The fatigue crack propagation threshold ΔKth is of paramount importance for any kind of damage tolerant design, but in contrast to this importance, the determination and application of the parameter ...is not on the firm ground as previously assumed. The paper discusses questions of its experimental determination as well as of its application to components. In both fields, new questions have been raised with the potential to challenge or modify long-standing knowledge. Against this background, the paper is an attempt to systematize the established knowledge as well as questions and open issues.
•Overview on the state-of-the-art of cyclic crack resistance curve.•Guideline for experimental determination of a cyclic R-curve.•Application of the cyclic R-curve in component and materials design.
...The so-called cyclic R curve, i.e. the crack size dependence of the fatigue crack propagation threshold in the physically short crack regime, is a key parameter for bringing together fatigue strength and fracture mechanics concepts. Its adequate determination is of paramount importance. However, notwithstanding this relevance, no test guideline is available by now and only very few institutions have spent research effort on cyclic R curves so far. The aim of the present paper is to give an overview on the state-of-the-art. Besides an introduction into the basic principles, the discussion will concentrate on the experimental determination on the one hand and questions of its application on the other hand.
A continuum ductile damage and failure model coupled with metal plasticity is presented, with the focus on capturing different failure mechanisms and prediction of the strain localisation. The onset ...of diffuse necking and local thinning is observed in the early stage of experiment and it is caused by the very low strain hardening capability of the material under study. Uniaxial tensile tests have been performed on notched and shear samples, specifically designed to cover different stress states by varying both the notch radius and orientation with respect to the sample axis. The experiments have revealed strain localisation caused by material softening. The latter is modelled by a coupled damage accumulation rule whose magnitude is dictated by the actual stress state and strain to fracture. Since the numerical model relies on the fracture strain determined by digital image correlation (DIC), which is dependent on the length scale imposed by the resolution of the DIC grid, an experimentally resolved length scale parameter l is introduced by nonlocal regularization of the material flow, based on large-deformation gradient theory. Apart from its common role of ensuring mesh independency, the length scale parameter obtains a new additional role: it makes the fracture locus calibration procedure robust under changes induced by different spatial averaging of the experimental fracture strain. The measured width of the strain localisation equals to the regularization length scale parameter l used in finite element (FE) simulations, thereby ensuring tight correspondence of physical reality and numerical model on the basis of a measurable quantity. As a result, the nonlocal regularization term prevents the strain in the numerical model to localise to a higher extent than experimentally observed. Eventually, the fracture locus has been constructed as a function of stress triaxiality ratio and Lode angle parameter. The FE-modelling methodology is calibrated by force-displacement curves and surface strain profiles prior to failure, obtained by DIC for a subset of sample geometries. The predictive capability of the proposed methodology is demonstrated by computing and comparing FE results to another experimentally characterized sample, not used in calibration procedure, i.e. dog-bone specimen. It undergoes a complicated loading path different from the paths used in the calibration procedure. It has been shown that regularization of strains and metal plasticity model supplemented by a damage variable are indispensable for an overall agreement of experimental and numerical results. The unique numerical model formulation has proven capable of capturing strongly different ductile failure modes, which are experimentally observed and further discussed.
•Generally applicable model covering a wide range of ductile damage mechanisms.•High predictive capability of the model despite a low number of material parameters.•Both ductile fracture at high and shear fracture at low triaxiality are captured.•Softening process dependent on the loading path to fracture.•Scale of the experimentally determined fracture strain included in the model.
While several different specimen geometries are typically required to calibrate a ductile fracture locus model, this article presents for the first time a calibration methodology that uses one single ...specimen geometry. This is accomplished by a computational framework that combines finite element modelling (FEM) and artificial neural network (ANN). The combinations of the model parameters are used to generate the training database. The local displacement fields and global force-displacement histories are extracted throughout the complete numerical experiment and passed to the ANN. Therefore, the influence of the local stress state on the evolution of the local deformation is implicitly taken into account. The trained ANN is verified by evaluating its predictability of material parameters of FE simulations unseen in the training stage. The experimental data obtained from the shear tensile test using Digital Image Correlation is introduced to the trained ANN to identify the parameter set that predicts the real mechanical response of the shear specimen. Three different ANN architectures with distinguished input representations are studied. It turns out that all of them can acceptably describe the experimental behaviour of not only the calibration specimen but also the specimens not used for training the model.
