LCF Regime Research Articles

Constitutive modelling and damage prediction of AlSi10Mg alloy manufactured by SLM technology with emphasis on ratcheting in LCF regime

Constitutive modelling and damage prediction of AlSi10Mg alloy manufactured by SLM technology with emphasis on ratcheting in LCF regime

Influence of the cutting method on the fatigue life and crack initiation of non-oriented electrical steel sheets

Influence of the cutting method on the fatigue life and crack initiation of non-oriented electrical steel sheets

On the effect of Cr/CrN nanolayered coating deposited by Arc-PVD method on axial fretting fatigue behavior of Al7075-T6 alloy

On the effect of Cr/CrN nanolayered coating deposited by Arc-PVD method on axial fretting fatigue behavior of Al7075-T6 alloy

Microstructure- and plasticity-based fatigue and defect tolerance assessment of age-hardenable Al-Si cast alloys in LCF and HCF regime

• Characterization and comparison of quasi-static and cyclic stress-strain behavior of AlSi7Mg0.3, considering the process-induced effects of casting process (gravity die, sand), HIP densification and solidification rates. • Correlation of quasi-static and cyclic deformation behavior with fatigue behavior by means of S-N (Woehler) curves and non-dimensional (relative) S-N curves based on the estimated fatigue limit of Murakami-Noguchi approach for fatigue damage tolerance assessment. • Understanding of the increased LCF damage tolerance and the similar HCF damage tolerance of sand castings compared to die castings. • Analysis of the contribution of the different microstructural constituents, i.e., primary Al matrix and Al-Si eutectic, on the cyclic deformation behavior by means of local cyclic indentation testing. Due to their excellent strength-to-weight ratio and near-net-shape manufacturing possibility, precipitation hardening Al-Si cast alloys are promising candidates for cost-efficient lightweight components in the automotive and railway industry. In this work, the microstructure and the fatigue properties of cast AlSi7Mg0.3-T6 (A356-T6) were analyzed, whereby the influence of a variation in solution annealing, hot isostatic pressing (HIP) densification, solidification rates as well as casting processes, i.e., gravity die and sand casting on the fatigue behavior and fatigue damage tolerance was investigated. Sand castings exhibit a decelerated fatigue crack initiation and enhanced LCF damage tolerance compared to die castings.

Generalization of the Weibull probabilistic compatible model to assess fatigue data into three domains: LCF, HCF and VHCF

• Fatigue models are classified in three classes according with their possible design application. • A new generalized parameter (GRV) is proposed to replace the stress range as driving force. • The probabilistic Castillo-Canteli fatigue model is revisited and improved to include the LCF regime. • The new probabilistic GRV -N field is applied to evaluate experimental results and discussed. In this work, three classes of fatigue models are reviewed according to the fatigue regimes commonly considered in the current components design. Particular attention is devoted to the so-called Class III fatigue models, covering the three fatigue regimes, namely, LCF, HCF and VHCF. The applicability and limitations of the proposed analytical sigmoidal solutions are discussed from the viewpoint of practical design. The compatible Weibull S-N model by Castillo and Canteli is revisited and improved by considering a new reference parameter GP = E· σ M · (dε/dσ)| M as the driving force alternative to the conventional stress range . In this way, the requirement, σ M ≤ σ u , according to the real experimental conditions, is fulfilled and the parametric limit number of cycles, N 0 , recovers its meaning. The probabilistic definition of the model on the HCF and VHCF regimes is maintained and extended to the LCF regime. The strain gradients may be calculated from the monotonic or cyclic stress–strain curve of the material although a direct derivation from the hysteresis loop is recommended. Some Class III fatigue models from the literature and another one improved by the authors are applied to the assessment of one experimental campaign under different stress ratios conditions and the results compared accordingly. Finally, the new probabilistic GP-N field is evaluated. The results confirm the practical confluence of the stress- and the strain-based approaches into a single and advantageous unified methodology.

Performance-Related Characterization of Forming-Induced Initial Damage in 16MnCrS5 Steel under a Torsional Forward-Reverse Loading Path at LCF Regime.

