Low Cycle Fatigue Conditions Research Articles

Low-cycle fatigue life of steel P92 under gradual loading

The aim of the study was to determine low-cycle fatigue life characteristics of P92 steel used in the power unit components that work under the highest effort. Steel service life was determined in the tests for total fixed strain Δεt ranges from 0.6 to 1.2 % and with application of two-stage sequence loading. Low-cycle fatigue tests were conducted at room temperature and at 550 °C on MTS-810 testing machine. The tests under sequence loading were carried out for two strain ranges: Δεt = 0.6 and 1.2 %. Sinusoidal load cycles of a coefficient R = −1 were used. Low-cycle fatigue life characteristics were developed with consideration of loading sequence impact. It was found out the fatigue life of an element under sequence loading is strictly correlated with its history of strain, which determines the degree of material microstructure damage. Higher degree of steel damage was found in two-stage tests, in which the larger range of strain Δεt = 1.2 % was used as the first one. Fractography analyses of scrap revealed the patterns of fatigue lines and bands typical for this process as well as numerous secondary cracks occurred to the boundaries of former austenite grains. Analytic method of predicting low-cycle fatigue life of an element under two-stage sequence loading is presented. For this purpose, an energy criterion for the material life developed for tests carried out in conditions of low-cycle fatigue was used.

Thermomechanical fatigue behavior and lifetime prediction of niobium-bearing ferritic stainless steels

In this study, the thermomechanical fatigue (TMF) and isothermal low-cycle-fatigue (LCF) behaviors of niobium-containing ferritic stainless steels are presented for the temperature range from 100°C to the maximum temperatures between 500 and 800°C; furthermore, we propose a new fatigue failure criterion to predict the fatigue lives of the components for different thermal cycle ranges using the TMF condition. Higher maximum temperatures during TMF cycle resulted in shorter TMF life. By modifying the Coffin–Manson equation using the temperature factor, we obtained a new parameter that was successfully correlated with the life under different maximum temperatures. The deformation responses during fatigue cycling and the fatigue microstructure were compared to elucidate the different fatigue behaviors under the TMF and LCF conditions.

TMF–LCF life assessment of a Lost Foam Casting A319 aluminum alloy

The LFC (Lost Foam Casting) process affects the microstructure, the mechanical properties, the damage mechanisms and the fatigue failure of the materials. The first purpose of this paper is to study the cyclic mechanical behaviors, damage and lifetime of the A319 aluminum alloy manufactured by the LFC process used in the automotive industry under TMF (Thermo-Mechanical Fatigue) and LCF (Low Cycle Fatigue) conditions. A second objective is to select an effective fatigue criterion which should be easy to apply for the design of structures submitted to complex multiaxial thermo-mechanical loadings. In this way, several energy-based criteria are used to predict fatigue failure. Good agreement between predicted fatigue lifetimes and experimental results was obtained for different TMF and LCF loading conditions.

0606 低サイクル疲労条件下におけるSCC 予き裂からのき裂進展挙動(GS07-2 材料力学・計算力学,オーガナイズドセッション 3:「熱・流体の可視化と計測」)

0606 低サイクル疲労条件下におけるSCC 予き裂からのき裂進展挙動(GS07-2 材料力学・計算力学,オーガナイズドセッション 3:「熱・流体の可視化と計測」)

Models of multiaxial fatigue fracture and service life estimation of structural elements

We study criteria and models of multiaxial fracture under the conditions of low-cycle fatigue (LCF). The model parameters are determined by using the data of uniaxial fatigue tests for different coefficients of the cycle asymmetry. A procedure for calculating the stress state of the compressor disk in a gas turbine engine (GTE) in the flight cycle of loading is outlined. The calculated stress state and models of multiaxial fatigue fracture are used to estimate the service life of the compressor disk. The results are compared with the observational data collected during the operation.

The effect of shot peening on notched low cycle fatigue

The improvement in low cycle fatigue life created by shot peening ferritic heat resistant steel was investigated in components of varying geometries based on those found in conventional power station steam turbine blades. It was found that the shape of the component did not affect the efficacy of the shot peening process, which was found to be beneficial even under the high stress amplitude three point bend loads applied. Furthermore, by varying the shot peening process parameters and considering fatigue life it has been shown that the three surface effects of shot peening; roughening, strain hardening and the generation of a compressive residual stress field must be included in remnant life models as physically separate entities. The compressive residual stress field during plane bending low cycle fatigue has been experimentally determined using X-ray diffraction at varying life fractions and found to be retained in a direction parallel to that of loading and to only relax to 80% of its original magnitude in a direction orthogonal to loading. This result, which contributes to the retention of fatigue life improvement in low cycle fatigue conditions, has been discussed in light of the specific stress distribution applied to the components. The ultimate aim of the research is to apply these results in a life assessment methodology which can be used to justify a reduction in the length of scheduled plant overhauls. This will result in significant cost savings for the generating utility.

