Thermomechanical Fatigue Tests Research Articles

Multiaxial isothermal and thermomechanical fatigue behavior of 316LN stainless steel

Effects of multiaxial loading on the isothermal fatigue (IF) and thermomechanical fatigue (TMF) behavior of 316LN stainless steel were comparatively studied. Results showed that the shape of axial stress-strain hysteresis loops was asymmetric in the tensile and compressive half-cycle while the shear hysteresis loops were symmetric. The kernel average misorientation value and low-angle grain boundaries fraction significantly increased, however, the fraction of Σ3 twin boundaries drastically decreased under multiaxial loading, where the coincidence site lattice value (Σ) represented the reciprocal density of coincident lattice sites between the two adjoining grains. The multiaxial non-proportional loading produced much more damage in comparison to uniaxial loading, consequently a significant reduction in fatigue life, regardless of the IF and TMF loadings. Furthermore, the IF life was the shortest, and the longest life occurred in out-of-phase TMF tests.

In-phase thermomechanical fatigue studies on P92 steel with different hold time

Abstract The effect of hold time with 0, 20, and 40 s on in-phase thermomechanical fatigue (TMF) behavior and life of P92 steel is investigated in this study. TMF tests are carried out under mechanical strain control with strain amplitudes of 0.4 0.4 , 0.6 0.6 , and 0.8 % 0.8\text{\%} , and temperature range of 550–650°C which is closely relevant to the operating condition in power plant. TMF tests are performed in a mechanical strain ratio of R = − 1 R=-1 and cycle time of 120 s. The fatigue life variation follows the sequence of N f 0 s < N f 20 s < N f 40 s {N}_{\text{f}}^{0\hspace{.1em}\text{s}}\lt {N}_{\text{f}}^{20\hspace{.1em}\text{s}}\lt {N}_{\text{f}}^{40\hspace{.1em}\text{s}} for the same mechanical strain amplitude. In addition, the influence of hold time on fatigue life decreases with the increasing strain amplitude. A continuous softening can be observed from the cyclic stress response under all test conditions. Fractographic and microstructural tests indicate that the fracture surfaces are characterized by a multi-source cracking initiation and an oxidation phenomenon. Furthermore, a modified Ostergren model is used to predict the fatigue life and achieves a good predicted result.

Thermomechanical fatigue behavior of CGI 450 and CGI 500 cast irons

The vermicular cast iron grade 450 is widely used in engineering applications, primarily by the automotive industry in the fabrication of internal combustion engines. However, in recent years, the vermicular cast iron grade 500 has been developed, and new studies searching for the replacement of the grade 450, with the aim of increasing the high-performance internal combustion engines life, as well as reducing their weight, are in development. Due to their operational characteristics, these engines are subjected simultaneously to thermal and mechanical cycles, which influence their useful life. Therefore, the characterization of the resistance to these types of loads for materials selection for these engines, is very important. Thus, the objective of this work was to study the thermomechanical fatigue life of two vermicular cast irons in conditions close to those to which internal combustion engines are subjected. The materials were characterized by chemical and metallographic analyses and tensile tests. The thermomechanical fatigue tests were carried out at temperatures ranging from 50 ºC to 420 ºC, and a dwell time of 180 seconds. The vermicular cast iron grade 500 exhibited higher tensile strength, with higher ductility when compared to the grade 450, as well as higher fatigue strength. These properties were related to the size and distribution of eutectic cells (graphite) in the matrix.

Plastic-Creep Fatigue Damage Rule for Aluminum Alloys Identified by Particle Swarm Optimization Method

Automotive engine components made of aluminum alloys are often subjected to cyclic loading in a high-temperature range that exceeds 1/2 of the melting temperature Tm of aluminum alloys. The fatigue life of an aluminum alloy subjected to such loading should be evaluated by a fatigue damage law considering not only plastic damage but also creep damage. Then, in this study, we attempted to derive the fatigue damage law for an Al-Si-Cu-Ni-Mg alloy by conducting plastic-creep separation strain analyses using low cycle fatigue (LCF) test data. The material parameters used in the damage law were estimated by employing the particle swarm optimization (PSO) method. The derived fatigue damage law was applied to the fatigue life evaluation for the thermo-mechanical fatigue (TMF) tests which closely relate to the strength reliability evaluation for the engine components in actual use. The results show that the fatigue damage law with the material parameters estimated by the PSO method can well evaluate the fatigue life of the TMF test even though it was derived by using the LCF test data.