Forming technology and in particular cold forward rod extrusion is one of the key manufacturing technologies with regard to the production of shafts. The selection of process parameters determines the global and local material properties. This particularly implies forming-induced initial damage in representation of pores. On this background, this study aims on describing the influence of these pores in the performance of the material 16MnCrS5 (DIN 1.7139, AISI/SAE 5115) under a torsional load path in the low cycle fatigue regime, which is highly relevant for shafts under operation conditions. For this purpose, the method of cyclic forward-reverse torsional testing was applied. Additionally, intermittent testing method and the characterization of the state of crack growth using selective electron microscopy analysis of the surface were combined. A first attempt was made to describe the influence of forming-induced initial damage on the fatigue performance and the crack growth mechanisms. The correlation of fatigue performance and initial damage was contiguous in the sense that the initial damage corresponds with a decrease of material performance. It was concluded that the focus of further investigations must be on small crack growth and the related material changes to identify the role of initial damage under cyclic loads.

Multiaxial Fatigue and Cracking Orientation of Forged AZ80 Magnesium Alloy

The effect of the multiaxiality and proportionality of loading on the cyclic behaviour and early cracking behaviour was studied for closed die forged AZ80 Mg alloy. Several different loading paths were presented, uniaxial and biaxial with varying levels of non-proportionality. In multiaxial loading the effect of proportionality is only present on the shear response of the material and increasing levels of non-proportionality is detrimental to the fatigue life. When compared with proportional loading, a 50% reduction in life was observed under 90° out of phase non-proportional loading. The pure axial cracking behaviour is dominated by transverse cracks along the plane of maximum normal stress, whereas in pure shear the material exhibited longitudinal cracking (shear dominated) behaviour in LCF regime and helical cracking (normal stress dominated) in the HCF regime. The synergistic effects of each individual uniaxial cracking modes contribution towards the combined macroscopic cracking behaviour in multiaxial loading was investigated, and was dominated by transverse cracking when the shear strain energy density (SED) contribution to the total SED was low, and mixed cracking when the shear SED contribution was high.

Anisotropy in the quasi-static and cyclic behavior of ZK60 extrusion: Characterization and fatigue modeling

Abstract The quasi-static and strain-controlled fatigue characteristics of ZK60 extrusion have been investigated along three different directions: the extrusion direction (ED), the radial direction (RD), and 45° to the extrusion direction (45°). The quasi-static response showed symmetric behavior for the samples tested along RD and 45°, whereas the ED samples manifested completely asymmetric behavior. Although the ED samples exhibited longer fatigue lives than the RD and 45° in the high cycle fatigue, the fatigue lives in the low cycle fatigue regime were similar. The texture measurement indicated a sharp basal texture along ED, explaining its asymmetric behavior. Higher tensile mean stress and less dissipated plastic energy per cycle for the ED samples, acting as two competing factors, were the principal reasons for exhibiting fatigue responses identical to those of RD and 45° in the LCF regime. The fracture surface in the ED direction was dominated by twin lamellae and profuse twinned grains, whereas that in RD was dominated by slip bands. Finally, Smith-Watson-Topper and Jahed-Varvani models were employed to predict the fatigue lives along all directions using a single set of material parameters.

Fatigue behavior of AA7075 aluminium alloy severely deformed by equal channel angular pressing

Abstract Fatigue failure is the main reason for the fracture of constructions; therefore, improving the fatigue strength of the engineering alloys such as Al-7075 aluminium alloy by some methods such as equal channel angular pressing (ECAP) is of great importance. According to this potential, fatigue behavior of Al-7075 alloy subjected to ECAP process was investigated in this research. The ECAP process was performed at ambient temperature up to 4 passes. Rotating bending fatigue test was conducted on ECAPed specimens and microstructure and fracture surface of the specimens were analyzed using transmission and scanning electron microscopes. Microstructural characterization revealed that the ultra-fine grains (UFG) with mean size of about 600 nm were formed during 4 passes of ECAP. Experimental results showed that fatigue strength of Al-7075 alloy in annealed and solution treated conditions increased significantly in both low (LCF) and high cycle fatigue (HCF) regimes after 1 pass of ECAP. Although, the obtained improvement by 1 pass of ECAP disappeared in LCF regime with increasing in the number of passes, this improvement sustained in HCF regime. Fractography analysis showed that fatigue fracture surface of 1 pass sample composed of granular and planer regions due to the bimodal microstructure consisting of fine and coarse grains. Granular region enhanced fatigue strength in HCF regime and planer region that was trace of shear bands formed during ECAP, improved fatigue strength in LCF regime. Fatigue fracture surface of 4 passes sample with UFG microstructure showed only granular fracture manner.

