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In this paper, the high-temperature constitutive behaviour of an additively manufactured ductile nickel-based superalloy is investigated and modelled, with application to thermomechanical fatigue, low-cycle fatigue and creep conditions at temperatures up to 800∘800∘C. Thermomechanical fatigue tests have been performed on smooth specimens in both in-phase and out-of-phase conditions at a temperature range of 100−800∘100−800∘C, and creep tests at 625∘625∘C, 700∘700∘C, 750∘750∘C and 800∘800∘C. Additionally, low-cycle fatigue tests at different strain ranges and load ratios have been performed at 700∘700, Funding Agencies|Linkoping University and Siemens Energy AB

In this study, a developed co-simulation method, which couples 1D-fluid and 3D-structural models, has been utilised to simulate wear in a hydraulic percussion unit. The effect of wear is generally detrimental on performance and lifetime for such units, but can also cause catastrophic failure and breakdown, requiring a total overhaul and replacement of core components. One experiment of standard straight impact was performed to investigate the tolerance against seizure. The percussion unit was operated at successively increasing operating pressures, and the level of wear was registered at each step, until seizure occurred. The co-simulation model was used to replicate the running conditions from the experiment to simulate the structural response to be used as input for the wear routine to calculate the wear depth. The wear pattern from the simulations corresponds well to the wear pattern from the experiment. Further, the effect of a misaligned impact on wear development was also studied, as this is a loading situation that typically occurs for hydraulic percussion units. The study demonstrates that the simulation method used has a potential for simulating wear and predicting seizure in hydraulic percussion units., Funding: Epiroc Tools & Attachments Division

The crack initiation life of a ductile additively manufactured nickel-based superalloy is studied and modeled for low-cycle fatigue and thermomechanical fatigue conditions up to 600 degrees C. Isothermal experiments were performed on smooth specimens at temperatures up to 600 degrees C with different applied strain ranges. Additionally, thermomechanical fatigue experiments at 100-450 degrees C and 100-600 degrees C were performed on smooth specimens under in-phase and out-of-phase conditions. A life prediction model accounting for the anisotropy was developed, where the temperature cycle is accounted with a Delta T$$ \Delta T $$-functionality, generating good agreements with the experiments. The model was also validated on notched specimens undergoing thermomechanical fatigue conditions at 100-500 degrees C using simplified notch correction methods., Funding Agencies|Linkoping University; Siemens Energy AB

The crack growth behaviour of the alloy CM 247 LC is investigated for out-of-phase TMF and isothermal tests at the same temperature as the minimum temperature in the TMF tests. The results suggest that it is possible to characterise crack growth behaviour if experimental corrections for crack closure are accounted for. The repli -cation of these experimental tests using a numerical FE-solver results in similar crack growth behaviour, sug-gesting that the main mechanism in place is plasticity-induced crack closure. A pragmatic analytical model to characterise crack closure including hold time effects is proposed. The comparison of the response from this model with the experimental and numerical results suggests that the proposed analytical model is capable to approximate crack closure effects for cases where substantial creep deformation is to be expected., Funding Agencies|Link oping University; Siemens Energy

Crack closure effects are known to have a large impact on crack growth behaviour. In this work, tests were performed on Inconel 792 specimens under TMF loading conditions at 100–850 °C with extended hold times at tensile stress. The effective stress-intensity range was estimated experimentally using a compliance-based method leading to the conclusion that crack closure appears to have a primary impact on the crack growth behaviour for this material under the conditions studied. The crack closure behaviour for the tests was successfully modelled using numerical simulations, including creep., Ytterligare forskningsfinansiär: Siemens Industrial Turbomachinery AB through “Turbines for Future Energy Systems” (Turbiner för framtidens energisystem), Grant No. 44100-1

With the improvement in technology, additive manufacturing using metal powder has been a go-to method to produce complex-shaped components. With complex shapes being printed, the residual stresses (RS) developed during the printing process are much more difficult to control and manage, which is one of the issues seen in the field of AM. A simplified finite element-based, layer-by-layer activation approach for the prediction of residual stress is presented and applied to L-shaped samples built in two different orientations. The model was validated with residual stress distributions measured using neutron diffraction. It has been demonstrated that this simplified model can predict the trend of the residual stress distribution well inside the parts and give insight into residual stress evolution during printing with time for any area of interest. Although the stress levels predicted are higher than the measured ones, the impact of build direction on the development of RS during the building process and the final RS distributions after removing the base plate could be exploited using the model. This is important for finalizing the print orientation for a complex geometry, as the stress distribution will be different for different print orientations. This simplified tool which does not need high computational power and time can also be useful in component design to reduce the residual stresses., Funding Agencies|Swedish Foundation for Strategic Research (Stiftelsen for Strategisk Forskning, SSF) within the Swedish national graduate school in neutron scattering (SwedNess) [GSn15-0008]

