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Microstructural factors affecting creep-fatigue behaviour of two-phase titanium alloys

Ziaja W.

Advances in Manufacturing Science and Technology

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2016

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Vol. 40, nr 1

67--74

EN

In the paper the dwell fatigue behaviour of two-phase Ti-6Al-2Mo-2Cr alloy (VT3-1) at elevated temperature was investigated. The reasons of high sensitivity of titanium alloys to dwell periods at high stress level during cyclic loading were summarized. The relations between morphology of the alloy microstructure and the effect of dwell periods at peak stress was established. The microstructure of the alloy was varied by means of changing conditions of heat-treatment. Dwell fatigue tests were carried out at the temperature of 400°C. Relative contributions of cyclic and creep processes to the overall damage were evaluated as a function of stress level.

PL

W pracy przedstawiono wyniki badań właściwości dwufazowego stopu tytanu Ti-6Al-2Mo-2Cr (WT3-1) w warunkach pełzania-zmęczenia. Przedstawiono źródła dużej wrażliwości stopów tytanu na okresy przestojów przy wysokim poziomie naprężenia w warunkach obciążenia cyklicznie zmiennego. Określono zależność między morfologią mikrostruktury stopu a wpływem okresów przestojów przy maksymalnym naprężeniu w cyklu obciążenia na trwałość materiału. Mikrostrukturę stopu kształtowano metodami obróbki cieplnej. Próbę zmęczeniową bez i z okresami przestojów przy maksymalnym naprężeniu prowadzono w temperaturze 400°C. Oszacowano względny udział zjawisk towarzyszących zmęczeniu i pełzaniu w procesie niszczenia materiału.

2

Lifetime prediction in creep-fatigue environment

Sabour M.H., Bhat R.B.

Materials Science Poland

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2008

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Vol. 26, No. 3

563--584

EN

The creep-fatigue interaction has been studied and innovative mathematical models are proposed to predict the operating life of aircraft components, specifically gas turbine blades subject to creep-fatigue at high temperatures. The historical evolution of the creep-fatigue lifetime prediction is given in order to place the present study in the context. A literature review of the life estimation under creep-fatigue environment is presented.

3

Creep fatigue of multi-part container during hot extrusion of copper - Simulation and experimental comparison

Krumphals F., Wlanis T., Redl C., Sommitsch C.

Computer Methods in Materials Science

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2007

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Vol. 7, No. 1

47-53

EN

Extrusion tools exhibit a complex strain-time pattern under a variety of cyclic loading conditions and thus are prone to failure by creep-fatigue interactions. Elevated temperature failure by creep-fatigue processes is time dependent and often involves deformation path dependent interactions of cracks with grain boundary cavities. The extrusion industry tries to accelerate the process by increasing the billet temperature and/or by accelerating the press speed that raise the loading of the tools. On the other side the tool steel producers develop enhanced more homogeneous and cleaner materials in order to increase tools lifetime. Finite element simulation of the extrusion process to get the temperature and stress evolution in the container, coupled with constitutive equations as well as lifetime consumption models in order to calculate both the inelastic strains and the tools lifetime, help to optimise the extrusion process and to compare the operating times of different hot work tool steels. Viscoplastic constitutive models were developed in the past to take into account the inelastic behaviour of the material during creep-fatigue loads. In the present study the Chaboche model was selected and calibrated to the material response of a hot work tool steel between 470°C and 590°C. To extend the prediction capability of Chaboche’s model for non-isothermal processes a temperature-rate term was added to the isotropic hardening rule. Additionally, a creep-fatigue lifetime rule for complex processes was investigated that is independent of single loading parameters, like stress or strain ranges or corresponding maxima, for the description of an entire cycle. Instead this rule evaluates the total damage in each time increment and accumulates that to the lifetime consumption. The present paper shows the development of temperatures, stresses and lifetime consumption during three copper extrusion cycles in a two-part container. The simulation of the heat treatment and the resulting state of the container used was the basis for the subsequent modelling of the cyclic loads during the press cycles. The numerical FEM extrusion simulation consists of the plastic simulation of the billet extrusion with rigid tools as well as of the subsequent simulation of several cycles of the same process, only considering the elastic container and using the time dependent temperature and pressure boundary conditions at the contact surface billet-liner. The reason for this procedure is the much shorter calculation time for the elastic container model with specified boundary conditions in comparison to the plastic extrusion process, especially for several extrusion cycles. Both a constitutive law and a lifetime consumption rule were coupled to the elastic container model in order to get the local inelastic strain rates and the damage rate, respectively. To verify the calculated temperature and pressure boundary conditions at the contact surface billet-liner, a experimental extruding plant was constructed. To obtain a pressure distribution, three holes at different levels were drilled into the container, with only a thin container wall thickness left. The pressure force is transmitted through a plug gauge with a ceramic temperature isolator to a load cell. The system plug gauge /load cell sustains the container wall against damage. The same drilled holes are also used for temperature measurements, measuring points are positioned near the inner wall of the liner, in the middle of the container and for heat control at the outer container region. For the chosen extrusion examples, the simulations led to maximum lifetime consumption in the region of relatively high both temperature and equivalent stresses. These results seem to be reasonable in comparison to real lifetime of copper extrusion containers.

PL

Artykuł przedstawia rozkład pól temperatur, naprężeń oraz zużycia materiału podczas trzech cykli wyciskania miedzi w dwuczęściowym pojemniku. Symulacja procesu obróbki cieplnej oraz ostateczny stan pojemnika zostały wykorzystane jako podstawa do opracowania kolejnych kroków zużycia materiału podczas cyklicznych etapów wyciskania. Symulacja numeryczna MES obejmuje modelowanie plastycznego wsadu i sztywnych narzędzi oraz kilku cykli tego samego procesu dla elastycznego pojemnika przy wykorzystaniu zależnych od czasu warunków brzegowych dla temperatury oraz nacisku. Powodem takiej procedury obliczeniowej jest krótszy czas symulacji niż w przypadku plastycznego procesu wyciskania. W celu weryfikacji obliczonych warunków brzegowych temperatury i nacisku na powierzchni styku, skonstruowano specjalną maszynę laboratoryjną do prowadzenia procesu wyciskania.