A comparative study on thermomechanical and low cycle fatigue failures of a single crystal nickel-based superalloy

The cyclic deformation and lifetime behaviors of a single crystal nickel-based superalloy CMSX-4 have been investigated under out-of-phase thermomechanical fatigue (OP TMF) and isothermal low cycle fatigue (LCF) conditions. OP TMF life exhibited less than a half of LCF life although smaller inelastic strain range and lower mean stress level during OP TMF were observed compared to those during LCF. During OP TMF cycling, the maximum tensile strain at the minimum temperature was found to accelerate the surface crack initiation and propagation. Additionally, the multiple groups of parallel twin plates near crack provided a preferential path for crack propagation.

Numerical modeling of elastoplastic deformation and damage accumulation in metals under low-cycle fatigue conditions

A mathematical model that describes the processes of fatigue damage accumulation in structural materials (metals and alloys) under multiaxial disproportionate combined thermomechanical loading is advanced from the standpoint of the damaged medium mechanics. Based on the results of basic experiments performed in a special way, an experimental-theoretical procedure for finding material parameters of the advanced constitutive relations of the damaged medium mechanics is put forward. The advanced version of the constitutive relations of the damaged medium mechanics is shown to adequately (qualitatively and quantitatively) reflect the main effects of elastoplastic deformation and damage accumulation in metals under low-cycle fatigue.

Fracture toughness analysis of specimens from granular alloy ÉP741NP under conditions of low-cycle fatigue using the finite element method

The possibilities of applying the procedures for calculating the fracture toughness in terms of the finite element method and fracture mechanics relationships are analyzed based on the experimental data obtained in low-cycle fatigue tests of specimens. The influence of defects available in the granular material on the life of specimens is estimated. Good agreement is obtained between the calculated results and the experimental ones, which has validated the applicability of the above computational procedures, as well as the possibility of their use for fracture toughness analysis and life prediction of real gas turbine engine parts. The peculiarities of the granular material fracture behavior revealed during this investigation can be taken into account in the mechanical design of new products of aircraft engineering.

Creep, plasticity, and fatigue of single crystal superalloy

Single crystal components in gas turbine engines are subject to such extreme temperatures and stresses that life prediction becomes highly inaccurate resulting in components that can only be shown to meet their requirements through experience. Reliable life prediction methodologies are required both for design and life management. In order to address this issue we have developed a thermo-viscoplastic constitutive model for single crystal materials. Our incremental large strain formulation additively decomposes the inelastic strain rate into components along the octahedral and cubic slip planes. We have developed a crystallographic-based creep constitutive model able to predict sigmoidal creep behavior of Ni base superalloys. Inelastic shear rate along each slip system is expressed as a sum of a time dependent creep component and a rate independent plastic component. We develop a new robust, computationally efficient rate-independent crystal plasticity approach and combined it with creep flow rule calibrated for Ni-based superalloys. The transient variation of each of the inelastic components includes a back stress for kinematic hardening and latent hardening parameters to account for the stress evolution with inelastic strain as well as the evolution for dislocation densities. The complete formulation accurately predicts both monotonic and cyclic tests at different crystallographic orientations for constant and variable temperature conditions (low cycle fatigue (LCF) and thermo-mechanical fatigue (TMF) tests). Based on the test and modeling results we formulate a new life prediction criterion suitable for both LCF and TMF conditions.

Fracture Mechanics Approach to Small Fatigue Crack Growth and Coalescence under Low Cycle Fatigue

Under low cycle fatigue condition, small cracks initiate on the specimen surface, and then they grow and coalesce with each other. The coalescence of small cracks is an important factor in evaluating fatigue life based on the fracture mechanics approach. However, surface observation of the coalescence is not sufficient in order to understand the mechanism and the influence on the fatigue life. Present study performed numerical simulations of the crack coalescence using the S-FEM based on the linear elastic fracture mechanics. The results revealed that the fatigue life was reduced when the main crack coalesces with a crack of comparative size, while the effect of fatigue life reduction was small when the main crack coalesces with a crack of less than half size. Experimental results were compared with the simulated results.

