Thermal Mechanical Fatigue Tests Research Articles

Strain-Controlled Thermal-Mechanical Fatigue Behavior and Microstructural Evolution Mechanism of the Novel Cr-Mo-V Hot-Work Die Steel.

In response to the intensifying competition in the mold market and the increasingly stringent specifications of die forgings, the existing 55NiCrMoV7 (MES 1 steel) material can no longer meet the elevated demands of customers. Consequently, this study systematically optimizes the alloy composition of MES 1 steel by precisely adjusting the molybdenum (Mo) and vanadium (V) contents. The primary objective is to significantly enhance the microstructure and thermal-mechanical fatigue performance of the steel, thereby developing a high-performance, long-life hot working die steel designated as MES 2 steel. The thermal-mechanical fatigue (TMF) tests of two test steels were conducted in reverse mechanical strain control at 0.6% and 1.0% strain levels by a TMF servo-hydraulic testing system (MTS). The microstructures of the two steels were characterized using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). The results indicate that throughout the entire thermomechanical fatigue cycle, both steels exhibit initial hardening during the low-temperature half-cycle (tension half-cycle) and subsequent continuous softening during the high-temperature half-cycle (compression half-cycle). Furthermore, under the same strain condition, the cumulative cyclic softening damage of MES 1 steel is more pronounced than that of the newly developed MES 2 steel. The number, width, and length of cracks in MES 2 steel are smaller than those in MES 1 steel, and the thermomechanical fatigue life of MES 2 steel is significantly longer than that of MES 1 steel. The microstructures show that the main precipitate phase in MES 1 steel is Cr-dominated rod-shaped carbide. It presents obvious coarsening and is prone to inducing stress concentration, thus facilitating crack initiation and propagation. The precipitate phase in MES 2 steel is mainly MC carbide containing Mo and V. It has a high thermal activation energy and is dispersed in the matrix in the form of particles, pinning dislocations and grain boundaries. This effectively delays the reduction in dislocation density and grain growth, thus contributing positively to the improvement in thermomechanical fatigue performance.

Thermal-Mechanical Fatigue Behavior and Life Assessment of Single Crystal Nickel-Based Superalloy

Thermal-mechanical fatigue (TMF) tests and isothermal fatigue (IF) tests were conducted using thin-walled tubular specimens under strain-controlled conditions. The results of TMF tests showed a strong correlation between mechanical behavior and temperature cycling. Under different phases of temperature and mechanical loading, the hysteresis loop and mean stress of the single crystal superalloy showed noticeable variations between the stress-controlled and strain-controlled conditions. In the strain-controlled TMF test, temperature cycling led to stress asymmetry and additional damage, resulting in a significantly lower TMF life compared to IF life at the maximum temperature. Moreover, the OP TMF life is generally lower than that of the IP TMF at the same strain amplitude. The Walker viscoplastic constitutive model based on slip systems was used to analyze the TMF mechanical behavior of the single crystal superalloy, and the change trends of the maximum Schmid stress, the maximum slip shear strain rate, and the slip shear strain range were analyzed, and their relationship with the TMF life was investigated. Finally, a TMF life prediction model independent of the loading mode and phase was constructed based on meso-mechanical damage parameters. The predicted TMF lives for different load control modes and phases fell within the twofold dispersion band.

Multidisciplinary approach for investigating the hot corrosion behavior of thermal barrier coatings

Herein, we propose a new multidisciplinary approach for investigating the corrosion behavior of thermal barrier coatings, combining the thermal-gradient mechanical fatigue method with hot corrosion tests. Corrosive salts (Na2SO4; V2O5) of varying concentrations (10−20 mg/cm2) were deposited on the surface of yttria-stabilized zirconia (YSZ) coatings to evaluate the microstructural changes occurring during the reaction, via thermal graded mechanical fatigue tests. The tests were conducted at a surface temperature of 1150 °C in 10 min cycles and underwent applied uniaxial tensile loads. The corrosion tests show that the ZrV2O7 plays an important role as an intermediate in the collapse of the lamellar structures during the reaction, which results from the repetitive precipitation of V2O5 and m-ZrO2 from the ZrV2O7. The microstructure of corroded YSZ coatings exhibited a different degradation mechanism under a thermomechanical environment, compared with the testing under isothermal atmospheric conditions.

