Fatigue Strength Of Components Research Articles

Parameters Influencing the Fatigue Behavior of Ti6Al4V

The fatigue strength of components is affected by loading and environmental conditions, their geometry, the material and the manufacturing process adopted for their production. As a consequence of local stress concentrations, fatigue cracks can propagate to failure from the defects created by the manufacturing processes or introduced during the service life, as occurs in impact-damage. Ti6Al4V titanium alloy is commonly used in the aeronautical sector and other engineering applications due to its high specific strength. The study of the fatigue behavior of Ti6Al4V is therefore critical for designing reliable and durable structures. The work reports a synthesis of the results present in the literature for the fatigue behavior of Ti6Al4V in inert and aggressive environments, in the presence of a deposited coating, in the case of impact-damage, when components are produced by additive manufacturing and in the absence of solution treatment and over-aging.

Fatigue strength evaluation of case‐hardened components combining heat‐treatment simulation and probabilistic approaches

AbstractIn order to raise the hardness and strength of the surface layer of mechanical components and induce favorable residual compressive stresses, case‐hardening procedures have become established in the heat treatment of steel. In this work, a calculation concept for the fatigue strength of components that have been case‐hardened through carburizing heat treatment is being developed. The residual stress and the load stresses in complex‐shaped, carburized materials are determined using a finite element (FE) model. The fatigue limit of the components is derived using probabilistic methods and taking into account hardness gradients, residual stresses, and non‐metallic inclusions. The model is validated with available axial bending fatigue test data and then used to predict the rotating bending fatigue limit of samples with various geometries and heat‐treatment conditions. This work demonstrates the capability of combining probabilistic and FE‐based modeling to represent complex interactions between variables that affect the fatigue of heat‐treated components, such as steel cleanliness, notch shape, case‐hardening depth, or loading conditions.

New possibilities for laser welding of highly loaded transmission components by strategic use of simulation methods

Laser welding in transmission manufacturing opens up completely new kinds of product solutions with excellent properties in terms of wear, corrosion resistance, and service life. Current welding designs are characterized in particular by difficult-to-weld material combinations (e.g., steel versus cast iron) and a high component stiffness, which is correlated with high residual welding stresses. The major challenge for these mass-produced components remains both their crack-free weldability and their complex cyclic load capacity [U. Stamm, “Rissfreies Laserstrahlschweißen von Mischverbindungen aus Gusseisen und Einsatzstahl, Jahresbericht Fraunhofer IWS 2006” (2006); X. Shu, “Untersuchungen zum Laserstrahlschweißen von Werkstoffkombinationen aus Gußeisen und Stahl,” Dissertation, Shaker, Band 4/94 (1994), ISBN: 3-8265-0098-9; G. Göbel, “Erweiterung der Prozessgrenzen beim Laserstrahlschweißen heißrissgefährdeter Werkstoffe,” Dissertation, Fraunhofer-IRB-Verlag (2007), ISBN: 978-3-8167-7671-0]. Therefore, this contribution presents practical solutions for weld-compatible joint constructions and the reduction of residual stresses on the basis of a representative transmission design. Specifically, a systematic study is being conducted to understand and qualitatively evaluate effective methods for reducing residual weld stresses in circumferential welds. The recommendations developed as part of this study take particular account of the influence of process modifications, material conditions, and geometric aspects on weldability and component distortion. Here, structural welding simulations are performed and verified by experimental welding trials, including metallographic examinations. To ensure the required component fatigue strength, a practical concept for determining Wöhler curves is presented, which is based on structural mechanical simulations and multiaxial fatigue strength tests on simplified test specimens. The adaptations developed in this way facilitate the production of difficult-to-weld and highly stressed transmission components. In particular, time-consuming and cost-intensive iterations of laser welding tests can be significantly reduced or even eliminated [Brenner et al., “Neuere Ergebnisse zum Schweißen von Eisenbasiswerkstoffen mit Faserlasern, 5. Laser-Anwenderforum,” Strahltechnik Band 28 (2006), pp. 139–148, ISBN: 3-933762-18-9, S]. Furthermore, the systematic investigations provide effective recommendations for phenomenological understanding and solving typical welding challenges in practice [J. Standfuß, “Ganzheitliche innovative fügetechnische Konzepte am Beispiel des PKW-Antriebsstranges,” Fraunhofer Verlag (2010), ISBN: 978-3-8396-0125-9]. This applies, in particular, to load-transmitting components in the fields of e-mobility, aerospace, and industrial engineering [Wagner et al., “Entwickeln und Auslegen von lasergeschweißten Getriebe-komponenten für die Luftfahrt,” DVS-Berichte Band 365 (2020), pp. 62–69, ISBN: 978-3-96144-098-6].