Fatigue Lives of Power Plant Structures Due to Load Sequence Effects Originating from Fluctuating Production of Renewable Energy

The change in operation of power plants due to the increasing use of renewable energies causes additional stresses to the components by a high number of smaller load cycles. This fact results in a demand for validated new approaches to estimate fatigue life especially for welded joints that are the weak parts within the piping. The components are operated in the LCF regime where short fatigue crack growth determines the life. Therefore, a non-linear fracture mechanics based approach is appropriate. For the development and validation of the model, an experimental campaign was performed including fatigue tests of smooth specimens with various microstructures of X6CrNiNb18-10 (AISI 347) as well as specimens containing mechanical and microstructural notches. Experiments are performed with all types of specimens with an increasing complexity from constant to variable amplitude loading, the latter also applied as pseudo-random sequence derived from measurements in power plants. The developed non-linear fracture mechanics based model uses the effective cyclic J-Integral normalized to the crack length to replace crack growth calculation by a linear damage accumulation. To consider the loading history an algorithm for the calculation of crack opening and crack closure is used. The advantages of this approach are evident when comparing calculated with experimentally determined fatigue lives. Remaining differences serve for further considerations of how to improve the life prediction for variable amplitudes.

Analysis of the energy dissipation in multiaxial fatigue tests of AISI 304L stainless steel bars

Abstract In the recent past, the specific heat loss per cycle (Q parameter) was used to synthesise in a single scatter band approximately 140 uniaxial fatigue test results obtained from constant amplitude, push-pull or torsional, stress- or strain-controlled fatigue tests on plain and notched AISI 304 L stainless steel specimens. It was also demonstrated that the Q parameter can be evaluated during a fatigue test by measuring the cooling gradient after a test stop at the hot spot region of the specimen surface. In the present contribution, the specific heat loss Q has been adopted for the first time to correlate the fatigue strength of AISI 304L specimens subjected to low-cycle multiaxial fatigue loadings. Completely reversed (R = -1) pure bending, pure torsion and combined bending and torsion tests were carried out on hourglass-plain specimens by using two servo hydraulic actuators. In-phase (ϕ = 0°) as well as out-phase (ϕ = 90°) multiaxial fatigue tests were performed by adopting two different biaxiality ratios. In addition, thin-walled tubular specimens were tested under completely reversed tension and torsion fatigue loadings for comparative purposes. Afterwards, all fatigue test results were expressed in term of specific heat loss and compared with the scatter band previously evaluated for plain and notched stainless-steel specimens subjected to uniaxial loading. In the LCF regime the scatter band relevant to uniaxial data correlated well the multiaxial fatigue data.

Short cracks growth in low cycle fatigue under multiaxial in-phase loading

Abstract Crack propagation in full plastic regions is one of the main aspects of fatigue life design for components subjected to high strain concentrations. Residual life assessment for those components, in which high stress concentrations cause cyclic yielding of the material, can be considered as a crack propagation problem by assuming crack growth from the first load cycle. The aim of this paper is to study the crack growth behaviour of short cracks in low cycle fatigue under a multiaxial loading condition. In particular, a series of experiments in LCF regime at room temperature was performed to determine crack growth during axial, torsional and axial-torsional tests. Crack advancement was checked with the plastic replica technique, during test interruptions. Experimental results were compared, in terms of crack growth rates and fatigue life assessment, with those analytically calculated, considering different multiaxial fatigue parameters introduced in an exponential crack growth law and an approach based on the multiaxial cyclic J-Integral concept.

