The results can provide a theoretical basis for predicting the thermal fatigue performance of asphalt mixtures under low temperature conditions more accurately.

Fatigue Evaluation of Pressure Vessel using Finite Element Analysis based on ASME BPVC Sec. VIII Division 2

A fatigue of a typical pressure vessel was evaluated using Finite Element Analysis based on ASME Boiler and Pressure Vessel Code Section VIII Division 2. The pressure vessel was subjected to thermal and pressure cyclic loading. A finite element code ANSYS ver. 14.5 was used to perform the linear elastic stress fatigue analysis of the vessel. The vessel was modeled as an 2D axisymmetric model. The fluctuation load of thermal, pressure, dead weight and pressure drop were considered in the analysis. The alternating stress was calculated using the result of Finite Element Analysis. Then from the fatigue curves of material, the permissible number of cycle corresponding to the alternating stress was determined. The fatigue damage was calculated by dividing the actual number of repetitions with the permissible number of cycle. If the accumulated fatigue damage was less than one then the design of the pressure vessel was accepted.

Statistical Determination of Fatigue Strength

A cyclic test method for the fatigue strength of metals is proposed. In this method, fatigue curves characterizing the stress dependence of the number of cycles to failure are plotted and statistically described. The number of cycles to failure is described by a Weibull distribution, using the lowest guaranteed values.

Corrosion fatigue crack initiation in ultrafine-grained near-α titanium alloy PT7M prepared by Rotary Swaging

The study focuses on corrosion fatigue processes taking place in an ultrafine-grained (UFG) near-α-titanium alloy Ti-2.5Al-2.6Zr (Russian industrial name PT7M) used in nuclear engineering. UFG structure formed with Rotary Swaging is found to increase resistance to corrosion fatigue. Parameters of the Basquin's equation are defined and the slope of the fatigue curve σa-lg(N) is shown to depend (nonmonotonic dependence) on the UFG alloy annealing temperature. This effect can be explained with the patterns of microstructural evolution in a UFG alloy PT7M during annealing: (1) reduced density of lattice dislocations, (2) precipitation and dissolution of zirconium nanoparticles, (3) release of α′′-phase particles causing internal stress fields along interphase (α-α′′)-boundaries, and (4) intensive grain growth at elevated annealing temperatures. It is shown that the fatigue crack closure effect manifested as changing internal stress fields determined using XRD method may be observed in UFG titanium alloys.

Low cycle fatigue and creep-fatigue response of the 316Ti stainless steel

SS 316Ti is widely used in bellows industry and is a good candidate material for high temperature bellows in sodium cooled fast reactor (SFR) systems. Typical operating temperature experienced by SFR systems is around 823K. Design of bellows for nuclear applications need to be in compliance with the standard design codes such as ASME section-III and RCC-MR. The fatigue data and cyclic stress strain curve of SS316Ti are not available in design codes such as RCC-MR or ASME section-III/NH. Hence, the material data required for high temperature design of bellows are generated experimentally. Initially, the basic tensile data such as yield strength, ultimate tensile strength and % elongation of the material were obtained from tensile testing at 823K. Low cycle fatigue tests were carried out in strain controlled mode on SS316Ti at 823K different strain ranges in air and variation of number of cycles with strain range was obtained. Creep-fatigue interaction (CFI) experiment was also conducted at 823K and strain amplitude of ± 0.4% with 1 minute hold time in peak tensile strain. The stress response (peak stress variation with number of cycles) of the material showed continuous hardening up to saturation followed by crack nucleation and final failure. The fatigue life was found to decrease with increase in strain range. The fatigue life decreased in presence of hold period in tension. The design fatigue curve for SS316Ti at 823K has been generated using the LCF data by incorporating factors of safety on strain and number of cycles. Cyclic stress strain curve was generated for the material at 823K. The tensile, LCF and CFI data generated will be useful in design of SS316Ti bellows for SFR systems.