A New Ductility Exhaustion Model for High Temperature Low Cycle Fatigue Life Prediction of Turbine Disk Alloys

Based on ductility exhaustion theory and the generalized energy-based damage parameter, a new viscosity-based life prediction model is introduced to account for the mean strain/stress effects in the low cycle fatigue regime. The loading waveform parameters and cyclic hardening effects are also incorporated within this model. It is assumed that damage accrues by means of viscous flow and ductility consumption is only related to plastic strain and creep strain under high temperature low cycle fatigue conditions. In the developed model, dynamic viscosity is used to describe the flow behavior. This model provides a better prediction of Superalloy GH4133's fatigue behavior when compared to Goswami's ductility model and the generalized damage parameter. Under non-zero mean strain conditions, moreover, the proposed model provides more accurate predictions of Superalloy GH4133's fatigue behavior than that with zero mean strains.

Relaxation and Stability of Welding Residual Stresses in High Strength Steel under Mechanical Loading

AbstractThe relaxation of near surface welding residual stresses in S1100QL under static and cyclic mechanical loadings was studied. By increasing static loads the residual stress relaxes continuously. Under cyclic loading the residual stresses behave depending on the initial status, local yield strength and maximum applied load. If the von Mises stress as a function of residual stress, load and mean stresses, exceeds the local yield strength, plastic deformation and thus relaxation occurs. The first load cycle is of importance in which a considerable portion of the initial residual stress field relaxes. Afterwards regardless of whether in high or low cycle fatigue conditions, the relaxation rates were the same and comparatively smaller than the first relaxation. When no relaxation in the first load cycle occurred, the number of cycles did not change the residual stress. The specimens were loaded until failure or a maximum of 2x106 numbers of cycles.

Inelastic cyclic behaviour of as‐received and pre‐corroded S500s tempcore steel reinforcement

PurposeSeismic loading can induce significant deformations to steel reinforcement. The recent approach suggested by Eurocode 8 indicates that steel reinforcement shall sustain repeated loading well within its elastic region, excluding by definition seismic loading. This paper aims to examine the behaviour of S500s steel reinforcement at strain ranges representing strains corresponding to small/medium earthquakes while significant attention has been paid to cases where the reinforcement has been corroded as this is most representative to aged buildings. The work concludes that the complex behaviour of steel reinforcement under low cycle fatigue conditions can be successfully treated via the use of the viscous stress. The latter is found to be independent to corrosion exposure while it holds the merits of ductility exhaustion on which most degradation models are based.Design/methodology/approachThis paper establishes a relationship between the cumulative effect of low cycle fatigue and that of the viscous stress.FindingsThe work identifies that the viscous stress follows an exponential growth behaviour which terminates at a plateau. The plateau value is found to be independent to corrosion exposure and strain rate and hence providing a strong potential for being a characteristic indicator of the behaviour of steel reinforcement under realistic inelastic loading.Research limitations/implicationsThe study is limited to S500s grade steel. Further study on different steel grades is necessary to increase the potential of viscous stress.Originality/valueThe significance of this paper is the introduction of viscous stress in an area where traditional approaches of cumulative damage are based on a large number of empirical parameters and assumptions.

Experimental and Analytical Evaluation of Reinforced Concrete Girders under Low-Cycle Shear Fatigue

Stirrup stresses in lightly reinforced concrete bridge girders may exceed the elastic limit while in service. Increasing load magnitudes and volume over time, multiple permit loads, and modern superloads creates the potential for low-cycle fatigue. In this study, six full-scale reinforced concrete girder specimens representative of those found in 1950s reinforced concrete deck-girder bridges were tested under low-cycle fatigue conditions. The specimens were tested in both T and inverted-T configurations. Other test variables included stirrup spacing, flexural reinforcing details, and spacing of supports. Results showed that low-cycle fatigue produced bond deterioration and cumulative plasticity of stirrups. Progressive fracture of stirrup reinforcement under low-cycle fatigue led to eventual specimen failure. A methodology for the analysis of low-cycle fatigue in girders is proposed using the finite element method to approximate stirrup stress ranges and a linear damage model to estimate the life of a conventionally reinforced concrete girder under repeated overloads. This methodology provided good correlation between the experimental and predicted number of fatigue cycles for lightly web-reinforced specimens.