Analysis of the effects of isothermal aging and Laser Shock Peening on the flow properties, fatigue crack growth, and fracture toughness of Inconel 718

Laser Shock Peening (LSP) is a surface treatment that improves the fatigue life and fatigue strength of components by inducing a shallow layer that is characterized by compressive residual stress and plastic deformation. This study investigates the use of LSP on Inconel 718 (IN718) on the fatigue crack growth (FCG) and fracture toughness (FT) of as-received (AR), solution-treated (SOL), and samples aged for 0.5, 4, and 100 h. Experimental results on aged samples established that the hardness of samples 0.5 and 4 h increased due to the precipitation of γ′ and γ″ phases while in the 100 h sample the hardness decreases due to the presence of coarser γ″ phase and intergranular δ-phase. These microstructural features impacted the plastic flow behavior of the different samples which was adequately described using a two-fitting approach to the Voce equation. While thermal aging accelerated the FCG of aged samples, the plastic flow and residual stress induced by the LSP significantly reduces the effect of thermal aging, by reducing the FCG rate and increasing the FT of the IN718 aged samples. Finally, results from EBSD analysis showed that the LSP-induced plastic strain depends on the microstructural transformations that occurred during the aging treatment.

A comparative analysis of step loading and staircase testing for fatigue strength estimation of an engine component

AbstractStaircase testing is a standard method for evaluating the fatigue strength of components. However, staircase testing assumes a normal distribution, while components can display bimodal behavior due to flaws in material, or issues during the manufacturing process. Three unique step loading data sets on different production crankshafts provide evidence that step loading reliably identifies material or manufacturing issues, which lower a component's fatigue strength. Staircase testing has an 87% or greater chance of overestimating the component's fatigue strength, which in turn overestimates the component's expected reliability. For example, a component with a 99.9% reliability based on staircase testing would only have a 74% reliability based on step loading. If a component contains an undetectable manufacturing defect, staircase testing has a 99% chance of overestimating the component's fatigue strength. Step loading reliably improves the estimation of a component's fatigue strength distribution while providing insights into a component's defect tolerance.

Extension of methods for estimating the fatigue strength of components made of ultra-high strength steels

Various high and ultra-high strength steels are investigated experimentally by tensile and fatigue testing. The results are used to extend existing methods for the estimation of cyclic material behaviour for cases in which a limited amount of experimental data is available. Furthermore, fatigue tests of component-like high strength steel specimens are performed, including the determination of residual stresses and fractographic investigations of failure-relevant inclusions. The results serve to adapt and apply a proof of structural durability based on the local strain approach for components made of ultra-high strength steels.

Innovative X-ray diffraction and micromagnetic approaches for reliable residual stress assessment in deep rolled and microfinished AISI 4140 components