High Temperature Fatigue of Aged Heavy Section Austenitic Stainless Steels.

This work investigates two austenitic stainless steels, Sanicro 25 which is a candidate for high temperature heavy section components of future power plants and Esshete 1250 which is used as a reference material. The alloys were subjected to out-of-phase (OP) thermomechanical fatigue (TMF) testing under strain-control in the temperature range of 100 C to 650 C. Both unaged and aged (650 C, 3000 h) TMF specimens were tested to simulate service degradation resulting from long-term usage. The scanning electron microscopy methods electron backscatter diffraction (EBSD) and energy dispersive spectroscopy (EDS) were used to analyse and discuss active failure and deformation mechanisms. The Sanicro 25 results show that the aged specimens suffered increased plastic straining and shorter TMF-life compared to the unaged specimens. The difference in TMF-life of the two test conditions was attributed to an accelerated microstructural evolution that provided decreased the effectiveness for impeding dislocation motion. Ageing did not affect the OP-TMF life of the reference material, Esshete 1250. However, the structural stability and its resistance for cyclic deformation was greatly reduced due to coarsening and cracking of the strengthening niobium carbide precipitates. Sanicro 25 showed the higher structural stability during OP-TMF testing compare with the reference material.

Influence of Ru on the thermomechanical fatigue deformation behavior of a single crystal superalloy

The deformation mechanisms of a single crystal nickel-base superalloy with and without Ru-doped have been investigated under out-of-phase thermomechanical fatigue. The Ru-doped alloy exhibits a thermomechanical fatigue life more than twice as high compared to the Ru-free alloy and a difference in thermomechanical fatigue behavior is also displayed. Microstructure studies by scanning electron microscopy and transmission electron microscopy revealed that the deformation mechanism of the Ru-free alloy in the initial stage is the movement of dislocations in the γ matrix. In the later stage of the thermomechanical fatigue test, large amounts of twins are formed in the material, and a large number of stacking faults and dislocations are sheared into the γ' precipitates. By comparing with the Ru-free alloy, the Ru-doped alloy has a higher matrix strength due to the solid solution strengthening effect of Ru, and is also prone to different deformation mechanisms. For example, the stacking faults are formed in the initial thermomechanical fatigue cycles and remain in the matrix throughout the entire thermomechanical fatigue process. The formation of twins, on the other hand, is suppressed by Ru addition. Such effects are believed to extend the thermomechanical fatigue life effectively.

Experimental characterization of the short crack growth behavior of a ductile cast iron (DCI GJS-500) affected by intergranular embrittlement at temperatures nearby 400 [formula omitted

In this investigation, the fatigue and crack growth behavior of the ductile cast iron material (DCI) GJS-500 was experimentally characterized by performing isothermal low cycle fatigue (LCF) tests, out-of-phase thermomechanical fatigue (OPTMF) tests as well as low cycle fatigue tests combined with the replica testing method (LCF-R) within the temperature range RT–500 °C. The studied material exhibits an embrittlement at temperatures nearby 400 °C, leading to a significant higher fatigue crack growth rate and a reduced lifetime, when compared with RT and 500 °C. A possible explanation for the observed higher fatigue crack growth rate coupled with a significant lifetime reduction, is intergranular embrittlement.