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•Machine learning supported material characterisation using one single specimen.•Training data includes displacement field evolution extracted from the finite element model.•Local stress state evolution is implicitly contained in the training of the artificial neural network.•Facilitated calibration of stress state dependent and regularized damage model.
A microstructural based creep model applied to alloy 718 Drexler, Andreas; Fischersworring-Bunk, Andreas; Oberwinkler, Bernd ...
International journal of plasticity,
June 2018, 2018-06-00, 20180601, Letnik:
105
Journal Article
Recenzirano
In this work a creep model based on the microstructural evolution of precipitation strengthening metals is presented. The model describes the influence of precipitates on the threshold stress as well ...as the power-law and the exponential creep rates depending on the aging condition. To show the predictive capabilities of this model, it was applied to the nickel based superalloy Alloy 718. This alloy is precipitation strengthened by the γ′ and γ’’ phases. Thermo-kinetic simulations based on a calibrated MatCalc routine were performed to determine the evolution of the volume fraction and mean radius of the precipitates. In addition, sequential creep and tensile tests were performed to characterize the mechanical material behavior. The simulated microstructural evolution and the corresponding measured mechanical properties were used to parametrize the creep model. Finally, the fully parametrized model was applied to simulate deformation mechanism diagrams for different aging conditions. From these diagrams the purely elastic, the power-law and the exponential deformation regimes can be estimated depending on the precipitate volume fraction and mean precipitate radius.
•A microstructurally based creep model accounting for the precipitation strengthening is proposed.•The creep model is applied to the widely used nickel-based superalloy Alloy 718.•Deformation mechanism diagrams for the half-aged and aged material were presented.
•Material model calibration independent of real loading scenario in the structure.•Width of the localisation band captured for failure zones vastly different in size.•Arbitrary mesh applicable in ...complex structural geometry.•High both qualitative and quantitative predictability of the failure scenario.
This article focuses on predicting the instant of failure in a real scale component of complex geometry and loading using a ductile damage model calibrated exclusively on small-scale laboratory specimens of relatively simple shape. The ductile behaviour of a strain hardened aluminium alloy AA1050, formed into a thin-walled component, is modelled by a coupled ductile fracture locus model presented in a recent study (Baltic et al., 2020). The component is exposed to high internal pressure and has a safety vent designed for safe pressure handling. The extensive plastic deformation in the safety vent leads to localised ductile failure occurring at a limit load. The pertaining material parameters were calibrated solely from basic ductile fracture experiments in the preceding work (Baltic et al., 2020), where the bottom section of the thin-walled component was machined into notched and shear samples to characterize different states of stress and to construct a well-defined fracture locus. Although the calibrated material model relies on the local fracture strain measurements, it involves a regularization as a function of the length scale defined as a width of the observed localisation band from Digital Image Correlation (DIC) analysis. In the current study the calibration on small-scale specimens is complemented by a large-scale specimen to determine the length scale correction crucial for capturing the correct width of the localisation band in the analysed structure. This is necessary because the failure initiation zones of the calibration specimens and the real size structure, i.e. their gauge lengths where the localisation band appears, vastly differ in size. Finite element (FE) model results are compared to measurements of the deformation of the aluminium component under pressure and maximum load prior to failure. The numerical and experimental results show an excellent agreement and consistent fracture predictions for various mesh discretizations.
