Filtern
Erscheinungsjahr
Dokumenttyp
- Zeitschriftenartikel (20)
- Beitrag zu einem Tagungsband (10)
- Forschungsdatensatz (5)
- Beitrag zu einem Sammelband (3)
- Vortrag (2)
- Sonstiges (2)
Sprache
- Englisch (42) (entfernen)
Schlagworte
- Additive manufacturing (5)
- Elastic modulus (4)
- Fatigue (4)
- Shear modulus (4)
- Ti-6Al-4V (4)
- Young's modulus (4)
- Cast iron (3)
- Thermal shock (3)
- AGIL (2)
- AISI 316L (2)
Organisationseinheit der BAM
- 5 Werkstofftechnik (12)
- 5.2 Metallische Hochtemperaturwerkstoffe (12)
- 9 Komponentensicherheit (6)
- 9.6 Additive Fertigung metallischer Komponenten (5)
- 5.5 Materialmodellierung (2)
- 8 Zerstörungsfreie Prüfung (2)
- 8.5 Röntgenbildgebung (2)
- 5.1 Mikrostruktur Design und Degradation (1)
- 5.6 Glas (1)
- 9.3 Schweißtechnische Fertigungsverfahren (1)
Eingeladener Vortrag
- nein (2)
Thermal barrier coatings allow increasing the operating temperature and efficiency of land-, sea-, or air-based turbines. As failure of the coating may result in serious damage of the turbine, reliable estimation of its lifetime is essential. To assess the lifetime, cyclic tests are conceived to combine thermal loading by heating the surface of the coating with laser irradiation and nondestructive methods for damage determination. Using laser irradiation allows a high reproducibility of the thermal load. The temperature of the sample surface during thermal loading is determined by an infrared-camera which also enables the possibility to detect damage in the coating via thermography. Additionally, four acoustic sensors, attached to the experimental setup, are used to detect damage in the sample and determine the source of acoustic events. Results of acoustic emission correlate well with thermographic images that visualize the formation and evolution of damage through delaminations in the samples.
Characterization of Ti-6Al-4V fabricated by multilayer laser powder-based directed energy deposition
(2022)
Laser powder-based directed energy deposition (DED-L) is increasingly being used in additive manufacturing (AM). As AM technology, DED-L must consider specific challenges. It must achieve uniform volume growth over hundreds of layers and avoid heat buildup of the deposited material. Herein, Ti–6Al–4V is fabricated using an approach that addresses these challenges and is relevant in terms of transferability to DED–L applications in AM. The assessment of the obtained properties and the discussion of their relationship to the process conditions and resulting microstructure are presented. The quality of the manufacturing process is proven in terms of the reproducibility of properties between individual blanks and with respect to the building height. The characterization demonstrates that excellent mechanical properties are achieved at room temperature and at 400 °C.
Modeling the lifetime reduction due to the superposition of TMF and HCF loadings in cast iron alloys
(2016)
The superposition of small amplitude, high frequency loading cycles (HCF) to slow, large amplitude loading cycles (TMF) can significantly reduce the fatigue life. In this work, the combined TMF+HCF loading has been experimentally investigated for a cast iron alloy. In particular, the influence of the HCF frequency of the HCF amplitude and of the location of the superposed HCF cycles has been assessed. It was observed that the HCF frequency has a limited impact on the TMF fatigue life. On the other side, the HCF-strain amplitude has a highly non-linear influence on the TMF fatigue life. A simple estimate for the fatigue life reduction due to the superposed HCF cycles has been derived from fracture mechanics considerations. It is assumed that the number of propagation cycles up to failure can be neglected after a threshold for the HCF loading has been reached. The model contains only two adjustable parameters and can be combined with any TMF life prediction model. The model predictions are compared with the test results for a large range of TMF+HCF loading conditions.
