Fatigue Strength Assessment Research Articles

A Probabilistic Fatigue Strength Assessment in AlSi-Cast Material by a Layer-Based Approach

An advanced lightweight design in cast aluminium alloys features complexly shaped geometries with strongly varying local casting process conditions. This affects the local microstructure in terms of porosity grade and secondary dendrite arm spacing distribution. Moreover, complex service loads imply changing local load stress vectors within these components, evoking a wide range of highly stressed volumes within different microstructural properties per load sequence. To superimpose the effects of bulk and surface fatigue strength in relation to the operating load sequence for the aluminium alloy EN AC 46200, a layer-based fatigue assessment concept is applied in this paper considering a non-homogeneous distribution of defects within the investigated samples. The bulk fatigue property is now obtained by a probabilistic evaluation of computed tomography results per investigated layer. Moreover, the effect of clustering defects of computed tomography is studied according to recommendations from the literature, leading to a significant impact in sponge-like porosity layers. The highly stressed volume fatigue model is applied to computed tomography results. The validation procedure leads to a scattering of mean fatigue life from −2.6% to 12.9% for the investigated layers, inheriting strongly varying local casting process conditions.

Laser welding of 16MnCr5 butt welds with gap: resulting weld quality and fatigue strength assessment

For laser welding, gaps should normally be avoided to ensure stable weld processes and high weld quality. Nonetheless, sometimes, gaps are resulting from non-optimal weld preparation in industrial applications. Within this investigation, the effects of a defocussed beam and laser beam oscillation on gap bridging abilities at reduced ambient pressure were investigated. For reference purpose, conventional laser welding with zero gap at ambient pressure was performed, too. The resulting weld geometry was investigated and correlated to gap bridging strategies and weld quality groups according to ISO 13919–1. The welds were characterized regarding their hardness and weld microstructure. Residual stress was determined by means of X-ray diffraction, and tensile tests as well as fatigue tests were conducted. The fatigue tests were evaluated by the nominal stress approach, the critical distance approach, and the stress averaging approach and correlated to weld quality measures. Resulting from this, fatigue resistance of laser welded butt welds with gap can be estimated by the FAT160 design S–N curve. The stress evaluation parameters for the determination of keff-values (ρ* = 0.4 mm, a = 0.1 mm) were confirmed.

Fatigue strength assessment of three-dimensional welded joints with the JV parameter

In this paper, the use of the J-integral (namely JV) is extended to three-dimensional sharp V-notches to evaluate the fatigue life of welded joints under complex loading. In order to simplify the procedure, JV is evaluated as a line integral on the free surface of the V-notch without defining a path contour inside the material. The length of the integration path depends on the critical characteristic length ρ* equal to one millimetre for welded joints made of steel. The JV is evaluated along the weld profile and the position of maximum JV is numerically calculated by means of the FE method. A relationship between the J integral and the JV is found in the case of mode III loadings. Finally, welded joints obtained with an RHS profile made of steel and subjected to tensile loading, bending and torque have been analysed.

Device for Torsional Fatigue Strength Assessment Adapted for Pulsating Testing Machines.

The torsional fatigue test determines the fatigue limit for a certain asymmetry coefficient of the cycle. The assessment of fatigue tests is performed on specialized machines. There are two types of torsion testing machines: universal machines that have the torsion component and specialized machines only for torsion testing. Nevertheless, no matter which proposed option we choose, the purchase prices for these testing machines or the values spent for self-management are quite high. This paper presented a device used for torsion fatigue testing, adaptable to a universal pulsating testing machine, designed to determine the torsion fatigue limit for different materials. The built device is simple and reliable, and therefore inexpensive. By using this device, we can determine the limit of the torsional fatigue after any stress cycle and we can use the parameters obtained from the universal machine to which it was attached. The torque and twisting angle of the test specimen during the test can be determined by calculation. The paper also presented an experimental method for determining shear strains based on calibration experiment, using a specimen on which strain gauges were mounted. The values taken from this calibration experiment were compared with those obtained from the theoretical calculation.

