Local Fatigue Strength Research Articles

Application of local fatigue strength approach to assess and optimise the impact of deep rolling on the fatigue performance of railway axles

Deep rolling is a frequently used post-treatment process to increase the service life of railway axles. Through the process application, the surface roughness is reduced, the surface is work-hardened and compressive residual stresses are introduced in the near surface region in a controlled manner. These three effects have a positive impact on service life, and in the investigated case of railway axles, the introduced residual stress state constitutes the most significant potential. The compressive residual stresses introduced near the surface, which are responsible for the positive lifetime effect, are inevitably balanced by compensating tensile residual stresses below the surface. Therefore, it is generally challenging to estimate the actual potential of the process application properly. Classical design methods consider only the condition at the surface and are thus not suitable for a surface layer-based analysis. In the present publication, the concept of local fatigue strength is applied for the fatigue strength assessment of deep rolled railway axles. The locally permissible fatigue strength at each surface-layer depth of the railway axle is determined as a function of local properties and compared with the locally occurring load. This allows a proper fatigue design in depth and, at the same time, the estimation of the critical point at which the potential of crack initiation and thus failure occurs. Based on the methodology presented, it is possible to determine the fatigue benefit achieved by deep rolling railway axles considering the influence of various deep rolling parameters, and to further systematically optimise the significant process parameters. An increase in fatigue strength of 4.7 % can be proven by deep rolling the railway axle with reference parameters compared with the not deep rolled state. By applying the proposed optimisation, fatigue strength can be further increased up to 10.4 %.

Effect of Surface Finishing State on Fatigue Strength of Cast Aluminium and Steel Alloys

The endurance limit of structural mechanical components is affected by the residual stress state, which depends strongly on the manufacturing process. In general, compressive residual stresses tend to result in an increased fatigue strength. Post-manufacturing processes such as shot peening or vibratory finishing may achieve such a compressive residual stress state. But within complex components, manufacturing-process-based imperfections severely limit the fatigue strength. Thus, the interactions of imperfections, residual stress state and material strength are key aspects in fatigue design. In this work, cast steel and aluminium alloys are investigated, each of them in vibratory finished and polished surface condition. A layer-based fatigue assessment concept is extended towards stable effective mean stress state considering the elastic–plastic material behaviour. Murakami’s concept was applied to incorporate the effect of hardness change and residual stress state. Residual stress relaxation is determined by elastic–plastic simulations invoking a combined hardening model. If the effective stress ratio within the local layer-based fatigue strength is evaluated as critical distance value, a sound calculation of fatigue strength can be achieved. Summing up, the layer-based fatigue strength design is extended and features an enhanced understanding of the effective stabilized mean stress state during cyclic loading.

A Study on Very High Cycle Fatigue Properties of Axle Steel with Induction Hardening on the Surface

Rotating bending fatigue tests were performed to investigate the effect of induction hardening (HQI) on the very high cycle fatigue (VHCF) properties of the axle steel. In addition, finite element analyses (FEA) and scanning electron microscope (SEM) observations were conducted to elucidate the fundamental behavior. The results obtained are summarized as follows; (1) It was found that the fatigue strength of the axle steel was improved by almost 300 MPa in VHCF region by HQI, (2) A lot of specimens of the HQI had fractured in VHCF region, therefore, the consideration of VHCF properties should be taken into account in the design of railway axles, (3) Crack initiation sites can be classified into three forms; the surface of the HQI layer in the minimum section, the base metal beyond the HQI layer in the axial direction, and the internal base material in the minimum section. These can be reconfirmed through the comparison between the local fatigue strength distribution and the bending stress distribution obtained by FEA.