Evaluation of plastic deformation and prediction of thermal mechanical fatigue life of an Al–Si alloy piston for diesel engines

AbstractIn this study, a three‐step prediction method was developed to predict the thermal mechanical fatigue (TMF) life of an Al–Si alloy piston: (1) establishing a relation between the plastic deformation and fatigue life under the temperature cycle of 100–350°C via a series of TMF tests of the aluminum alloy specimens; (2) computing the dangerous plastic deformation of the piston under the thermal shock condition of interest by numerical simulation; (3) estimating the life of the piston through taking the computed plastic deformation into the established relation. The effectiveness of the proposed prediction method was verified by thermal shock tests of the piston, and the prediction error is no more than 25%. The mechanism of piston cracking can be summarized as follows: the inhomogeneous plastic deformation induced higher tensile stress around primary Si particles and protrusions at the rim and then microcracks were produced at the protrusions under the dangerous tensile stress, and finally, macrocracks occurred. The life of the piston under the thermal shock load and combustion pressure is speculated to be shorter than that under the thermal shock load. Considering the effect of stress relaxation on plastic deformation, the long full‐load operation of the engine contributes to the piston cracking in T direction, while the long idling operation reduces the piston cracking tendency in T direction and increases the piston cracking tendency in F and R directions.

High temperature corrosion influence on deformation and damage mechanisms in turbine blades made of IN-792 during service

A metallographic study is performed on a turbine blade from a land-based gas turbine in an industrial environment. The aim of the present work is to study the degradation of the turbine blade. The gas turbine has been running for about a month before shut down, which has established steady-state temperature gradients around the cooling holes, causing tensile stresses as a result of cold spots. The tensile stresses cause creep damage, which, in turn, plastically deformed the material, generating substructures and twins near the crack. Furthermore, by comparing substructures from the turbine blade formed during service with substructure from test bars subjected to thermal mechanical fatigue testing gives a strong indication that the damage of the turbine blade is not caused by thermal mechanical fatigue. The turbine blade is also exposed to chemical degradation by type I hot corrosion and internal corrosion/nitridation. Type I hot corrosion has formed Ti-sulfides in grain boundaries and nearby surroundings. Ti-sulfides are also found ahead of the crack tip region. The internal corrosion/nitridation has established TiN, AlN and simultaneously formed a depletion zone near the crack.

Effect of temperature range on thermal-mechanical fatigue properties of P92 steel and fatigue life prediction with a new cyclic softening model

P92 steel is a typical steel used in the main steam pipeline of an ultra-supercritical power plant. In most cases, the main load consists of the thermal-mechanical fatigue (TMF) load due to the fluctuation of temperature and pressure during operation. TMF tests at different temperature ranges and isothermal fatigue (IF) tests at different temperatures were performed in this study. The effects of different temperature ranges on the fatigue life of P92 steel were analyzed. It was found that the TMF life decreased first and then increased, as a result of an increase in the temperature range and decrease in the mean temperature. The cyclic softening of the TMF and IF was also investigated. In addition, it was found that the typical characteristics of dynamic strain aging (DSA) occurred in the TMF and IF process. This is considered to be an important reason for the decrease in the TMF life of P92 steel. A new cyclic softening model was established to describe the cyclic softening behavior of P92 steel, and the effect of the load conditions on the cyclic softening was reflected in this model.

Low cycle fatigue of alloys in hot section components: progress in life assessment

Hot section components are exposed to high temperature operation and thermal transients during service life. The research of improved performances leads to consider the risk of crack initiation due to isothermal (LCF) or thermal-mechanical (TMF) low cycle fatigue loading in component design. This article gives a short survey of progress in LCF and TMF life estimation procedures due to the description of metallic alloys behavior (superalloys, cast iron, stainless steels). Assessing the lifetime under cyclic loading conditions requires constitutive models accounting for viscoplasticity at service temperatures or during transients. These models can be validated at the laboratory scale using thermal mechanical fatigue tests. Crack initiation life prediction models can be quite simple or need to account for creep-fatigue interactions or environmental effects. Examples illustrate advances in materials studies and models over the years in aero-engine and automotive applications.

Hot corrosion influence on deformation and damage mechanisms in turbine blades made of IN-792 during service

Abstract The deformation and damage mechanisms of a gamma-prime hardened superalloy is investigated in the current paper. Two turbine blades made of precision cast polycrystalline superalloy IN-792 have been examined after service exposure under engine conditions typical for industrial gas turbines. This study is compared to a previous study with focus on deformation and damage mechanisms in IN-792 during thermal mechanical fatigue testing performed under laboratory conditions. The failure of the two turbine blades is explained as a combination of two damage mechanisms, mechanical and chemical damage. In the current investigation, type I hot corrosion and creep are the two dominant damage mechanisms. The type I hot corrosion is confirmed by the presence of Ti-sulfides and sulfur in free form at the grain boundaries, which has caused embrittlement and loss of resistance to crack growth. In turn, this has shortened the turbine blade life dramatically and intercrystalline failure is the dominant damage mechanism. Almost all cracks have propagated intercrystalline in the two turbine blades. In the previous study, mechanical damage mechanism is the dominant mechanism and for the highest temperature also oxidation give is contribution. In the previous study, almost all cracks propagated transcrystalline. When exposed to laboratory conditions, the areas around cracks are more plastically deformed compared to the area around the cracks in the turbine blades. In the two studies, dynamic recrystallization has occurred at the grain boundaries.