Comparison between PSM and IBESS approaches for the fatigue life estimation of weldments

In the framework of the fatigue assessment of welded components, several methods are available in design standards such as the nominal stress, hot-spot stress, notch stress and linear elastic fracture mechanics approaches. The present paper aims at comparing two advanced local approaches for the fatigue strength assessment of different welded joints made of steel. The first one is IBESS which is based on short crack fracture mechanics. The second one is the PSM which instead involves the strain energy density approach. Both methods will be briefly presented, and the fatigue life prediction results discussed. The results obtained for the joint geometries considered in this work show advantages and drawbacks of the approaches, which are thoroughly analysed as well.

Evaluation of geometrical notch and quality effects in the fatigue strength assessment of ultra-high-strength steel cut edges

Experimental fatigue tests were performed for the S960 and S1100 steel grades laser cut notched geometries in this study. Fatigue test results were analyzed with the conventional nominal stress method in order to compare the fatigue performance to current standards. A finite element method -based local stress method was employed to consider the geometrical notch and surface roughness, together with a multiparametric 4R method to obtain the local cyclic behavior at the notch root, considering residual stresses, local material properties, 3D-scanned surface quality, and geometry-related notch stress. The nominal stress method with the standard S-N curves showed both unconservative and conservative results for the low-quality and high-quality cut edges, respectively, while the 4R method significantly improved the accuracy of the fatigue strength predictions.

Design recommendations for container ship side-shell structure under fatigue strength assessment

The aim of the work is to evaluate various designs in order to arrive at some guidelines for choosing a good standard design by means of a fatigue damage analysis in the connection between longitudinal stiffener and transverse web frame on container ship side-shell structure located in the upper wing torsional box. Design proposals and recommendations are developed, based on numerical and experimental analysis, which allow, from different structural configurations formed by combinations between cut-outs and collar plates, to establish more and less adequate structural designs in one of the ships with the greatest future projection. In the first phase, the cut-out without influence of the longitudinal stiffener is studied and the stress state and fatigue assessment are determined. In the second phase, the longitudinal stiffener is taken into account under the presence of the collar plate in the model and different geometrical alternatives (thickness, ducktail, overlap and radius of curvature) and their influence on the determination of stress and fatigue life are studied. From the different configurations, the most appropriate combination is established and structural solutions are proposed in order to improve the design.

On the interaction of axial and bending loads in the weld root fatigue strength assessment of load-carrying cruciform joints

Current design standards and recommendations incorporate the nominal stress system for assessing the fatigue strength capacity of load-carrying cruciform (LCX) joints with the fillet welds and failing from the weld root. Thus far, bending-loaded joints have not been addressed in these standards. The aim of the present study is to investigate the fatigue performance of LCX joints subjected to cyclic axial and bending loads. Firstly, fatigue test data sets of such joints subjected to axial loading and bending loads in the adjoined plate component are extracted from the literature, and statistical analyses are carried out to evaluate the fatigue strength capacity using the nominal weld stresses (NWSs). Secondly, experimental fatigue tests are carried out on LCX joints made of ultra-high-strength steel (UHSS) grade using constant amplitude loading and subjected to combined axial and bending load to study the load interaction effects on the fatigue strength capacity. The results showed that the FAT36 detail category for the weld root failures is applicable for bending-loaded joints when applying NWSs calculated on the basis of effective throat thickness of weld and assuming linear-elastic stress distribution over the joint section. The effective notch stress analyses showed unconservative results for the tested joints, when applying FAT225 design curve with the reference radius of rref = 1.0 mm.

