The guideline “Analytical Strength Assessment” is a stress-based framework for evaluating the fatigue strength of safety-relevant components. However, it possesses significant weaknesses when dealing with multiaxial nonproportional stresses, often leading to results that are unnecessarily conservative and which waste valuable component potential. This study addresses these shortcomings by proposing two new calculation procedures designed to be compatible with the existing framework of the guideline: a normal stress-based approach using modified scaled normal stresses and a shear stress-based approach inspired by the Findley parameter. Both methods utilize a critical plane approach in combination with a nonproportionality measure and a multiaxiality grade to adjust the S-N curves and mean stress correction.The accuracy of these proposals is validated using a comprehensive database containing fatigue lives for various materials, including steel, cast iron, and aluminium, under both proportional and nonproportional multiaxial loading. The evaluation confirms that the current guideline is highly conservative for nonproportional loading and shows that changes to the algorithm of the guideline can increase accuracy for both nonproportional and proportional stresses, thereby making significantly higher fatigue performance accessible

The low-cycle fatigue behavior of high-manganese twinning-induced plasticity (TWIP) steel is examined for specimen axial orientations of 0 degrees, 45 degrees, and 90 degrees relative to the rolling direction at a temperature of 449 K. The study aims at a total strain amplitude range (Delta epsilon/2) of 0.6 % to 2.4 %, considering both pre-deformed (P-type) and non-pre-deformed (F-type) loading conditions. The fatigue behavior is compared to our recent work at room temperature (295 K, Song et al. Int J Fatigue 2023), to obtain a more comprehensive analysis of the varying deformation mechanisms with different temperatures. Experimental results show that elevated temperature significantly increases the ductility of the material while reducing its strength. Applying a severe plastic pre-strain at high temperature (449 K) will dramatically decrease fatigue life, which is markedly different from the behavior at room temperature. All these characteristics are related to the loading orientations and strain amplitudes. Additionally, a life-prediction model for high-Mn steel that considers different deformation mechanisms at varying temperatures is also proposed. The model demonstrates fatigue life predictions across 77 loading cases, encompassing five orientations, seven strain amplitudes, three types of loading modes, and two temperatures. The comparison of experimentally and analytically determined lives shows the comprehensive descriptive capacity of the proposed model.

Most commonly used guideline-based procedures for assessing the fatigue strength of welded joints are limited to the high cycle fatigue (HCF) regime. Although calculation methods for the low cycle fatigue (LCF) regime also exist, they are often either too complex for industrial practice or only suitable for rough estimation of fatigue strength. Fatigue tests were conducted on welded joints of varying sheet thicknesses made from S960M and X6CrNiTi18-10 materials, resulting in fatigue lives within the LCF regime. The test results were used to assess the effectiveness of two approaches for enhancing elastic stress analysis: the Ke-factor method and the local strain approach according to FKM-guideline nonlinear extended to welds and considering elastic–plastic material behavior. The influence of the mismatch of welded joints on critical stress conditions leading to failure was also investigated.

In fracture mechanics, phase field theories have been introduced for the first time for brittle materials, in order to regularise the sharp crack topology. Especially, the crack surface part of the total energy functional is regularised by using a phase field variable. In the present work, a phase field model for ductile fracture in the framework of non-conventional thermodynamics is studied. In contrast to brittle fracture, the physical mechanisms for fracture are supposed to be driven by plastic deformation.The aim of the paper is to highlight, by analysing numerical examples, important features of the model that affect the predicted material responses. The analysis refers to assumptions commonly adopted in phase field theories and continuum damage mechanics and comprises the numerical robustness of the related finite element integrations.

The paper addresses the numerical calculation of multi-axial fatigue crack propagation in ductile materials under proportional and non-proportional cyclic loading conditions. The analysis is based on a phase field fracture theory proposed in earlier work, which describes crack propagation on the basis of the evolution of plastic strain. A plane stress formulation of the theory is developed and used to analyze both length scale effects and comparison with experimental results. The considered loading histories comprise proportional and non-proportional cyclic tension/compression and shear loadings. Generally, the predicted responses are in good agreement with the experimental results for proportional loading histories. Qualitative differences seem to exist, however, in what concerns the number of predicted cracks for non-proportional cyclic loading conditions.