Thermal-mechanical fatigue behavior and lifetime prediction of P92 steel with different phase angles

The influence of phase angle varying from 0° (in-phase, IP), 90° (clockwise diamond, CD) and 180° (out-of-phase, OP) on thermal-mechanical fatigue behavior and lifetime of P92 steel has been investigated in present paper. Thermal-mechanical fatigue (TMF) tests were conducted under mechanical strain control whose amplitudes were 0.4%, 0.6% and 0.8% with thermal cycles between 550 °C and 650 °C. For comparison, isothermal fatigue (IF) tests also have been achieved at maximum circular temperature of 650 °C under the three mechanical strain amplitudes. Both thermal-mechanical and isothermal fatigue tests were finished with the equal mechanical strain ratio of Rε = −1 and cycle period of 120 s. Results show that the sequence of failure lifetime is NfOP<NfIP<NfCD<NfIF for a given mechanical strain amplitude. Fractographic and microstructural investigations show that cracks initiate and propagate transgranularly for both TMF and IF tests and the creep relaxation and oxidation phenomenon contribute a lot in the process of microstructural damage. Three life prediction models were selected and compared to predict failure lifetime, in which the Ostergren model related to phase angle was modified to achieve a better predictive precision.

Thermomechanical and Thermal Gradient Mechanical Fatigue Lifetime of Thermal Barrier Coating Systems

Detailed damage analyses of an Y2O3 stabilized ZrO2 top coat (TC)–MCrAlY bond coat (BC)–superalloy thermal barrier coating (TBC) system during thermomechanical fatigue (TMF) and thermal gradient mechanical fatigue (TGMF) tests had been performed in present work. During tests, the lifetime of TBCs was strongly dependent on the strain ranges, pre-oxidation time and the thermal gradient in TBCs. Cracks were initiated in the TGO layer, propagated along the TC/TGO or TGO/BC interface, forming the delamination cracks. When the delamination cracks connected with the segmentation cracks which were initiated and propagated in TC, the TBCs spalled. The failure mechanism and stress were analyzed, which were significantly helpful to establish the TMF/TGMF lifetime prediction model for the TBCs.

Constitutive modeling of a directionally solidified nickel-based superalloy DZ125 subjected to thermal mechanical creep fatigue loadings

A transversely isotropic continuum elasto-viscoplasticity model, which was developed from Chaboche’s unified constitutive model, was formulated to capture the thermal mechanical creep fatigue deformation behavior of a directionally solidified nickel-based superalloy. A fourth-order tensor was introduced to model material anisotropy. In order to model the tertiary creep behavior, the Kachanov damage evolution equation was coupled into the stress tensor. Based on the test results of uniaxial tensile, fatigue, and creep loadings at isothermal temperature conditions, the material parameters are obtained. Thermal mechanical fatigue (TMF) and creep–fatigue interaction test results were used to verify the robustness of the model. Additionally, strain–temperature-dependent stress–strain responses under TMF loadings were analyzed using the present model. Under strain-controlled conditions, both of the stress ranges and mean stresses are strongly influenced by the strain–temperature phases, a key parameter for TMF tests.

Alternative Approach for Strain Measurement and Control in Fatigue Tests at Elevated Temperature

Abstract For strain-controlled fatigue tests, especially at elevated temperature, implementation of knife-edges and punching dimples within the gauge length of a specimen is a common practice to mount the extensometer. Premature failure on the test section of the specimens might be introduced by implementing these techniques. In this paper, an alternative approach is introduced to control and measure the strains in fatigue tests. Two dimples outside the gauge-length section are punched and used to mount the extensometer. The correlation between the controlled (measured) strain and the strain within the gauge-length section is established. For the isothermal fatigue tests (strain cycling at constant elevated temperature), the correlation can be established either through analytical analysis or finite-element analysis (FEA). By comparing the hysteresis loops obtained through the correlation and the one directly measured within the gauge length, it is found that the established correlation obtained from FEA has better agreement with the latter result. In addition, this technique can also be used for thermal-mechanical fatigue (TMF) tests (both temperature and strain cycling). However, it is found that the correlation developed through FEA does not give accurate result. Therefore, the interconnection should be established through a cyclic test, in which two extensometers are mounted on the specimen, one measures the strain within the gauge length and the other measures the strain outside the gauge length.