Multi-domain optimization of cast iron components in wind turbines

The continuously rising demand for renewable energies leads to increased installations of wind turbines with higher power. While the current power-to-weight ratio of up to 20 metric tons of cast iron per megawatt is stagnating, cast iron components of modern wind turbines are facing new challenges in terms of weight, manufacturability, and castability. These challenges can be addressed by systematically using multi-domain optimization approaches to reduce component weight and increase local component utilization.In order to meet the requirements for modern cast iron components, this multi-domain approach must employ methods from casting simulation, micromechanical analysis, topology optimization, and strength assessment. Here, casting simulation is used to determine local microstructure descriptors, which are subsequently used in micromechanical shakedown analysis to estimate the local microstructure-dependent fatigue strength. In parallel to the fatigue strength estimation, topology optimization is performed iteratively in combination with a castability analysis. The component strength is evaluated using a strength assessment approach based on the previously determined local material properties in combination with the topology optimized component.In this study, the overall concept of the proposed multi-domain approach is presented and requirements for the application of such an approach are formulated. The use case of this study is a planet carrier of a wind turbine gearbox manufactured from austempered ductile cast iron ADI-GJS-1050‑6. For this use case, a weight reduction of 17% was achieved while maintaining the required stiffness, such that the microstructure variance along the component was significantly reduced. Furthermore, the potentials and limitations of the presented approach are outlined and discussed in the context of the design of heavy-section castings.

Study of Local Fatigue Methods (TCD, N-SIF, and ESED) on Notches and Defects Related to Numerical Efficiency

The fatigue strength of structural components is strongly affected by notches and imperfections. Both can be treated similarly, as local notch fatigue strength methods can also be applied to interior defects. Even though Murakami’s √area approach is commonly used in the threshold-based fatigue design of single imperfections, advanced concepts such as the Theory of Critical Distances (TCD), Notch Stress Intensity Factors (N-SIF), or Elastic Strain Energy Density (ESED) methods provide additional insight into the local fatigue strength distribution of irregularly shaped defects under varying uniaxial load vectors. The latter methods are based on the evaluation of the elastic stress field in the vicinity of the notch for each single load vector. Thus, this work provides numerically efficient methods to assess the local fatigue strength by means of TCD, N-SIF, and ESED, targeting the minimization of the required load case count, optimization of stress field evaluation data points, and utilization of multi-processing. Furthermore, the Peak Stress Method (PSM) is adapted for large opening angles, as in the case of globular defects. In detail, two numerical strategies are devised and comprehensively evaluated, either using a sub-case-based stress evaluation of the defect vicinity with an unchanged mesh pattern and varying load vector on the exterior model region with optimized load angle stepping or by the invocation of stress and strain tensor transformation equations to derive load angle-dependent result superposition while leaving the initial mesh unaltered. Both methods provide numerically efficient fatigue post-processing, as the mesh in the evaluated defect region is retained for varying load vectors. The key functions of the fatigue strength assessment, such as the evaluation of appropriate planar notch radius and determination of notch opening angle for the discretized imperfections, are presented. Although the presented numerical methods apply to planar simulation studies, the basic methodology can be easily expanded toward spatial fatigue assessment.

On the use of milled shell‐only specimens to study the effect of fiber orientation on the fatigue behavior of a short fiber‐reinforced polyamide

AbstractThe interpretation of the fiber orientation effects on the fatigue behavior of short fiber‐reinforced polymers using specimens extracted from injection‐molded plates is hampered by the typical shell‐core‐shell structure, which affects the stress distribution through the sample's thickness. In this work, local fatigue strength values are obtained using specimens machined to leave only the outer shell layers, thus having a more homogeneous fiber orientation. Results of the finite element analysis of integer and shell‐only specimens, accounting for locally measured fiber orientations, are compared. Conclusions are drawn about the role of the core layer and the limitations of milled specimens in the evaluation of the fatigue strength.

