A fracture mechanics based model is proposed for calculating the fatigue life of smooth and notched tubular specimens under torsional and combined axial--torsional loading. Equivalent initial damage size (EIDS) values are calculated by applying the model to experimental data obtained by testing of smooth and notched PBF-LB Ti-6Al-4V specimens subjected to torsional and axial-torsional loading. A three-parameter Weibull distribution fitted to the resulting EIDS values is statistically consistent with the distribution previously derived from axial loading tests, indicating transferability across loading conditions. The model accurately predicts the fatigue life for specimens exhibiting planar fatigue crack growth, irrespective of loading type, but underpredicts the fatigue life of notched specimens displaying factory-roof crack morphology at intermediate stress levels.

The present study investigates the role of process-induced defects on the fatigue performance of high-strength Al2139 alloy fabricated by laser-based powder bed fusion of metals (PBF-LB/M). Specimens of two surface conditions: as-built and machined, underwent high-cycle uniaxial fatigue testing to evaluate the combined influence of surface state and defect characteristics. Fractographic examination using scanning electron microscopy identified gas pores and lack-of-fusion (LoF) defects as the dominant crack initiation sites. The LoF defects were further classified into regular LoF defects with relatively compact morphology and shallow, irregularly shaped LoF defects exhibiting larger projected areas.Despite the removal of surface roughness effects, machined specimens exhibited greater scatter in the S–N response and, in several cases, shorter fatigue lives than as-built counterparts tested at identical stress levels. Premature failures in the machined group were consistently associated with shallow, irregular LoF defects with a larger effective size that were exposed at the surface by the machining operation. This concludes that defect morphology and projected area govern fatigue performance more critically than nominal surface finish.Defect severity was quantified using Murakami’s √area parameter. An equivalent defect radius was calculated by idealising defects as semicircular surface cracks on a cylindrical specimen to determine the corresponding stress intensity factor (SIF). A clear correlation between SIF and fatigue life was established, enabling a defect-based life prediction method. The proposed framework provides a quantitative basis for defining allowable defect sizes at given stress levels and contributes to defect-tolerant design strategies for PBF-LB/M Al2139 components.

This study explores two different data augmentation approaches to generate artificial data for the training process of machine learned constitutive models. The cyclic Ramberg-Osgood model was employed by predicting the stress amplitude at 1% strain amplitude in the materials. A baseline model using 96 materials in the training process was first defined. Ten materials out of the 96 were sampled to be augmented. The first augmentation approach was to perform a linear numerical scaling of sampled materials to generate artificial copies. However, this approach lack the real physical behaviour of the material. Thus, to account for the physical behaviour, an augmentation based on empirical statistical distributions of the material behaviour of the sampled materials was made. This generates improved artificial data with a reliable outcome. Hence, a robust, accurate and efficient training procedure was obtained compared to used experimental data.

Numerous methods have been suggested to quantify fatigue-initiating defect size on fracture surfaces, the most prevalent are based on the Murakami-Endo /area-parameter. However, there is an ambiguity in how systemically determine defect areas. For instance, in literature on high-cycle fatigue of ductile cast irons the several different methods have been suggested: (i) the traced contour, (ii) the convex hull, (iii) the minimum circumscribed circle, and (iv) the minimum bounding rectangle. This work focuses on comparing and evaluating these methods by assessing the fatigue-initiating defect area distributions, and the influence on fatigue assessment using the /area-parameter. To this end, very high cycle fatigue data on ductile cast irons with different microstructures is used, where complex shaped defects are the root-cause for fatigue failures. It is shown that there is a significant difference in the area distributions, originating from the applied area measurement method. In addition, to enable and include fatigue assessment of high strength ausferritic ductile irons, two improved Murakami-Endo type models are proposed, which show satisfactory prediction capabilities over a wide range of ductile cast iron microstructures. To further evaluate the different area measurement methods, the suggested models are validated against ductile cast iron high-cycle fatigue data from literature having artificial defects and notches. Finally, it is concluded that the traced contour defect measurement method yields the best agreement between artificial and natural defects, and overall, the least prediction errors.

