Fatigue Life Curves Research Articles

Prediction of Strain Fatigue Life of HRB400 Steel Based on Meso-Deformation Inhomogeneity

The relationship between strain fatigue life and evolution of meso-deformation inhomogeneity was studied, through the cyclic process of numerical simulation of crystal plasticity compared with the fatigue test of steel hot-rolled ribbed-steel bar 400 (HRB400). The statistical characterization parameters at grain level, including the standard deviation of the dot product of longitudinal stress and strain, the product of the macro stress and the standard deviation of the longitudinal strain, and the product of the macro stress ratio and the standard deviation of the longitudinal strain, were proposed and respectively applied to measure the meso-deformation inhomogeneity of materials. These parameters take the effect of peak stress into account, distinct from the pure strain statistical parameters. The numerical results demonstrate that the low-cycle fatigue life curves of materials are predictable using the new parameters as FIPs (fatigue indicator parameters), and the predictions are more rational than by utilizing the FIPs without considering the peak stress effect.

Tensile Fatigue of Poly(Vinyl Alcohol) Hydrogels with Bio‐Friendly Toughening Agents

AbstractInspired by the avoidance of toxic chemical crosslinkers and harsh reaction conditions, this work describes a poly(vinyl alcohol)‐based (PVA) double‐network (DN) hydrogel aimed at maintaining biocompatibility through the combined use of bio‐friendly additives and freezing–thawing cyclic processing for the application of synthetic soft‐polymer implants. This DN hydrogel is studied using techniques that characterize both its chemical and mechanical behavior. A variety of bio‐friendly additives are screened for their effectiveness at improving the toughness of the PVA hydrogel system in monotonic tension. Starch is selected as the best additive for further tensile testing as it brings about a near 30% increase in ultimate tensile strength and maintains ease of processing. This PVA–starch DN sample is then studied for its tensile fatigue properties through cyclic, strain‐controlled testing to develop a fatigue life curve. Though an increase in monotonic tensile strength is observed, the PVA–starch DN hydrogel does not bring about an improvement in the fatigue behavior as compared to the control. Although synthetic hydrogel reinforcement is widely researched, this work presents the first fatigue analysis of its kind and it is intended to serve as a guide for future fatigue studies of reinforced hydrogels.

Torsional fatigue in bamboo fibers reinforced epoxy resin composites

Natural fibers are low-priced, abundant and renewable. The application of natural fiber to reinforced polymers materials can reduce cost, decrease weight and increase the mechanical properties of the composites. Many studies were carried out in natural fibers reinforced polymer composites using different natural fibers, one of those is bamboo fibers. Nevertheless, torsional fatigue tests have not been applied to predict the mechanical behavior of the bamboo fibers reinforced polymer composites. In this study, bamboo fiber reinforced epoxy resin composites were molded by compression in a rectangular mold. These specimens were machined according to the schematic of thin-walled tubular specimen suggested by ASTM-2207–15 and tested in torsional fatigue loading with R = −1. A mechanical torsion fatigue testing machine was constructed to perform torsion test with constant amplitude of the twist angle. An angle measuring device was constructed to measure the angular displacement in the useful region of the specimens (gage length). Torsional fatigue life curves were plotted for pure epoxy resin, unidirectional and discontinuous bamboo fibers reinforced epoxy resin composites. Pure epoxy resin specimens presented the longest torsional fatigue life and the poorest results were found in discontinuous bamboo fibers composites. The experimental results suggest that the epoxy resin is strongly related to the torsional fatigue behavior of the composite materials. The fracture surface of test specimens often showed that cracks occurred in the interfacial region of the composites.

Low cycle fatigue properties of hydrogenated welding sheets of Zr–Sn–Nb alloy using funnel-shaped flat specimens

Low cycle fatigue tests on the hydrogenated welding seam of Zr–Sn–Nb alloy at room temperature and 360 °C had been carried out by using the funnel-shaped flat specimens. The relationships between nominal stress & strain directly measured across the funnel and local stress & strain at the root of the funnel are given by considering cyclic plasticity correction. The results show that the fatigue resistance of welding seam at room temperature is only slightly better than that at 360 °C. Probabilistic fatigue life curves are obtained by using a two-parameter power function.

