Fatigue Strength Reduction Factor Research Articles

Investigation of the High-Temperature Low-Cycle fatigue failure characteristics of P91 steel weld joints and their fatigue strength reduction factors under various load control regimes

Low cycle fatigue tests are conducted on the base metal, weld metal, a welded joint under the condition that the strain control region is the joint (WJWJ) and a welded joint under the condition that the strain control region is the base metal (WJBM) of P91 steel at 500℃ using different total strain ranges. The cyclic stress response is highest for the weld metal, followed by the base metal, and is relatively similar for WJWJ and WJBM. At 2.0 % total strain range conditions, the materials generally exhibit non-Masing behavior and dynamic strain hardening aging. At all total strain ranges, WJWJ failed in the heat-affected zone, and WJBM broke in the base metal. For the difference in fatigue life between base metal and welded joints, the fatigue strength reduction factor of P91 steel welded joints at 500 °C is given as a suggested value. The fatigue behavior of P91 steel welded joints is verified based on a hybrid hardening model and a modified damage model. This study presents a theoretical framework for analyzing the fatigue strength reduction factors of weldments and comparatively analyzes the effects of different geometrical features.

Actual weld profile fatigue performance by digital prototyping of defected and un‐defected joints

AbstractThis paper considers the effects of the actual weld profile on the fatigue behavior of welded joints by means of the implicit gradient approach. Different types of geometrical approximation are considered in the analysis. In order to define the fatigue strength reduction factor of the actual weld profile, FE analyses have been considered based on a three‐dimensional scan of the welded joints. Furthermore, the fatigue behavior of butt‐welded joints with a discontinuous undercut is considered, and a comparison has been made with experimental data taken from the literature.

Multiaxial fatigue life prediction model considering stress gradient and size effect

Stress gradient effect (SGE) and notch size effect (NSE) have important influences on the fatigue performance of engineering components. In this paper, a new multiaxial fatigue life prediction model is developed by considering the effects of both. Firstly, with the help of the coordinate transformation principle (CTP), the location of the dangerous critical plane is found according to the energy-critical plane (ECP) method, and the equivalent stress gradient factor (SGF) within the fatigue damage zone is calculated by analyzing the equivalent stress distribution on the critical plane. Secondly, considering the size of the notch, the effect of stress concentration at the notch is analyzed. Meanwhile, a size influence factor is introduced to characterize the NSE and construct a new fatigue strength reduction factor. Further, combining with the Manson-Coffin equation (MC equation), a new fatigue life prediction model for multiaxial notched components is proposed by considering SGE and NSE on fatigue damage. Finally, the experimental data of fatigue specimens with different notch sizes processed by En8 and GH4169 materials are used for model validation and comparison with other three classical models, and the results show that the prediction life of the new model for proportional and non-proportional loads is within two-fold dispersion band and the prediction ability is better than the other three classical models.

Notches, nominal stresses, fatigue strength reduction factors and constant/variable amplitude multiaxial fatigue loading

This paper deals with the reformulation of the Modified Wöhler Curve Method to make it suitable for being used along with nominal stresses to design notched components against constant/variable amplitude multiaxial fatigue loading. The accuracy of this design methodology is checked against a large number of experimental results taken from the literature. The outcomes of the performed validation exercise strongly support the idea that the proposed approach can safely be used to perform the multiaxial fatigue assessment of notched structural components. This holds true also when the necessary fatigue strength reduction factors are estimated via standard, classic formulas.