In this study, a previously developed co-simulation method has been expanded to also simulate the dynamic behaviour of sealing gap regions in hydraulic percussion units. This approach is based on a 1D system model representing the fluid components and a 3D finite element model representing the structural parts of a hydraulic hammer. The sealing gap is a fundamental feature of a percussion unit, where the reciprocating motion of the piston is generated by the valve mechanism of the sealing gap. When the gap is closed it will prevent fluid flow between regions of different pressure levels. However, a small leakage flow through the gap will always occur which size depends on the clearance and the position of the piston. The method proposed here will take the structural motion and deformation into consideration when calculating the leakage flow. The deformed state of the structure is approximated by a cylindrical surface, in a least square manner, and communicated through the co-simulation interface to the fluid simulation module, and then used when calculating the leakage flow. This method aims at a more accurate simulation of the leakage flow that will not only yield a more realistic description of the mechanism on the local level, but also a more accurate estimation of global parameters such as overall performance and efficiency. The results indicate that the simulated leakage flow will decrease when dynamic gaps are used in comparison to static gaps, which is a consequence of the deformed structure that will generate smaller clearances. The leakage flow for the dynamic gaps will even be lower than for the static perfectly concentric case, mainly due to the reduction of clearances. The results also indicate that the dynamic eccentricity does not have a major influence on the leakage flow. The outcome from this study highlights the potentials of the described co-simulation approach for analysing the dynamics of the sealing gaps in a hydraulic percussion unit (i.e. gap

Single-crystal nickel-base superalloys frequently experience two distinct fatigue crack growth modes. It has been observed that, under certain conditions, cracks transition from a path perpendicular to the loading direction to a crystallographic slip plane. As crystallographic cracking is associated with an increased fatigue crack growth rate, it is important to be able to predict when this transition occurs. In this work three different criteria for crystallographic cracking based on resolved anisotropic stress intensity factors are evaluated in a three-dimensional finite element context. The criteria were calibrated and evaluated using isothermal fatigue experiments on two different specimen geometries. It is suggested by the results, that a threshold value of a resolved shear stress intensity factor can act as a conservative criterion indicating cracking mode transition. Further, a trend hinting towards a loading frequency dependency could be observed., Funding Agencies|Linkoping University; Siemens Industrial Turbomachinery AB

Fatigue crack growth rate in Room Temperature and Out-of-Phase Thermo-Mechanical Fatigue notched specimen experiments on the nickel-base alloy IN792 is studied. It is shown that it is possible to explain the observed higher crack growth rate in OP TMF compared to RT testing for the same external load, if plasticity induced crack closure is considered. Modeling utilizes node-release finite element simulations with a temperature dependent yield stress, resulting in different yield stress in tension and compression. It is shown that a simple extension of the analytical Newman crack closure equation can describe the opening level in the performed experiments., Funding Agencies|SIEMENS Energy AB; Swedish Energy AgencySwedish Energy Agency [44100-1]

Crack propagation rates from isothermal and in-phase high temperature thermomechanical fatigue crack propagation tests for Inconel 792 resulted in a rather large scatter band when plotted against the linear-elastic stress intensity function Delta K. Previous observations indicated that the residual strains generated during the first few cycles in the uncracked specimen can explain, to a large extent, the differences between the tests. An effective Delta K-eff was calculated for each test based on non-linear FE simulations. When the crack propagation rates for each test were plotted against the calculated Delta K-eff, the curves collapsed to a smaller scatter band., Funding Agencies|Swedish Energy AgencySwedish Energy Agency; Siemens Industrial Turbomachinery AB through "Turbines for Future Energy Systems" (Turbiner for framtidens energisystem) [44100-1]

Materials in steam turbine rotors are subjected to cyclic loads at high temperature, causing cracks to initiate and grow. To allow for more flexible operation, accurate fatigue models for life prediction must not be overly conservative. In this study, fully reversed low cycle fatigue tests were performed on a turbine rotor steel called FB2. The tests were done isothermally, within temperature range of room temperature to 600 °C, under strain control with 0.8-1.2 % total strain range. Some tests included hold time to calibrate the short-time creep behaviour of the material. Different fatigue life models were constructed. The life curve in terms of stress amplitude was found unusable at 600 °C, while the life curve in terms of total strain or inelastic strain amplitudes displayed inconsistent behaviour at 500 °C. To construct better life model, the inelastic strain amplitudes were separated into plastic and creep components by modelling the deformation behaviour of the material, including creep. Based on strain range partitioning approach, the fatigue life depends on different damage mechanisms at different strain ranges. This allowed the formulation of life curves based on plasticity or creep domination, which showed creep domination at 600 °C, while at 500 °C, creep only dominates for higher strain range.