In this work, AI algorithms are utilized to predict fatigue life based on known stress and strain states, representative of a small-data paradigm with limited experimental data. The work also explores variable safety factors of fatigue models, derived from the model uncertainty induced by the available training data. The approach enables adjustment of fatigue safety margins based on model confidence, reducing unnecessary conservatism against failure while maintaining structural reliability.A physics-informed neural network (PINN) model was trained on experimental data with physical principles modelled mathematically, such that physically feasible predictions are made possible outside of the training data. By utilizing these functions, the PINN model mitigates the influence of limiting experimental data. Predictions of the elastic and plastic deformation from the material testing gives an underlying understanding of the material behaviour for fatigue evaluation. Based on the probability intervals from the fatigue model, a variable safety factor can be estimated, which allowed for a smaller safety factor compared to conservative conventional methods.

We propose a fracture mechanics based method for determination of equivalent initial damage size (EIDS) distribution in as-built additively manufactured (AM) Ti-6Al-4V notched geometries. The crack growth model is shown to correctly capture the effect of the stress raisers and load ratio on the fatigue life of notched specimens. Results of constant amplitude fatigue tests on notched round bar specimens, with two different stress concentration factors and at multiple load ratios, are fitted to a three-parameter Weibull distribution. Based on the surface roughness measurements performed in this study and in the literature, maximum surface valley depth is found to be a reasonable estimation of the median EIDS.

The very high cycle fatigue performance of four ductile cast irons, a solid solution strengthened ferritic, ferritic-pearlitic, and two austempered ductile cast irons, was investigated by ultrasonic fatigue testing under fully reversed loading conditions. The step-stress fatigue testing method incorporating re-use of runouts was employed, with a 3x108$$ 3\times 1{0}<^>8 $$ cycles to failure runout criteria. The guarantee of failed specimens enables extensive fractography and characterization of fatigue-initiating defects, facilitating extreme value analysis of defect distributions. Furthermore, the fatigue and defect data were used for evaluation of stress-cycle-defect relationships as well as fatigue strength distributions. The obtained relationships between applied stress and fatigue-initiating defect size were used to adjust the data with respect to a given defect size, to compare and evaluate fatigue-initiating defect size equivalent fatigue strength distributions, emphasizing the role of the different matrix microstructures.

This study evaluates the Ottosen-Stenstr & ouml;m-Ristinmaa (OSR) incremental fatigue damage model for predicting fatigue life in powder bed fusion with laser beam (PBF-LB) Ti6Al4V notched specimens. To fit the OSR model, we conduct constant-amplitude tension-compression fatigue tests on PBF-LB Ti6Al4V specimens with as-built surface. Our results highlight a relatively low scatter in fatigue life data for PBF-LB Ti6Al4V across different studies, a critical factor for reliable design against fatigue failure. The study suggests that the stress gradient effect is influenced by the as-built surface, which carries load differently from the target build geometry due to surface undulations. The OSR model effectively captures the characteristics of W & ouml;hler curves for various notch geometries and stress ratios. We validate the OSR model with out-of-phase tension-torsion tests, demonstrating that it provides safe fatigue life predictions for nonproportional loads. Overall, our findings show that the OSR model offers conservative fatigue life predictions for PBF-LB Ti6Al4V, underscoring its practical utility and reinforcing the suitability of PBF-LB Ti6Al4V for aircraft applications.

To account for or neglect the defect position, i.e. the fatigue initiating defect location, both radially and axially, is evaluated for hourglass-shaped ultrasonic fatigue specimen. The commonly used analytical equations to calculate the stress is compared against a finite element (FE) based approach, which is able to fully considering the stress state at the defect position. Notably, the effects on several common fatigue analyses are evaluated: the fatigue strength distribution, the stress–life and the stress–defect relationships. Fracture mechanical assessment is also performed, for a comprehensive VHCF characterization of the EN-GJS-500-7 ductile cast iron used in the study. The VHCF properties are characterized up to 3⋅108 cycles, using the Step-Stress fatigue testing method under fully reversed loading. The FE-model and Weibull distribution as the choice of fatigue strength distribution, enables size effect evaluation by Weakest-link effective volume with the highly stressed volume method as benchmark. The work shows that it is imperative to use the local stress state at the defect position, as the distribution of failures can diverge largely from the center of the specimen, and that neglecting this causes systematic error and flawed potentially results.