Effects of voids and raster orientations on fatigue life of notched additively manufactured PLA components

In this study, the fatigue life of notched polylactic acid (PLA) samples fabricated through the fused deposition modeling (FDM) technique was studied experimentally and numerically. The volumetric method based on the theory of critical distance was employed for fatigue life predictions. The effects of influential process parameters including raster orientation and FDM-induced defects such as voids or gaps inside the parts were examined on the fatigue strength reduction factors and fatigue lives. Circular and elliptical-shaped notch geometries were considered with various dimensions. Fatigue tests were conducted on notched and un-notched samples at the load ratio of 0.1. Predicted results were compared with experimental fatigue test data. Results revealed that the raster orientation parameter had a substantial impact on fatigue strength reduction factors and fatigue lives. The stress concentrations induced by the FDM process on the surfaces and inside the parts for the samples with 90° raster angles acted similar to the sharp notches, resulting in no substantial difference in fatigue life of notched and un-notched 3D-printed samples. In contrast, un-notched 3D-printed specimens with 0° raster orientations possessed higher fatigue lives as compared to the notched samples. While the volumetric approach efficiently predicted the fatigue lives of the samples with 90° raster orientations, it moderately underpredicted the fatigue lives of the samples with 0° raster angles.

Effect of machine processing scratch on fatigue strength of notched specimen of aluminum alloy

The purpose of this study is to investigate effect of machine processing scratches on fatigue strength of a notched specimen of aluminum alloy. In this study, strain-controlled fatigue tests were carried out for two types of notched specimens with machine processing scratches. The fatigue strengths of notched specimens decreased by more than the elastic stress concentration factor due to the notch. To discuss the fatigue strength reduction, the specimen and fracture surfaces were observed carefully. The observations showed that the fatigue cracks initiated from the machine processing scratches and they are aligned around the notch root due to the stress concentration. As a result, the fatigue strength reduction factor became greater than elastic stress concentration factor.

Fatigue Characterization on a Cast Aluminum Beam of a High-Speed Train Through Numerical Simulation and Experiments

The cast aluminum beam is a key structure for carrying the body-hung traction motor of a high-speed train; its fatigue property is fundamental for predicting the residual life and service mileage of the structure. To characterize the structural fatigue property, a finite element-based method is developed to compute the stress concentration factor, which is used to obtain the structural fatigue strength reduction factors. A full-scale fatigue test on the cast aluminum beam is designed and implemented for up to ten million cycles, and the corresponding finite element model of the beam is validated using the measured data of the gauges. The results show that the maximum stress concentration occurs at the fillet of the supporting seat, where the structural fatigue strength reduction factor is 2.45 and the calculated fatigue limit is 35.4 MPa. Moreover, no surface cracks are detected using the liquid penetrant test. Both the experimental and simulation results indicate that the cast aluminum beam can satisfy the service life requirements under the designed loading conditions.

Fatigue performance evaluation of plate and shell heat exchangers

Fatigue life of plate and shell heat exchangers has been determined by experiments. Tests occurred with the aid of a pneumatic-hydraulic setup in which plate pack samples were internally pressurized by water. The samples were exposed to cyclic pressure loads varying from ambient to service pressure in the range 1.0–1.4 MPa, typical operating pressure range of crude oil production sites. Stress determination was possible due to strain gauge measurements. A heterogeneous stress field occurs along the corrugated plate due to its complex geometry. The external welded region is the preferential failure location. An average stress-life curve has been provided with 50% failure probability in accordance to ISO-12107. Fatigue strength reduction factors at the critical welded locations were determined via elastic stress analysis as proposed by ASME procedure. PSHE useful life increases with decreasing spacing between plates.

Fatigue Strength Reduction Factors Based on Strain Energy Density Applied to Sharp and Blunt Notches under Multiaxial Loading

