Strain Range Partitioning Research Articles

Influence of 650 °C Thermal Aging on Microstructure and Creep–Fatigue Behaviors of P92 Steel

Abstract In this paper, P92 steel was subjected to thermal aging treatment at 650 °C for 800 h, and then basic mechanical and creep–fatigue test was performed. The creep–fatigue cycle response trend is consistent before and after aging. Subsequently, microscopic observation shows that P92 steel after aging still has typical lamellar martensite and prior austenite grains. The thermal aging of 650 °C resulted in more precipitates of martensite lath, obvious lath boundary, coarsening of martensite lath, and decreased dislocation density. Furthermore, thermal aging results in the increase of precipitates (Laves phase) and martensite width of P92 steel. The fine Laves phase located on the grain boundary can effectively nail the grain boundary and play the role of precipitation strengthening. Besides, the Laves phase located on the dislocation has the effect of diffusion strengthening, which prevents dislocation slip and improves the creep–fatigue resistance of P92 steel. Finally, four creep–fatigue life model parameters of aging P92 steel were obtained according to the test, including strain range partitioning (SRP), strain energy partitioning (SEP), frequency separation life model (FSL), and strain energy density exhaustion model (SEDE). The prediction results of the four models fall within the double tolerance zone. The SRP and SEP are found to be conservative, while the FSL and SEDE are recommended herein due to their suitability of predicting creep–fatigue life of aging P92 steel.

Grain boundary sliding model for assessing creep-fatigue life of Sn37Pb eutectic solder

This study discusses the creep-fatigue life prediction of eutectic Sn37Pb solder. Creep-fatigue lives were experimentally obtained at 313 K using fast-fast, slow-slow, slow-fast, fast-slow, and strain-hold waveforms. Conventional four creep-fatigue life prediction rules, time fraction rule, strain range partitioning method, frequency modified fatigue life, and ductility exhaustion model was used to predict the creep-fatigue lives. However, not all the rules were able to properly estimate them. Since the grain boundary sliding rates were closely related to creep-fatigue lives, a new life prediction method based on grain boundary sliding rates was proposed.

Low cycle fatigue life modelling using finite element strain range partitioning for a steam turbine rotor steel

Materials made for modern steam power plants are required to withstand high temperatures and flexible operational schedule. Mainly to achieve high efficiency and longer components life. Nevertheless, materials under such conditions experience crack initiations and propagations. Thus, life prediction must be made using accurate fatigue models to allow flexible operation. In this study, fully reversed isothermal low cycle fatigue tests were performed on a turbine rotor steel called FB2. The tests were done under strain control with different total strain ranges and temperatures (20 °C to 625 °C). Some tests included dwell time to calibrate the short-time creep behaviour of the material. Different fatigue life models were evaluated based on total life approach. The stress-based fatigue life model was found unusable at 600 °C, while the strain-based models in terms of total strain or inelastic strain amplitudes displayed inconsistent behaviour at 500 °C. To construct better life prediction, the inelastic strain amplitudes were separated into plastic and creep components by modelling the deformation behaviour of the material, including creep. Based on strain range partitioning approach, the fatigue life depends on different damage mechanisms at different strain ranges at 500 °C. This allows for the formulation of life curves based on either plasticity-dominated damage or creep-dominated damage. At 600 °C, creep dominated while at 500 °C creep only dominates for higher strain ranges. The deformation mechanisms at different temperatures and total strain ranges were characterised by scanning electron microscopy and by quantifying the amount of low angle grain boundaries. The quantification of low angle grain boundaries was done by electron backscatter diffraction. Microscopy revealed that specimens subjected to 600 °C showed signs of creep damage in the form of voids close to the fracture surface. In addition, the amount of low angle grain boundaries seems to decrease with the increase in temperature even though the inelastic strain amplitude was increased. The study indicates that a significant amount of the inelastic strain comes from creep strain as opposed of being all plastic strain, which need to be taken into consideration when constructing a life prediction model.

Thermo-Mechanical Fatigue Life Evaluation for Ni-23Cr-7W Alloy by Strain Range Partitioning Method

Thermo-Mechanical Fatigue Life Evaluation for Ni-23Cr-7W Alloy by Strain Range Partitioning Method

Low cycle fatigue modelling of a steam turbine rotor steel

Materials in steam turbine rotors are subjected to cyclic loads at high temperature, causing cracks to initiate and grow. To allow for more flexible operation, accurate fatigue models for life prediction must not be overly conservative. In this study, fully reversed low cycle fatigue tests were performed on a turbine rotor steel called FB2. The tests were done isothermally, within temperature range of room temperature to 600 °C, under strain control with 0.8-1.2 % total strain range. Some tests included hold time to calibrate the short-time creep behaviour of the material. Different fatigue life models were constructed. The life curve in terms of stress amplitude was found unusable at 600 °C, while the life curve in terms of total strain or inelastic strain amplitudes displayed inconsistent behaviour at 500 °C. To construct better life model, the inelastic strain amplitudes were separated into plastic and creep components by modelling the deformation behaviour of the material, including creep. Based on strain range partitioning approach, the fatigue life depends on different damage mechanisms at different strain ranges. This allowed the formulation of life curves based on plasticity or creep domination, which showed creep domination at 600 °C, while at 500 °C, creep only dominates for higher strain range.