In this paper, the high-temperature constitutive behaviour of an additively manufactured ductile nickel-based superalloy is investigated and modelled, with application to thermomechanical fatigue, low-cycle fatigue and creep conditions at temperatures up to ${800<^> \circ }$800 circle C. Thermomechanical fatigue tests have been performed on smooth specimens in both in-phase and out-of-phase conditions at a temperature range of $100 - {800<^> \circ }$100-800 circle C, and creep tests at ${625<^> \circ }$625 circle C, ${700<^> \circ }$700 circle C, ${750<^> \circ }$750 circle C and ${800<^> \circ }$800 circle C. Additionally, low-cycle fatigue tests at different strain ranges and load ratios have been performed at ${700<^> \circ }$700 circle C, and tensile tests have been performed at ${600<^> \circ }$600 circle C, ${700<^> \circ }$700 circle C and ${800<^> \circ }$800 circle C. A clear anisotropic mechanical response is obtained in the experiments, where the anisotropic effects are larger at high stress levels in creep loadings. To capture this behaviour, a rate-dependent strain based on a double-Norton model has been adopted in the model, by which the creep and mid-life response of the thermomechanical fatigue tests can be simulated with good accuracy.

In this study, a developed co-simulation method, which couples 1D-fluidand 3D-structural models, has been utilised to simulate wear in a hydraulic percussion unit. The effect of wear is generally detrimental on performance and lifetime for such units, but can also cause catastrophic failure and breakdown, requiring a total overhaul and replacement of core components. One experiment of standard straight impact was performed to investigate the tolerance against seizure. The percussion unit was operated at successively increasing operating pressures, and the level of wear was registered at each step, until seizure occurred. The co-simulation model was used to replicate the running conditions from the experiment to simulate the structural response to be used as input for the wear routine to calculate the wear depth. The wear pattern from the simulations corresponds well to the wear pattern from the experiment. Further, the effect of a misaligned impact on wear development was also studied, as this is a loading situation that typically occurs for hydraulic percussion units. The study demonstrates that the simulation method used has a potential for simulating wear and predicting seizure in hydraulic percussion units.

In warm pre-stressing (WPS), the fracture resistance of cracked steel components is raised when subjected to certain temperature-load histories. WPS's beneficial effects enhance safety margins and potentially prolong fatigue life. However, understanding and predicting the WPS effects is crucial for employing such benefits. This study utilised pre-cracked compact tension specimens made from steam turbine steel for WPS and baseline fracture toughness testing. Two typical WPS cycles were investigated (L-C-F and L-U-C-F), and an increase in fracture resistance was observed for both cycles. The WPS tests were simulated using finite element analysis to understand its effects and predict the increase in fracture resistance. A local approach was followed based on accumulative plastic strain magnitude ahead of the crack tip. Since cleavage fracture is triggered by active plasticity, the WPS fracture is assumed when accumulated plasticity exceeds the residual plastic zone formed at the crack tip due to the initial pre-load.

The crack initiation life of a ductile additively manufactured nickel-based superalloy is studied and modeled for low-cycle fatigue and thermomechanical fatigue conditions up to 600 degrees C. Isothermal experiments were performed on smooth specimens at temperatures up to 600 degrees C with different applied strain ranges. Additionally, thermomechanical fatigue experiments at 100-450 degrees C and 100-600 degrees C were performed on smooth specimens under in-phase and out-of-phase conditions. A life prediction model accounting for the anisotropy was developed, where the temperature cycle is accounted with a Delta T$$ \Delta T $$-functionality, generating good agreements with the experiments. The model was also validated on notched specimens undergoing thermomechanical fatigue conditions at 100-500 degrees C using simplified notch correction methods.

A general testing and analysis framework for the Step-Stress fatigue testing method is identified, utilizing interval-censored data and maximum likelihood estimation in an effort to improve estimation of fatigue strength distribution parameters has been performed. The Step-Stress method’s limitations are characterized, using a simple material model that considers cumulative damage to evaluate load history effects. In this way, the performance including cumulative damage was evaluated and quantified using a probabilistic approach with Monte-Carlo simulations, benchmarked against the Staircase method throughout the work. It was found that the Step-Stress method, even when cumulative damage occurs to a wide extent, outperforms the Staircase method, especially for small sample sizes. Furthermore, positive results reaches further than the increase performance in estimating fatigue strength distribution parameters, where improvements in secondary information, i.e. S-N data gained from failure specimens, are shown to be distributed more closely to the fatigue life region of interest.

