Temperature Low Cycle Fatigue Behaviour Research Articles

Low cycle fatigue behavior and life prediction of a directionally solidified alloy

Low cycle fatigue behavior and life prediction of a directionally solidified alloy

Influence of copper content on the high temperature tensile and low cycle fatigue behavior of cast Al-Cu-Mn-Zr alloys

The relationship between low cycle fatigue (LCF) and monotonic tensile fracture strain was investigated for cast Al-Cu-Mn-Zr alloys containing 6 and 9 wt% Cu at 250 °C. The 9% Cu alloy consisted of larger size and volume fraction of brittle intermetallic grain boundary particles that fractured during tensile tests to reduce the fracture strain by 50% compared to the 6% Cu alloy. LCF life was similar between the two alloys and weakly influenced by the particles. LCF life and fracture strain were poorly correlated due to the dominant influence of particles on monotonic tensile crack but not on fatigue crack.

Predicting damage and failure under thermomechanical fatigue in hot forging tools

Finite element (FE) model is developed to examine thermomechanical fatigue behavior of a hot work tool steel. Experimental results for fatigue properties of 9Cr steel were used. An approach, employing a local ductile damage initiation and evolution model, using the hysteresis stress–strain energy concept combined with element removal, has been used to predict the failure. FE analysis of cyclic loading was based on a nonlinear kinematic hardening criterion employing the Chaboche constitutive equations. Parameters of fatigue damage are calculated based on experimental test data at 300 and 620 °C. Using these parameters, simulation of high temperature low cycle fatigue behavior for standard specimens according to test conditions is carried out and the results of LCF simulation were in good agreement with the experimental. Then, a set of thermomechanical fatigue simulations for hot forging die was performed and fatigue crack initiation and growth in forging die are predicted.

Elevated temperature low cycle fatigue behaviour of Haynes 282 and its correlation with microstructure – Effect of ageing conditions

In this paper, total strain-controlled high temperature (760 °C) low cycle fatigue (HTLCF) behaviour (at 0.5% strain amplitude) of a new γ′-strengthened wrought Ni based superalloy (Haynes 282) for various ageing conditions is presented for the first time, emphasizing the microstructural effects. Haynes 282 is a promising material for hot gas path components of ultra-supercritical steam turbines due to its adequate fatigue property at elevated temperatures, along with high creep and oxidation resistance. Various microstructures were generated by varying ageing time (1 day, 6 days and 9 days) and temperature (650 °C and 760 °C). While continuous cyclic hardening behaviour was witnessed for lower aging temperature (650 °C), predominantly cyclic softening after initial hardening through few cycles was observed for higher aging temperature (760 °C), due to formation of more number of larger size voids and secondary cracks in the later specimen during the testing (as characterized by X-ray tomography). Underlying deformation mechanisms (characterized through TEM) and fracture modes (characterized through SEM, EDS) were correlated to alloy's microstructures under various HTLCF conditions; distinct differences were observed between the two ageing temperatures. While 650 °C, 9 days ageing condition showed primarily signatures of γ′ shearing, 760 °C, 9 days ageing condition showed primarily the Orowan mechanism of deformation. For 650 °C ageing condition, predominantly intergranular fracture mode has been observed, which is attributed to the presence of brittle primary MC particles on the boundaries. Whereas for the 760 °C ageing condition, the fracture mode remained predominantly transgranular because it involves less crack deflection through its path. Plastic strain energy density based life prediction model were used to predict fatigue life. Close agreement between calculated and measured life indicates validity of these models for the present material.

Effect of heat treatment on the high temperature fatigue life of single crystalline nickel base superalloy additively manufactured by means of selective electron beam melting

The high temperature low cycle fatigue behavior of specimens manufactured from a single crystalline nickel base superalloy processed by selective electron beam melting (SEBM) has been investigated with respect to the effect of different heat treatments. The fatigue lifetime of heat treated material was significantly higher than that of as-built material. Applying hot isostatic pressing (HIP) with an integrated heat treatment resulted in even longer fatigue life. Lifetime limiting crack initiation occurred at interfaces of melting layers, at micro-porosity generated during solidification or, in HIP treated samples, at precipitates which formed at the location of collapsed pores.

