VHCF Regime Research Articles

Very high cycle fatigue of austenitc stainless steels and their welds for reactor internals at ambient trmperature and 300 °C

The fatigue assessment of safety relevant components is of importance for ageing management with regard to safety and reliability of nuclear power plants. Austenitic stainless steels are often used for reactor internals due to their excellent mechanical and technological properties as well as their corrosion resistance. During operation reactor internals are subject to mechanical and thermo-mechanical loading which induce low cycle (LCF), high cycle (HCF) and even very high cycle (VHCF) fatigue. While the LCF behavior of austenitic steels is already well investigated the fatigue behavior in the VHCF regime has not been characterized in detail so far. Accordingly, the fatigue curves in the applicable international design codes have been extended from originally 106 to the range of highest load cycles up to 1011 load cycles by extrapolation. Nevertheless, the existing data base for load cycles above 107 is still highly insufficient. The aim of the cooperative project of the Institute of Materials Science and Engineering (WKK) at RPTU Kaiserslautern-Landau, Materials Testing Institute (MPA) Stuttgart and Framatome GmbH, Germany is to create a comprehensive database up to the highest load cycles N = 2·109 for austenitic stainless steels and their welds at ambient and elevated temperature.

Orientation dependence of nickel-based single crystal superalloys in VHCF regime

Aimed at research the deformation orientation in VHCF regime under different temperatures, fatigue behavior of [111]-oriented DD6 single crystal superalloy at 1000 °C and 760 °C was studied. Using the stress range criterion, the [111] orientation shows higher fatigue resistance. This difference decreases with lower stress ranges and almost vanishes near 109 cycles. In contrast, the [111] orientation has lower fatigue resistance than the [001] orientation by using the total strain range criterion. Dislocations are restricted from shearing the γ’ phase due to the lower octahedral resolved shear stress in the [111]-oriented, decreasing the deformation in γ’ phase. Combined with distinctive cross-slip behavior, this further increased the deformation along the γ-channel, significantly enhancing deformation inhomogeneity between the γ’ phase and γ-channels. At elevated temperatures, thermally activated dislocation climb and higher climb stress factor lead to a greater reduction in fatigue performance for [111] orientation at lower stress level.

Evaluation of matrix crack growth in interlaminar toughened quasi-isotropic carbon-fiber reinforced plastic laminates up to the very-high cycle regime by ultrasonic fatigue testing

It is time-consuming to conduct conventional hydraulic fatigue testing up to the very-high cycle fatigue (VHCF, N (number of cycles) ≥ 108) regime. Ultrasonic fatigue testing has been proposed as an accelerated alternative method. In this study, ultrasonic fatigue testing was conducted on interlaminar toughened quasi-isotropic carbon-fiber reinforced plastic (CFRP) laminates to evaluate the characteristics and mechanism of fatigue up to the VHCF regime. Hydraulic fatigue tests were conducted at the same stress ratio as a comparison. The resulting S–N diagrams showed a lower fatigue strength and different fatigue mechanisms for specimens subjected to ultrasonic fatigue tests compared with those subjected to hydraulic fatigue tests. The observed growth of the matrix crack density in each laminate layer was used to perform a shear-lag analysis to calculate the energy release rate considering off-axis matrix cracks. The results suggested different damage growth behaviors for specimens subjected to ultrasonic and conventional fatigue tests.

Notch effect in very high-cycle fatigue behaviour of a structural steel

The presence of a notch or a detail which leads to stress concentration due to complex geometries has a considerable impact on the fatigue life of engineering structures. Therefore, on the one hand, the notch effect must be investigated and considered in the fatigue assessment of these structures. On the other hand, in the last decades, an interest in studying the very high-cycle fatigue regime (VHCF) has been perceived, due to the extension of the service life of engineering components beyond the 10 million cycles. Thus, the notch effect in the VHCF regime was analysed for the S690 structural steel. Therefore, an analytical approach to design notch specimens was developed and a numerical simulation was performed to assess it. Then, an experimental campaign with smooth and notched specimens with two different stress concentration factors was performed in an ultrasonic fatigue machine to assess the VHCF behaviour. The mechanisms of failure and fracture surfaces were also investigated. It was observed a fully notch sensitivity as well as the dominance of surface crack initiation for the S690 steel in the VHCF regime.

