In-phase Thermomechanical Fatigue Tests Research Articles

Microcrack initiation mechanisms of 316LN austenitic stainless steel under in-phase thermomechanical fatigue loading

In-phase thermomechanical fatigue tests with various mechanical strain amplitudes (0.6% to 1.2%) at temperature interval of 250–450 °C were carried out on 316LN austenitic stainless steel. The principal deformation and damage mechanisms include dynamic strain aging (DSA) and oxidation. The operation temperature range of DSA is about 400–600 °C determined by monotonic tensile tests. The distinct serrated flow in hysteresis loops and significant cyclic hardening are representative manifestations of the occurrence of DSA which promotes formation of well-developed planar slip bands and intense persistent slip markings (PSMs) characterized by surface extrusions and intrusions. Conspicuously localized oxidation at sensitive grain boundaries and along PSMs is observed with an iron oxide composition. Moreover, the degree of oxidation is positively correlated to strain amplitude. The oxidation plays a significant role in microcrack nucleation and growth which is further enhanced by the strong interaction between oxidation and DSA. The oxidation-assisted intergranular cracking is controlled jointly by a combination of effects including thermal mismatch, slip bands impingement and deformation incompatibility while two different types of transgranular cracks along oxidized PSMs are proposed depending on the particular locations where brittle oxide cracking occurs.

Thermomechanical fatigue behavior of annealed Cu-Cr-Zr-Ti alloy in argon atmosphere

Strain controlled thermomechanical fatigue (TMF) behavior of Cu-Cr-Zr-Ti alloy was studied in the temperature range of 375–525°C in high purity argon atmosphere under both in-phase (IP) and out-of-phase (OP) loading. In the case of OP TMF tests, the material exhibited continuous hardening without any saturation of stress. On the other hand, it showed cyclic hardening for the first few cycles followed by saturation of stress in IP TMF tests. The material exhibited superior lives under OP TMF loading compared to that under IP TMF loading especially at lower strain amplitudes. This may be attributed to the fact that the material experienced peak tensile stress at lower temperatures in OP TMF tests, where the resistance to crack initiation is expected to be higher. Microstructural observations and fractographic studies indicated mixed mode of fracture involving Intergranular facets and transgranular cracking in both IP and OP TMF tests.

Creep-fatigue interaction and thermo-mechanical fatigue behaviors of thermoplastics and their composites

Creep-fatigue interaction and thermo-mechanical fatigue (TMF) behaviors of injection molded thermoplastic composites were investigated. Talc-filled polypropylene and short glass fiber reinforced polypropylene, polyamide-6.6, and a blend of polyphenylene-ether and polystyrene were used for the experimental study. Trapezoidal load signal with hold time periods was used for the creep-fatigue testing. Effects of temperature, frequency, load level, and hold-stress position on creep-fatigue interaction behavior were studied. In-phase TMF tests were conducted on polyamide-based composites for the temperature variation between 85 and 120°C. Significant non-linearity was observed for the interaction of creep and fatigue damage. The applicability of Chaboche non-linear creep-fatigue interaction model to predict creep-fatigue and TMF lives for thermoplastic composites was investigated. A frequency term was added to the model to consider the beneficial effect of increased frequency observed for some thermoplastics. More than 90% of the life predictions by the Chaboche model were within a factor of 2 of the experimental life for both creep-fatigue and TMF test conditions.

Fatigue of the Near-Alpha Ti-Alloy Ti6242

Ti6242 is the workhorse of high-temperature Ti-alloys in the high pressure compressor of aero engines. In this study the influence on isothermal fatigue of different load controls, i.e. stress, total strain and plastic strain control at different temperatures and environments was investigated. The alloy had a bi-modal microstructure (some 30 vol.% primary alpha), which yields a good balance between fatigue and creep properties. In addition thermomechanical fatigue (TMF) tests were also performed. Modelling lifetime on the basis of a Basquin–Coffin–Manson relationship revealed only marginal scattering in the temperature range between 350°C and 650°C. Increasing the temperature led to a decrease in lifetime. This can be attributed to increased oxidation and creep. The latter one is clearly seen in isothermal tests under stress control. Tests in vacuum resulted in longer lifetimes. In-phase TMF tests exhibited a longer lifetime than out-of-phase tests, with a factor of about 4. Lifetime and stress response of in-phase tests are similar to the corresponding lifetime of an isothermal test at the maximum temperature. This similarity can be considered as a starting point for modelling TMF behaviour on the basis of isothermal fatigue.

Creep–fatigue interaction of the ODS superalloy PM 1000

This study reports on the creep–fatigue interaction of the oxide dispersion strengthened Ni-base superalloy PM 1000. Fully reversed symmetrical push–pull isothermal fatigue, thermomechanical fatigue (TMF), slow-fast and tensile hold time tests were conducted in the temperature range from 450 to 1050 °C. TMF tests resulted in unexpectedly low fatigue lives. Grain boundary cavitation observed in the in-phase TMF tests indicated that creep damage plays an important role under TMF loading conditions. Similarly, fatigue life dependence on wave-shape demonstrated that creep–fatigue tests exhibit the shortest cyclic life due to the additional creep damage in the tensile-going part of the cycle. Damage mechanisms were observed to vary substantially depending on the test mode. In the creep–fatigue tests, the grain boundaries of fine equiaxed grains triggered multiple internal crack initiation, thereby pointing to the adverse effects of these microstructural features. Cavitation occurs by a local stress directed diffusion and growth of voids, as evidenced by dispersoid free zones in areas near the transverse grain boundary cavities. Time-dependent damage occurring at and above 850 °C in the creep–fatigue tests is mostly due to creep whereas environmental attack plays only a minor role.