The computational efficiency in analysing cyclically loaded structures is a highly prioritised issue for the gas turbine industry, as a cycle-by-cycle simulation of e.g. a turbine disc is far too time consuming. Hence, in this paper, the efficiency of two different procedures to handle computational expansive load cases, a numerical extrapolation and a parameter modification procedure, are evaluated and compared to a cycle-by-cycle simulation. For this, a local implementation approach was adopted, where a user-defined material subroutine is used for the cycle jumping procedures with good results. This in contrast to a global approach where the finite element simulation is restarted and mapping of the solution is performed at each cycle jump. From the comparison, it can be observed that the discrete parameter modification procedure is by margin the fastest one, but the accuracy depends on the material parameter optimisation routine. The extrapolation procedure can incorporate stability and/or termination criteria., Funding Agencies|Cleansky [686600]

Cracks in single-crystal nickel-base superalloys have been observed to switch cracking mode from Mode I to crystallographic cracking. The crack propagation rate is usually higher on the crystallographic planes compared to Mode I, which is important to account for in crack growth life predictions. In this paper, a method to evaluate the crystallographic fatigue crack growth rate, based on a previously developed crystallographic crack driving force parameter, is presented. The crystallographic crack growth rate was determined by evaluating heat tints on the fracture surfaces of the test specimens from the experiments. Complicated crack geometries including multiple crystallographic crack fronts were modelled in a three dimensional finite element context, The data points of the crystallographic fatigue crack growth rate collapse on a narrow scatter band for the crystallographic cracks indicating a correlation with the previously developed crystallographic crack driving force., Funding Agencies|Linkoping University; Siemens Industrial Turbomachinery AB

In this study, a previously developed co-simulation approach has been adopted to simulate the responses of an existing hydraulic hammer product. This approach is based on a 1D system model representing the fluid components and a 3D finite element model representing the structural parts of the hydraulic hammer. The simulation model was validated against four experiments with different running conditions. The corresponding set-ups were analysed using the co-simulation method in order to evaluate the overall responses. A parameter study was also performed involving the working pressure and the restrictor diameter, with the objective to validate that a parameter change in the simulation model will affect the input and output power in the same direction as in the experiments. The experimental responses used in the validation were time history data of fluid pressure, component position and acceleration, and structural stresses. The experiments result in high frequency and high amplitude excitations of the hydraulic hammer and thus require a model with a high resolution of the model dynamics. The conclusion of the validation is that the simulation model is able to replicate the experimental responses with high accuracy including the high frequency dynamics. The favourable outcome of the validation makes the described co-simulation approach promising as an efficient tool for a wide range of other applications where short time duration mechanisms need to be studied., Funding Agencies|Epiroc Hydraulic Attachment Tools Division

Fatigue crack propagation tests under both isothermal and non-isothermal thermomechanical fatigue conditions have been performed on wrought Haynes 230, a ductile combustor material. A number of specimens were thermally aged by pre-straining and subsequent furnace exposure for 3000 h at 600 degrees C. The tests were performed both under load and strain control, between room temperature and 600 degrees C. The thermally aged notched specimens show a decrease in the crack initiation life, similar to results previously reported for smooth test specimens at room temperature. For the crack growth rates, the effects of thermal ageing were less pronounced than for crack initiation. Further, the tests have been simulated using the finite element method to calculate the crack driving force, where the plasticity induced crack closure is handled with a full history description. A temperature dependent linear kinematic hardening plasticity law has been adopted for describing the material behaviour between room temperature and 600 degrees C. A post-processing tool was used in which the plasticity induced crack opening level was calculated, followed by a calculation of the effective Delta J range for each crack length. The adopted procedure yields good correlation between the different tests, under both isothermal and non-isothermal conditions., Funding Agencies|Siemens Industrial Turbomachinery AB, Finspang, Sweden

The inherent anisotropy of single-crystal nickel-base superalloys brings many difficulties in terms of modelling, evaluation and prediction of fatigue crack growth. Two models to predict on which crystallographic plane cracking will occur is presented. The models are based on anisotropic stress intensity factors resolved on crystallographic slip planes calculated in a three-dimensional finite-element context. The developed models have been compared to experiments on two different test specimen geometries. The results show that a correct prediction of the crystallographic cracking plane can be achieved. This knowledge is of great interest for the industry and academia to better understand and predict crack growth in single-crystal materials., Funding Agencies|Swedish Energy Agency; Siemens Industrial Turbomachinery AB through the Research Consortium of Materials Technology for Thermal Energy Processes [KME-702]