Architected Tunable Structure for Improved Capability in Extreme Environments

Abstract A novel patterned-void structure is developed to improve the fatigue life compared with conventional circular cooling holes typically used in gas turbine components exposed to high temperatures. The distinctive S-shape of the voids and their specific arrangement enable manipulation of the structure's macroscopic stiffness and Poisson's ratio. An investigation of the isothermal and thermomechanical fatigue (TMF) properties of the proposed structure is carried out in strain-controlled conditions. The testing is performed on tubular specimens machined from a Nickel-based superalloy commonly used in gas turbine combustion systems (HAYNES 230®). The isothermal fatigue tests, performed at 300 °C, 600 °C, and 800 °C, demonstrated an increase in crack-initiation life of the proposed structure by a factor of up to 28 compared with the standard circular holes. The thermomechanical fatigue tests, performed across temperature ranges 300 °C–750 °C and 300 °C–850 °C, and using in-phase (IP) and out-of-phase (OP) strain ratios, demonstrated an increase in crack-initiation life by a factor of up to 16. The life after crack initiation (crack-propagation mode) was also shown to be longer for the proposed structure, which is attributed to a crack-arresting behavior inherent to the structure.

ToF-SIMS Analysis of Demineralized Dentin Biomodified with Calcium Phosphate and Collagen Crosslinking: Effect on Marginal Adaptation of Class V Adhesive Restorations

This study aimed to assess the effect of biomodification before adhesive procedures on the tooth-restoration interface of class V restorations located in caries-simulated vs. sound dentin, and the quality of dentin surface by time-of-flight secondary ion mass spectrometry (ToF-SIMS). Class V cavities located on cervical dentin were prepared on the buccal surfaces of extracted human molars under the simulation of intratubular fluid flow. Two dentin types, i.e., sound and demineralized by formic-acid, were biomodified with 1% riboflavin and calcium phosphate (CaP) prior to the application of a universal adhesive (Clearfil Universal Bond) in etch and rinse or self-etch mode, and a conventional micro hybrid composite (Clearfil APX). Restorations were subjected to thermo mechanical fatigue test and percentages of continuous margins (% CM) before/after fatigue were compared. Bio modification of dentin surfaces at the molecular level was analyzed by Time-of-Flight Secondary Mass Spectometry (ToF-SIMS). % CM were still significantly higher in tooth-restoration interfaces on sound dentin. Meanwhile, biomodification with riboflavin and CaP had no detrimental effect on adhesion and in carious dentin, it improved the % CM both before and after loading. Etching carious dentin with phosphoric acid provided with the lowest results, leading even to restoration loss. The presence of molecule fragments of riboflavin and CaP were detected by ToF-SIMS, evidencing dentin biomodification. The adhesive interface involving carious dentin could be improved by the use of a collagen crosslinker and CaP prior to adhesive procedures.

Experimental investigation of the damage characteristics of two cast aluminium alloys: Part I – Temperature dependent low cycle and thermomechanical fatigue behavior

Detailed material investigations of the fatigue behavior of two cast aluminium alloys used in combustion engines are presented. The network of intermetallic phases of both aluminium alloys is characterized by means of detailed energy dispersive X-ray spectroscopy. In order to investigate the temperature-dependent fatigue behavior of the materials, tensile, low cycle and thermomechanical fatigue tests are performed over a wide temperature and loading range. The influence of the temperature dependence on the experimental results is discussed.

Multiaxial low-cycle thermo-mechanical fatigue of a low-alloy martensitic steel: Cyclic mechanical behaviour, damage mechanisms and life prediction

Axial–torsional Low-Cycle Fatigue tests (LCF) and Thermo-Mechanical Fatigue tests (TMF) were performed on a low-alloy martensitic steel for temperatures between 300°C and 600°C, as both proportional and non-proportional. The results show that non-proportional loading leads to an increase in fatigue damage and to a decrease in the observed lifetime. The TMF damage model is proposed here in order to incorporate the effects of multiaxial loading and varying temperatures. This damage model is based on a critical plane approach, and it incorporates the fatigue, oxidation and creep forms of damage. Finally, the proposed model is validated for a large experimental database.