Fatigue Fracture of Metals under Complex Stress State with Consideration of Structural Changes

A phenomenological model of brittle fatigue fracture in metals and alloys is discussed in the case of proportional (simple) loading. The model is formulated in the framework of the physico-mechanical approach as a system of hypotheses concerning the probability of defect evolution at various scale-structural levels, such as micro- and macrocracks. The described constitutive relations are based on this model. A number of fatigue curves are obtained for various limit states of metals.

Experimental SAC305 Shear Stress–Strain Hysteresis Loop Construction Using Hall's One-Dimensional Model Based on Strain Gages Measurements

Temperature-induced solder joint fatigue is a main reliability concern for aerospace and military industries whose electronic equipment used in the field is required to remain functional under harsh loadings. Due to the RoHS directive, which eventually will prevent lead from being utilized in electronic systems, there is a need for a better understanding of lead-free thermomechanical behavior when subjected to temperature variations. Characterizing solder joints properties remains a challenge as viscoplastic behavior during thermal cycling is complex, and their small dimensions prevent direct measurements from being performed. This paper reports the experimentation based on strain gage measurements, allowing the construction of the shear stress–strain hysteresis loop corresponding to Sn3.0Ag0.5Cu (SAC305) solder joints behavior during thermomechanical loading. This methodology, initially developed in 1984 by Hall for Sn60Pb40 interconnects, allows the measurement of the strain energy density dissipated during temperature cycles. Custom daisy-chained 76 I/O ceramic ball grid array (CBGA76) components were designed and assembled on flame retardant (FR-4) multilayered printed circuit boards (PCB). Four strain gages were specifically placed at the center of the assembly on top and bottom faces of both PCB and CBGA76 component. The assembly was subjected to temperature cycles and the SAC305 solder joints shear stress–strain hysteresis loop was plotted. The correlation between the measured strain energy density and measured lifetime corresponds to one point of the energy based fatigue curve for SAC305 solder joints. The hysteresis loop also provides the necessary data to derive SAC305 solder joints constitutive laws.

Reference strength values to design against static and fatigue loading polylactide additively manufactured with in‐fill level equal to 100%

The aim of this paper is to provide a quantitative examination of the state‐of‐the‐art knowledge of the static and fatigue behaviour of additively manufactured (AM) polylactide (PLA). To this end, existing literature was reviewed, and a number of data sets were extracted and re‐analysed in terms of static strength and standard S‐N curves. As long as objects are 3D‐printed flat on the build plate, printing direction appears to have little effect on the mechanical behaviour of AM PLA, therefore stress/strain analysis can be performed effectively by simply treating this polymer as a linear‐elastic, homogenous, and isotropic material. If static strength cannot be determined experimentally, a conservative reference value of 22 MPa is suggested as being used in situations of practical interest. As far as fatigue is concerned, findings from post‐processing reveal that non‐zero mean stresses can be modelled by simply using the maximum stress in the cycle. According to the statistical re‐analysis discussed in the paper, a reference fatigue curve for the design of AM PLA subjected to uniaxial cyclic loading (for a probability of survival larger than 90%) can be defined by taking the negative inverse slope equal to 5.5 and the endurance limit (at 2·106 cycles to failure) equal to 10% of the material ultimate tensile strength.

Nominal and local stress quantities to design aluminium-to-steel thin welded joints against fatigue

Welding aluminium to steel to make mechanical joints is possible, but there is, to date, no accepted method for performing the fatigue assessment of such hybrid connections. In this context, the present investigation aims at checking the accuracy of nominal stresses, effective notch stresses, notch-stress intensity factors, and the Modified Wöhler Curve method (applied in conjunction with the Theory of Critical Distances) in estimating fatigue lifetime of butt, cruciform, lap and tee aluminium-to-steel thin welded joints. EWM coldArc® welding technology was used to manufacture the welded specimens that were used for this validation exercise. The samples being tested in the structural laboratory of the University of Sheffield, UK, were manufactured by using AA1050 aluminium and EN10130:1991 steel with main plates thicknesses of 1 mm or 2 mm. The results from this experimental/theoretical investigation demonstrate that all the design methodologies being investigated can be used to perform the fatigue assessment of aluminium-to-steel thin welded joints provided that suitable reference/calibration fatigue curves are used. In the present paper, some quantitative recommendations are given for use in situations of practical interest of the design techniques being considered.