Influence of elevated temperature on the very high cycle fatigue properties of bearing steels

The influence of temperature on the very high cycle fatigue properties of three high carbon chromium bearing steels (52100, 52100-VIM/VAR and M50-VIM/VAR) is investigated. Ultrasonic fatigue tests up to more than 1010 cycles are performed at 22 °C, 200 °C and 300 °C. At all temperatures, the dominant failure mechanism is interior defect-initiated cracking. Nonmetallic inclusions are the origin of fracture for 52100 and carbides for 52100-VIM/VAR and M50-VIM/VAR. The fatigue strength (fracture probability of 50 % at 2 × 109 cycles) at 300 °C decreases by 56–57 % compared with 22 °C for both 52100 steels and by 41 % for M50. 52100 steels soften at elevated temperature due to low tempering temperature, but M50 is thermally stable. The hardness decrease at elevated temperature is more pronounced for 52100 steels than for M50.The results are evaluated applying fracture mechanics principles. Based on the area-parameter model, an equation to predict the fatigue strength in the VHCF regime at different temperatures is proposed.

Experimental investigation and crystal plasticity simulation for the fatigue crack initiation of the equiaxed Ti-6Al-4V alloy in the very high cycle regime

This work investigated the fatigue crack initiation mechanism of the equiaxed Ti-6Al-4V alloy in the very high cycle fatigue (VCHF) regime. Ultrasonic fatigue experiments were carried out on Ti-6Al-4V specimens and a detailed analysis of the failure mechanism was conducted by means of scanning electron microscopy (SEM). The localized cyclic plastic response of the alloy was investigated using the crystal plasticity finite element (CPFE) method to further clarify the fatigue crack initiation process in the VHCF regime, and the CPFE models with varying transformed β(βt) phase volume fractions were presented to investigate the impact of βt phase on the VHCF behavior. The simulation results indicate that the proposed CPFE model can effectively captures the microstructural plastic deformation mechanisms, such as stress concentration near the phase boundaries and plastic accumulation. And the βt phase significantly affects the VHCF behavior of the alloy.

Fatigue behavior and lifetime assessment of an austenitic stainless steel in the VHCF regime at ambient and elevated temperatures

AbstractWhile the LCF behavior of austenitic steels used in nuclear power plants is already well investigated, the VHCF regime has not been characterized in detail so far. For this, fatigue tests on the metastable austenitic steel AISI 347/1.4550 were performed with a servo‐hydraulic testing system at test frequencies up to 980 Hz and with an ultrasonic fatigue testing system at a test frequency of 20,000 Hz. To compare these test results to the ASME standard fatigue curve (total strain amplitude vs. load cycles to failure), a fictitious‐elastic and an elastically plastic assessment method was used. The elaborated elastic–plastic assessment method generates good results, while a purely elastic assessment in the VHCF regime, commonly used in literature, leads to significantly nonconservative results. Moreover, phase transformation from metastable austenite into stable α′‐martensite can take place, and no specimen failure occurs in the VHCF regime. Consequently, for this material, a real endurance limit exists.

Experimental Study of VHCF Fractographic Features of Conventionally and Additively Manufactured Steels

Materials produced by additive manufacturing (AM) have been extremely related to literature. However, there is still unconsolidated knowledge about the fatigue life and respective mechanisms of initiation of cracks predominant in the VHCF regime for these materials. What has been observed in materials produced by conventional routes is that fatigue cracks tend to nucleate from intrinsic defects of the material located internally or in subsurface regions. The change in the evolution process of fatigue cracks leads to the formation of a characteristic morphology on the fracture surface, known as “fish-eye”. Another widespread aspect observed on the fracture surfaces is the formation of a fine granular area (FGA) nearby the initiation sites. This work aims to investigate the mechanisms of crack nucleation in VHCF of two distinct materials: conventional steel, DIN 34CrNiMo6 and AISI 316L stainless steel produced by L-DED. The ultrasonic tests were carried out at a frequency of 20±0,5 kHz and R= -1. The S-N curves were obtained and fracture surfaces were analyzed, fish-eye and FGA formation was verified. FGA sizes were compared to values estimated by empirical equations. FGA and fish-eye sizes were related to stress amplitude and maximum stress intensity factor (SIF).

Parametric method for the assessment of fatigue damage for marine shaft lines

An original parametric method is introduced for the assessment of the fatigue life of marine shaft lines. The particularity of the method lies in the fact it introduces a relevant load modelling around a limited number of parameters specific to marine shaft lines while depicting the loading complexity (multiaxiality, mean stress effect, non-proportionality of the loading path). The method is designed to allow for assessments, in both in-use and pre-design phase shafts, towards a particular fatigue damage mode. The observations made in this study show two damage modes. On one hand, a damage mode issued from multiaxial cycles and associated with ship maneuvers, corresponding to HCF regime. On the other hand, there is a damage mode in the VHCF regime resulting from bending stress cycles due to the structure rotary bending.