High-cycle fatigue and fracture behavior of double-side friction stir welded 6082Al ultra-thick plates

For welded structures of precipitation-strengthened aluminum alloys, achieving sound joint with good fatigue property is not an easy task, especially for thick plates. In the present work, ultra-thick 6082Al-T4 alloy plates with 80 mm in thickness were double-side friction stir welded and heat-treated. The high cycle fatigue (HCF) property and fracture behavior of the joint were investigated using trisection specimens in thickness, and the key factors that affect the fracture behavior were addressed. High quality joint was achieved with the three-layered specimens fractured along the lowest hardness zone (LHZ) in the static tensile testing. Further, the fatigue life curves exhibited almost the same distribution tendency, and the same fatigue limit (110 MPa) and fatigue ratio (0.49) were achieved in the three-layered specimens. During fatigue testing, the middle and lower layers fractured along the LHZ, which was similar to those during the static tensile tests, while abnormal fracture occurred in the nugget zone (NZ) of the upper specimen. Further examination revealed that the residual stress in the NZ of the upper layer was tensile stress (+9 MPa) whereas it was compressive stress in the lower layer (–31 MPa). The tensile residual stress together with the external loaded cyclic tensile stress resulted in more serious fatigue damage in the upper layer and led to the abnormal fracture in the NZ.

Experimental Investigation of the Fatigue Behavior of Basalt Fiber Reinforced Polymer Grid‐Concrete Interface

Fatigue behavior is an important factor for mechanical analysis of concrete members reinforced by basalt fiber reinforced polymer (BFRP) grid and polymer cement mortar (PCM) and plays a critical role in ensuring the safety of reinforced concrete bridges and other structures. In this study, on the basis of the static loading test results of concrete specimens reinforced by BFRP grid and PCM, a series of fatigue tests with different loading levels were conducted on interfaces between BFRP grid and concrete to investigate the fatigue behavior of BFRP grid‐concrete interfaces. The test results indicate that with high loading level, the fatigue failure mode of interface is interfacial peeling failure while it transforms to the fatigue fracture of the BFRP grid under low loading level. The fatigue life (S-N) curves of BFRP grid‐concrete interface are obtained and fitted in stages according to different failure modes, and the critical point of the two failure modes is pointed out. The relative slip evolution of interface during fatigue is further revealed in different stages with two failure modes, and the law of interface strain is studied with the increase of fatigue times. The relation of effective bonding length of interface and fatigue times is also described.

Development of a fatigue life prediction methodology for welded steel semi-trailer components based on a new criterion

This paper presents a procedure developed to predict the fatigue life in components made of steel, based on the mechanical properties of the base material and Thermally Affected Zones (TAZs) owing to welding. The fatigue life cycles of the studied components are obtained based on a certain survival probability provided by a Weibull distribution. This procedure is thought to be applied on semi-trailer components, and therefore it is proposed for the steels that are typically used in its manufacturing. A criterion for the adjustment of the exponent and the stress stroke of the fatigue life curve in welded joints is proposed in which the parameters that define the alternating stress versus the number of cycles to failure (S-N) curve are obtained exclusively from the ratio between the base material yield stress of a given steel and the strength of its Thermally Affected Zone. This procedure is especially useful for steels that do not have a complete characterization of their fatigue parameters. These developments are implemented in a subroutine that can be applied in commercial codes based on Finite Element Method (FEM) to obtain a fatigue life prediction. Finally, a numerical-experimental validation of the developed procedure is carried out by means of a semi-trailer axle bracing support fatigue analysis.

Weibull and Bootstrap-Based Data-Analytics Framework for Fatigue Life Prognosis of the Pressurized Water Nuclear Reactor Component Under Harsh Reactor Coolant Environment