Creep-fatigue and cyclically enhanced creep mechanisms in aluminium based metal matrix composites

An aluminium (Al, 2024T3) matrix composite reinforced with continuous alumina (Al2O3) fibres is investigated under tensile off-axis constant macro stress and thermal cyclic loading. The micromechanical approach to modelling and three different fibre cross-section geometries have been employed. The effect of creep is included by considering three dwell times at the peak temperature of the thermal loading history. The presence of the hold time gives rise to different sources of failure such as cyclic enhanced creep and creep ratchetting. These failure mechanisms are carefully discussed and assessed. The linear matching method framework has been used for the direct evaluation of the crucial parameters for creep-fatigue crack initiation assessment at the steady cycle. A detailed representation of the steady-state hysteresis loops is provided by using the strain range partitioning and a method for dealing with multiaxiality is reported with regard to the algebraic sign of the Mises-Hencky equivalent stress and strain. All the results obtained have been benchmarked by fully inelastic step-by-step (SBS) analyses. The design of a long fibre metal matrix composite should consider not only the detrimental effect of their dissimilar coefficient of thermal expansion, but also the state of stress at the interface between the matrix and fibre.

Creep-Fatigue Behaviors and Life Assessments in Two Nickel-Based Superalloys

The aim of this paper is to investigate different factors, including dwell time, strain range, and strain ratio on creep-fatigue endurances in nickel-based Inconel 718 and GH4169 superalloys. We also summarize classic approaches for life assessments based on the generalizations of Coffin–Manson equation, linear damage summation (LDS), and strain-range partitioning (SRP) method. Each approach does have some degree of success in dealing with a specific set of creep–fatigue data. In order to evaluate the prediction capabilities of the validated approaches, a Bayesian information criterion (BIC) allowing for maximum likelihood and principle of parsimony is used to select the best performing model.

Thermo-mechanical fatigue behavior of nickel-based 23Cr-45Ni-7W alloy for boiler pipes and tubes

This study focuses on thermo-mechanical fatigue (TMF) behaviors of nickel-based 23Cr-45Ni-7W alloy for pipes and tubes in high-efficiency power boilers. TMF tests (maximum temperature = 700 °C; minimum temperature = 100 °C) were conducted under in-phase (IP) and out-of-phase (OP) conditions. The lives under the IP condition is 0.30–0.44 times of that under the OP condition when same total strain range, Δεt, is applied. The TMF properties obtained via the tests are compared with the results of isothermal fatigue (ITF) tests conducted at 700 °C and bithermal fatigue (BTF) tests with maximum and minimum temperatures of 700 °C and 100 °C, respectively. The fracture morphology for each type of TMF, ITF, and BTF is similar under the test conditions of this study. Intergranular cracking is predominant under application of cyclic tensile creep strain during the fatigue tests, and transgranular cracking is predominant under application of cyclic compressive creep strain during the fatigue tests. The inelastic strain range–fatigue life (Δεin–Nf) relation in the TMF test and the inelastic strain range components–life (Δεij–Nij) relation determined using the strain range partitioning method show a good agreement within a factor of 1.5 scatter band. These results indicate the possibility of predicting the TMF life of the 23Cr-45Ni-7W alloy using the results of the ITF test by considering the amount and direction (tension or compression) of creep strain applied to the alloy.

Creep–fatigue endurance of 304 stainless steels

In this paper, the effects of different factors on the combined creep–fatigue endurance of 304 stainless steel as well as the strength and limitation of the different approaches for creep–fatigue life prediction are reviewed. The factors, which influence on the creep–fatigue endurance, include hold time, strain range, temperature, and stress relaxation behavior. Some relationships between time or number of cycles to failure and the individual factors are summarized from the available data. A phenomenological approach is proposed to describe the synergistic effects of different factors. The approaches used for creep–fatigue life prediction range from purely empirical to mechanistic models based on microstructural observations. Each approach does have some degree of success in dealing with a specific set of creep–fatigue data. The comparison of the prediction capacities of linear damage summation method, strain-range partitioning method, and damage rate approach is made for the same set of creep–fatigue data through Bayesian information criterion analysis. The damage rate model exhibits higher accuracy than other two models.