Corrosion fatigue assessment of creep-resistant magnesium alloys DieMag422 and AE42

Abstract Corrosion fatigue behaviors of creep-resistant magnesium alloys DieMag422 and AE42 were characterized by means of lifetime- and process-oriented approaches. Fatigue properties of both materials were assessed in constant amplitude tests at several adjusted corrosion rates. Deterioration and cracking mechanisms were identified and monitored by means of corrosion potential response analysis during constant amplitude tests. Reduction of corrosion fatigue strength could be quantitatively correlated with adjusted corrosion rates. Analysis of corrosion potential response revealed characteristic deterioration and cracking mechanisms for LCF and HCF regime.

Fatigue behavior and retained austenite transformation of Al-containing TRIP steels

Abstract In material selection for the design of advanced lightweight automotive steel components, fatigue performance is of particular significance. High strength TRIP steels offer very good cyclic behavior, especially under cyclic plastic strains, which is assisted by the Transformation Induced Plasticity effect (TRIP). In the present study the TRIP effect has been quantified under both elastic (HCF regime) and plastic (LCF regime) cyclic strains for two Al-containing TRIP steels with similar chemical composition and different initial retained austenite (RA) content. The results illustrate that transformation behavior differs for the two materials under elastic and plastic cyclic straining and fatigue behavior is in both cases linked to the amount of RA transformation. The latter is discussed in the paper considering relevant RA microstructural aspects like content and particle size. In the investigation the fatigue crack initiation resistance of the materials has been experimentally evaluated, resulting in different damage tolerance ability for the two steels. A numerical simulation is developed to determine the local strains at the notch tip under monotonic loading conditions and is used with the LCF material characteristics to discuss the differences obtained in crack initiation resistance.

Fatigue crack growth behaviour in the LCF regime in a shot peened steam turbine blade material

In this study, short fatigue crack initiation and early growth behaviour under low cycle fatigue conditions was investigated in a shot peened low pressure steam turbine blade material. Four different surface conditions of notched samples have been considered: polished, ground, T0 (industry applied shot peened process) and T1 (a less intense shot peened process). Fatigue crack aspect ratio (a/c) evolution in the early stages of crack growth in polished and shot peened cases was found to be quite different: the former was more microstructure dependent (e.g. stringer initiation) while the crack morphology in the shot peened cases was more related to the shot peening process (i.e. surface roughness, position with respect to the compressive stress and strain hardening profiles). Under similar strain range conditions, the beneficial effect of shot peening (in the T0 condition) was retained even at a high strain level (??11=0.68%), Nf, ground< Nf, T1 < Nf, polished < Nf, T0. The a/c evolution effects were incorporated in K-evaluations and used in calculating da/dN from surface replica data. Apparent residual stress (based on crack driving force ?K difference) was applied to describe the benefit of shot peening and was seen to extend significantly below the measured residual stress profile, indicating the importance of the strain hardening layer and stress redistribution effects during crack growth.

Fatigue life and cyclic material behavior of butt-welded high-strength steels in the LCF regime*

Abstract Fatigue life approach methods based on general local concepts for welded joints assign universal material properties to geometrically well-defined weld toes or weld roots, in order to evaluate local stresses and strains, ignoring any metallurgical discrepancies. In this study, analysis of the main influences on these concepts, linear damage accumulation and cyclic material behavior, of the high-strength and ultra-high-strength steels S960QL, S960M and S1100QL was performed for the base material and for butt-welded specimens. Results of stress-controlled tests under constant amplitude loading (CAL) and variable amplitude loading (VAL) show that a linear damage accumulation with a damage sum of Dal = 0.5 is conservative. Effects of the geometrical and metallurgical notch on the cyclic material behavior are shown by strain controlled testing of both base material and welded specimens. Cycles to crack initiation, presented in strain-life curves, are consequently reduced in the as-welded state. Cyclic stress-strain curves show a cyclic softening of the base material as well as of the welded joints. Furthermore, the advantages of the ultra-high-strength steel S1100QL disappear in welded connections.