Very high cycle fatigue assessment at elevated temperature of 100 μm thin structures made of high-strength steel X5CrNiCuNb16-4

Many components and structures are exposed to very high number of cycles and challenging environmental conditions during operation. This study contributes to a better understanding of the very high cycle fatigue (VHCF) properties of high-strength steel X5CrNiCuNb16-4 at room temperature (RT) and 350 °C. For this purpose, conventional specimens and thin-walled structures are extensively examined with novel high-frequency fatigue testing techniques at elevated temperature. Tests with unnotched specimens at 350 °C show a 21.7% reduction in fatigue strength for 107 cycles and a different failure mechanism compared to RT. In contrast, no temperature influence is observed for mildly notched specimens and even a higher local fatigue strength is found for sharply notched specimens at 350 °C. The decrease in fatigue strength for 109 cycles is more pronounced at 350 °C (−10%) than at RT (−5%), and it is proven that notched specimens adequately represent the VHCF behaviour of structures. The transferability of specimen results to components and structures is given great attention. A new proposal for the VHCF strength assessment of structures with high stress gradients is presented, which is based on specimen results, an extended material-mechanical support factor and a VHCF reduction factor. The prediction model gives conservative fatigue strength estimates for 109 cycles with a maximum deviation of 5.8%. This demonstrates that even complex shaped structures can be successfully evaluated with suitable specimens and methods.

On the Transferability of Fatigue and Cyclic Deformation Data to 100 µm Thin Structures

The fatigue properties and cyclic deformation behaviour of materials are usually determined using cylindrical specimens (e.g., Ø7.5 mm). Since the transferability to very small dimensions has not been comprehensively considered so far, this study investigates the transferability of specimen data from high-strength steel X5CrNiCuNb16-4 to real structures characterised by a wall thickness of 100 µm. Regarding fatigue, extensive calculations demonstrate that for notched specimens and thin structures, both the material-mechanical support factor concept according to the FKM guideline and the point method of the Theory of Critical Distance (TCD) make correct predictions of the local fatigue strength, with a maximum deviation from experimental values of less than 5%. However, the study points out that the TCD is only conditionally applicable for thin-walled structures, as the material parameter a0 must be significantly smaller than the wall thickness. Regarding the deformation behaviour, the material reveals special flow characteristics in the first hysteresis. Nevertheless, a combined hardening approach is suitable for modelling. The validation of the model by a plastic deformation of the structure seems plausible, although geometric influences prove to be dominant. In conclusion, even 100 µm thin structures can be evaluated using conventional specimen tests and established assessment or modelling methods.

Effect of the Torsion Box Dimensions on Local Stress Distribution and Fatigue Strength Assessment of a Container Ship

The aim of this paper is to estimate the influence of the dimensions of the torsion box (height and width), of a 2400 TEU feeder-class container ship, on local stress distribution and assessment of local fatigue strength by using a numerical approach based on the fatigue limit-state and ultimate limit-state in the midship region. In terms of the fatigue limit-state, the effect of the dimensions of the torsion box is obtained by geometrical modifications, in the connection between the side shell longitudinal stiffeners with the transverse web frame for nine structural details, referring to different arrangements of strengthening elements (brackets and flat bars). The process of comparing the different elements determines the most effective combination. This structural influence on the local stress distribution is assessed, along the longitudinal plates between ordinary stiffeners bounding the perimeter of the torsion box, by calculating the hull girder stresses, local buckling stresses and shear stress distribution induced by vertical and horizontal shear forces, Saint Venant and warping torques, and finally the shear stresses induced by a warping moment. Scantling criteria are obtained allowing for a better design of this very important region in container ships.

