Fatigue design of structures under thermomechanical loadings

ABSTRACTThis paper presents a global approach to the design of structures that experience thermomechanical fatigue loading, which has been applied successfully in the case of cast‐iron exhaust manifolds. After a presentation of the design context in the automotive industry, the important hypotheses and choices of this approach, based on a thermal 3D computation, an elastoviscoplastic constitutive law and the dissipated energy per cycle as a damage indicator associated with a failure criterion, are first pointed out. Two particular aspects are described in more detail: the viscoplastic constitutive models, which permit a finite element analysis of complex structures and the fatigue criterion based on the dissipated energy per cycle. The FEM results associated with this damage indicator permit the construction of a design curve independent of temperature; an agreement is observed between the predicted durability and the results of isothermal as well as non isothermal tests on specimens and thermomechanical fatigue tests on real components on an engine bench. These results show that thermomechanical fatigue design of complex structures can be performed in an industrial context.

Viscoelastic material behaviour of PBT-GF30 under thermo-mechanical cyclic loading

Reduced softening of EUROFER 97 under thermomechanical and multiaxial fatigue loading and its impact on the design rules

The scope of this paper is the investigation of damage mechanisms at microscopic scale on a short glass fiber reinforced polybutylene terephthalate (PBT-GF30) under thermo-mechanical cyclic loading. In addition the principal mechanisms are verified through micro mechanical FE models. In order to investigate the fatigue behavior of the material both isothermal strain controlled fatigue (ISCF) tests at three different temperatures and thermo-mechanical fatigue (TMF) tests were conducted on plain and notched specimens, manufactured by injection molding. The goal of the work is to determine the damage mechanisms occurring under TMF conditions and to compare them with the mechanisms occurring under ISCF. For this reason fracture surfaces of TMF and ISCF samples loaded at different temperature levels were analyzed using scanning electron microscopy. Furthermore, specimens that failed under TMF were examined on microsections revealing insight into both crack initiation and crack propagation. The findings of this investigation give valuable information about the main damage mechanisms of PBT-GF30 under TMF loading and serve as basis for the development of a TMF life estimation methodology.

Failure of thermal barrier coating systems under cyclic thermomechanical loading

Experimental fatigue lifetime of coated and uncoated aluminum alloy under isothermal and thermo-mechanical loadings

A gas turbine blade operates under severe conditions including high temperature, high speed rotation, and frequent starts and shutdowns. Under such operating conditions, a blade is exposed to a thermo mechanical fatigue environment. These conditions shorten the life of the turbine parts and reduce the reliability of the gas turbine. Therefore, research on the characteristics of blade material is necessary. In this study, thermo-mechanical fatigue (TMF) tests were conducted for IN738LC, which is base a material for turbine blades. Tests were conducted under both the IP (in-phase) and OP (out of-phase) conditions, and tests results were compared in terms of fatigue life. After obtaining the ε-N curves, additional TMF tests were conducted according to variations in fatigue life. The purpose of the TMF test is to observe the change in mechanical properties of materials damaged during TMF and how these relate to fatigue life. For this purpose, an indentation test was conducted and the relationship between hardness and fatigue life were obtained.

In this article, out-of-phase thermo-mechanical fatigue (TMF) behaviours of light alloys were investigated in comparison to their high temperature low cycle fatigue (LCF) behaviours. For this objective, strain based fatigue tests were performed on the A356 aluminium alloy and on the AZ91 magnesium alloy. Besides, TMF tests were carried out, where both strain and temperature changed. The fatigue lifetime comparison demonstrated that the TMF lifetime was less than that one under LCF loadings at elevated temperatures for both light alloys. The reason was due to severe conditions in TMF tests in comparison to LCF tests. The temperature varied in TMF test but it was constant under LCF loadings. As the other reason, the tensile mean stress occurred under TMF loadings, in comparison to the compressive mean stress under LCF loadings. At high temperatures, the cyclic hardening behaviour occurred in the AZ91 alloy and the A356 alloy had the cyclic softening behaviour.

Two unique facilities for isothermal low-cycle fatigue (LCF) and in-phase (IP) or out-of-phase (OP) thermo-mechanical fatigue (TMF) testing of tubular specimens under boiling water reactor (BWR) and pressurized water reactor (PWR) coolant conditions were set up and successfully tested. The systems allow both strain- and stress-controlled fatigue experiments with very small strain amplitudes or complex stress/strain profiles with superimposed rather rapid temperature changes (100°C to 340°C) under flowing conditions. The present article introduces the new test facility and briefly discusses the first results of these experiments. In contrast to air, a strong effect of temperature on LCF and TMF lives was observed in deoxygenated and hydrogenated high-temperature water environment, where both lives decrease with increasing temperature above 100°C. The TMF life is between that of the isothermal LCF tests at minimum and maximum temperature. The IP TMF life is shorter than that of OP TMF and close to the LCF life at maximum temperature. The OP TMF life is close to that of LCF at minimum temperature. In a hydrogenated environment, the LCF and TMF lives seem to be slightly shorter than predicted by the NUREG/CR-6909 approach of the United States Nuclear Regulatory Commission (NRC) Regulatory Guide 1.207. Significant environmental effects were observed at 100°C, which is significantly below the temperature threshold of 150°C.

Self-etch vs etch-and-rinse adhesives: effect of thermo-mechanical fatigue loading on marginal quality of bonded resin composite restorations

In this article, the thermomechanical fatigue damage evolution of fiber-reinforced ceramic–matrix composites subjected to multiple loading sequences is investigated. The damage evolution models considering synergistic coupling effects of thermomechanical fatigue loading sequences, thermal cyclic temperature, and multiple thermomechanical fatigue damage mechanisms (i.e. matrix multi-cracking, fiber/matrix interface debonding/sliding/wear) are developed. The effects of composite material properties, peak stress, and matrix crack spacing on the thermomechanical fatigue damage evolution of fiber-reinforced ceramic–matrix composites are discussed. Comparisons of damage evolution between the in-phase and the out-of-phase thermomechanical fatigue loading under multiple loading sequences are performed. The thermomechanical fatigue hysteresis energy, hysteresis modulus, peak strain, and interface debonded length of cross-ply ceramic–matrix composites subjected to different loading sequences are predicted. The damage under thermomechanical fatigue multiple loading sequences is much more serious than that under thermomechanical fatigue single peak stress level.

Effects of surface roughness on thermo-mechanical fatigue life of a P91 power plant steel

High-temperature fatigue behavior of a near-γ titanium aluminide was studied over the temperature range from 500–750 °C under both isothermal and thermomechanical fatigue (TMF) conditions. Cyclic stress-strain (CSS) response was observed to change drastically with test temperature. In isothermal tests conducted below the ductile-to-brittle transition temperature (DBTT) initial cyclic hardening occurred. By contrast, in isothermal fatigue tests performed above the DBTT the material displayed pronounced cyclic saturation throughout the test. Still, modeling of CSS behavior under TMF conditions based solely on isothermal input data is feasible, as corresponding microstructures evolve in both types of tests. Environmental degradation plays a key role in this material, and the effects were assessed based on fatigue tests conducted in air and high-vacuum, respectively. Moreover, the material displays large mean stress effects below the DBTT, and thus, out-of-phase TMF tests were found to be most detrimental, when the temperature range encompassed the DBTT.

Thermomechanical fatigue in the automotive industry

A Study on Fatigue Life Prediction of Ni-base Superalloy