Thermomechanical fatigue behaviour and damage mechanisms in a 9% Cr steel: Effect of strain rate

Thermomechanical fatigue (TMF) loading is inevitable during the operation of ultra-supercritical power plants. In the present work, TMF tests are carried out at strain rates of 5 × 10−5/s to 4 × 10−4/s and in-phase (IP) and out-of-phase (OP) to investigate the cyclic deformation and damage mechanisms of P92 steel. The results reveal that the increase in strain rate leads to a significant improvement in fatigue life under both IP-TMF and OP-TMF, in which OP-TMF induces more severe damage to fatigue life than IP-TMF. It was found that the fatigue life reaches a maximum after a strain rate of 1 × 10−4 s−1. It is important to note that the action of dynamic strain ageing (DSA) is strain rate-, fatigue cycle- and phase angle-dependent. It was also demonstrated that the effect of the strain rate on the fatigue life is a combined effect of DSA, fatigue cracks, creep voids and oxidation. Additionally, the predictive abilities of various traditional life prediction methods under various loading conditions are comparatively assessed.

Investigation of Thermal Gradient Mechanical Fatigue Test Methods for Nickel-based Superalloys

Temperature gradients significantly affect the material fatigue process. A reliable and robust test procedure is needed for quantifying the effects of temperature gradients on the evolution of fatigue damage in nickel-based superalloys. The present study aims to develop a radiation heating system for universal material testing machine for simulating thermal gradient mechanical fatigue in turbines. The developed heating system mainly consists of halogen lamps, reflectors, cooling subsystems, and control units. Based on extensive experimental and computational investigations, a thermal model is developed for accurate thermo-mechanical fatigue testing under given temperature gradients in the heating system, and it is applied to variable temperature fatigue tests. The detail procedure for determining heat transfer coefficients is presented based on experimental and computational results. TGMF tests are successfully performed with the developed radiation heating system. The temperature gradients are found to reduce the TGMF life significantly in comparison to that of the TMF life. It is confirmed that the developed thermal gradient mechanical fatigue methodology can be applied to different thermo-mechanical fatigue tests with various temperature gradients.

On the correlation between microstructural parameters and the thermo-mechanical fatigue performance of cast iron

Strain-controlled out-of-phase thermo-mechanical fatigue tests at 100–500 °C and various strain ranges were conducted on five cast iron grades, including one lamellar, three compacted and one spheroidal graphite iron. Investigations of graphite morphology and matrix characteristics were performed to associate parameters, such as geometrical features of graphite inclusions and the matrix microhardness, to the thermo-mechanical fatigue performance of each grade. From this, thermo-mechanical fatigue life as a function of maximum stress at half-life, is found to decrease consistently with increasing average graphite inclusion length irrespective of the graphite content. In contrast, no evident correlation between the fatigue life and the matrix microhardness is observed.

Life assessment of a low-alloy martensitic steel under isothermal low-cycle fatigue-creep and thermo-mechanical fatigue-creep loading conditions

Strain-controlled Low-Cycle Fatigue tests (LCF) and Thermo-Mechanical Fatigue tests (TMF) were performed on a low-alloy martensitic steel in the temperature range between 300 °C and 600 °C, with and without a strain dwell. TMF tests were performed for three different phase angles between thermal strain and mechanical strain: as in-phase, as out-of-phase and as counter clockwise-diamond. The results show that the phase angle that is employed has a significant influence on the dominant damage mechanism and on the observed lifetime. Finally, a modified damage model is proposed in order to take into account the effect of the phase angle.

Influence of thermomechanical fatigue loading conditions on the nanostructure of secondary hardening steels

Abstract Dual hardening steels reach their well-balanced mechanical properties in terms of strength and toughness through the combination of secondary hardening carbide and intermetallic particle precipitation. This characteristic profile makes them well suited for hot-work applications. In this study, out-of-phase thermomechanical fatigue tests, recreating operating conditions present during hot-work applications, were performed on a dual hardening steel and a 5% Cr martensitic hot-work tool steel. Via high resolution analysis utilizing atom probe tomography and transmission electron microscopy, the behaviour of the different precipitate populations under combined thermal and mechanical loading conditions were compared. Coarsening of the different precipitates and partial dissolution of the intermetallic compounds was observed. It could be shown that with rising maximum fatigue test temperature, the dual hardening steel reaches an increased lifetime caused by its higher tempering resistance.