Fatigue strength assessment of TIG-dressed high-strength steel cruciform joints by nominal and local approaches

According to the IIW recommendation, the fatigue strength of welded steel joints is defined as independent of the base material in case of the as-welded condition. However, post-treatment techniques can improve the fatigue performance of welded structures, especially for increased base material strengths. Therefore, this paper investigates the effect of TIG dressing, as common post-weld treatment method, on the fatigue strength of high-strength steel S700 cruciform joints. The statistically evaluated fatigue test results reveal a significant increase of the nominal fatigue strength from FAT 90 for the as-welded up to 182 MPa for the TIG-dressed state. The experiments are further compared to recommended and suggested design curves applying both nominal as well as local stress approaches. Focusing on the TIG-dressed state, the suggested increase in nominal stresses is well validated leading to a conservative assessment. In addition, the proposed slope in the finite life region with a value of m1 = 4 shows a sound fit to the statistically evaluated value of m1 = 4.7 for the test results. The local fatigue strength estimation is performed based on a recent proposal using the theory of critical distances. Therefore, linear elastic numerical analysis of the investigated specimens is performed. Again, the resulting S-N curves agree well to the experiments validating the proposed local design approach.

Probabilistic modelling of pitting corrosion and its impact on stress concentrations in steel structures in the offshore wind energy

Supporting structures for offshore wind turbines and the appropriate transformer platforms are highly susceptible to corrosion. Especially the phenomenon of pitting corrosion is very crucial, since this leads to local stress concentrations and hence has an impact on the fatigue life of marine structures. Despite corrosion protection systems, corrosion cannot be completely avoided, which can lead to pitting corrosion on the steel surface. Stress concentrations from pitting corrosion are not explicitly considered within the design of offshore structures. This is only possible by using local fatigue concepts, which requires a realistic modelling of corroded steel surfaces and the corresponding stress concentrations. Hence, in this work, three different models for evaluating the stress concentrations were developed: single-pit model, double-pit model and random fields model. The results were compared to the stress concentrations of a real corroded surface, which was analysed with the reverse engineering method. In all models, the probability-based determination of the stress concentration was the main focus in order to consider the characteristic scatter in pitting corrosion. The comparison of the three models with the real surface shows a good agreement with the double-pit model. It becomes evident, that the single-pit and the random fields model are not sufficient for modelling the stress concentrations. However, derived stress concentrations from the double-pit model and the real surface model can be utilized for risk and reliability-based analysis with local fatigue strength concepts.

Investigation of Fatigue Damage of Tempered Martensitic Steel during High Cycle Fatigue and Very High Cycle Fatigue Loading Using In Situ Monitoring by Scanning Electron Microscope and High‐Resolution Thermography

Herein, the fatigue damage mechanisms of a low‐alloyed 0.5 wt% carbon steel (50CrMo4) are examined in the high cycle fatigue and very high cycle fatigue regime, taking into account different strength conditions. For this purpose, the heat treatment of the material is conducted using two different batches at two different tempering temperatures, which lead to hardnesses of 37HRC and 57HRC, respectively. The fatigue tests are conducted accounting for different test frequencies of f = 95 Hz and f = 20 kHz, as well as size effects. It is found that the fatigue behavior of the tempered steel 50CrMo4 can be subdivided into type I (surface cracks) and type II (internal cracks) VHCF damage behavior depending on the material strength condition. With the aid of in situ test equipment, the influence of microstructural banding on the local fatigue strength is demonstrated by high‐resolution thermography. By implementing an ultrasonic testing machine in a high‐resolution scanning electron microscope, the development of a fine granular area (FGA) at an artificial defect in vacuum atmosphere is demonstrated.

Effect of surface nanocrystallization on fatigue properties of Ti–6Al–4V alloys with bimodal and lamellar structure

High-energy shot peening (HESP) was employed to obtain a nanocrystalline surface layer in Ti–6Al–4V alloy with bimodal and lamellar structure. The microstructure and residual stress after HESP treatment were characterized by transmission electron microscope (TEM) and X-ray diffractometer (XRD), respectively. The fracture morphology of fatigue was analyzed by scanning electron microscope (SEM). The results showed that the average grain sizes of Ti–6Al–4V alloy with bimodal and lamellar structure were 18.9 nm and 17.1 nm at the topmost surface layer, respectively. The maximum compressive residual stress of the bimodal structure was −906MPa, which was higher than that of the lamellar structure (−859 MPa). The fatigue limits of the HESP-treated Ti–6Al–4V alloy with the bimodal and lamellar structure were increased by 13.5% and 32.8%, respectively. Finally, a Goodman equation was employed to calculate the local fatigue strength and analyze the residual stress-fatigue strength correlation. Highly defected substructure, reduced effective slip length and improved yield strength were important factors for the improvement of fatigue strength. The synergy effect of surface gradient nanostructure and compressive residual stress improved the fatigue limit of the HESP-treated Ti–6Al–4V alloy with bimodal and lamellar structure.