Finite Element Calculation of the Polymer Stamp Material Redistribution under Restrictions on Fatigue Life

The numerical method of stamp topological optimization taking into account fatigue strength is presented in the work. It is proposed to take into account the restrictions on the stress state in accordance with the curve of the dependence of the maximum stresses on the number of loading cycles in the ESO topological optimization method. An approach to the selection of the evolutionary coefficient with a step-by-step increase in the rejection coefficient is proposed when constructing an iterative scheme for the rejection of elements by the method of topological optimization. The calculation of the stamp optimal topology with a decrease in volume due to the removal and redistribution of material was carried out in the study. The new geometric model of the optimal topology stamp is based on the predicted distribution of elements with a minimum stress level. The verification calculation of the stress state of the stamp of optimal topology with an assessment of fatigue strength was carried out in the work. The numerical calculation was carried out using the finite element method in the Ansys software package. The minimized stamp volume decreased by 35% according to the calculation results. The results of the study can be further applied in the development of topological optimization methods and in the design of stamping tools of optimal topology.

Automated implementation of the Peak Stress Method for the fatigue assessment of complex welded structures

Dealing with the fatigue design of complex welded structures, the Peak Stress Method (PSM) is a rapid approach to the fatigue strength assessment, which is based on the singular, linear-elastic opening, in-plane and out-of-plane shear peak stresses calculated at the weld toe or weld root adopting coarse FE meshes. By adopting the averaged Strain Energy Density (SED) as a fatigue strength criterion, a design stress, the so-called equivalent peak stress, has been defined as a proper combination of the local peak stresses and adopted in conjunction with a reference design curve to estimate the fatigue lifetime of welded structures. To furtherly enhance the applicability to complex welded structures, the PSM has recently been calibrated by using 10-node tetra elements, which allow to easily discretize complex geometries. Moreover, an interactive analysis tool has been developed within Ansys® Mechanical, in order to automate the application of the PSM to complex both aluminum and steel 3D welded structures subjected to multiaxial fatigue loads. The automated PSM tool has been developed taking advantage of the integrability between Ansys® Mechanical and modern programming languages, enforcing PSM compatibility requirements and automating the implementation tasks and phases of the method. In this perspective, the present work deals with the employment of the PSM tool for the fatigue strength assessment of some complex welded details adopted in modern roller-coaster structures. Due to the geometrical complexity of large-scale structures, private companies usually employ finite element beam models in order to evaluate nominal stresses, that must be compared with appropriate fatigue strength references (FAT classes) in design standards. The assessment procedure proposed in this work aims instead at defining appropriate stress-based FAT classes from the results of automated PSM analyses on the considered joint geometries. Moreover, the proposed approach allows to compensate the limited number of FAT classes and details available in International Standards and Recommendations, when dealing with complex joint geometries.

A novel methodology for fatigue assessment of Ductile Cast Iron (DCI) with solidification defects

In the present research work, the fatigue strength assessment of a DCI containing solidification defects is theoretically carried out by means of a procedure implementing: (i) a defect content analysis, (ii) the √area-parameter model, and (iii) the multiaxial critical plane-based criterion by Carpinteri at al. An experimental campaign available in the literature, performed on DCI specimens under multiaxial fatigue loading, is analysed. The comparison between the obtained results and the experimental data shows a quite satisfactory agreement, highlighting the criterion accuracy.

Probabilistic assessment of fatigue crack propagation with accounting for effects of crack retardation due to overloads

The paper presents a probabilistic approach to fatigue strength assessment based on the Wheeler model that provides the opportunity to account for crack retardation due to overload in a probabilistic formulation. The probabilistic assessment is performed by Monte Carlo simulations based on random sampling. It consists of running deterministic assessments a sufficiently large number of times with different combinations of randomly sampled input data in order to build a statistical picture of the output quantities. A computer code in Matlab was developed to obtain the prior assessment of the probability of fatigue failure of a pipeline structural component. The Bayesian inference is used to update the prior assessment failure probability assessments as soon data from in-service technical inspections become available

Numerical calculation of homogenized effective material properties of the single layer over arbitrary fiber distributions