The residual stress state in the subsurface is a key element of surface integrity. It is well known to have a significant impact on a component's properties in terms of fatigue behavior and resistance to wear and corrosion. For this reason, adjusting residual stresses during manufacturing is a major challenge in modern production engineering, to improve and ensure a component's fatigue strength. In this context, hydrostatic deep rolling of the workpiece surface using adapted parameters enables the targeted induction of compressive residual stresses into subsurface layers. Due to specific properties regarding subsurface and topography for functional components in tribological applications, a further machining operation by microfinishing following deep rolling seems to be purposeful. In particular with regard to the production of components exposed to periodic load changes when used, the process combination can enable a substitution of the typically required conventional subsurface zone hardening. With the aim of economical process design, the corresponding parts can be manufactured with significantly reduced time and costs. Efficient and well-founded methods for monitoring the resulting influence on the subsurface zone properties are essential for a reproducible and target-oriented process design. The prevailing method for the non-destructive assessment of residual stresses in both academia and industry is X-ray diffractometry using the sin2 ψ-method. However, this method is time-intensive and requires complex instrumentation. Thus, efforts have been undertaken in past decades to develop alternative methods for the efficient and reliable characterization of residual stresses. In this research, the applicability of the cos α-method in X-ray diffractometry and a micromagnetic approach for residual stress assessment was investigated, analyzing deep rolled and microfinished AISI 4140 specimen conditions. In addition to the diffractometric and micromagnetic measurements, metallographic and topographic analyses of machined surfaces were carried out. Deep rolling was found to induce significant compressive residual stresses of up to −1100 MPa. After microfinishing of the deep rolled surfaces, favorable compressive residual stresses remain in the subsurface, reaching approximately up to −750 MPa. Based on this, the production of tailored surfaces with respect to a suitable combination of topography and subsurface is possible. For all surface states investigated, a good agreement between the two approaches in X-ray diffraction was found. Magnetic Barkhausen noise (MBN) measurements prove to be well applicable for an efficient and holistic assessment of surface integrity in the subsurface of deep rolled and microfinished AISI 4140 specimens.

On the use of the theory of critical distances with mesh control for fretting fatigue life assessment in complete and nearly complete contacts

In this work, the theory of critical distances using point method and mesh control approach for fretting fatigue life prediction in complete and nearly complete contacts is analysed. The approach is applied to thirty experimental tests found in the literature, where almost nine hundred simulations are carried out. The work concludes that the theory of critical distances using mesh control is able to predict the fretting fatigue strength of components using an optimum mesh size of 1.6 times the critical distance for all contact configurations analysed. Due to the use of an element size of around the critical distance length, the computational cost can be reduced up to 98% in comparison with the traditional theory of critical distances.

Analysis of the Influence of Surface Modifications on the Fatigue Behavior of Hot Work Tool Steel Components.

Hot work tool steels (HWS) are widely used for high performance components as dies and molds in hot forging processes, where extreme process-related mechanical and thermal loads limit tool life. With the functionalizing and modification of tool surfaces with tailored surfaces, a promising approach is given to provide material flow control resulting in the efficient die filling of cavities while reducing the process forces. In terms of fatigue properties, the influence of surface modifications on surface integrity is insufficiently studied. Therefore, the potential of the machining processes of high-feed milling, micromilling and grinding with regard to the implications on the fatigue strength of components made of HWS (AISI H11) hardened to 50 ± 1 HRC was investigated. For this purpose, the machined surfaces were characterized in terms of surface topography and residual stress state to determine the surface integrity. In order to analyze the resulting fatigue behavior as a result of the machining processes, a rotating bending test was performed. The fracture surfaces were investigated using fractographic analysis to define the initiation area and to identify the source of failure. The investigations showed a significant influence of the machining-induced surface integrity and, in particular, the induced residual stress state on the fatigue properties of components made of HWS.

August Wöhler – founder of fatigue strength research

AbstractThe year 2014 marked the 100th anniversary of August Wöhler's death. On that occasion the authors wrote a book about his scientific work, his work on the safety of components and structures in the emerging industrial society, his life and his significance for the historical development in the field of fatigue strength of components and structures [1]. Those investigations were accomplished in cooperation with the DVM (German Association for Materials Research and Testing). As national and international events have shown [2], that book is of great interest. Therefore, an English version suggested itself, not least because of the 200th anniversary of Wöhler's birth in 2019.August Wöhler was an interesting and unique character, influenced by the 19th century Industrial Revolution, especially railway development. The spread of that new technology caused numerous problems. Fractures occurred on axles, wheels and rails at stresses below the component's static fracture strength. Through systematic tests, August Wöhler was able to prove that repeated stresses far below the static strength might lead to fractures. The present article gives a brief overview of Wöhler's life and achievements.