In the present work, the high and very high cycle fatigue (HCF/VHCF) properties of pearlitic steel R350HT are investigated. Servo-hydraulic (cycling frequency between 15 Hz and 60 Hz) and ultrasonic ...(19 kHz) fatigue tests were performed with different surface conditions (fine turned, mechanically polished and electropolished). At a load ratio of R = −1, solely failures in the HCF regime were found for all surface conditions, although runout specimens were loaded with more than 1010 cycles, and crack initiation was exclusively at the surface. At R = 0.1, in contrast, mechanically polished specimens showed internal failures for lifetimes between 106 and 109 cycles. The origin of fracture in the interior was identified as distributed ferritic phases embedded in the harder pearlitic microstructure. Crack initiation from ferritic zones could be explained by their larger size relative to inherent defects such as nonmetallic inclusions and precipitates. The absence of turning grooves in combination with compressive residual stresses near the surface of mechanically polished specimens led to an increase in HCF strength compared with fine-turned and electropolished specimens and facilitated VHCF failure from the interior. No frequency effect was observed when crack initiation originated from turning grooves or artificial surface defects, but slightly higher fatigue strengths were determined for electropolished specimens at 19 kHz than at conventional cycling frequency. This can be explained by the strain-rate sensitivity of ferrite where fatigue cracks preferentially initiate. Additional tests with specimens containing artificially introduced micro-holes were performed to systematically study material's defect tolerance and to quantify the mean-stress sensitivity.
•Evidence of interior crack initiation at ferrite is provided for the first time.•Fish-eye fracture occurred at positive load ratio and not at R = −1.•A frequency effect was observed when ferrite is the origin of fracture.•The effective sizes of ferrite zones served to predict the fatigue limit.
•Determination of the stress intensity factor for w-shape ESE(T) specimens by 3D finite element simulations.•Stress intensity factor solution for a wide range of a/W ratios.•Excellent prediction of ...the stress intensity factor even for small crack lengths.•Investigation of the biaxiality and T-stress for w-shape ESE(T) specimens.
The evolution of the irradiation embrittlement of nuclear pressure vessel steels is determined experimentally using special specimen geometries. Such sub-sized w-shape eccentrically-loaded single edge cracked tension specimens are employed due to mounting and handling requirements. Stress intensity factor solutions are available in the literature for this geometry; however, they are inaccurate for smaller crack lengths. Therefore, new approximate solutions are presented in this work to determine the stress intensity factor for such a w-shape tension specimen for a wide range of crack lengths (0.05 < a/W < 0.9). Furthermore, the T-stress constraint is investigated for this special specimen design and compared with comparable designs provided in standards.
•Crack propagation tests on railway axles with variable load sequences.•Real time crack growth measurement during fatigue tests.•Residual stress measurement during crack propagation.•Influence of ...residual stresses on residual lifetime.
A wheel-set axle is located in the unsprung part of a rail vehicle and takes a supporting role in the running gear. If the wheel-set axle fails, the train will inevitably derail. Depending on the time of failure, this can lead to catastrophic consequences. For this reason, a wheel-set axle is a highly safety-critical component of railway vehicles. Detailed knowledge of crack propagation in wheel-set axles is therefore of great importance for a modern safety concept. To analyse the material behaviour under external loads approximated from operating conditions, experimental investigations were carried out on 1:1 specimens. This article presents the results of these experimental investigations carried out within the framework of the research project “Eisenbahnfahrwerke 3 (EBFW 3)” – “Probabilistic Fracture Mechanical Concept for the Assessment of Railway Wheel-sets”.
In many applications metal foils are part of multifunctional composites exhibiting a wide range of (thermo-) mechanical fatigue loading. Data about low cycle fatigue of metallic foils subjected to ...tension-compression fatigue is sparse due to geometric limits. In this contribution, detailed information about design, tension-compression fatigue testing and data extraction of a layered sample are given. Fatigue failure of both copper and Pre-Preg are analysed using scanning electron microscopy and computed tomography. Numerical models are used to calculate the elastic and plastic strains for the copper foils within the composite sample, allowing a correlation to the observed fatigue life.
•Fatigue behaviour is crucial to microelectronics, aviation and spacecraft.•A sample enabling R-1 fatigue testing of metallic foils was designed.•Using synchrotron radiation, neglectable residual deviatoric stresses were found.•SEM and CT analysis of fatigued samples were performed.•Finite elements method allowed fatigue life prediction.
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