An experimental investigation of fatigue performance and crack initiation characteristics for an SLMed Ti-6Al-4V under different stress ratios up to very-high-cycle regime

The fatigue behavior of SLMed Ti-6Al-4V has been intensively investigated in recent years. However, the effect of stress ratio has not been comprehensively studied, especially in VHCF regime. In this paper, the fatigue performance and crack initiation characteristics up to VHCF regime for an SLMed Ti-6Al-4V at various stress ratios (R = −1, −0.5, 0.1 and 0.5) were investigated and the effects of stress ratio were substantially discussed. The stress amplitude decreases with the increase of stress ratio with the order of: σa (R = −1) > σa (R = −0.5) > σa (R = 0.1) > σa (R = 0.5). Internal crack initiation is dominant in HCF and VHCF regimes under the four stress ratios, although surface crack initiation rarely occurs in HCF regime at R = −1 and 0.5. Whether the crack initiates from subsurface or from interior is affected by the applied maximum stress, and the crack initiation is in relation not only to defect size but also to defect location. The nanograins in RA are evidently formed in VHCF regime at R = −1, −0.5 and 0.1, but not at R = 0.5, which is well explained by the NCP model.

Crack initiation mechanism of titanium alloy in very high cycle fatigue regime at 400℃ considering stress ratio effect

Fatigue behavior of titanium alloy with various stress ratios was investigated at 400℃ in VHCF regime. Cracks initiation depends on the formation of facets and is related to the maximum shear stress. The formation of facets is induced by basal or prismatic slips with the high Schmid factor inside αp grains. The morphology of αp grains will affect the crack initiation. The high-density GNDs are found to be accumulated along with the basal or prismatic slips, then form subgrain boundaries (sub-GBs) inside αp grains. With the cyclic loading, the created sub-GBs will further form facets, thus initiating the microcracks.

Influence of the stress gradient at the notch on the critical distance and life prediction in HCF and VHCF

The stress gradient associated with the notch geometry was integrated into the critical distance estimate procedure and formed the Stress Gradient Method (SGM) proposed in this study. The critical distance was discovered to be highly related to the product of the fatigue life and the stress gradient. The fatigue life of varied notched specimens was then pr edicted using SGM and three other typical critical distance determination methods. The results showed that using the SGM was beneficial for enhancing the prediction accuracy of the theory of critical distance (TCD) in the HCF regime. Furthermore, it is acceptable for applying the SGM in the VHCF regime with mean stress.

Creep-fatigue voids and sub-grain boundaries assisted crack initiation for titanium alloy in VHCF regime with high mean stress at 400°C

Very high cycle fatigue failure of bimodal titanium alloy at 400 °C and high mean stress has been investigated. Many circular creep-fatigue voids (CFVs) were found underneath the crack initiation zone, which were distributed in the β transformed matrix and near the grain boundaries (GBs) of primary α grains. By the energy dispersive spectroscopy (EDS) element mapping, the oxygen element was found to be enriched around the CFVs. In addition, many sub-GBs of small-angle GBs could be observed inside the primary α grains. Based on these, a new mechanism of CFVs and sub-GBs assisted crack initiation and early propagation in bimodal titanium alloys was proposed in very high cycle fatigue regime.

Experimental insight on the fatigue resistance of FV520B-I stainless steel under corrosive environments

Corrosion fatigue is the main concern for FV520B-I steels in broad industries. However, the corrosion fatigue behavior has not been elucidated clearly. In this paper, the designed corrosion fatigue tests in air, water, 3.5% and 5% NaCl solution are carried out. It is found that with the transformation of test environments from the air to NaCl solution, the fatigue life decreases from VHCF to HCF; and in the water, it spans two ranges. With the comprehensive combination of test data and the Basquin formula, the fatigue S-N formula in the different environments are achieved, the undetermined formula parameters for different test environments have been estimated, which can reflect the influence of different corrosive mediums on the fatigue resistance. The observations of post-mortem specimens clearly show that, in the water, both the internal failure and surface failure are detected when the fatigue life reaches VHCF regime, and with the fatigue life decreases to HCF regime, the fatigue failure is mainly from the surface. While, in the NaCl solution, and corrosion fatigue failure only tends to originate from the surface crack, obvious corrosion traces are observed around the region of crack initiation sites.