The effect of platinum-aluminide coating features on high-temperature fatigue life of nickel-based superalloy Rene®80

Low cycle fatigue is the most important failure mode in Aviation/Industrial engine rotary turbine parts. In this paper, the influence of Pt-aluminide coating parameters on high temperature low cycle fatigue behavior of superalloy Rene?80 which is used to manufacture turbine blades, has been investigated. For this purpose, initial platinum layers of different thicknesses (2?m and 8?m) were coated on fatigue specimens. Then the aluminizing process was performed with two conditions of low temperature-high activity and high temperature-low activity. The results of microstructure investigations performed by scanning electron microscope and X-ray diffraction phase analysis indicated a three-layer structure for the coating (bi-phase (Ni,Pt)Al+PtAl2, single-phase (Ni,Pt)Al and interdiffusion zone) with different chemical compositions at both thicknesses of the platinum layer and using both aluminizing methods. Also, the results of low cycle fatigue tests at 871?C, R=0 and strain rate of 2?10 -3 s-1 showed a decline in fatigue properties in coated specimens as compared to uncoated sample, at total strains of 0.4, 0.8, and 1.2%. This reduction was lower in the low temperature-high activity with platinum layer thickness of 2?m, while it was more significant in the high temperature-low activity with the platinum layer thickness of 8?m. The fractography studies on coated and uncoated specimens indicated a mixed mode of ductile and brittle fracture.

High temperature low cycle fatigue and creep-fatigue behavior of a casting Al-9Si-CuMg alloy used for cylinder heads

The low-cycle fatigue test and the compression creep-fatigue test of the cast Al-9Si-CuMg alloy for cylinder heads were carried out at high temperature. The cyclic mechanical response characteristics of the material under two kinds of test loads were analyzed. The microstructural evolution process and fracture mechanism of the material in the process of failure were studied, and the damage modes of the material under two kinds of loads were summarized. The results showed that, the stress-strain response (hysteresis loop) of the cast Al-9Si-CuMg alloy exhibited cyclic stability under high temperature low-cycle fatigue loading at 200℃, and the crack occurred at the holes and defects. When the temperature was raised to 350℃, the voids and defects in the material would no longer be the crack source, and the cyclic stress-strain response of the material would also show the characteristics of continuous softening. When coupled with the effect of compressive creep, the yielding radius and elastic modulus of the cyclic stress-strain response were almost unchanged, but the yield center moved upward significantly. Microscopic observation indicated that, to some extent, the effect of compressive creep at high temperature could prevent the initiation and propagation of the large-sized crack within the material, and prevent the rapid development of damage under low-cycle fatigue loading. The eutectic silicon particles inside the material firstly cracked under the compressive creep-fatigue load and formed many tiny cracks or cavities. These tiny cracks extended in the aluminum matrix and connected with the neighboring cracks until they broke.

Fatigue damage characterisation of MarBN steel for high temperature flexible operating conditions

This article is concerned with the high temperature low cycle fatigue behaviour of a new nano-strengthened martensitic-ferritic steel, MarBN. A range of strain-controlled, low cycle fatigue tests are presented on MarBN at 600 ℃ and 650 ℃, and compared with previously published data for a current state-of-the-art material, P91 steel, including microstructural analysis of the fracture mechanisms. A modified Chaboche damage law, incorporating Coffin–Manson life prediction, is implemented within a hyperbolic sine unified cyclic viscoplastic constitutive model. Calibration and validation of the model with respect to the effects of strain-rate and strain-range is performed based on an optimisation procedure for identification of the material parameters. The cyclic viscoplasticity model with damage successfully predicts fatigue damage evolution and life in the cyclically softening materials, MarBN and P91.