AbstractIn general, the fatigue life of a safety critical pressure component is estimated using best-fit fatigue life curves (S-N curves). These curves are estimated based on underlying in-air condition fatigue test data. The best-fitting approach requires a large safety factor to accommodate the uncertainty associated with large scatter in fatigue test data. In addition to this safety factor, reactor component fatigue life prognostics requires an additional correction factor that in general is also estimated deterministically. This additional factor known as the environmental correction factor Fen is to cater the effect of the harsh coolant environment that severely reduces the life of these components. The deterministic Fen factor may also lead to further conservative estimation of fatigue life leading to unnecessary early retirement of costly reactor components. To address the above-mentioned issues, we propose a data-analytics framework which uses Weibull and Bootstrap probabilistic modeling techniques for explicitly quantifying the uncertainty/scatter associated with fatigue life rather than estimating the lives based on a best-fit based deterministic approach. We assume the proposed probabilistic approach would provide the first hand information for assessing the maximum and minimum effects of pressurized water reactor water on the reactor component. In the discussed approach, in addition to the probabilistic fatigue curves, we suggest using a probabilistic environment correction factor Fen. We assume the probabilistic fatigue curve and Fen would capture the S-N data scatter associated with the bulk effect of material grades, surface finish, strain rate, etc. on the material/component fatigue life.

Static and Fatigue Characteristics of Pinned Metal-Composite Joints

Metal-composite joining methods currently rely almost exclusively on the adhesive bonding, the application of mechanical fasteners or a combination of the methods, the pros and cons of which are well known. In this article, a manufacturing-oriented solution for increasing the static and fatigue characteristics of metal composite bolted T-joints loaded in the out-of-plane direction by reinforcing the composite basement with a thin metal plate having die cut pins is proposed. Static load–displacement and fatigue life curves were obtained experimentally for different types of joints. The results obtained showed that, for a fiberglass/epoxy laminate, the solution proposed increased its static and fatigue failure loads by 64 and 50% respectively. It was also found that pinned adhesive joints were more effective than the conventional bolted ones at high-cycle loadings (the number of cycles exceeded 104).

A Durability Prediction for the Magnesium Alloy AZ31 based on Plastic and Total Energy

Magnesium has a good strength-to-weight ratio. As a result, magnesium alloys are gradually becoming an integral part of lightweight designs that are especially important to the automotive and aerospace industries. The magnesium alloy AZ31 is particularly suitable for forming. Furthermore, as its cyclic response is asymmetric, the accumulated energy during loading is a convenient basis for durability predictions of a product that is made of the AZ31 alloy. In this article, we show how the inverse-power law model can be combined with a Weibull’s probability density function to describe the shape of the fatigue-life curves and their scatter. Various load histories were chosen to characterize the response of the AZ31 alloy under different cyclic loadings. Both the plastic and the total strain energy densities were calculated for all the load histories, and then, a suitable dependence in terms of energy fatigue-life curves was determined. The proposed model is appropriate for modeling the low-cycle fatigue life of the AZ31 alloy. With the application of a linear damage-accumulation rule, it is also possible to predict the fatigue life for the non-constant amplitude loading within one decade of accuracy.

Study on Fatigue Properties of Copper Alloy Netting Structure

Aiming at the damage and failure problem of copper alloy netting structure, the ultimate strength and fatigue performance of the net structure were studied by test method. Based on the research on the ultimate strength of copper wire, through a series of fatigue tests on copper wire and net structure, the fatigue life and failure modes of copper wire and net structure under different loads are analyzed, and their fatigue life curves are also drawn. The results show that the fatigue strength of copper wire and net structure considering corner processing is lower than that of copper wire not considering corner processing, which indicates that corner processing has a great influence on the fatigue strength of actual copper net structure. Compared with the fatigue strength value of 32.8 MPa of the copper net, the fatigue strength value of the net structure decreases to a certain extent (about 14.3%), which indicates that the assembly process of the copper net has certain influence on the fatigue life of the net structure.

Evaluation of fatigue life and fatigue limit of circumferentially-notched Type 304 stainless steel in air and hydrogen gas based on crack-growth property and cyclic stress-strain response