The present work shows on the one hand the influence of symmetrical and random fiber distributions on the numerical determination of the effective homogenized material parameters of the unidirectional layer. On the other hand, the stress paths of symmetric and arbitrary RVE systems are analyzed and evaluated. In a fatigue-relevant consideration, the developed RVE systems map the damaging force paths between fiber and matrix. This numerical calculation of the stress on the matrix material is intended to simplify the fatigue strength assessment workflow of CFRP structures. The goal is to replace the currently required orthotropic Wöhler curves with a Wöhler curve for the matrix material. In investigations on pure resin samples it could be shown that the strength of these pure resin samples, corresponded to the matrix range of the numerical simulations of the RVE systems.

Modelos fractomecánicos para la estimación del crecimiento de fisuras cortas y la evaluación de la resistencia a la fatiga

The fatigue strength assessment of metallic components containing manufacturing defects is currently analysed by applying fracture mechanics-based methodologies. This work begins by dealing with the recently published paper entitled “Short crack propagation analysis and fatigue strength assessment of additively manufactured materials: an application to AISI 316L”, Int J Fatigue 151 (2021) 106396, by Bergant, Werner, Madia, Yawny and Zerbst, where IBESS approach and Chapetti´s short crack growth threshold models were implemented for assessing the fatigue strength of laser powder bed fusion processed AISI 316L stainless steels. The application of the Chapetti´s model is carried out here in the way its author thinks it should be made, and results show clear differences when comparing with the results of the referenced paper. Analyses of the sources of discrepancy are also carried out. Some discussions associated with other recent applications of the available fracture mechanics models and hypotheses, or their combinations, are added in order to optimize future analysis when using them for short crack growth threshold estimations. Later, several analyses are made by making general observations associated with the prediction models, their hypotheses, their combinations and their relationship to the Kitagawa-Takahashi diagram. Finally, it is shown that when applying the fracture mechanics models special attention is necessary when comparing intrinsic strengths of the analysed material and that of material-defect combinations, particularly when the defects used in the analysis are artificial and/or are relatively large comparing with the microstructural dimension.

Implementation of the Peak Stress Method for the automated FEM-assisted design of welded joints subjected to constant amplitude multiaxial fatigue loads

The Peak Stress Method (PSM) is a local approach to the fatigue strength assessment of welded structures. Starting from FE-calculated opening, in-plane shear and out-of-plane shear peak stresses at the weld toe and weld root, the PSM defines an equivalent peak stress for estimating the fatigue failure location and fatigue life of welded structures, in compliance with properly defined design curves. An interactive tool has been developed in Ansys® Mechanical to automate all calculation tasks required to apply the PSM to generic welded structures. The developed application allows to identify and analyse all weld toe and weld root lines of the structure in a full-automated way, performing fatigue life estimation on each analysed node. Finally, fatigue life results can be visualized directly on the model’s geometry through dedicated contour plots. In this work, some joint geometries taken from the literature and related to steel welded joints subjected to multiaxial fatigue loads are re-analysed taking advantage of two design procedures: (i) manual application of the PSM, (ii) automated implementation of the PSM. Taking advantage of the tool developed in Ansys®, a remarkable reduction in analysis time and effort can be achieved, with respect to manual PSM analyses.

Review of methods for the high-cycle fatigue strength assessment of steel structures subjected to sub-zero temperature

In recent years, there has been a growing interest in exploring the resources of the Arctic regions and an increase in shipping in regions with seasonal sub-zero temperatures. Such low temperatures impose challenging requirements for the structural integrity of steel structures and their weldments. Hence, specific knowledge of the behavior of materials and weldments and how to account for temperature effects is crucial to ensure safe operations. A number of recent studies have shown that fatigue strength at sub-zero temperatures is a major knowledge gap. This is exacerbated by the fact that this effect is not addressed in international standards. In this context, this study presents a comprehensive review of the current state of the art of high-cycle fatigue strength assessment methods for steel structures subjected to sub-zero temperature. The main objective is to provide an overview of how to account for temperature effects on fatigue strength when designing steel structures.