Real mechanical assemblies favor the initiation and propagation of fatigue cracks due to stress concentration phenomena arising from the geometrical features such as notches, corners, holes, welding toes, etc. Classical fatigue analysis of notched specimens is done using an empirical formula and a fitted fatigue strength reduction factor, which is experimentally expensive and lacks physical scene. In the present paper, a simple and meaningful methodology is proposed to assess notched components against multiaxial fatigue. In this method, by precisely defining a finite-size volume surrounding the fatigue crack initiation site (notch tip), over which the strain energy is averaged, the morphological effect on the process zone is fully addressed. Such a method takes into account the effect of combination of different modes (I, II, III) and the load ratio. In order to implement it for components with a sharp or blunt notch, it is enough to analyze a linear elastic finite element model and to know only the properties of materials obtained from simple uniaxial tests. New relationships for determining an effective (tensile-type) stress and fatigue strength reduction factors are derived for notched specimens. The accuracy of the proposed model is validated by experimental data available in the literature, related to tubular specimens weakened with sharp/blunt notches under combined bending-torsion loading. Such a situation widely appears in equipment used for various branches of industry such as piping, automotive, power plant, drilling, etc.

New fatigue life design approach for metal sheets with discontinuities

The existing references of Wöhler data curves (S-Nf fatigue life) for fully reversed cycling loading fail to incorporate the discontinuities effect on metal sheets fatigue life. Experimental results show that the discontinuities, i.e., holes and notches weaken metal sheets and reduce fatigue life. The notch fatigue strength reduction factor (Kf) cannot incorporate the discontinuity effect into the fatigue life prediction models. This paper investigates a new method to study the discontinuity effect on metals fatigue life by applying the Discontinuity Stress Triaxiality (DST) factor to fatigue life prediction models. Three discontinuity geometries were studied in order to investigate the different effect of different radius. A simulation using ABAQUS FEA was used to export the stress triaxiality at the notch root element. The results show that this new method can help in predicting fatigue life for high-strength metals and metals with sharp notches, unlike the Kf factor. Also, the theoretical results show good correlation with the experimental results.

Fatigue life evaluation of an ultrafine-grained pure aluminum

The aim of this study is an experimental and numerical investigation of the fatigue behavior of a notched ultrafine-grained pure aluminum processed by strip cyclic extrusion-compression method. In this regard, the fatigue experiments were conducted for the unprocessed and strip cyclic extrusion-compression processed specimens under various cyclic loads. In the numerical analyses, a dislocation dynamic constitutive material model which tracks the microstructure evolution was implemented for numerical estimation of the values of fatigue strength reduction factor via the volumetric approach. Considering the three-dimensional effect near the plate hole, the variation of the fatigue notch factor through the thickness of the plate was investigated and the obtained results showed that maximum fatigue strength reduction factor was occurred in the middle of the plate due to the symmetry of specimen geometry and loading condition. The investigation reveals a good agreement between the numerical and experimental lives. The results showed although the smooth processed specimens have higher fatigue strength in comparison of the unprocessed ones, the notched processed specimens have lower fatigue strength in comparison of the unprocessed ones.

Study on Application of Fatigue Strength Reduction Factor and Stress Concentration Factor in Fatigue Analysis of Screw Threads

Taking the reactor pressure vessel as an example, the theoretical relationship of fatigue strength reduction factor (Kf) and stress concentration factor( Kt ), the reasonable values of Kf and Kt, the principles of equivalency of Kf and Kt in engineering project and the methods and techniques of how to use Kf and Kt in fatigue analysis are presented. An example of fatigue analysis for screw threads of reactor pressure vessel (RPV) stud with certain Kf is given. Consequently, definitions and differences of Kf and Kt are classified. Opinions on the application of Kf to high strength alloy stud are given, which can be used as a reference to the stud selection and the structure design in the engineering application.

Surface Topography Effects on the Fatigue Strength of Cast Aluminum Alloy AlSi8Cu3

Abstract This article investigates the effect of the cast surface topography on the fatigue strength of the cast aluminum alloy AlSi8Cu3, which is equivalent to EN AC-46200 and A380 according to ASTM. Fatigue tests utilizing small-scale specimens under bending at a load stress ratio of R = 0 reveal a significant influence of the cast surface layer, leading to a reduction of the nominal fatigue strength at ten million load cycles by about one half. Fracture surface analysis highlights that crack initiation occurs because of both surface roughness and micropores within the cast surface layer. A numerical simulation procedure to assess the influence of the surface roughness on the fatigue strength is presented. Thereby, a three-dimensional mesh of an optically scanned surface topography is set up and a local fatigue strength reduction factor is numerically computed based on the critical distance approach by Taylor. The results reveal that the presented method slightly overestimates the fatigue strength compared to the experiments; however, the determined values lead to a sound accordance if the scatter of the fatigue test data points is considered.