Fatigue tests were performed using circumferentially-notched, round bar specimens with a stress concentration factor, Kt, of 6.6 for Type 304, metastable, austenitic stainless steel. The tests were carried out in ambient air and in 0.7 MPa hydrogen gas at room temperature. In a relatively higher stress amplitude regime (i.e., the amplitude resulting in Nf < 105), the hydrogen gas environment caused a marked degradation in fatigue life. In contrast, in a relatively lower stress amplitude regime (i.e., the amplitude resulting in Nf > 105), it appeared that fatigue life did not differ between air and hydrogen gas. It was also confirmed that the fatigue limit appeared not have been degraded in the hydrogen environment though there was a slight difference between the data obtained in two environments. The fatigue life curve and fatigue limit were predicted by assuming that the notch was equivalent to a circumferential crack. Consequently, there was a significant disparity between the prediction and the experimental results. As a result of microscopic observations of the fracture process in combination with elastic-plastic finite element analyses, these discrepancies were attributed to (i) complex cyclic plastic deformation behavior under large- and small-scale yielding conditions within the vicinity of the notch root, (ii) the retardation of crack initiation in the finite life regime, and (iii) the absence of non-propagating cracks at the fatigue limit, all of which are typical characteristics of metastable austenitic stainless steel.

Fatigue Life Prediction of Steam Generator Tubes by Tube Specimens with Circular Holes

Heat exchangers manufactured from Inconel 690 tubes are widely used for steam generators in nuclear power plants. Inconel 690 tubes have suffered failures of fatigue fracture due to flow induced vibration. It is difficult to obtain the fatigue life of the tube directly since the conventional fatigue test would potentially cause end fatigue failure due to the stress concentration at the clamp end. In this study, a thin-walled Inconel 690 tube with circular hole is designed to deduce the fatigue life of smooth tube based on the notch fatigue life prediction technology. Firstly, the local stress and strain distributions around the hole based on the finite element analysis are discussed. Local stress-strain is calculated and compared with Neuber’s ruler. Meanwhile, fatigue life tests using tube specimens with circular holes are carried out. Finally, based on the best-fitted fatigue life curve of Inconel 690 alloy, the fatigue life of tube specimen is estimated from the local strain according to Neuber’s ruler. The results show that the local stress and strain estimated by Neuber’s ruler are basically consistent with those obtained by finite element analysis. Compared with the average fatigue life of nickel-based alloy, the new predicted equivalent fatigue life of heat Inconel 690 transfer tube with a hole is higher. The Inconel 690 heat transfer tube has better fatigue performance.

A comparison between S-N Logistic and Kohout-Věchet formulations applied to the fatigue data of old metallic bridges materials

A new formulation of a Logistic deterministic S-N curve is applied to fatigue data of metallic materials from ancient Portuguese riveted steel bridges. This formulation is based on a modified logistic relation that uses three parameters to fit the low-cycle- (LCF), finite-life- and high-cycle-fatigue (HCF) regions. This model is compared to the Kohout-Věchet fatigue model, which has a refined adjustment from very low-cycle fatigue (VLCF) to very high-cycle fatigue (VHCF). These models are also compared with other models, such as, Power law and fatigue-life curve from the ASTM E739 standard. The modelling performance of the S-N curves was made using the fatigue data considering the stress fatigue damage parameter for the materials from the Eiffel, Luiz I, Fao and Trezoi riveted steel bridges. Using a qualitative methodology of graphical adjustment analysis and another quantitative using the mean square error, it was possible to evaluate the performance of the mean S-N curve formulation. The results showed that the formulation of the S-N curve using the Logistic equation applied to the metallic materials from the old bridges obtained superior performance to the analysed models, both in the estimation of fatigue behaviour in the low-cycle fatigue (LCF) region and in the lowest mean square error.

Low Cycle Fatigue Behavior of CP-Ti at Different Temperatures

Low cycle fatigue (LCF) tests are performed on CP-Tiat different temperatures (293K,423K and 523K). It is found that the fatigue life of CP-Tidecreases with temperature. A short cycle hardening phenomenon occurs at the beginning of cyclic deformationat 293K and 423K, followed by cyclic softening untilfailure. At 523K, cycle hardening isexhibited throughout the entire cycle until thefracture. The fatigue-life curves obtained from the tests are constructed using Coffin-Manson-Basquin model. According to the relationship between the four parameters of Coffin-Manson-Basquin model and temperature, the temperature-based life prediction model is further proposed. Scanning electron microscopy observation of fatigue fractures showsthat the fatigue cracks of CP-Tiat 423K and 523K under different strain amplitudes initiate on the surface of fatigue specimens and extend to the fracture zone by the transgranular mode.