Fatigue behaviour of coke drum materials under thermal-mechanical cyclic loading

Coke drums are vertical pressure vessels used in the delayed coking process in petroleum refineries. Significant temperature variation during the delayed coking process causes damage in coke drums in the form of bulging and cracking. There were some studies on the fatigue life estimation for the coke drums, but most of them were based on strain-fatigue life curves at constant temperatures, which do not consider simultaneous cyclic temperature and mechanical loading conditions. In this study, a fatigue testing system is successfully developed to allow performing thermal-mechanical fatigue (TMF) test similar to the coke drum loading condition. Two commonly used base and one clad materials of coke drums are then experimentally investigated. In addition, a comparative study between isothermal and TMF lives of these materials is conducted. The experimental findings lead to better understanding of the damage mechanisms occurring in coke drums and more accurate prediction of fatigue life of coke drum materials.

Evaluation on Thermo-Mechanical Fatigue Life of IN738LC Using Finite Element Analysis

In this paper, thermo-mechanical fatigue tests were performed for the nickel-based super alloy IN738LC, after which the thermo-mechanical fatigue life was evaluated using finite element analysis. Nickel-based super alloy is used as the main material of turbine blades, which are important equipment in thermal power generation plants. In general, such materials receive three types of damage under thermo-mechanical fatigue loading. In the case of low-cycle fatigue behavior in which large plastic deformation mainly occurs, the lifetime can be decided by its relationship with the plastic strain amplitude. In order to obtain the plastic strain amplitude from the measured strain amplitude, a hysteresis loop should be derived. However, low-cycle fatigue tests are difficult. Moreover, precise experimental techniques are required to obtain the hysteresis loops. In this study, after thermo-mechanical fatigue tests were performed, thermal mechanical fatigue tests on IN738LC were simulated using finite element analysis. The results of analysis were verified by comparing with the hysteresis loops of an experiment

Numerical computation of the energetic criterion at isothermal and thermal–mechanical cyclic tests under generalized plasticity of the F17TNb stainless steel

The aim of this work is to investigate the energetic integral J criterion in generalized plasticity of the F17TNb at isothermal and Thermal Mechanical Fatigue (TMF) tests using a numerical simulation. An optimization of the constitutive law based on mechanical results was carried out using ABAQUS standard solver. Isothermal tests showed that ΔJ evolution is higher at lower temperature (300°C) and a has a linear dependence at log–log presentation with da/dN at 300°C. TMF tests simulation showed that Out of Phase (OP) configuration is more damaging than In Phase (IP) tests. A comparison between OP and isothermal tests highlighted the greater value of ΔJ at isothermal tests.

Thermal–mechanical and isothermal fatigue of 304L stainless steel under middle range temperatures

The present study investigates the crack initiation in a 304L stainless steel under thermal fatigue using volume element tests designed to assess the endurance to engineering crack initiation in real structures under middle range temperature and fairly large number of cycles. The inelastic cyclic strain is significant in most testing conditions for this alloy, even for long tests. Regarding tests, thermal–mechanical fatigue life is compared with low cycle fatigue tests under isothermal conditions. Noteworthy, throughout the different studied ranges of applied temperature cyclic behavior of the alloy has shown an initial hardening followed by a cyclic softening. In addition, no clear effect in lifetime for the high strain range has been discovered. In fact, when exposed to various increasing temperature levels, the material endurance tends to decreases for low strain range (correspond to high number of cycle). A different behavior in cyclic hardening tests is identified between the In-Phase thermal–mechanical fatigue tests and the Out-of-Phase tests at temperature levels ranges between 90 and 165 °C. In-Phase thermal–mechanical test increases lifetime with respect to the Out-of-Phase test. The fracture surfaces for all tested conditions are characterized by a fatigue striation.