The Ottosen–Stenström–Ristinmaa (OSR) incremental fatigue damage model is adapted for fatigue-life assessment of integral airframes milled from 7050-T7451 aluminum plates. For validation, variable-amplitude high-cycle fatigue experiments are conducted for circumferentially notched, axisymmetric specimens, and for a geometry similar to an aircraft fuselage frame, with flanges, stiffeners, and web panels. We also describe how the parameters of the OSR model can be modified to account for surface roughness, and for setting acceptable failure probability.

Using the stress intensity factor to describe the stress field around a crack has become widely adopted due to its simplicity. The stress intensity factor depends on the applied nominal stress, the crack length, and a geometrical factor. Geometrical factors can be obtained from handbook solutions or, for complicated cases, through finite element simulations. Carefully defining the geometrical factor with realistic boundary conditions is vital to obtain accurate values for the stress intensity factor. For fatigue life predictions, even a small error in the stress intensity factor may get amplified as the total fatigue life is computed through integration over thousands of crack growth increments. A commonly used specimen geometry for fatigue crack growth testing is the single-edge cracked specimen. For such a specimen, the crack on one side of the geometry introduces bending, which, to some degree, is constrained by the grips that hold the specimen in the testing rig. The effect of bending on the geometrical factor, and consequently on the stress intensity factor, is generally overlooked due to the assumption that the test rig grips are infinitely stiff. Not considering the bending effects could lead to an inaccurate evaluation of the stress intensity factor, especially for long crack lengths. This work investigated the effect of bending on the stress intensity factor for a single-edge cracked specimen. Different grip dimensions were studied to understand the degree of bending and its impact on the stress intensity factor. The work resulted in recommendations for accurately evaluating the stress intensity factor for single-edge cracked specimens.

The crack growth behaviour of the alloy CM 247 LC is investigated for out-of-phase TMF and isothermal tests at the same temperature as the minimum temperature in the TMF tests. The results suggest that it is possible to characterise crack growth behaviour if experimental corrections for crack closure are accounted for. The replication of these experimental tests using a numerical FE-solver results in similar crack growth behaviour, suggesting that the main mechanism in place is plasticity-induced crack closure. A pragmatic analytical model to characterise crack closure including hold time effects is proposed. The comparison of the response from this model with the experimental and numerical results suggests that the proposed analytical model is capable to approximate crack closure effects for cases where substantial creep deformation is to be expected.

The Ottosen–Stenström–Ristinmaa (OSR) incremental fatigue damage model, based on an moving endurance surface centered around a backstress, is adapted for high-cycle fatigue at stress-raisers in AA7050-T7 specimens. Fatigue experiments are carried out for circumferentially-notched, axisymmetric specimens subjected to constant-amplitude (CA) load. The OSR model parameters are fitted to CA fatigue data, showing fair agreement for one set of model parameters across different stress ratios, stress concentration factors, uniaxial stress and biaxial stress. To demonstrate predictive capability, the fatigue life is integrated for an aircraft load spectrum (TWIST), and compared with experimental fatigue life data for holeplate specimens in the literature.

In this study, the mechanical response of an additively manufactured nickel-based combustor alloy, subjected to thermomechanical fatigue (TMF) loadings has been investigated and modelled. TMF tests were performed in both in-phase and out-of-phase conditions with different strain ranges and temperature ranges of 100 degrees C-450 degrees C and 100 degrees C-600 degrees C, respectively. The smooth specimens were manufactured in two different orientations to study the influence of anisotropy, and the specimens were machined to final dimensions with conventional techniques. A constitutive model with focus on describing the mid-life behaviour was developed where the total inelastic strain was divided into one plastic (rate-independent) and one creep (rate-dependent) part, to be able to describe both the rate-dependent effects from TMF conditions as well as rate-independent responses. A cycle jumping procedure was used, which enables to simulate the mid-life response of the material for TMF as well as low-cycle fatigue conditions within three simulated loading cycles.

Exploring crack growth behaviour is needed to establish accurate fatigue life predictions. Cracked specimens were tested under strain-controlled out-of-phase thermomechanical fatigue conditions. The tests included dwell times and three different minimum temperatures. Higher minimum temperature gave faster crack growth rates while the additions of dwell times showed no effects. Crack closure was observed in all the tests where the addition of dwell times and change in minimum temperature displayed little to no effect on crack closure stresses. Finite element models with a sharp stationary crack and material parameters switching provided acceptable predictions for the maximum, minimum, and crack closure stresses.