The characteristics of fatigue under isothermal and thermo‐mechanical load in Cr–Ni–Mo cast hot work die steel

ABSTRACT High temperature isothermal fatigue (IF) and in‐phase thermo‐mechanical fatigue (TMF) tests in load control were carried out in cast hot work die steel. At the same load amplitude, the fatigue lives obtained in the in‐phase TMF tests are lower than those obtained in the isothermal tests. Observations of fracture surface and the response of stress–strain reveal that cyclic creep in the tensile direction occurs and the intergranular cracks dominate in TMF tests, whereas cyclic creep in the compressive direction occurs and the path of the crack growth is mainly transgranular in IF tests. A model of life prediction, based on the Chaboche law, was discussed. Damage coefficients that are functions of the maximum temperature and the variation of temperature are introduced in the model so as to evaluate TMF lives in load control. With this method, the lifetime prediction gives results corresponding well to experimental data.

High temperature fatigue behaviour of TZM molybdenum alloy under mechanical and thermomechanical cyclic loads

High temperature isothermal mechanical fatigue and in-phase thermomechanical fatigue (TMF) tests in load control were carried out on a molybdenum-based alloy, one of the best known of the refractory alloys, TZM. The stress–strain response and the cyclic life of the material were measured during the tests. The fatigue lives obtained in the in-phase TMF tests are lower than those obtained in the isothermal mechanical tests at the same load amplitude. It appears that an additional damage is produced by the reaction of mechanical stress cycles and temperature cycles in TMF situation. Ratcheting phenomenon occurred during the tests with an increasing creep rate and it was dependent on temperature and load amplitude. A model of lifetime prediction, based on the Woehler–Miner law, was discussed. Damage coefficients that are functions of the maximum temperature and the variation of temperature are introduced in the model so as to evaluate TMF lives in load control. With this method the lifetime prediction gives results corresponding well to experimental data.

Modeling fiber breakage in a metal-matrix composite

An analysis is presented of the effects of a broken fiber in a representative volume element (RVE) of a metal-matrix composite. Results from a simple model show that strain accumulation from occasional fiber breaks during in-phase thermomechanical fatigue tests can be considerably larger than the corresponding percentage reduction in stiffness, thereby corroborating experimental observations. Detailed finite element analysis of the RVE assuming elastic-plastic matrix material behavior supports the results of the simple model and provides further details of the relationships between strain increase, stiffness decrease, debond length along fiber/matrix interface, and opening displacement due to a broken fiber. The load transfer from the matrix to the fiber at the tip of the interface crack is found to occur through a concentrated shear load, in contradiction to the common assumption of a finite shear-lag distance.

Thermo-Mechanical Fatigue of Mar-M247: Part 1—Experiments

Isothermal fatigue tests, out-of-phase and in-phase thermo-mechanical fatigue tests were performed on Mar-M247 nickel-based superalloy. The experiments were conducted in the temperature range 500°C to 871°C. Results indicate that the lives differ with strain-temperature phasing and with strain rate. The results of out-of-phase thermo-mechanical tests correspond well with strain-life data of isothermal tests conducted at the peak temperature (871°C). However, the in-phase thermo-mechanical results differed depending on the strain amplitude. Significant surface and crack tip oxidation and gamma prime depletion has been observed based on metallographic and Auger Spectroscopic analyses. These changes were measured as a function of time. The environment induced changes significantly influenced the fatigue lives in isothermal and out-of-phase thermo-mechanical fatigue cases. In these cases transgranular cracking was observed. Grain boundary crack nucleation and grain boundary crack growth dominated the in-phase thermo-mechanical fatigue cases. Based on these observations the requirements for a life prediction model are outlined. The life prediction model and the predictions are given in Part 2 of this paper.

Thermomechanical fatigue, oxidation, and creep: Part i. Damage mechanisms

Isothermal fatigue tests and both out-of-phase and in-phase thermomechanical fatigue tests were performed in air and in helium atmospheres. A wide range of temperatures from 20 ‡C to 700 ‡C was considered in these tests on 1070 steel specimens. A procedure for inert atmosphere testing using encapsulated specimens is described. Results indicate that the fatigue lives are 2 to 12 times greater in helium than in air. Interrupted tests were performed to characterize the pro-gression of damage in the material. Results indicate that oxidation-induced crack nucleation and crack growth are detrimental at high temperatures for isothermal and out-of-phase thermome-chanical fatigue tests. In these tests, transgranular cracking is observed. However, creep-induced intergranular cracking is the dominant damage mechanism during in-phase thermomechanical fatigue tests.