Failure mechanisms and design of dissimilar welds of 9%Cr and CrMoV steels up to very high cycle fatigue regime

Cross-weld and pure base metal specimens machined from dissimilar welded joints of 9%Cr and CrMoV steels were cyclically loaded up to very high cycle fatigue at 500 °C under load ratio of zero. Results showed fatigue strength of the welds was greatly weakened, lower than base metals. A transition of failure mechanisms from geometrical soft zone in heat-affected zone of CrMoV steel to micro-defects induced cracking at weld metal was responsible for decreasing fatigue strength reduction factor with the increasing of fatigue lifetime. A sound fatigue design requires careful selection of filler metal and consideration of soft zone and micro-defects.

Failure mechanisms and fatigue strength reduction factor of a Cr-Ni-Mo-V steel welded joint up to ultra-long life regime

It is known that welded joint is much “weaker” than base metal due to discontinuities of geometry, materials and residual stresses. It seems current international design rules do not adopt a uniform approach to weld efficiency, which is often defined as the ratio of the strength of a welded joint to the strength of base metal, in their guidance for creep and fatigue design of welds. This appears to be a great barrier for the application of nuclear welded structures which has a prolonged design lifetime of 60 years. In this work, fatigue strength reduction factor of a Cr-Ni-Mo-V steel welded joint, machined from welded steam turbine rotors for nuclear power plant, was investigated by performing axially push-pull cyclic loads tests with both cross-weld and pure base metal specimens up to very high cycle fatigue regime under ultrasonic frequency at ambient temperature. The effects of residual stress, strain localization, and microdefects in mismatched steels on failure mechanisms of welds were discussed thoroughly. Results show that fatigue strength reduction factor is varied in the range of 0.95-0.975, and is found to be dependent on fatigue lifetime for the first time. It is indicated that variation of fatigue strength reduction factor are associated with transition of crack initiation from specimen surface in high cycle fatigue regime to interior micro-defects in very high cycle fatigue regime. Comparing existing codes and standards for fatigue design of welds with experimental data indicates the over-conservativeness of present code-based design method. This implies a micro-defect based fatigue design approach is required for long life safe and reliability of weldments.

Fatigue assessment of welded joints in API 579-1/ASME FFS-1 2016 - existing methods and new developments

The 3rd Edition of API 579-1/ASME FFS-1 2016 Fitness-For-Service includes a new Part 14 dedicated to fatigue assessment. An important section in this part covers the fatigue assessment of welded joints. In this paper, an overview of the fatigue methods for welded joints is provided and extensions are recommended. First, an overview is given of the classical fatigue method used in the ASME B&PV Code based on smooth bar fatigue curves in conjunction with a fatigue strength reduction factor. In addition, the mesh insensitive structural stress method is outlined using an equivalent stress parameter based on fracture mechanics considerations in conjunction with a master S-N curve based on the analysis of over 2000 high and low cycle S-N test data. The resulting master S-N curve approach is applicable to high cycle fatigue and low cycle fatigue if a Neuber correction is introduced. In this paper, a new structural strain method is presented to extend the early structural stress based master S-N curve method to the low cycle fatigue regime in which plastic deformations can be significant while an elastic core is present. With this new method, some of the inconsistencies of the pseudo-elastic structural stress procedure can be eliminated, such as its use of Neuber’s rule in approximating structural strain beyond yield. The earlier mesh-insensitive structural stress based master S-N curve method can now be viewed as an application of the structural strain method in the high cycle regime, in which structural strains are linearly related to traction-based structural stresses according to Hooke’s law. Thus, both low cycle and high cycle fatigue behavior can now be treated in a unified manner. In the low-cycle regime, the structural strain method characterizes fatigue damage directly in terms of structural strains that satisfy a linear through-thickness deformation gradient assumption, material nonlinear behavior, and equilibrium conditions. A PVRC Joint Industry Project is currently sponsoring work on the structural strain method that will lead to its incorporation in the next edition of API 579-1/ASME FFS-1.