Fatigue life assessment in the very high cycle regime of AISI 316L stainless steel processed by L-DED additive manufacturing

Additive manufacturing technologies have arisen great industrial interest for manufacture of components and final parts intended for applications in several sectors of the industry. Most of these components are designed to have a service life greater than 107 cycles, making the analysis of the fatigue behaviour in the very high cycle regime (VHCF) an essential design criterion. The 316L stainless steel is one of the most processed and discussed in the literature. However, there is still no consolidated knowledge about the fatigue life of this material manufactured by different available techniques and their respective mechanisms of initiation of cracks predominant in the VHCF regime. The present work is intended for an experimental study of the VHCF failure mechanism of AISI 316L steel after additive manufacturing by laser directed energy deposition. In order to also identify the effect of post-processing steps, two different conditions of the material (as built and heat treated) were analysed. The specimens were submitted to ultrasonic tests with a target number of 109 cycles, at a frequency of 20±0,5 kHz and R=-1. The fracture surfaces were analysed later and the influence of heat treatment on the population of metallurgical defects, “fish-eye” formation and material's fatigue life were verified.

Internal Crack Initiation and Growth Starting from Artificially Generated Defects in Additively Manufactured Ti6Al4V Specimen in the VHCF Regime.

The aim of the present work was to investigate the ‘fine granular area’ (FGA) formation based on artificially generated internal defects in additively manufactured Ti6Al4V specimens in the early stage of fatigue crack growth in the ‘very high cycle fatigue’ (VHCF) regime. Fatigue tests were performed with constant amplitude at pure tension-compression loading (R = −1) using an ultrasonic fatigue testing setup. Failed specimens were investigated using optical microscopy, high-resolution ‘scanning electron microscopy’ (SEM), and ‘focused ion beam’ (FIB) techniques. Further, the paper introduces alternative proposals to identify the FGA layer beneath the fracture surfaces in terms of the ‘cross section polishing’ (CSP) technique and metallic grindings with special attention paid to the crack origin, the surrounding microstructure, and the expansion of the nanograin layer beneath the fracture surface. Different existing fracture mechanical approaches were applied to evaluate if an FGA formation is possible. Moreover, the results were discussed in comparison to the experimental findings.

A new method for ultrasonic fatigue testing of equibiaxial and pure shear cruciform specimens

Mechanical and material testing and analysis are of indisputable importance. As the variety of materials and/or the complexity of products grows, so does the need for new material characterisation methods. In this study, experimental results from a new method for ultrasonic fatigue testing of equibiaxial and pure shear cruciform specimens is presented and validated. The method uses specially designed cruciform specimens’ geometries for ultrasonic fatigue testing machines. Three different cruciform geometries were tested. The responses from the specimens were first studied by an adapted modal calculation method FDD. The new method allows determining fatigue strength of cruciform specimens in the VHCF regime with very satisfactory results.

Proposal of Fatigue Limit Design Curves for Additively Manufactured Ti-6Al-4V in a VHCF Regime Using Specimens with Artificial Defects

Cantilever-type rotating bending fatigue tests were conducted under a very high cycle fatigue regime using conventionally manufactured Ti-6Al-4V specimens having drilled artificial defects with different sizes. The relationship between fatigue limit and defect size was defined as a fatigue limit design curve considering the transition from the fracture-mechanics dominating area to the fatigue-limit dominating area. A conventional Murakami’s equation was applicable as a design curve of additively manufactured Ti-6Al-4V with defects at 107 cycles. However, conventional equation gave un-conservative predictions for the fatigue limit at 108 cycles. Therefore, two kinds of modified Murakami’s equation were proposed as fatigue limit design curves for the very high cycle fatigue regime. Simple parallel shift of Murakami’s equation gave a conservative fatigue limit, whilst better result was obtained by changing the slope of Murakami’s equation. The proposed design curve was valid for the defect sizes ranging from 10 to 500 μm.

On the fatigue behavior of differently deep rolled conditions of SAE 1045 in the very-high-cycle fatigue regime

Very-high-cycle fatigue (VCHF) tests were performed on differently deep rolled conditions of SAE 1045 steel. The dominant failure mode in the VHCF regime is linked to internal material defects leading to the well-known fish-eye fracture appearance. Despite lower residual stress stability, superior fatigue strength was observed for room-temperature deep rolled conditions as compared to counterparts deep rolled at elevated temperature. Crack initiation and advance is promoted by relatively higher and more stable tensile residual stresses in the core of the high-temperature deep rolled specimen, i.e., within sample volumes where cracks are initiated, eventually resulting in the inferior fatigue strength seen.

Fatigue Voids and Sub-Grain Boundaries Assisted Crack Initiation with High Stress Ratio in Vhcf Regime for Titanium Alloy at 400℃

Fatigue Voids and Sub-Grain Boundaries Assisted Crack Initiation with High Stress Ratio in Vhcf Regime for Titanium Alloy at 400℃