Comparative Study of Microstructure and High Temperature Low Cycle Fatigue Behaviour of Nickel Base Superalloys Inconel 713LC and MAR-M247

The present work is focused on the study of microstructure and low cycle fatigue behavior of the first generation nickel-base superalloy IN 713LC (low carbon) and its promising second generation successor MAR-M247 HIP (hot isostatic pressing) at 900 °C. Microstructure of both alloys was studied by means of scanning electron microscopy (SEM). The microstructure of both materials is characterized by dendritic grains, carbides and casting defects. Size and morphology of precipitates and casting defects were evaluated. Fractographic observations have been made with the aim to reveal the fatigue crack initiation place and relation to the casting defects and material microstructure. Low cycle fatigue tests were conducted on cylindrical specimens in symmetrical push-pull cycle under strain control with constant total strain amplitude and strain rate at 900 °C in air. Hardening/softening curves, cyclic stress-strain curve and fatigue life data of both materials were obtained. Cyclic stress-strain curve of MAR M247 is shifted approximately to 120 MPa higher stress amplitudes in comparison with IN 713LC. Significantly higher fatigue life of MAR-M247 has been observed in Basquin representation. On the other hand IN 713LC shows prolonged lifetime compared with MAR-M247 in the Coffin-Manson representation. Results obtained from high temperature low cycle fatigue tests are discussed.

Understanding Low Cycle Fatigue Behavior of Alloy 617 Base Metal and Weldments at 900 °C

In order to better understand the high temperature low cycle fatigue behavior of Alloy 617 weldments, this work focuses on the comparative study of the low cycle fatigue behavior of Alloy 617 base metal and weldments, made from automated gas tungsten arc welding with Alloy 617 filler wire. Low cycle fatigue tests were carried out by a series of fully reversed strain-controls (strain ratio, Rε = −1), i.e., 0.6%, 0.9%, 1.2% and 1.5% at a high temperature of 900 °C and a constant strain rate of 10−3/s. At all the testing conditions, the weldment specimens showed lower fatigue lives compared with the base metal due to their microstructural heterogeneities. The effect of very high temperature deformation behavior regarding cyclic stress response varied as a complex function of material property and total strain range. The Alloy 617 base weldments showed some cyclic hardening as a function of total strain range. However, the Alloy 617 base metal showed some cyclic softening induced by solute drag creep during low cycle fatigue. An analysis of the low cycle fatigue data based on a Coffin-Manson relationship was carried out. Fracture surface characterizations were performed on selected fractured specimens using standard metallographic techniques.

Mechanical properties and fatigue behaviour of railway wheel steels as influenced by mechanical and thermal loadings

During the operation of trains, high thermal loads are evolved because of frictional heating on recurring acceleration, braking, curving and occasional full slippage. Furthermore, since long-term block braking may heat the wheel rim to over 500°C, it is relevant to examine the high temperature performance of wheel material as well as the decrease in strength after thermal exposure. This work concerns the elevated temperature low cycle fatigue behaviour, and the deterioration of microstructure caused by pre-deformation and heat treatment. The materials examined are two similar medium carbon steels UIC ER7T and ER8T (~0.55wt% C), heat treated in regular wheel production to a near pearlitic microstructure with some 5–10% pro-eutectoid ferrite in the wheel tread surface. For the study of microstructure deterioration, specimens were extracted from virgin wheels and pre-strained either monotonically to 6.5% strain or cyclically, to imitate plastic deformation developing in the wheel tread surface during operation. Both un-deformed and pre-strained materials were heat treated at various temperatures from 250°C to 650°C for various time durations, and thereafter the change in room temperature hardness was measured and the microstructural degradation was analysed. Hardening due to strain ageing was observed around 300°C while microstructure degradation caused softening at higher temperatures. Spheroidisation of the pearlite started to become visible at 450°C for the un-deformed material and at around 400°C for the pre-strained. The elevated temperature low cycle fatigue tests showed a similar increase in strength around 300°C due to dynamic strain ageing, and decreasing strength at higher temperatures due to increased thermal activation and microstructure degradation. Hold times showed that viscous effects exist already at 250°C giving stress relaxation, but the influence of hold times on further cycling is small regarding stress–strain relations.