Cyclic deformation and fatigue behavior of additively manufactured 17–4 PH stainless steel

Static and fatigue experiments were conducted on conventionally manufactured (CM) and additively manufactured (AM) 17–4 PH stainless steels in ambient air. The fatigue experiments were tested under the fully reversed strain-controlled condition with the strain amplitudes ranging from 0.15% to 1.0%. The two manufacturing conditions result in similar tensile strengths but the AM 17–4 PH stainless steel has a significantly lower ductility than that of the CM steel. The fatigue limit of the CM 17–4 PH stainless steel is approximately 640 MPa and the fatigue limit of the AM steel is approximately 300 MPa. The strain-life fatigue curves of both materials display a kink point from low cycle fatigue regime to high cycle fatigue regime. Defects and porosity from the additive manufacturing process contribute to the weakened fatigue properties and ductility of the AM stainless steel.

Relation between Internal Friction and Fatigue

Solids show the feature of internal friction and the feature of fatigue. As a cause for the both features we assume the migration of atomic items. The driving forces for the migration of the atomic items can be mechanical stress gradients and temperature gradients. In an earlier publication formulas have been derived for the internal friction and for the fatigue. From these formulas follows a relation between internal friction and fatigue. The mechanical stress gradients are the cause of the background of internal friction which shows a monotonous rise with temperature. Temperature gradients in the solid can be produced by total internal friction. In contrast to the background of internal friction the temperature dependence of the total internal friction shows maxima and minima. Because of the relations between internal friction and fatigue the temperature dependence of fatigue shows that the fatigue is caused by temperature gradients. The relation between internal friction and fatigue also shows the influence of the internal friction on the fatigue curve.

Study on fatigue self-healing properties of basalt fiber asphalt mixture

Asphalt concrete pavement will age gradually and produce fatigue damage under the repeated load and environment. Under the repeated action of driving load and the continuous influence of external environment, it will gradually age and produce fatigue damage. It is found that the asphalt mixture has self-healing characteristics, and it is of positive significance to improve the service life of asphalt pavement by giving full play to the self-healing characteristics of asphalt mixture. With the extensive application of basalt fiber asphalt mixture, its self-healing characteristics have become an urgent problem to be studied. In this paper, according to the four-point bending fatigue test of basalt asphalt mixture, the slope ratio of fatigue curve is selected as the recovery. The effects of basalt fiber asphalt mixture on its self healing ability under three factors, such as damage degree, healing time and healing temperature, were analyzed by orthogonal test, and compared with matrix asphalt mixture and SBS modified asphalt mixture. The results showed that the self healing capacity of asphalt mixture was inversely proportional to the damage degree and time. When the damage degree is more than 50%, the healing performance of the matrix asphalt mixture decreases obviously, and the SBS modified asphalt mixture almost does not heal, while the healing performance of the basalt fiber asphalt mixture is the best.

A finite element based approach for fatigue life prediction of headed shear studs

Steel shear studs in bridges are subjected to rapidly fluctuating stresses causing fatigue failure. Research on fatigue of shear studs mainly focused on tests. Both AASHTO and Canadian design curve for fatigue resistance of shear studs are based on the tests conducted in the mid1960s by Slutter and Fisher. This paper presents a finite element based approach using push-out specimen for fatigue life estimation of headed shear stud connectors. Both crack initiation and crack propagation life are estimated and an excellent correlation is found when compared against test results. In addition, since a significant amount of push-out tests data on headed shear studs are now available, this paper evaluates the fatigue design curves of different standards, with special focus given to evaluation of the value of constant amplitude fatigue limit (CAFL) given in the current AASHTO and Canadian code (CSA S6-14). The regression analysis also shows that the current fatigue curves in different codes can be used for shear studs as large as 31.8 mm. Thus, restriction of use of studs larger than 25 mm (1 in.) in different bridge codes (CSA S6, Eurocode 4, and AASHTO) can be waived.

