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Cast iron materials are used as materials for cylinder heads for heavy duty internal combustion engines. These components must withstand severe cyclic mechanical and thermal loads throughout their service life. While high-cycle fatigue (HCF) is dominant for the material in the water jacket region, the combination of thermal transients with mechanical load cycles results in thermomechanical fatigue (TMF) of the material in the fire deck region, even including superimposed TMF and HCF loads. Increasing the efficiency of the engines directly leads to increasing combustion pressure and temperature and, thus, lower safety margins for the currently used cast iron materials or alternatively the need for superior cast iron materials. In this paper (Part I), the TMF properties of the lamellar graphite cast iron GJL250 and the vermicular graphite cast iron GJV450 are characterized in uniaxial tests and a mechanism-based model for TMF life prediction is developed for both materials. The model can be used to estimate the fatigue life of components by means of finite-element calculations (Part II of the paper) and supports engineers in finding the appropriate material and design. Furthermore, the effect of the elastic, plastic and creep properties of the materials on the fatigue life can be evaluated with the model. However, for a material selection also the thermophysical properties, controlling to a high level the thermal stresses in the component, must be considered. Hence, the need for integral concepts for material characterization and selection from a multitude of existing and soon-to-be developed cast iron materials is discussed.
The following contribution deals with the growth of cracks in low-cycle fatigue (LCF) and thermomechanical fatigue (TMF) tested specimens of Inconel 718 measured by using the replica method. The specimens are loaded with different strain rates. The material shows a significantly higher crack growth rate if the strain rate is decreased. Electron backscatter diffraction (EBSD) is adopted to identify the failure mechanism and the misorientation relationship of failed grain boundaries in secondary cracks. The analyzed cracks propagated mainly transgranular but also intergranular failure can be observed in some areas. It is found that grain boundaries with coincidence site lattice (CSL) boundary structure are generally less susceptible for intergranular failure than grain boundaries with random misorientation. For modeling the experimentally identified crack behavior an existing model for fatigue crack growth based on the mechanism of time dependent elastic–plastic crack tip blunting is enhanced to describe environmental effects based on the mechanism of oxygen diffusion at the crack tip. For the diffusion process the temperature dependent parabolic diffusion law is assumed. As a result, the time dependent cyclic crack tip opening displacement (DCTOD) is used as representative value to describe both mechanisms. Thus, most
of the included model parameters characterize the deformation behavior of the material and can be determined by independent material tests. With the determined material properties, the proposed model describes the experimentally measured crack growth curves very well. The model is validated based on predictions of the number of cycles to failure of LCF as well as in-phase and out-of-phase TMF tests in the temperature range between room temperature and 650 °C.
In this paper, the multiaxial formulation of a mechanism-based model for fatigue life prediction is presented whichcan be applied to low-cycle fatigue (LCF) and thermomechanical fatigue (TMF) problems in which high-cycle fa-tigue loadings are superimposed. The model assumes that crack growth is the lifetime limiting mechanism and thatthe crack advance in a loading cycleda/dNcorrelates with the cyclic crack-tip opening displacement ΔCTOD.The multiaxial formulation makes use of fracture mechanics solutions and thus, does not need additional modelparameters quantifying the effect of the multiaxiality. Furthermore, the model includes contributions of HCF on ΔCTODand assesses the effect of the direction of the HCF loadings with respect to LCF or TMF loadings inthe life prediction. The model is implemented into the finite-element program ABAQUS. It is applied to predictthe fatigue life of a thermomechanically loaded notched specimen that should represent the situation between theinlet and outlet bore holes of cylinder heads. A good correlation of the predicted and the measured fatigue lives isobtained.
In this paper the fatigue life of three cast iron materials, namely EN-GJS-700, EN-GJV-450 and EN-GJL-250, is predicted for combined thermomechanical fatigue and high cycle fatigue loading. To this end, a mechanism-based model is used, which is based on microcrack growth. The model considers crack growth due to low frequency loading (thermomechanical and low cycle fatigue) and due to high cycle fatigue. To determine the model parameters for the cast iron materials, fatigue tests are performed under combined loading and crack growth is measured at room temperature using the replica technique. Superimposed high cycle fatigue leads to an accelerated crack growth as soon as a critical crack length and thus the threshold stress intensity factor is exceeded. The model takes this effect into account and predicts the fatigue lives of all cast iron materials investigated under combined loadings very well.