Estimation of Component Fatigue Lives under Service Load-Time Functions Using an Advanced Transfer Methodology for the Transition from Specimen to Component Related Material Behaviour

Due to steadily shortening product development cycles, the design and validation of new products mainly takes place in a virtual environment. The structural durability and a reliable lightweight design is the key target to be achieved during the product development process. A fatigue assessment has therefore to be included in the product design process from the very beginning. Because prototypes are usually not available during early product development stages, it is necessary to estimate the component behaviour by transferring material properties, which have been determined by fatigue tests on material specimens with standardized geometry, to the component geometry. In order to enable a reliable statement about the structural durability based on these simplified tests, different influencing factors have to be considered. Some of the major influencing factors on fatigue are the component size and geometry, the boundary conditions of the loading and the load-time function, and a possible location and load-history dependency of the material behaviour. The presented work will discuss those influences and how they can be treated by simplified approaches during the assessment of the component fatigue strength.

Influence of the deep hole drilling process and sulphur content on the fatigue strength of AISI 4140 steel components

When using quenched and tempered steels in the automotive industry or other industrial applications the fatigue behavior is of elementary importance. The surface and subsurface integrity of machined parts is known as one key factor related to the fatigue strength of components and is strongly influenced by the machining process. Especially for the deep hole single lip drilling, a large part of the cutting force is transferred to the bore wall and the surface and subsurface zone are influenced by the interaction between the tool and the workpiece material.In this study, the approach of inserting residual stresses into the bore wall and influencing the microstructure of the bores subsurface in a way, that the components can withstand a higher mechanical and dynamic load, is investigated in detail. In a close cooperation between the authors the parameters of the single-lip drilling process and the workpiece material are correlated with the produced surface quality and subsurface microstructure as well as the resulting fatigue strength of the components. For the comparative characterization of the fatigue behavior, established test procedures of destructive material testing in combination with new non-destructive test methods, such as the Barkhausen noise analysis, are applied and developed further. The results of the fatigue tests in correlation with the results of the Barkhausen noise analysis are used to design a model for predicting the fatigue strength of drilled components.

On the effect of weld defects on the fatigue strength of beam welded butt joints

Modern guidelines concerning the fatigue strength of welded components are often based on the nominal stress concept. These guidelines are valid for fusion welded steels but do not distinguish between different welding processes. However, different welding processes can lead to significant differences in the resulting weld geometry. This is particular true for arc welded components in comparison to beam welded components. Furthermore, it is well known, that the geometry of welded joints effects the fatigue strength severely. Therefore, it can be expected that beam welds behave differently to arc welds in fatigue tests. In this study, electron beam and laser beam welded samples of different thicknesses made from fine-grained steels were tested in fatigue tests under a constant amplitude loading. In order to assess the effects of weld defects on the fatigue strength, samples with defined weld defects (e.g. axial misalignment) were included in the study as well. The weld geometry of each sample was measured and evaluated according to the quality groups in ISO 13919-1. Additional numerical notch stress calculations were performed. Finally, a correlation between the quality groups according to ISO 13919-1 and the nominal cyclic stress at 2*106 load cycles with a survival probability of 97,5% was proposed. This correlation allows users of beam welding processes to predict the fatigue strength of components under the condition that certain quality levels according to ISO 13919-1 are met.