Creep-fatigue life assessment for three kinds of solders

This paper presents a creep-fatigue life evaluation for three kinds of solders of Sn–37Pb, Sn– 3.5Ag and Sn – 3.0Ag – 0.5Cu. Strain controlled push – pull creep-fatigue tests were performed for solid bar specimens under fast–fast, fast–slow, slow–fast and strain–hold strain waveforms and the effect of strain waveforms on creep-fatigue lives is discussed. Sn– 3.5Ag and Sn– 3.0Ag – 0.5Cu solders showed stronger creep-fatigue resistance compared to Sn –37Pb solder. The applicability of the linear damage rule, the strain range partitioning method and the grain boundary sliding model is discussed for predicting creep-fatigue lives of the three solders. The strain range partitioning method and the grain boundary sliding model satisfactorily predicted creep-fatigue lives within a small scatter.

Prediction of fatigue life of high-heat-load components made of oxygen-free copper by comparing with Glidcop

Following a successful study on the prediction of fatigue life of high-heat-load components made of Glidcop, the thermal limitation of oxygen-free copper (OFC), which is used more commonly than Glidcop, has been studied. In addition to its general mechanical properties, the low-cycle-fatigue (LCF) and creep properties of OFC were investigated in detail and compared with those of Glidcop. The breaking mode of OFC, which was observed to be completely different from that of Glidcop in a fatigue fracture experiment, clarified the importance of considering the creep-fatigue interaction. An additional LCF test with compressive strain holding was conducted so that the creep-fatigue life diagram for out-of-phase thermal fatigue could be obtained on the basis of the strain-range partitioning method. The life predicted from elasto-plastic creep analysis agreed well with that determined from the void ratio estimated in the fatigue fracture experiment.

Creep-fatigue life assessment for three kinds of solders

This paper presents a creep-fatigue life evaluation for three kinds of solders of Sn–37Pb, Sn– 3.5Ag and Sn – 3.0Ag – 0.5Cu. Strain controlled push – pull creep-fatigue tests were performed for solid bar specimens under fast–fast, fast–slow, slow–fast and strain–hold strain waveforms and the effect of strain waveforms on creep-fatigue lives is discussed. Sn– 3.5Ag and Sn– 3.0Ag – 0.5Cu solders showed stronger creep-fatigue resistance compared to Sn –37Pb solder. The applicability of the linear damage rule, the strain range partitioning method and the grain boundary sliding model is discussed for predicting creep-fatigue lives of the three solders. The strain range partitioning method and the grain boundary sliding model satisfactorily predicted creep-fatigue lives within a small scatter.

Creep-Fatigue Life Evaluation of Sn-3.5Ag-0.5Cu-Ni-Ge Solder at Low Temperatures

This paper describes creep-fatigue lives of Sn-3.5Ag-0.5Cu-Ni-Ge solder at low temperatures. Push-pull creep-fatigue tests under four types of strain wave form were carried out at 253K and 273K to study an application of creep-fatigue life evaluation method for Sn-3.5Ag-0.5Cu-Ni-Ge. There were effects of temperatures on stress amplitude, and tension stress amplitude at 253K was larger than that at 273K in fast-fast wave form test. The higher stress amplitude reduced the creep-fatigue lives. The creep-fatigue lives with slow strain rate were shorter than the creep-fatigue lives with fast strain rate. A linear damage rule and a strain range partitioning method estimated the creep-fatigue lives in a large scatter. A grain boundary sliding model was a suitable parameter for creep-fatigue life estimation for Sn-3.5Ag-0.5Cu-Ni-Ge.

Creep-Fatigue Life Evaluation for Materials Subjected to Nonproportionally Combined Tension and Torsion

The author and his colleagues have proposed in their recent works the creep-fatigue life prediction method based on the strain range partitioning concept for nonproportionally combined tension and torsion using new inelastic strain range parameters and have been studying their applicability to the materials subjected to nonproportional axial-torsional loading at elevated temperatures. In the present paper are summarized the results of in-phase and 90 deg out-of-phase axial-torsional straining tests conducted on thin-walled tubular specimens of SUS304, 316LC and 2.25Cr-1Mo steels.

Isothermal, Thermo-Mechanical and Bithermal Fatigue Life of Ni Base Alloy HR6W for Piping in 700 °C USC Power Plants

Three types of high temperature fatigue tests are carried out for HR6W, which is one of the candidate materials for piping in an advanced ultra super critical (A-USC) power boiler operating over 700°C steam condition. The first test is the isothermal fatigue test at 700°C, the second one is the thermo-mechanical fatigue (TMF) test between 100°Cand 700°C under in-phase and out-of-phase conditions, the final one is the bithermal fatigue (BTF) test at 100°C and 700°C. The relations between the partitioned inelastic strain range determined by the strain range partitioning method and the life in those tests show good agreement, while the relations between the total strain range and the life show some differences.