Fatigue Strength and Life Prediction of a MAR-M247 Nickel-Base Superalloy Gas Turbine Blade with Multiple Carbide Inclusions

Casting of a gas turbine blade from MAR-M247 nickel-base superalloy is followed by multiple carbide precipitates as a result of solidification. The microstructure of carbide was established with scanning electron microscopy and energy spectrum analysis. A finite element model simulating the blade with a random carbide precipitate is constructed using ANSYS software, and stress-strain evaluation is performed. The blade operation conditions are realized in a vibratory-tensile combined test setup with stress measurements using strain gauges arranged on different parts of the blade. The respective von Mises equivalent stresses obtained in simulation and test measurements are found to be comparable and can be used to assess the blade fatigue life based on the available fatigue curves and Palmgren–Miner rule of damage accumulation. Simulation and experimental results are the basis for plotting the relationship between the volumetric ratio of precipitates in the blade and its fatigue life. The results obtained at room temperature can; be extrapolated to the elevated ones to provide more reliable prediction of the superalloy blade fatigue and creep-fatigue life values.

Discussion on crack growth behavior in large-scale fatigue tests of carbon and low-alloy steel plates based on fracture surface observation

In Japan, the Design Fatigue Curve (DFC) Phase 1 and Phase 2 subcommittees, organized under the Atomic Energy Research Committee of the Japan Welding Engineering Society, have proposed new best-fit fatigue curves and design fatigue curves for carbon, low-alloy, and austenitic stainless steels. To investigate the correlation between the fatigue life by the proposed best-fit curves and that of the actual components, the authors carried out reversed four-point bending fatigue tests for large-scale specimens of carbon steel and low-alloy steel plates with 90 mm thickness and 150 mm width. As a result, the fatigue lives given by the best-fit curve were found to correspond to approximately 3-4-mm-deep crack initiation lives in large-scale specimens. The result implies that only several-millimeter crack initiation defines a large structure. It would be reasonable to consider the crack growth behavior after reaching the design fatigue life. During the fatigue tests, the crack growth behavior was monitored with a plastic replica and a beach mark method. Since the main crack grew with the coalescing of micro cracks, the applicability of fracture mechanics method is uncertain. In this study, a crack growth analysis was performed on the cracks observed in large-scale specimens using a crack growth law specified in a fitness-for-service (FFS) code or literature. With respect to the surface crack length, some experimental data surpassed the analysis results, which might be attributed to crack coalescence. The crack depth data that are not easily affected by the coalescence was plotted within the upper limit of data scatter. The analysis results enabled the prediction of the total fatigue life of the large-scale specimens as the sum of the crack initiation life by the best-fit curve and the growth life from a 3-mm-deep crack.

Experimental fatigue curves to perform the fatigue assessment of aluminium-to-steel thin welded joints

Welding of different materials, especially aluminium alloys to steel, has always been a challenge because of the significant differences in their mechanical, thermo-physical and metallurgical properties that result in the formation of hard and brittle intermetallic phases in the welding region. In this context, this paper experimentally investigates the fatigue behaviour of aluminium-to-steel thin welded joints. The purpose of this investigation is to verify the accuracy of the nominal stress approach in estimating the fatigue life of aluminium-to-steel welded joints. EWM coldArc® welding technology was used to make the welded samples used for this validation exercise. The results of this experimental/theoretical study show that the nominal stress approach can be used to evaluate the fatigue of thin aluminium-to-steel welded joints, provided that the appropriate reference/calibration fatigue curves are used.