Effect of the casting process on microstructure and lifetime of the Al-piston-alloy AlSi12Cu4Ni3 under thermo-mechanical fatigue with superimposed high-cycle fatigue loading

For the present research work, the well-known Al-piston-alloy AlSi12Cu4Ni3 was manufactured in three different processes (gravity die casting – GDC, low pressure die casting – LPDC, and high pressure die casting – HPDC) and T5 heat treated. The microstructure of the material from each process was analyzed, specimens were tested in OP-TMF loading with and without superimposed high-frequency fatigue and lifetimes were compared.The microstructure of GDC specimens shows a homogeneous distribution of primary Si and intermetallic phases. The LPDC material also shows a homogeneous microstructure over the whole sample. However, the Al-mixed-crystal formed bigger dendrite arms compared to the GDC material. The HPDC material shows a gradient in the microstructure getting finer from the center to the outer shape. In the very fine microstructure in the outer regions of the specimen no primary Si was formed and the Al-mixed-crystal built a globular–dendritic structure, surrounded by a eutectically solidified melt. Porosity was higher in the LPDC and HPDC compared to the GDC material.To simulate the thermally induced loading at the “hot side” of a piston during start–stop, strain controlled out-of-phase thermal–mechanical fatigue (TMF) tests with superimposed high-cycle fatigue (HCF) loading were performed. The TMF-cycles were carried out with a minimum temperature Tmin=200°C and a maximum temperature Tmax=440°C. The mechanical strain amplitude εa,tme of the TMF cycles was kept equal to 50% of the thermal strain amplitude εath and the amplitude of the superimposed HCF cycles εa,tHCF was varied between 0.03% and 0.05%.The lifetime of the specimens produced in the GDC-process represents the current state of the art. Compared to that, the specimens manufactured in LPDC and HPDC reached nearly the same TMF lifetimes as the GDC samples. The maximum stress of the LPDC specimens is approximately equal to that observed at the GDC material. However, during the very first TMF cycles, HPDC-specimens show higher maximum stress than the reference material. This is attributed to the very fine, nearly defect free microstructure at the outer shape of the HPDC material. Afterwards, the maximum stress of the HPDC samples is decreasing faster than that of the GDC material due to early formation of crack networks starting from fine pores in the HPDC microstructure. In TMF/HCF-testing the HPDC material shows the same effect. Furthermore, the HPDC samples show pronounced swelling during temperature cycling and TMF testing. The root cause was identified as the high internal pressure of air encased in the pores formed during HPDC.

On TGO creep and the initiation of a class of fatigue cracks in thermal barrier coatings

The initiation of a class of fatigue cracks observed in thermal barrier coatings (TBCs) subjected to thermal gradient mechanical fatigue testing is investigated. The coating system is based on a NiCoCrAlY bond coat and a partially yttria stabilized zirconia top coat. To explain the development of the cracks of interest, the thermo-mechanical response of the bond coat and the thermally grown oxide (TGO) is examined and quantified through finite element analyses. The models include non-linear and time-dependent behavior such as creep, TGO growth stress, and thermo-mechanical cyclic loading. The simulations suggest that stress-redistribution due to creep can lead to tensile stresses in the TGO during TGMF testing that are large enough to initiate the cracks investigated.

Development of Thermal Mechanical Fatigue Testing Machine for Solders

This paper describes development of a thermal mechanical fatigue testing machine for solders. Several researches studying on creep, low cycle fatigue and creep-fatigue for solders at constant temperature have been reported recently, but no or small number of studies on thermal mechanical fatigue of solders have been reported. It might be resulted from that suitable thermal mechanical fatigue testing machines, which can perform the test at temperatures applicable to solders, have not been developed. In this study, the thermal mechanical fatigue testing machine which can cover the test temperatures between 233K (–40°C) and 423K (150°C) is developed. Using the developed testing machine, thermal mechanical fatigue test for Sn-3.5Ag lead-free solder was carried out and an applicability of the testing machine is investigated. This study also discusses the obtained thermal mechanical fatigue life in comparing with data in low cycle fatigue test at constant temperature.

ガスタービン動翼材の疲労特性におよぼす耐食コーティングの影響

Isothermal low-cycle fatigue tests and thermal-mechanical fatigue tests with linear out-of phase cycles were carried out using cylindrical specimens made of Ni-base superalloy and coated with several kinds of corrosion-resistant coatings. In some tests, fatigue loading was interrupted several times and coating cracks were observed by means of replica technique. As a result, three types of coating cracks were observed, namely, multiple parallel long cracks, multiple parallel short cracks and non-parallel cracks. Multiple parallel long cracks are caused from lack of ductility of coating materials. This type of coating cracks initiate in an early stage of fatigue loading and has a strong effect to reduce fatigue life. Multiple parallel short cracks are caused by fatigue of coating layer. These cracks propagate into the substrate before cracks initiate in the substrate and reduce fatigue life of the coated specimen. Non-parallel coating cracks are caused by propagation of cracks initiated in the substrate and have little effect to reduce fatigue life.