Experimental study of weld fatigue strength reduction for a stainless steel piping component

An experimental mean curve and a design fatigue curve corresponding to 95% survival probability were derived from realistic fatigue experiments on a non-welded water pressurized piping component with primarily focus on high cycle fatigue. The components were subjected to a synthetic variable amplitude bending deformation. Comparison with the results obtained for a similar piping component with a circumferential butt weld allowed the determination of an experimental fatigue strength reduction factor. Comparison with the fatigue procedure and design curve in ASME BPVC Section III allowed to quantify its conservatism with regards to accounting for the presence of a weldment and more generally transferability.

Strain energy-and stress-based approaches revisited in notch fatigue of ductile steels

The constant amplitude, zero-mean stress, axial-fatigue behaviour of plain and bluntly notched AISI 304 L stainless steel specimens is investigated in terms of strain energy density. Concerning plain material, it was found that at the fatigue knee the plastic strain energy density is 1.49 times higher than the elastic strain energy density. In the authors’ opinion, the presence of plasticity at the fatigue knee is responsible for the unsuitableness of classical stress - based approaches to synthesise the fatigue behaviour of this material. On the contrary, the elastic-plastic strain energy density was found an efficient parameter to rationalise in a single scatter band fatigue data of plain and bluntly notched specimens. Based on this result, the classic stress-and the point stress-based approaches were revisited taking into account the presence of plasticity at the fatigue knee, by introducing an equivalent fully elastic material having a linear elastic strain energy density at the fatigue knee equal to that of the actual material. Accordingly, a coefficient of plasticity Kp was successfully introduced to modify the classical definition of fatigue strength reduction factor, Kf.

Fatigue evaluation of the API specification 12F shop welded flat bottom tanks

Shop-built storage tanks are widely used in several industries all over the world. These equipment are fabricated with relatively small dimensions and capacities to facilitate their transportation to production fields. Particularly, API 12F shop-welded, flat bottom tanks are a group of standard equipment with specific sizes and capacities that are commonly used in the upstream, exploration, and production segments of oil and gas projects. The extensive utilization of this equipment has raised the need to investigate their behavior under different load cases and determine their service life due to cyclic loading.Throughout this investigation, a fatigue evaluation was performed following the guidelines of the ASME BPVC Section VIII, Division 2, design-by-analysis rules. The thirteen API 12F tanks were separated in three different groups according to their diameters. Also, different thicknesses as well as pressure cycles including internal pressure and vacuum were considered for the evaluation of each group.An elastic stress analysis using finite elements was conducted on shell models, axisymmetric models and solid submodels to determine the stress components and stress tensor range as well as obtain the effective alternating equivalent stress. Moreover, the fatigue penalty factor and a fatigue strength reduction factor were defined in accordance with the ASME code specifications. Hence, the protection against failure from cyclic loading of these equipment was determined using smooth bar design fatigue curves and the permissible number of operational cycles and location of the most critical joint were computed for each API 12F tank.

Methodology of strength calculation under alternating stresses using the diagram of limiting amplitudes

The work proposes the algorithm to calculate strength under alternating stresses using the developed methodology of building the diagram of limiting stresses. The overall safety factor is defined by the suggested formula.Strength calculations of components working under alternating stresses in the great majority of cases are conducted as the checking ones.It is primarily explained by the fact that the overall fatigue strength reduction factor (Кσg or Кτg) can only be chosen approximately during the component design as the engineer at this stage of work has just the approximate idea on the component size and shape.