The “Guideline non-linear” of the German Research Association for Mechanical Engineering provides a fatigue strength assessment for machine components based on the local strain approach. Currently, this assessment is limited to homogeneous components and without the possibility to consider residual stresses. The methods are thus inadequate for surface-hardened components, where the presence of material inhomogeneity and the introduced residual stresses significantly impacts fatigue life and the potential failure origin. In the following paper, an adaptation of the proof of structural durability of the guideline is shown, that firstly includes a two-point assessment and the estimation of load cycles to crack initiation at two failure-relevant points simultaneously. This enables the consideration of inhomogeneous material states, since the failure of surface-hardened components may initiate from the notch root as well as from the transition area between the low-strength core material and the high-strength surface layer. Secondly, the consideration the residual stress state for both failure-relevant points is also introduced as part of the adapted fatigue strength assessment.

In this paper two different kinds of notch approximation methods are presented. The proposed methods are known as the incremental notch approximation method (e. g. Neuber method [1] or ESED [2, 3]) and the structural yield surface approach (SFF) (after Kottgen et al. [3]), while two different approaches of the SFF are used. All methods are capable to handle multiaxial cyclic loading. For a validation, the methods are compared to the results of a finite element analysis (FEA) of a notched component. The presented comparison is made for a nonproportional loading case. One objective of the work is to find out, which is suitable to extend it to thermomechanical loads as a next step.

This study investigates multiaxial fatigue behavior of ductile construction steel S355 and cast iron EN-GJS-500-14. Fatigue lives and crack initiation orientations are measured. Three critical plane models are evaluated for predicting fatigue life and crack initiation angles. While all models achieve decent accuracy in predicting fatigue life, crack initiation angle predictions are less satisfactory. Weak correlation is observed between the quality of fatigue life and crack orientation prediction, particularly for the Smith-Watson-Topper (SWT) and Fatemi-Socie (FS) models. The SWT model performs better in predicting fatigue life and crack angles for cast iron, while the FS model shows higher accuracy for S355. The short-crack model (SC) provides the best overall fatigue life prediction but crack orientation estimation is not superior.

The FKM-Guideline "Analytical Strength Assessment" contains an algorithm for both static and the fatigue strength assessment of components made of steel, cast iron, and aluminium alloys. Designed to be advantageous for industrial applications, this guideline expresses the design stress using a straightforward linear-elastic material behaviour. Starting with the material strength properties specified in technical standards, the relevant component properties for both static and fatigue strengths can be determined by considering all influencing factors. The safety level required for assessment can be adjusted using experience-based safety factors. The outcomes of the analytical strength assessment provide a measure of the degree of utilisation. While the FKM-Guideline has been successfully applied since 1994, continuous further development is necessary to ensure it remains at the forefront of industry standards. Key development areas include the improvement of the critical strain curve, introduction of a coordinate-invariant fatigue strength assessment procedure, conducting investigations on the accuracy of the calculation according to the FKM-Guideline as a precondition for the development of a probabilistic safety concept, and extension of the FKM-Guideline to additional material groups such as high-strength steel.

In this research paper we evaluate the potential of an ultra-high-strength steel for use in cyclically loaded, inner-pressure bearing components. Component-like specimens with intersecting boreholes were subjected to different grades of autofrettage pressure. The study investigates the material behaviour and describes it using a simple approach. Numerical analysis is conducted to determine the effects of autofrettage on endurance limits, crack arrest occurrences and the length of arrested cracks. The numerical results are compared to results from component-like specimen testing and the overall calculation procedure is found to be accurate. This research provides valuable insights into using ultra-high-strength steel for such applications.

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.

In the present work, a phase field approach for ductile fracture and fatigue failure in the framework of non-conventional thermodynamics is proposed. In contrast to brittle fracture, the physical mechanisms for fracture are supposed to be initiation, growth and coalescence of voids, driven by plastic deformation. Thus, the aim of the paper is to demonstrate how well-established ingredients of plasticity, continuum damage mechanics and phase field theories can be fit in the adopted framework. The main features of the proposed theory are that damage evolution is coupled to the evolution of plastic strain, that the appropriate modelling of yield stress effects in the free energy function is of central importance and that fatigue failure can be captured without employing fatigue degradation functions. Various numerical examples and comparisons with experimental data demonstrate the capabilities of the model.

The low-cycle fatigue performance of high-manganese twinning-induced plasticity (TWIP) steels with the specimen axial direction of 0 degrees, 22.5 degrees, 45 degrees, 67.5 degrees, and 90 degrees to the rolling direction is investigated in the range of the total strain amplitudes (Delta epsilon/2) of 0.6 % similar to 4.4 % and two types of pre-deformed conditions (i.e., tension and tension-unloading). Experimental results show that the dependence of the fatigue life on the specimen orientation is obvious for lower strain amplitudes, and it gradually reduces with the increase of the applied strain level. The 0 degrees orientations show the highest sensitivity to the pre-deformation process, and the tension-unloading pre-deformation exhibits the most severe change of the texture at fracture.