Investigation and simulation of the age-dependent behaviour of an aluminium alloy under thermomechanical loading

A constitutive material model, which predicts the material behaviour of an Al–Si–Mg–Cu alloy under thermomechanical load, is proposed. Moreover, an ageing model is used to simulate the ageing behaviour, which also affects the hardening parameters. A new strategy for determining the material parameters is proposed, and a new testing concept for investigating the ageing properties of materials is presented. Thermomechanical fatigue tests are used to validate the material model. The results of the simulations show that it is possible to determine the material parameters based on low-cycle fatigue, relaxation, and ageing tests such that a cycle-by-cycle simulation of thermomechanical fatigue tests is possible. Finally, a comparison of the simulation and test results show a good agreement.

On the Dominance of Creep–Fatigue Interaction Over Oxidation in Thermomechanical Fatigue Behavior of a Diffusion Aluminide Coated Near α Titanium Alloy

In the present study, in-phase (IP) and out-of-phase (OP) mechanical strain controlled thermomechanical fatigue tests at two strain rates, viz. 6.67 × 10–5 s−1 and 3.33 × 10–4 s−1, have been conducted on a near α titanium alloy applied with a diffusion aluminide coating. Microstructural examination of failed specimens revealed the most likely operative mechanisms causing reduction in fatigue life. While lowering the strain rate was found to cause creep–fatigue interaction in IP-TMF loading, severity of oxidation increased marginally in case of OP-TMF loading. The alloy exhibited lowest fatigue life under IP-TMF loading at 6.67 × 10–5 s−1 which was attributed to the dominance of creep–fatigue interaction over other operative mechanisms.

Experimental and computational study on thermo‐mechanical fatigue life of aluminium alloy piston

AbstractTo assess the life of a new diesel aluminium alloy piston under thermal shock loads, thermo‐mechanical fatigue (TMF) testing was conducted to characterise the TMF properties of the piston alloy, and an empirical model based on the constraint ratio concept was proposed to predict the TMF life of the piston. Considering that the empirical model required expensive experimental support, a platform with high‐frequency induction heating was established to simulate the force on the piston under thermal shock loads to calculate the piston life using the thermal shock test. Additionally, a finite element method was developed to compute the distributions of temperature, strain, and stress during this process. The characteristics of crack initiation and propagation in TMF test rods and piston mock‐ups were also investigated. The results showed that the TMF test rod suffered brittle fracture with brittle quasi‐cleavage features. The microcracks mainly occurred in primary Si particles due to stress concentration around the primary Si particles induced by the difference between the thermal expansion coefficients of Si and Al. From a macro perspective, the piston initially cracked at the rim above the pinhole, where the stress is larger than that along other directions. From a micro perspective, the protrusions of various sizes on the piston rim were induced by the compression stresses at high temperature. The piston cracks usually initiate around primary Si particles, propagate along the edge of primary Si in a straight line, bifurcate and then stop at a certain depth. If the piston was only heated, cracks or plastic deformations were not produced. The piston life can be assessed using the proposed empirical model based on the constraint ratio concept or thermal shock testing based on the developed platform. The difference between the predicted and experimental life was not greater than 7%.

Influence of specimen diameter size on the deformation behavior and short-term strength range of an aluminum alloy

Abstract Components often manifest varied local behavior due to their manufacturing process. In order to be able to determine local material behavior in the best possible way, it is necessary to take specimens from the area under investigation. Due to constant developments in efficiency and lightweight construction, it is difficult to produce standard-compliant specimens from the examined area in a component. For this reason, specimens with smaller dimensions are often taken. Through the investigation of the influence of size in the area of high-cycle fatigue, it is well known that the size of a test specimen influences its lifespan. Not so much is known about the influence of specimen size on the behavior of material in the field of low-cycle fatigue (LCF). In this work, tensile, LCF and thermomechanical fatigue tests are performed using AlCu4PbMgMn with varied specimen geometries, the smallest test diameter being 3 mm, the largest 7.5 mm. The results of the tensile test show that the mean values of tensile strength for both diameters is within one percent. At LCF load and thermomechanical load, there are no or only slight deviations in deformation behavior. The low cycle fatigue behavior at RT is identical for both diameters. However, the results show that stress-strain behavior is the same for both test diameters, and fatigue behavior is the same, except in tests with high strain amplitudes and temperature.