Influence of discontinuities on fatigue strength using the example of additively manufactured specimens made of AlSi10Mg

Fatigue strength is largely affected by the influence of discontinuities: geometric notches like holes and corners in components, surface notches (roughness), pores and inclusions in the material. Therefore fatigue of structures and materials is a local problem and fatigue strength calculations are best carried out using local approaches such as local stress concept, local strain concept or fracture mechanical concepts. In this investigation, the influence of discontinuities on low cycle and high cycle fatigue strength of materials was examined on the example of additively produced material specimens made of aluminium AlSi10Mg by using a fracture mechanics parameter. Results of fatigue tests on hourglass-specimens made from four different series of 3-D printed specimens are presented. The examination of the fracture surfaces revealed that damage always started from pores located near the surface of the specimens. The fractography gave information about geometry, size and position of the crack-causing pores. The influence of such pores on the fatigue strength of the printed aluminium was investigated by using a modification of Murakami’s √area parameter. The common scatter band of the four test series could thus be reduced significantly.

Fatigue Behavior of Thick-Walled Nodular Cast Iron Containing Dross

Abstract In large castings made of nodular cast iron, dross is not easy to detect with nondestructive testing (NDT) methods and reduces the static and cyclic fatigue strength of the material significantly, as shown in the example in figure 1. This causes a significant amount of work in foundries in removing dross or rejecting the complete component if a secure assessment of the component’s lifetime cannot be given. The aim of the reported work was thus to find appropriate methods to determine the characteristics of dross, such as its stiffness and density, in thick-walled nodular cast iron and to set up an assessment concept for the lifetime determination of components affected by dross. For the investigations, large cast blocks of EN-GJS-400-15 were cast with fields of dross in the upper side of the blocks. These blocks were investigated with different NDT methods, such as mechanized ultrasonic testing and magnetic particle inspection, to obtain information on the position and the characteristics of the dross. Subsequently, fatigue specimens and tensile specimens were removed from the cast blocks from the dross regions and from the baseline material without dross and were tested under strain control to determine the cyclic strain-based material behavior of the different specimens, with varying forms of dross. Additionally, all fatigue specimens were investigated with regard to their dross distribution using X-ray computer tomography, sulfur tests, and magnetic particle inspection. During the investigations, special attention was paid to the material stiffness, because this is significantly influenced by dross. Also, previous results on shrinkages in nodular cast iron showed good correlation between the material stiffness and the density measured in the specimen’s test volume. This correlation offers a link to NDT and a possibility to assess the influence of dross on the local fatigue strength.

Fatigue strength assessment of ground fillet-welded joints using 4R method

In the present paper, a fatigue analysis for welded joints made of mild, and high- and ultra-high-strength steel grades and post-weld-treated with grinding and combined grinding and peening methods is carried out using literature data. The 4R method was used to re-analyze the data points considering material strength, residual stress, applied stress ratio, and weld toe radius. The use of the 4R method, instead of local fatigue strength assessment approaches, enabled residual stresses to be taken into consideration and accurately predicted the fatigue strength of joints in the as-welded and post-weld-treated conditions, including grinding and combined grinding and peening methods.