Tensile creep behavior and strain-rate sensitivity of superalloy GH4049 at elevated temperatures

According to the working temperature and load of superalloy GH4049 in an aero-engine, we systematically study its high temperature mechanical behaviors that have crucial effects on component life. Creep tests are performed in the stress range of 137–600MPa and the temperature range of 700–900°C. The strain-rate sensitivities (SRSs) under simple tension, fatigue and creep conditions are also analyzed in detail at 600°C, 768°C and 832°C, the working temperatures at typical positions on the turbine blade. Through creep experiments of GH4049 under single- and multistage loading conditions, we obtain the relationship between the minimum creep rate and the rupture time, the effect of high temperature strain recovery after creep and the effect of warm-working at high temperatures on the subsequent room-temperature strength. In the SRS analysis, we achieve the SRS indexes under different experimental conditions. Meanwhile, we generalize the SRS definition from simple tension to low-cycle fatigue (LCF) and multistage creep conditions, and verify the validity of these generalizations. The verifications illustrate that the SRS definitions in this paper can quantitatively describe the strain-rate dependence behaviors under LCF and multistage creep. All these fundamental works establish the basis of complex mechanical behavior simulations for GH4049 and the basis of the analysis of structural integrity and reliability.

Behavior of alloy Ti – 24% Al – 15% Nb – 1% Mo under conditions of low-cycle fatigue at high temperatures

An ingot of alloy Ti – 24% Al – 15% Nb – 1% Mo with a mass of 600 kg was obtained by vacuum-arc melting with consumable electrode. The chemical composition of the alloy (wt.%) was as follows: 12.55 Al, 27.36 Nb, 1.84 Mo, 0.15 Fe, 0.016 C, 0.0140 N, 0.0051 H, remainder Ti. The ingot was forged for bars 220 mm in diameter at a temperature of the - and (2 + B2)-ranges. Then the bars were cut into washers that were subjected to upsetting. After this the forged bars were rolled at a temperature from the two-phase range and used for making ring preforms 760 600 150 mm in size. Specimens 20 mm diameter and 100 nm long were cut from the rings in the longitudinal direction and heat treated in the following mode: hardening for solid solution at 980°C (1 h) and aging in the lower (2 + O + B2)-region. This yielded a triplex structure in the alloy. A specimen for LCF tests is presented in Fig. 1. The tests were performed in an MTS servohydraulic installation, which makes it possible to perform cyclic loading with control of the deformation. The deformation amplitude was changed within 0.4 – 1.2%, the coefficient of cycle asymmetry R = – 1, the stress frequency was 0.1 – 0.33 Hz, and the cycle shape was triangular. We also performed ten

Reliability-Based Design Optimization for Durability of Ground Vehicle Suspension System Components

The effect of materials processing- and component manufacturing-induced uncertainties in material properties and component shape and size on the reliability of component performance is investigated. Specifically, reliability of a suspension system component from a high-mobility multipurpose wheeled vehicle which typically can fail under low-cycle strain-based fatigue conditions is analyzed. Toward that end, the most advanced reliability-based design optimization methods available in the literature were combined with the present understanding of low-cycle fatigue durability and applied to the component in question. This entailed intricate integration of several computational tools such as multibody vehicle dynamics, finite-element simulations, and fatigue strain-life assessment/prediction techniques. The results obtained clearly revealed the importance of consideration of material property uncertainties in attaining vehicle performance of critical structural components in complex systems (e.g., a vehicle).

Modeling the influence of high dose irradiation on the deformation and damage behavior of RAFM steels under low cycle fatigue conditions

A viscoplastic deformation damage model developed for RAFM steels in the reference un-irradiated state was modified taking into account the irradiation influence. The modification mainly consisted in adding an irradiation hardening variable with an appropriate evolution equation including irradiation dose driven terms as well as inelastic deformation and thermal recovery terms. With this approach, the majority of the material and temperature dependent model parameters are no longer dependent on the irradiation dose and only few parameters need to be determined by applying the model to RAFM steels in the irradiated state. The modified model was then applied to describe the behavior of EUROFER 97 observed in the post irradiation examinations of the irradiation programs ARBOR 1, ARBOR 2 and SPICE. The application results will be presented and discussed in addition.

21316 円孔板に対する荷重制御型高温低サイクル疲労・クリープ疲労寿命予測(材料強度と構造設(2),OS.5 材料強度と構造設計)

The purpose of this study is to obtain the fundamental feature of perforated plate under the load-controlled low-cycle fatigue and creep-fatigue condition. The hysteresis-loop and the crack propagation were measured during the test to examine the relation between crack propagation and the history of strain. And the Stress Redistribution Locus (SRL) method is applied for estimation of stress concentration in order to predict fatigue and creep-fatigue life for the specimens. It was found that the strain varies with increasing number of cycle. In the latter of cycle the strain increased sharply. It was shown that the life evaluation can be more accurate using the strain obtained by applying SRL method. Additionally, it was shown that the life evaluation using crack initiation cycle is more accurate than that using failure cycle.