Fatigue strength assessment of high-speed railway axle EA4T steel with foreign object damage

Axle is easy to be hit by ballast and other foreign object rolled up due to aerodynamic effect to form the typical foreign object damage (FOD) on the surface, leading to deteriorate the axle structural integrity and reduction of fatigue performance. In order to evaluate the effects of surface damage and strengthening treatment on the fatigue performance, the EA4T axle steel specimens with or without shot peening (SP) were induced FOD by a compressed-gas gun. Meanwhile, the strengthening mechanism of shot peening were investigated by both nanoindentation technique and X-ray diffraction. Then the probabilistic fatigue S-N curves (P-S-N) were acquired for unSPed and SPed specimens subjected to FOD or not. In addition, the fatigue limit of FODed axle estimated by Murakami equation was corrected by size effect, surface quality, loading mode factors. Finally, the fatigue limits with 97.5% reliability were adopted to construct the modified Kitagawa-Takahashi (K-T) diagrams for FODed axle with or without shot peening treatment, which can be used in the service performance evaluation of foreign object damaged axle.

Statistical analysis of sub-zero temperature effects on fatigue strength of welded joints

Ships and offshore structures in Arctic environments are exposed to severe environmental actions and sub-zero temperatures. Thus, the design of such structures has to account for the Arctic environment and must be cost-efficient at the same time. A vital part of the design process is to ensure that fatigue-induced failure does not occur in the lifetime of the structure. While effects of high temperatures on material behavior are well covered in international standards and guidelines, there is no comprehensive guidance for sub-zero temperature fatigue strength assessment. Additionally, stress-life (S–N) test data of welded joints at sub-zero temperatures is particularly scarce. Hence, this study presents an extensive review of recent test results of various weld details tested in the range of − 50 to 20 °C. This data could build the basis for future considerations of temperature effects in fatigue design guidelines and recommendations. For this purpose, the fatigue test results are submitted to a rigorous statistically assessment—including a summary of the limitations of current design guidelines with respect to sub-zero temperature effects.

Structural performance of laser welded edge joints made of high-strength steel

Laser welding is a widely-used fusion welding process in industry. However, laser welding is not a common welding process in the manufacture of industrial crane structures. There has been a remarkable increase in the available strength classes of steel grades over the last 10 years, such that strengths of up to 1200 MPa are now commercially available. This enables the use of thinner materials in welded products and at the same time, has opened up new possibilities for using laser welding more widely in the manufacture of steel structures. This study focuses on the static and fatigue strength of the laser-welded joints. Details investigated are edge joint with flange preparation between two rectangular tubes and edge joint between flat bar and rectangular tube. A novel fatigue strength assessment concept, the FATmod method, is applied to assess the theoretical fatigue performance of the joint in comparison with the effective notch stress method with a FAT630 design curve. The FATmod method is based on the local stress ratio at a fatigue-critical point of the joint and the analysis considers the strength of the material, surface quality and applied stress ratio in the assessment of fatigue. The study shows that the samples failed from the base material side in static tests and the FATmod method developed was found to agree well with the test results.

Fatigue strength assessment of as-welded and HFMI treated welded joints according to structural and local approaches

High-frequency-mechanical-impact (HFMI) is a post-weld treatment for improving the fatigue strength of welded structures, by introducing compressive residual stresses, localized strain hardening and enlarged weld toe radii. The Peak Stress Method (PSM) is extended for the first time to the fatigue strength assessment of HFMI treated steel joints, by means of a dedicated analysis procedure and new fatigue design curves. Experimental data generated from joints tested under uniaxial loading, both in the as-welded and HFMI treated conditions, have been taken from the literature and have been successfully analysed by using the PSM and the structural hot-spot stress (SHSS) approaches.