Fatigue Behavior of Precipitation Hardening Alloys in the LCF and VHCF Regime

LCF/HCF strength of precipitation hardening alloys is primarily controlled by its heat treatment condition. However, for a nickel-based superalloy and a wrought aluminium alloy it will be shown, that the VHCF behavior cannot solely be explained by the precipitation morphology. Damage accumulation is dominated by microstructure related slip localization, grain morphology and microstructural flaws. In contrast to the LCF behavior, the prediction of cyclic strength in the VHCF regime requires a detailed analysis of the competing microstructural crack initiating characteristics. Hence, new fatigue life prediction models have to be developed, which consider a statistical analysis of the failure-relevant inhomogeneities. In the case of the two materials studied, VHCF behavior is dominated by isolated and inhomogeneously distributed irreversible slip accompanied by a low dislocation density. The formation of single slip bands in favorably oriented grains in Nimonic 80A results in a decrease of the VHCF strength for the peak-aged condition. The overaged condition shows better VHCF strength due to a more homogeneous distribution of slip bands, as dislocations pile up at the overaged precipitates due to the very low strain amplitudes, while in the LCF regime the Orowan mechanism results in a weaker cyclic strength compared to the peak-aged condition. Crack initiation of the aluminium alloy EN AW-6082 on the one hand depends on the size and distribution of primary intermetallic particles of the Al-Fe-type acting as local stress raiser embedded in a strong matrix in case of the peak-aged condition. In contrast, such stress peaks are less failure-relevant due to the softer solid solution depleted matrix. Hence, the heat treatment alone does not define VHCF behavior.

Geometrical Influence of a Butt Weld in the Low Cycle Fatigue Regime

This contribution deals with the low cycle fatigue behaviour of butt-welded joints of austenitic stainless steel (Type 347). The influence of the geometrical notch of a butt weld on the fatigue life in LCF regime is of particular interest in this context. For the purpose of comparison different weld geometries – as-welded and machined – are investigated. Fatigue tests reveal that the weld notch does not always constitute the failure location in the LCF regime. Accompanying numerical simulations are based on 3D scans with a resolution of 30 Pm in order to receive accurate local strain amplitudes at the weld notch or rather the failure location. The combination of experiments and numerical simulations results in local strain life curves.

A methodology to predict residual life in LCF regime

In this paper a methodology to calibrate a nonlinear constitutive model, implemented in a finite element commercial code and able to correctly predict the cyclic behaviour of material, is proposed. This model allows to take into account all the main phenomena that occur when a metallic material is subjected to cyclic loadings, in particular cyclic hardening or softening, Bauschinger effect, ratcheting and shakedown. High Cycle Fatigue and Low Cycle Fatigue tests under different strain levels have been performed on an aluminum alloy for high temperature applications, and the obtained experimental data have been used to determine the four constitutive parameters of the isotropic and kinematic hardening parts of the model. The comparison between the number of stabilized cycles predicted by numerical simulations and experimental data has been very satisfying. Using a self-made post-processing software, the Basquin-Manson-Coffin, Neu-Sehitoglu, Chaboche and Skelton damage models have been applied to determine the residual life of the specimen and to compare the results.

Comparing fatigue behavior of titanium and nickel-based alloys

Abstract Titanium alloys are in competition with nickel-based alloys particularly in aerospace applications, where the lower density of titanium alloys is of advantage. For these applications fatigue strength is of primary importance. When discussing the influence of microstructure on fatigue resistance, it is important to distinguish between fatigue crack initiation, the propagation of microcracks and the propagation of macrocracks. In terms of microstructure, this paper addresses those parameters, which can be readily varied during the processing of a part: i.e. in titanium alloys the choice between a duplex (bi-modal) and a fully lamellar microstructure, in a given nickel alloy it is usually the grain size and the population of the hardening γ′ precipitates. Titanium alloys with duplex microstructures are found to be superior to lamellar structures with respect to fatigue at high stress amplitudes (LCF regime) and to microcrack propagation. At low stress amplitudes and with respect to macrocrack propagation, the tendency is reversed. In nickel alloys, a dominating effect of crack nucleation at hard particles is observed, which masks the positive effects of increased yield stress by grain refinement or γ′ modifications. Both variables also have little influence on microcrack propagation, while increasing the grain size tends to reduce the propagation rate of macrocracks like in most other metallic materials.