Special Testing Equipment and Validation of Measurement Methodologies for High Temperature Low Cycle Fatigue Testing of Miniature Metallic Specimens

A technique for high temperature low cycle fatigue testing of metallic materials has been developed, to determine fatigue behaviour through the testing of miniature specimens. The miniature specimen geometry was specifically designed, such that it could be manufactured from a small volume of material removed by chain-drilling extraction. An extensometry method to measure and control strain at the specimen shoulders during testing was adopted. This was undertaken to minimise the deleterious contact effects that can occur via extensometry attached at the gauge length of specimens, hence leading to premature failure and inaccurate fatigue data. By the application of this technique, the high temperature low cycle fatigue behaviour of 2.25Cr-1Mo steel was successfully characterised at 540 °C, under a fully reversed strain-controlled regime. The fatigue properties of the steel obtained from testing miniature specimens were shown to correlate well with existing literature for the material under comparable conditions, as determined by the testing of conventional standard-sized specimens.

Lifetime Improvement of AlSi6Cu4 Cylinder Head Alloy

Efficiency increase of combustion engines in automotive development is directly related to improved material performance at high temperatures. In particular, the interaction of complex thermal and mechanical forces under service loading is one of the major factors affecting the lifetime. In this paper the aluminium cylinder head alloy AlSi6Cu4 is characterized concerning the high temperature low cycle fatigue (LCF) and thermo-mechanical fatigue (TMF) behaviour. Experiments were performed in air at temperatures between 200 and 400°C. The influence of the initial heat treatment and microstructure, testing temperatures, strain amplitudes and strain rates is reported. Two heat treatment modifications (i.e. overaged condition, T7, and a modified combined heat treatment, hot iso-static pressing (HIP), prior to peak hardening according to T6) are compared to the T6 standard used in service. The strain amplitudes ranged between 0.2 to 0.7%. For the standard T6 heat treatment LCF results at 400°C indicate increased ductility and longer lifetimes compared to 200°C. For the overaged microstructure and the modified treatment HIP + T6 under TMF exposure clear lifetime enhancements are observed which are significantly highest for the HIP + T6 version. Thus, the elimination of almost any microporosity reduces local stress concentrations and early crack initiation promoting failure. Special attention is focussed on microstructural changes during high temperature exposure.

High Temperature, Low Cycle Fatigue Characterization of P91 Weld and Heat Affected Zone Material

The high temperature low cycle fatigue behavior of P91 weld metal (WM) and weld joints (cross-weld) is presented. Strain-controlled tests have been carried out at 400 °C and 500 °C. The cyclic behavior of the weld material (WM) and cross-weld (CW) specimens are compared with previously published base material (BM) tests. The weld material is shown to give a significantly harder and stiffer stress–strain response than both the base material and the cross-weld material. The cross-weld tests exhibited a cyclic stress–strain response, which was similar to that of the base material. All specimen types exhibited cyclic softening but the degree of softening exhibited by the cross-weld specimens was lower than that of the base material and all-weld tests. Finite element models of the base metal, weld metal and cross-weld test specimens are developed and employed for identification of the cyclic viscoplasticity material parameters. Heat affected zone (HAZ) cracking was observed for the cross-weld tests.

A Unified Viscoplastic Model for High Temperature Low Cycle Fatigue of Service-Aged P91 Steel

The finite element (FE) implementation of a hyperbolic sine unified cyclic viscoplasticity model is presented. The hyperbolic sine flow rule facilitates the identification of strain-rate independent material parameters for high temperature applications. This is important for the thermo-mechanical fatigue of power plants where a significant stress range is experienced during operational cycles and at stress concentration features, such as welds and branched connections. The material model is successfully applied to the characterisation of the high temperature low cycle fatigue behavior of a service-aged P91 material, including isotropic (cyclic) softening and nonlinear kinematic hardening effects, across a range of temperatures and strain-rates.