Corrosion fatigue properties of narrow gap laser welded 301L stainless steel

In this paper, the corrosion-fatigue behavior of 301L stainless steel (SS) welded joints using narrow gap laser wire welding under the different stress ratio and concentrations of Cl[Formula: see text] has been investigated. Corrosion fatigue life curve (S-N) under different conditions was tested and fitted to obtain the fatigue limit. The microstructures and fracture surfaces of specimens were examined by optical microscopy (OM) and scanning electron microscopy (SEM). EBSD results have shown that chromium depletion in the weld heat-affected zone (HAZ) contributes to the decrease of corrosion fatigue properties in the HAZ.

Experimental and numerical investigation of static and fatigue behaviors of composites honeycomb sandwich structure

Static and fatigue behaviors of Nomex honeycomb sandwich structures were characterized by compression and bending experiments. And the finite elemental numerical simulation research was carried out based on the test data. The load level for fatigue test was determined and the necessary stiffness information for the construction of finite element model was provided by the study of static load performance. The fatigue failure mode and damage mechanism of Nomex honeycomb sandwich structures were analyzed by comparing the fatigue life curves and experimental phenomena of bending fatigue and compression tests. This study presented a method to predict the fatigue life of honeycomb sandwich structures and its feasibility was verified by experiments.

Self-piercing riveting of aluminium–lithium alloy sheet materials

The self-piercing riveting (SPR) of 1420 aluminium–lithium alloy sheets is investigated. Static tests with lap-shear loads and fatigue tests using the tension–tension loading mode were applied to obtain the joint performances and failure modes. The mechanism analysis regarding failure and fretting was employed using a scanning electron microscope (SEM) and an energy dispersive X-ray spectroscopy (EDX). The 1420 Al–Li alloy sheets can be joined effectively using SPR. The fatigue load–fatigue life curve with a 95% confidence level is obtained using a variation coefficient method. The joints failed in the rivet pulled out from the locked sheet in static trials, and the locked sheet fractured during the fatigue tests. The fretting wear can be observed at the contact interface between the rivet tail and the locked sheet or between both sheets at different load levels. The fretting region changes at a high or low load level, and it is highly relevant to the fatigue rupture paths.

Low‐cycle fatigue life of thin‐plate auxetic cellular structures made from aluminium alloy 7075‐T651

AbstractIn the article, the low‐cycle fatigue life durability of thin‐plate auxetic cellular structures is compared with the thin‐plate specimens of standard shape. Both the re‐entrant auxetic cellular structures and the standard specimens were cut from a 2‐mm‐thick strip of aluminium alloy 7075‐T651. First, a fatigue life curve and a cyclic curve were determined for the standard specimens. A special antibuckling device was applied to prevent the bending of the specimens. The same experimental arrangement was then applied to determine the low‐cycle fatigue life of auxetic structures. In the continuation, the most appropriate method was selected to calculate the fatigue life on the basis of the measured fatigue life curves. Abaqus and SIMULIA fe‐safe software were applied for this purpose. The best predictions for the standard specimens were obtained with the Brown‐Miller method. Finally, the selected method was applied to predict the low‐cycle fatigue life of re‐entrant auxetic cellular specimens.

Probabilistic Fatigue and Reliability Simulation

This paper demonstrates the advantages of using ‘probabilistic fatigue simulation’ and ‘stochastic’ design over the traditional ‘deterministic’ design approach. It demonstrates how ‘Monte Carlo’ simulation with ‘Latin hypercube sampling’ are effective for obtaining simulated reliability tests based on standard FEA models. Particular attention is paid to recommendations for addressing the effects of fatigue endurance and static failure in the reliability analysis. It also describes how design ‘robustness’ is ensured by using a ‘factorial sampling’ technique in conjunction with a ‘response surface’ model. The requirements for a ‘Reduced Order Model’, obtainable from FEA, are discussed. Recommendations are made to address the effects of fatigue endurance, static failure and non-linearities in the fatigue life curves during Weibull analysis. A case study demonstrates how the methods are applied to an air-cooled intercooler. A comparison is made between measured reliability test results and simulations performed using both deterministic and stochastic approaches. The simulation offers excellent correlation with the experimental measurements. No further improvements in the model were deemed necessary and the model is considered a suitable platform for performing additional design simulations and for extrapolation of the reliability statistics.