Validation Study on the Statistical Size Effect in Cast Aluminium

Imperfections due to the manufacturing process can significantly affect the local fatigue strength of the bulk material in cast aluminium alloys. Most components possess several sections of varying microstructure, whereat each of them may inherit a different highly-stressed volume (HSV). Even in cases of homogeneous local casting conditions, the statistical distribution parameters of failure causing defect sizes change significantly, since for a larger highly-stressed volume the probability for enlarged critical defects gets elevated. This impact of differing highly-stressed volume is commonly referred as statistical size effect. In this paper, the study of the statistical size effect on cast material considering partial highly-stressed volumes is based on the comparison of a reference volume V 0 and an arbitrary enlarged, but disconnected volume V α utilizing another specimen geometry. Thus, the behaviour of disconnected highly-stressed volumes within one component in terms of fatigue strength and resulting defect distributions can be assessed. The experimental results show that doubling of the highly-stressed volume leads to a decrease in fatigue strength of 5% and shifts the defect distribution towards larger defect sizes. The highly-stressed volume is numerically determined whereat the applicable element size is gained by a parametric study. Finally, the validation with a prior developed fatigue strength assessment model by R. Aigner et al. leads to a conservative fatigue design with a deviation of only about 0.3% for cast aluminium alloy.

Probabilistic Surface Layer Fatigue Strength Assessment of EN AC-46200 Sand Castings

The local fatigue strength within the aluminium cast surface layer is affected strongly by surface layer porosity and cast surface texture based notches. This article perpetuates the scientific methodology of a previously published fatigue assessment model of sand cast aluminium surface layers in T6 heat treatment condition. A new sampling position with significantly different surface roughness is investigated and the model exponents a 1 and a 2 are re-parametrised to be suited for a significantly increased range of surface roughness values. Furthermore, the fatigue assessment model of specimens in hot isostatic pressing (HIP) heat treatment condition is studied for all sampling positions. The obtained long life fatigue strength results are approximately 6% to 9% conservative, thus proven valid within an range of 30 µm ≤ S v ≤ 260 µm notch valley depth. To enhance engineering feasibility even further, the local concept is extended by a probabilistic approach invoking extreme value statistics. A bivariate distribution enables an advanced probabilistic long life fatigue strength of cast surface textures, based on statistically derived parameters such as extremal valley depth S v i and equivalent notch root radius ρ ¯ i . Summing up, a statistically driven fatigue strength assessment tool of sand cast aluminium surfaces has been developed and features an engineering friendly design method.

Adjustment of Lifetime-Increasing Surface Layer States by Complementary Machining∗

Abstract During manufacturing of metallic components, thermo-mechanical loadings induce surface layer states like topography or residual stresses, which influence the component properties like fatigue strength. In order to optimize the component properties, a mechanical surface treatment can be carried out after the machining process. In this work, the influence of the process parameters processing velocity and penetration depth on the resulting tool wear during external longitudinal turning of AISI 4140q&t by Complementary Machining is analyzed. The process strategy Complementary Machining (CM) combines machining and mechanical surface treatment using the cutting tool. The mechanical surface treatment takes place after the machining in the opposite machining direction. The results of this study show that the process variables have an influence on the tool wear and thus directly influence the resulting topography. The fatigue analysis shows that the fatigue strength after Complementary Machining is comparable to that of shot peening. Furthermore, the concept of local fatigue strength is used to show the extent that residual stress reduction as a result of cyclic loading affects the fatigue strength.

Local Fatigue Strength Evaluation of Shot Peened 40Cr Notched Steel

Fatigue life prediction for the notched components is an essential step within the design process of machines. Fatigue strength and life prediction of 40Cr notched steel before and after shot peening were studied. Fatigue fracture of specimens treated by three shot peening intensity parameters was discussed. The life prediction considering residual stress, work hardening and surface roughness caused by shot peening was analyzed. The results indicated that fatigue strength was obviously improved after shot peening and the improvement effect was gradually enhanced with the increase of shot peening intensity. The predicted values based on Rz coefficient showed a good correspondence with the experimental data.