High temperature fatigue behaviors of a cast ferritic stainless steel

Abstract This work investigated the high temperature low cycle fatigue (LCF) behavior of ACI HB20-type, cast ferritic stainless steel. In addition to the preliminary results from tension test, isothermal strain controlled fatigue tests were carried out at 600 °C and 800 °C. The total strain amplitudes applied to the samples were from ±0.25% to ±0.65% with a strain rate of 10 −3 /s. The fatigue lifetime curves at each temperature were obtained based on Basquin and Coffin–Manson equations. Also, these temperature dependent plots were re-constructed into a general superposition curve and compared to other heat resistant stainless steels. Additionally, microstructure and fractured surface after LCF test were analyzed using a scanning electron microscopy suggesting that the crack initiated at the grain boundary of the surface and propagated intergranularly as well as transgranularly. This is attributed by the formation of Cr and Nb carbide along the grain boundaries.

Influence of Al-Si Diffusion Coating on Low Cycle Fatigue Properties of Cast Superalloy Inconel 738LC at 800 °C

High temperature low cycle fatigue behaviour of cast nickel-based superalloy Inconel 738LC in as-received condition and coated with an Al-Si diffusion layer was studied. The Al-Si protective layer was deposited on the gauge section of cylindrical specimens using the slurry technique. Fatigue tests were performed on cylindrical specimens under total strain control in symmetrical cycle at 800 °C in air. The coating has a beneficial effect on fatigue life in the low amplitude domain. The stress response of the coated material is higher for high amplitudes in comparison with the uncoated one. Fracture surfaces and sections parallel to the specimen axis have been examined to study fatigue damage mechanisms.

High temperature low cycle fatigue behaviour of hot isostatically pressed superalloy Udimet 720 LI

Nickel base superalloy Udimet 720 LI was processed through a powder metallurgy (P/M) hot isostatic pressing (HIP) route. The effects of this consolidation technique (HIPing) on low cycle fatigue (LCF) properties were evaluated and are reported in this paper. Total strain controlled LCF tests have been conducted in air at 550°C at a nominal strain rate of 6.67 × 10−3 s−1. The as-HIPed Udimet 720 Li exhibits stable behaviour followed by very mild cyclic hardening at total strain amplitudes ± 0.4–0.5%. However, cyclic hardening has been observed at total strain amplitudes ± 0.6–1.2%. The cyclic hardening is attributed to dynamic strain aging (DSA). The dual slope behaviour in a Coffin-Manson plot is attributed to a change in fracture mechanism.

High temperature fatigue resistance of an ACI HH50-type cast austenitic stainless steel

A study was undertaken to examine the high temperature low cycle fatigue behavior of an ACI HH50-type cast austenitic stainless steel. The samples were cast and machined into cylindrical specimens and then subjected to isothermal strain controlled fatigue tests at 500, 650, and 800 °C with total strain amplitudes from ±0.30% to ±0.80% at a strain rate of 3.3 × 10 −4/s. The cyclic stress responses and fatigue resistances were analyzed by obtaining the fatigue parameters of both Basquin and Coffin-Manson equations. Additionally, a general superposition curve containing all of test temperatures was obtained using a modified fatigue equation based on the constitutive equation for the energy criterion of fatigue failure in terms of the thermally activated dislocation motions. The temperature dependent fatigue parameters that were obtained from this equation enables compare high temperature fatigue resistances of alloys. The ACI HH50-type alloy showed superior fatigue properties in comparison with the ACI HF30 and AISI 304 stainless stills.

High temperature low cycle fatigue behaviour of P92 tungsten added 9Cr steel

Modified 9Cr-1Mo steel is being used extensively as structural material for steam generator components of liquid metal cooled fast breeder reactor and fossil fired power plants. The selection of this material is primarily based on a good combination of mechanical properties, high thermal conductivity, low thermal expansion coefficient and good resistance to stress corrosion cracking in water-steam and sodium environment systems compared to austenitic stainless steels. A further improvement of the thermal efficiency of modern steam turbines can be achieved by increasing the steam operating temperature. It has been found that the substitution of W for Mo enhances the high temperature tensile strength, fracture toughness and creep strength. This had led to the development of P92 Steel.