09.17: Fatigue life of marked steel components

ABSTRACTSteel components have to be identifiable and traceable during the whole manufacturing chain. The choice of the identification method is not specified consistently in international rules and standards. The European standard for the execution of steel structures EN 1090‐2 specifies that the use of durable marks may not result in producing damages without saying which damages are meant. Hard stamped marks are only permitted for steel grades up to S355 and only in areas where no effect on the fatigue life is expected. Otherwise the use of soft stamps should be verified. In other international standards hard stamped marks are explicitly required. These contradictions should be resolved.In terms of durability and liability, markings should be resistant against particular manufacturing processes such as sandblasting, hot‐dip galvanizing or coating. For this reason, the following marking methods are used in practical applications: hard stamping, scribing, plasma marking and needling.Nowadays the machinery manufacturers have developed machines in which the marking process can be automatically integrated in the manufacturing process of steel components so that manual layout scribing is not required anymore and components can be marked on the fly. However, the effect of the installed notch due to the marking process on the fatigue strength of the components has not been investigated yet. As a result, a classification of the resulting notched details according to the European detail categories of EN 1993‐1‐9 is in principle not possible.For these reasons, the influence of durable marking methods on the fatigue strength of steel components needs to be clarified by experimental fatigue tests and numerical calculations which have been performed at the Institute for Metal and Lightweight Structures of the University of Duisburg‐Essen. Within these investigations the different marking methods hard stamped, scribed, plasma marked and needled as well as unmarked structural details have been examined. The experimental investigations have been carried out considering three different steel plate thicknesses 15 mm, 25 mm and 40 mm and two different steel grades S355J2 and S460N.As expected the marked specimens show a decrease of the loading cycles until failure even though there are differences in fatigue life due to the different markings and their characteristic notch geometries.

Fatigue strength evaluation of small defect at stress concentration

The effect of individual large notches on the fatigue strength of components is one of the oldest and most studied topics in the history of metal fatigue. When a small defect is present at the notch root, both the stress concentration of the main notch and the effect of the small defect interact and simultaneously influence the fatigue strength. The effect of the main notch can be evaluated from the viewpoint of stress concentration and stress gradient. Both have a strong influence on the fatigue notch factor. The area parameter model has been successfully applied to fatigue limit evaluation of materials containing small defects under uniform stress condition. If a small defect is present at the notch root, the effect of stress gradient must be also considered in the application of the model. In the present study, the fatigue tests and fatigue crack growth analyses are carried out for specimens containing a small defect with the size area = 46.3µm at the root of notch with 1mm depth and root radius of 1.0mm or 0.3mm. Fatigue limit predictions are made based on the area parameter model and the stress intensity factor analyses for a small crack subject to a steep stress gradient. Existing fatigue notch effect methods are reviewed and used in fatigue limit predictions for comparison. Moreover, new fatigue notch effect method based on the area parameter model is proposed. The greatest advantage of the proposed method is that it can predict fatigue limit using easily obtainable parameters and without requiring fatigue tests or troublesome analyses. Suggestions for the extension of the proposed method to practical engineering problems are also made.

Reliability modelling for rotorcraft component fatigue life prediction with assumed usage

ABSTRACTFatigue life is a random variable. Thus, the reliability of a conservative fatigue life prediction for a component in the helicopter dynamic system needs to be substantiated. A standard analytical substantiation method uses averaged manoeuvre loads instead of seeing manoeuvre loads as a random variable whose distribution is estimated with limited precision. This simplification may lead to inaccuracies. A new simulation-based method is developed to conservatively predict fatigue life, while also accounting for the full random distribution and uncertainty of manoeuvre loads. Both methods fully account for uncertain fatigue strength but assume that the mission profile is known or can at least be conservatively estimated. Simulations under synthetic but realistic engineering conditions demonstrate that both methods may be used for accurate substantiation of conservative fatigue life predictions. The simulations also demonstrate that, under the tested conditions, uncertainties from manoeuvre loads may be neglected in fatigue life substantiations as the resulting error is not significant with respect to uncertainties in component fatigue strength.