Evaluation of the effect of strain hold time on nonprroportionlly combined axial/torsional fatigue life of SUS304 and 316LC steels

The present paper investigates whether or not the life prediction method proposed by the authors based on the strain range partitioning concept for SUS304 and 316LC steels subjected to nonproportionally combined push-pull and cyclic torsion under triangular strain waveforms can be used to explain the effect of strain hold time on nonproportionally combined axial/torsional fatigue life of both materials. The 90 deg out-of-phase straining axial/torsional fatigue tests were conducted on both materials using trapezoidal strain waveforms at 1023 K in air. The results were analyzed by the proposed method.

Investigation and modeling of low cycle fatigue behaviors of two Ni-based superalloys under dwell conditions

The ability to predict fatigue behaviors of turbine-disk-materials under operating conditions is an important aspect of designing a safe turbine engine. Studies of two Ni-based superalloys, powder metallurgy (PM) FGH95 and Cast GH4169, have been undertaken to investigate their low cycle fatigue (LCF) behaviors with different dwell conditions reflecting the fatigue–creep interaction at high temperature of 650 °C. Based on the deformation behaviors obtained by the tests, the effects of dwell on mean stress and shape of hysteretic loop at half life have been analyzed and considered to be introduced in developing a new fatigue model. For the purpose, two groups of parameters are defined in order to introduce the effects of mean stress and shape of hysteretic loop, especially due to their changes caused by dwell. Finally, an energy-based fatigue model is given with modifying of the original energy-type damage parameter. As a comparison, four typical models of LCF are used to model the experimental results, which are the Strain Range Partitioning method (SRP), the Frequency Separation method (FS), the Damage Rate method (DR) and the Ostergren method. The experimental data of LCF of cast and wrought (CW) Rene95, and PM Rene95 from open resources are also used to verify abilities of the newly-developed model. The results show that the model can describe fatigue behaviors well, and the scatter bands are within ±2, even considering the dwell effects. The accuracy and adaptability of predicting LCF life of the model are better than those of the four existing models.

Effect of Nonproportional Loading on Creep-Fatigue Life of SUS304 Steel Subjected to CC-Type Combined Push-Pull Loading and Cyclic Torsion at 973K in Air

This paper investigates the applicability of the procedure proposed by one of the authors for estimating the nonproportional loading effect on PP test lives to the estimation of the creep-fatigue lives of SUS304 austenitic stainless steel subjected to nonproportionally combined push-pull and cyclic torsion under CC type strain waveform. Fully reversed strain-controlled CC tests were conducted on thin-walled tubular specimens of SUS304 steel at 973K in air under in-phase and 90deg out-of-phase straining conditions. The partitioned inelastic strain range versus life equations, Δecc versus Ncc relationships, obtained from three different proportional loading conditions, such as push-pull, cyclic torsion, and in-phase combined push-pull and cyclic torsion, are found to be correlated with one another by the multiaxiality factor. By using the multiaxiality modified Δepp versus Npp and Δecc versus Ncc relationships, the 90deg out-of-phase straining CC test lives were estimated by the proposed procedure, where the strain range partitioning was done by superposing the hysteresis loops of PP tests on those of CC tests. As the results, the estimated lives are found to agree well with the experimental lives in adopting the superposition method rather than the rapid loading method.

Creep-Fatigue Life Assessment for Sn-3.0Ag-0.5Cu Solder

This paper describes the creep-fatigue life assessment of Sn-3.0Ag-0.5Cu solder under pull-push loading using fast-fast, fast-slow, slow-fast and strain-hold strain waveforms. Strain controlled creep-fatigue tests were carried out using solid bar specimen, and the effect of strain waveform on the creep-fatigue life was discussed. Creep-fatigue damage was evaluated by the linear damage rule, the frequency modified fatigue life, the ductility exhaustion model, the strain range partitioning method and the grain boundary sliding model. The strain range partitioning method and the grain boundary sliding model only predicted the creep-fatigue lives within a small scatter.

Life Estimation of SUS304 Steel Subjected to Nonproportionally Combined Push-Pull and Cyclic Torsion at 973K

The present paper summarizes the fully reversed strain-controlled creep-fatigue tests conducted on thin-walled tubular specimens of SUS304 austenitic stainless steel at 973K in air under push-pull, cyclic torsion, in-phase straining and 90deg out-of-phase straining of push-pull and cyclic torsion. It is shown that, as the results of analysis of the experimental data by the strain-range partitioning methodand the critical plane model parameter, a new inelastic-strain based parameter was proposed for life estimation of SUS304 subject to nonproportionally combined push-pull and cyclic torsion by the strain-range partitioning method.