Fenomenologické modelování porušení se zaměřením na teplotní únavu

This works presents original research data for low-cycle fatigue and thermo-mechanical fatigue of SiMo 4.06 cast iron. The cast iron was subjected to a various strain rates and strain amplitudes in the temperature range of 20°C-750°C under low-cycle fatigue and thermo-mechanical fatigue loading conditions. The experiments were carried out on the newly in-house designed test stand for performing uniaxial low-cycle and thermo-mechanical fatigue tests. A unified viscoplastic material is implemented by the means of user defined material subroutine for Abaqus commercial finite element software. The anisothermal multiaxial formulation of unified viscoplastic material model used here is based on hyperbolic sine flow rule and the model incorporates isotropic hardening and non-linear kinematic hardening. The model is calibrated systematically with temperature on the basis of obtained experimental data. A new method is applied in order to calibrate the temperature dependent viscoplastic material model parameters. Numerical simulations of cyclic mechanical behaviour of SiMo 4.06 follows. Finally, a novel energy based fatigue criterion is proposed in order to take into account effect of mean stress, which usually appears during thermo-mechanical loading. A good correlation was achieved between the predicted results and the observed results.Tato disertační práce dokumentuje původní výsledky výzkumu nízkocyklové a teplotně-mechanické únavy tvárné litiny SiMo 4.06. Byly provedeny rozsáhlé mechanické zkoušky tohoto materiálu pro různé rychlosti a hladiny amplitud deformace, pro teploty v rozsahu 20°C-750°C. Mechanické zkoušky byly provedeny na nově navrženém zkušebním zařízení, které je určeno pro jednoosé zkoušky nízkocyklové a teplotně-mechanické únavy. Dále byl implementován unifikovaný viskoplastický materiálový model jako uživatelská funkce pro konečně-prvkový řešič Abaqus. Unifikovaný materiálový model je implementován pro případ multiaxiálního zatěžování za předpokladu proměnné teploty, konstitutivní vztah pro funkci tečení je založen na hyperbolickém sinu, model zahrnuje isotropní i nelineární kinematické zpevnění. Kalibrace modelu je provedena pro získané experimentální výsledky. Teplotně závislé parametry modelu jsou kalibrovány systematicky s využitím nových postupů. Zkalibrovaný materialový model je použit pro numerické simulace cyklického mechanického chování zkoumaného materiálu. Nakonec je navrženo nové kritérium pro hodnocení životnosti při nízkocyklové teplotně-mechanické únavě. Navržené kritérium je založeno na disipované hysterezní energii a zahrnuje vliv středního napětí, které je často pozorováno během mechanických zkoušek prováděných za proměnných teplot. Výsledky simulací dobře korelují s výsledky z experimentů

THERMO-MECHANICAL FATIGUE ANALYSIS OF A STEAM TURBINE SHAFT

Increasing demands on the flexibility of steam turbines due to the use of renewable energy sources substantially alters the fatigue strength requirements of components of these devices. Rapid start-ups as well as the increased number of the load cycles applied to the turbines must be handled by design methodologies. The goal of the work presented in this paper was to provide a computational framework applicable to the thermo-mechanical fatigue (TMF) prediction of steam turbine shafts. The so-called Damage Operator Approach by Nagode et al. has been implemented to the software codes and applied to fatigue analysis of the thermo-mechanical material response computed numerically by the finite element analysis. Experimental program conducted in order to identify the material thermo-mechanical behavior and to verify numerical simulations is introduced in the paper. Some results of TMF prediction of a sample steam turbine shaft are shown

Data Extraction from PDF Documents

The work focuses on extracting information from medical records saved in PDF format, which were created by heart pacemakers during regular patient monitoring in the hospital. The result of this work is a desktop application written in Java that retrieves and analyzes data from records using PDFBox and pdf2dom libraries. The output of the application is a CSV file, which represents the acquired values in table form, as well as extracted images that are saved to a user-defined output folder. Application testing on records from three different companies proved that record extraction is highly reliable (with overall precision and recall metrics reaching almost 100 % in every test), provided that the application arguments are correctly set

Use of Prandtl operators in simulating the cyclic softening of Inconel 718 under isothermal low-cycle fatigue loading

In this article, a new approach is proposed for modelling the stressstrain response of the Inconel 718 super-alloy under isothermal Low-Cycle Fatigue (LCF) loading. The proposed constitutive model is based on the Prandtl operator approach, in which a set of modifications is introduced in order to simulate strain range dependent cyclic softening. A new simulation capability is introduced by evolving the yield strains of the individual hysteresis operators with an accumulated plastic strain. In addition, the effect of the strain range dependency of cyclic softening is introduced into the proposed constitutive model by coupling its parameters with the concept of the plastic strain memory surface. These introduced modifications preserve the main advantages of the Prandtl operators, such as a small number of model parameters, their fast determination from the cyclic stressstrain curve, and a high computational speed, when used to simulate complex non-linear mechanical behaviour. Finally, the prediction capability of the proposed model is illustrated by various strain controlled tests performed at 500 °C, including block spectrum loading and variable strain amplitude loading

An approximate method for calculating elastic-plastic stress and strain on notched specimens

The paper deals with an approximate method for calculating elastic-plastic stresses and strains on the surface of notched samples. The method is based on the Abdel-Karim-Ohno cyclic plasticity model. The plane stress condition is considered within the evaluation. The output of the approximation on several multiaxial axial-torsion load paths is compared to our own experimental results. Experiments were carried out on samples of two notch types manufactured from the 2124-T851 aluminum alloy. Strain distribution in the notch area was measured by digital image correlation. The comparison between computational solution and measured response shows that the new method allows for obtaining reasonably good approximation, even for relatively complicated multiaxial load cases.Web of Science154art. no. 143

Thermo-mechanical fatigue prediction of a steam turbine shaft

The increasing demands on the flexibility of steam turbines due to the use of renewable energy sources substantially alters the fatigue strength requirements of components of these devices. This paper presents Thermo-Mechanical Fatigue (TMF) design calculations for the steam turbine shaft. The steam turbine shaft is exposed to complex thermo-mechanical loading conditions during the operating cycle of the turbine. An elastic-plastic structural Finite Element Analysis (FEA) of the turbine shaft is performed for the turbine operating cycle on the basis of calculated temperature fields obtained in a previous transient thermal FEA. The temperature dependent material parameters, which are used in the elastic-plastic FEA, are obtained from the uniaxial tests. Consequently, the TMF is predicted for the steam turbine shaft. Several fatigue criteria are used for the identifications of the critical domain and for the TMF damage assessment of the turbine shaft

Thermo-mechanical fatigue prediction of a steam turbine shaft

The increasing demands on the flexibility of steam turbines due to the use of renewable energy sources substantially alters the fatigue strength requirements of components of these devices. This paper presents Thermo-Mechanical Fatigue (TMF) design calculations for the steam turbine shaft. The steam turbine shaft is exposed to complex thermo-mechanical loading conditions during the operating cycle of the turbine. An elastic-plastic structural Finite Element Analysis (FEA) of the turbine shaft is performed for the turbine operating cycle on the basis of calculated temperature fields obtained in a previous transient thermal FEA. The temperature dependent material parameters, which are used in the elastic-plastic FEA, are obtained from the uniaxial tests. Consequently, the TMF is predicted for the steam turbine shaft. Several fatigue criteria are used for the identifications of the critical domain and for the TMF damage assessment of the turbine shaft