Damage initiation and structural degradation through resonance vibration: Application to composite laminates in fatigue

The definition of failure is fundamental to the characterisation of fatigue strength of components and structures and is often expressed as a percentage of stiffness degradation. This article proposes a new method to capture damage initiation and structural degradation during a fatigue test, by exploiting resonance vibrations. The method entails monitoring dynamic parameters, including the phase angle between excitation and response, which suddenly changes as soon as the overall stiffness changes as a consequence of damage. In this paper, a theoretical approach based on equations of motion is presented in order to describe the possible scenarios in which a component undergoes structural degradation. In the proposed approach, an abrupt change in the time history of the stiffness of the component is assumed and analysed in both the quasi-static case at low excitation frequencies and the dynamic case, when the excitation frequency is close to the resonance frequency. The resulting analytical solution shows that any small stiffness variation is amplified by the dynamic response of the component as a large phase change, even when it is too small to be captured by the quasi-static response. The idea is successfully applied to capture the initiation of delamination in CFRP laminates, and the relative S-N curve to initiation is drawn. The experimental evidence of a critical event in the fatigue life of a component is found both in the dynamic phase response and in the base acceleration. The critical event is captured from a sudden change in the dynamic parameters of the structure as soon as a delamination is initiated. Thermography, C-scan and micrograph confirmed that the onset of delamination occurs at the moment of the critical event, after an initial stage of microcracking.

Multiaxial high-cycle fatigue criteria and life prediction: Application to gas turbine blade

A recent work conducted by the authors (Maktouf and Saï, 2015) demonstrated that the root cause of the premature blade failure was caused by high-cycle fatigue (HCF) mechanism initiated at a localized carbon-rich area inducing grain boundary brittleness. The blade was subject to multiaxial cyclic loadings during its service life and any attempt to assess component fatigue strength leads to the question of choosing an appropriate fatigue design criterion. In this paper several multiaxial fatigue models are applied as post-processing step of the Finite Element Analysis (FEA) output results and the estimated fatigue lifetimes were assessed under different loading conditions. The material fatigue parameters, required as an input to the selected fatigue models were determined through a series of bending and torsion tests on specimens made of aged Inconel 718. A numerical post-processing algorithm was developed for Fatemi-Socie fatigue criterion and included as additional post-computation model in the used computer aided fatigue damage evaluation tool. The authors point out that the majority of the multiaxial fatigue studies available in the literature are conducted mainly for correlating the experimental laboratory results on specimens while they have been used in the frame of this study to investigate their application to an industrial case.

High cycle fatigue behavior of V-microalloyed medium carbon steels: A comparison between bainitic and ferritic-pearlitic microstructures

High cycle fatigue behavior of two V-microalloyed medium carbon steels with ferritic-pearlitic and bainitic microstructures were studied by using rotating bending fatigue and fatigue crack growth (FCG) testing. The results show that fatigue strength σw is a power function of Kt with its coefficient equals to smooth fatigue strength. This function provides a simple method for predicting notch fatigue strength of components. Bainitic steel shows higher σw, lower fatigue notch sensitivity and FCG rate than those of ferritic-pearlitic steel. It is concluded that bainitic steel is a promising candidate to manufacture automobile security components in terms of fatigue properties.

Fatigue strength of marked steel components

AbstractAccording to EN 1090‐2, steel components have to be identifiable and traceable throughout the whole production chain. The choice of identification method is not specified consistently in international rules and standards. In terms of durability and liability, markings should be resistant to particular fabrication processes such as sandblasting, hot‐dip galvanizing or coating. The methods are hard stamping, scribing, plasma marking and needling. The effect of the notch caused by the marking process on the fatigue strength of the components has not yet been investigated in detail. As a result, a classification of the notch details in the European detail categories of EN 1993‐1‐9 is, in principle, impossible. For these reasons, the influence of durable marking methods on the fatigue strength of steel components needs to be clarified by experimental fatigue tests currently being performed at the Institute for Metal and Lightweight Structures, University of Duisburg‐Essen. Part of this investigation involves examining the different surface conditions of hard‐stamped, scribed, plasma‐marked and needled specimens. The experimental investigations are being carried out considering two different steel grades, S355J2 and S460N, and three different steel plate thicknesses, 15, 25 and 40 mm.