The local strain approach has been successfully applied in evaluating the fatigue life of notched components under multiaxial loading. In the present work, this approach is combined with the critical plane method. First, it is shown that by considering non-proportional strain hardening in the estimation of local stresses and strains, the accuracy of the results under non-proportional loading can be improved regardless of the applied multiaxiality hypothesis. Three different critical plane hypotheses, SWT (Smith Watson Topper, based on normal strain), FS (Fatemi Socie, based on shear strain) and a short-crack model are used to calculate the fatigue life of notched specimens and compare it to results from an extensive database of test results. The accuracy of the SWT approach is highly dependent on the loading situation. For tests under non-proportional and pure torsional loading, excessively long fatigue lives are predicted. A simple proposal is made for the FS approach to be applied in critical planes with multiple shear strain components. The FS and short-crack approaches predict fatigue lives with high accuracy regardless of the applied load.

The approaches used to calculate the fatigue life of components must inevitably consider multiaxial stresses. Compared to proportional loading, the calculation of nonproportional loading is particularly challenging, especially since different materials exhibit the effects of nonproportional hardening and shifts in fatigue life. In this paper, the critical plane approach of scaled normal stresses, first proposed by Gaier and Dannbauer and later published in a modified version by Riess et al., is investigated in detail. It is shown that, on the one hand, compatibilities exist or can be established with known proportional strength criteria that can account for the varying ductility of different materials. Furthermore, it is demonstrated that the scaled normal stress approach can be formulated in such a way that different strength criteria can be used therein. As an example, the generally formulated approach for scaled normal stresses is applied to test results from ductile cast iron material EN-GJS-500-14. Different correction factors accounting for nonproportional loading are investigated. Through appropriate parameterization of one of the studied corrections, proportional and nonproportional test results were observed to fall within one common scatter band.

For the dimensioning of components made of low and medium-strength steels, there are established estimation methods for the material properties as well as transfer functions to transmit these parameters to components. However, for ultra-high strength steels, this level of knowledge is still insufficient and has not yet been specified in guidelines. In the present work, tensile and fatigue tests are performed with different high-strength and ultra-high-strength steels, which provide the data basis for an extension of the estimation methods. This is intended to enable the estimation of the cyclic material behaviour of these kind of steels on the basis of a limited amount of experimental input data and then to use these material parameters as input for the proof of structural durability based on the local strain approach for components made of ultra-high strength steels.

This work is primarily concerned with the fatigue life of high-pressure bearing components with intersecting holes, typically used in Diesel engine fuel injection systems. The investigation focuses on specimens with orthogonally intersecting holes that have undergone the process of Autofrettage (single mechanical overload), which is typically used to extend the fatigue life of components loaded by cyclic internal pressure. The Autofrettage process induces advantageous, life-time prolonging residual compressive stresses in the highly stressed areas of the components. The resulting residual stress distribution thus influences the fatigue failure and especially the crack propagation behaviour of the components. In previous works, fracture mechanics based approaches were used to describe the crack propagation behaviour for autofrettaged specimens made of the quenched and tempered steel 42CrMo4. Results showed that crack arrest has to be taken into account when calculating fatigue lives of autofrettaged specimens as the endurance limit is otherwise underestimated. As efforts are made to increase the injection pressures of fuel injection systems, in this work, the benefit of using ultra high strength steel for the application described is investigated. In order to achieve reliable results, material testing with samples made of the ultra-high-strength steel W360 was performed. The resulting test data were used to describe the initial loading and cyclic loading behaviour of the material with a suitable material model. Finite element analysis was then performed to simulate the Autofrettage process and subsequent cyclic loading. Based on the simulation results, possible crack initiation was determined. For predicted crack initiation, the simulated residual stress distribution was used to investigate the crack propagation behaviour with fracture mechanics based approaches of different complexity in order to identify possible crack arrest or crack propagation. Calculated results were compared to experimental test data from component-like specimens. The comparison to the test results showed an overestimation of the predicted fatigue lives. The modelled material behaviour and consequently the residual stress distribution from the simulation models was identified as the decisive factor for the deviation. Still, the comparison showed that the fracture mechanics based approaches are capable of describing the crack arrest and propagation behaviour reliably. Further investigation regarding the modelling of the material behaviour with focus on the Autofrettage process is still required. (C) 2022 The Authors. Published by Elsevier B.V.