This paper reports on the evolution of cracks in the cylinder heads of a large V8 Diesel engine during cyclic engine tests. The observations are compared with the predictions of a lifetime model for combined thermo-mechanical (TMF) and high cycle fatigue (HCF) loading, which is based on a fracture mechanics analysis of microcrack growth in viscoplastic solids and assumes that the crack advance per cycle is proportional to the cyclic crack tip opening displacement. Since the material of the cylinder heads, the cast iron EN-GJV450, exhibits the typical features of cast iron, namely pressure dependence of the yield stress, dilatancy and tension-compression asymmetry, the Gurson model is applied and combined with the viscoplastic Chaboche model. This constitutive model together with the lifetime model is implemented into a finite element code as a user defined material routine. Published model parameters for the considered cast iron are used to carry out the simulation of the engine test. This simulation comprises a CFD analysis to determine the heat transfer coefficients, a thermal analysis of the load cycle and the mechanical analysis. The thermal analysis reproduces the temperatures at various measuring points sufficiently accurately. Finally, the mechanical analysis predicts the location and orientation of the cracks in the valve bridges correctly in all cases. However, the lifetime predictions are rather conservative compared to the tests (by a factor of 1 to 5 in lifetime). This is discussed and explained by the fact that the cracks were detected in the tests only when they had already spread over a substantial fraction of the valve bridge width. To describe this situation a long-crack analysis would be necessary, which is not yet included in the applied lifetime model.
In dem abgeschlossenen Vorhaben „Entwicklung von Rechenmodellen zur Lebensdauervorhersage von Motorbauteilen unter thermisch-mechanischer Ermüdungsbeanspruchung“ der Forschungsvereinigung Verbrennungskraftmaschinen e. V. (FVV) wurde am Fraunhofer Institut für Werkstoffmechanik IWM in Freiburg ein Materialmodell zur Lebensdauervorhersage thermomechanisch belasteter Komponenten entwickelt. Das Modell basiert auf einem viskoplastischen Verformungsmodell für Eisengusswerkstoffe und einem mechanismenbasierten Modell für Mikrorisswachstum zur Lebensdauervorhersage.
Warmumformwerkzeuge unterliegen während des Betriebes komplexen thermischen und mechanischen Beanspruchungen. In kritischen Bereichen können dadurch lokal Spannungen entstehen, die die Fließgrenze überschreiten. Bei der Serienproduktion führt dies zu zyklischen plastischen Verformungen und zur thermomechanischen Ermüdung, welche die Lebensdauer der Warmumformwerkzeuge maßgeblich bestimmen kann. Zur Bewertung der thermomechanischen Ermüdung der Warmumformwerkzeuge gibt es jedoch heute keine etablierten Konzepte, da dieser Aspekt erst durch die Notwendigkeit einer höheren Ressourcen- und Energieeffizienz und optimierter Produktionsprozesse (beispielsweise im Rahmen von Industrie 4.0) eine höhere Aufmerksamkeit erreicht. In dieser Arbeit wird zum einen die aktuell industriell angewandte Vorgehensweise zur Auslegung von Warmumformwerkzeugen hinsichtlich der Lebensdauer erläutert. Des Weiteren wird ein Überblick über existierende Plastizitätsmodelle und Lebensdauermodelle gegeben. Dabei wird zwischen rein phänomenologischen und mechanismenbasierten Modellen unterschieden. Aus der betriebenen Recherche wird ersichtlich, dass weiterer Forschungsbedarf auf diesem Gebiet notwendig ist.
A complete thermomechanical fatigue (TMF) life prediction methodology is developed for predicting the TMF life of cast iron cylinder heads for efficient heavy duty internal combustion engines. The methodology uses transient temperature fields as thermal loads for the non-linear structural finite-element analysis (FEA). To obtain reliable stress and strain histories in the FEA for cast iron materials, a time and temperature dependent plasticity model which accounts for viscous effects, non-linear kinematic hardening and tensioncompression asymmetry is required. For this purpose a unified elasto-viscoplastic Chaboche model coupled with damage is developed and implemented as a user material model (USERMAT) in the general purpose FEA program ANSYS. In addition, the mechanismbased DTMF model for TMF life prediction developed in Part I of the paper is extended to three-dimensional stress states under transient non-proportional loading conditions. The material properties of the plasticity model are determined for lamellar graphite cast iron GJL250 and vermicular graphite cast iron GJV450 from isothermal and non-isothermal uniaxial tests. The methodology is applied to obtain a TMF life prediction on two cast iron cylinder heads for heavy duty diesel engine applications made from both cast iron materials. It is shown that the life predictions using the developed methodology correlate very well with observed lives from two bench tests in terms of location as well as number of cycles to failure.
A crack opening stress equation for in-phase and out-of-phase thermomechanical fatigue loading
(2016)
In this paper, a crack opening stress equation for in-phase and out-of-phase thermomechanical fatigue (TMF) loading is proposed. The equation is derived from systematic calculations of the crack opening stress with a temperature dependent strip yield model for both plane stress and plane strain, different load ratios and different ratios of the temperature dependent yield stress in compression and tension. Using a load ratio scaled by the ratio of the yield stress in compression and tension, the equation accounts for the effect of the temperature dependent yield stress and the constraint on the crack opening stress. Based on the scaling relation established in this paper, Newman's crack opening stress equation for isothermal loading is enabled to predict the crack opening stress under TMF loading.