Thermal barrier coatings (TBCs) are used to increase the operating temperature of land-, sea-, or air-based turbines. As failure of the coating may result in serious damage of the turbine, reliable estimation of the lifetime is essential. Most experiments to assess the lifetime or to determine parameters for simulations of the behavior of TBCs are done by burner-rig-tests, where the operating conditions are simulated by cyclic heating of the surface and cooling of the backside of a coated sample.
In this work a possibility is presented to do comparable experiments by heating the surface with laser irradiation instead of a burner. For this purpose a Nd:YAG-laser with a maximum output power of 1?kW and a wavelength of 1064?nm is used. The laser spot can be moved by integrated optics across the sample surface to achieve homogeneous heating of the coating. Cooling of the backside is done by air. The temperature of the sample surface is determined by an infrared-camera which also enables the possibility to detect failures in the coating via thermography. Additionally, acoustic sensors attached to the sample holder are used to detect failures in the sample. The investigated ceramic material (yttria stabilized zirconia) has a very low absorption coefficient at the used laser wavelength. Therefore, a pre-treatment of the samples was needed to increase the absorption coefficient to be able to heat up the samples.
In this paper, the experimental setup and first experimental results are presented.
The determination of critical failure parameters during thermal shock requires a time and space resolved temperature measurement of the sample. High frequency pyrometry is a suitable technique for such a measurement task, which usually requires the knowledge of the optical properties of the investigated material. Another challenge is the infrared transparency of materials to be checked in the wavelength range of the pyrometer. The thermal shock disks are very thin, due to the need of homogeneous temperature distribution and to maintain a two-dimensional problem. To allow a pyrometric temperature measurement a universal calibration method for high frequency infrared cameras has been developed. Pyrometry in various media was demanded, as well as the estimation of optical properties (reflection, transmittance and emission) for the selected ceramics to be tested.
The thermal shock behavior of three commercial-advanced ceramics (SSiC, MgO–PSZ, Al2O3) is characterized in air and vacuum applying a laser thermal shock. The available testing system permits the reproducible setting of defined temperature profiles in thin disks and allows a heating-up thermal shock in various media. Due to the accurate determination of the time- and space-resolved temperature distribution, the local stress state can be calculated as a function of time. It is shown that the thermal shock strength is highest for SSiC and lowest in Al2O3 with MgO–PSZ in between. The approach presented in this work allows quantifying the stress state at failure in terms of tangential tensile stress. The investigated environment does not affect the thermal shock resistance under the studied experimental conditions.
The fatigue behaviour of cast iron is usually investigated on machined specimens. Components of cast iron, however, have a casting skin; therefore the investigation of the influence of the casting skin on the lifetime is of interest. To study this influence isothermal fatigue tests were carried out on heat-resisting spheroidal graphite cast iron EN GJS SiMo 4.05 in 4–point-bending set-up at 400 °C. Specimens with and without casting skin were investigated comparatively. The number of cycles to failure was significantly lower for specimens with casting skin. Metallographic investigations underline the reduction of lifetime caused by casting skin.
The continuing increase of steam parameters of fossil fuelled high efficiency power plants and new combustion concepts for the capture and storage of carbon dioxide lead to harsher service conditions for the components and structural materials of such facilities. The present work introduces a test concept that allows testing of candidate materials under simultaneous mechanical and corrosive loading. The material's reaction can be directly investigated under simulated temperature, load and corrosion conditions of modern installations. First results obtained for different heat resistant steels suggest a strong influence of the environmental medium on the fatigue and creep behaviour. Such findings complement the data that is available from the classical qualification process of the materials and may support the material selection for new power plant installations.
Experimental and analytical investigation of the TMF-HCF lifetime behavior of two cast iron alloys
(2017)
The combined loading Thermomechanical Fatigue (TMF) with High Cycle Fatigue (HCF) has been experimentally investigated for two cast iron alloys. Both alloys contain globular graphite nodules but the first one has a ferritic structure while the second one has an austenitic crystal structure. In particular, the influences of the HCF frequency, of the HCF loading amplitude and of the location of the superposed HCF cycles have been investigated. It was observed that the HCF frequency has a limited impact on the fatigue life. On the other side, the HCF-strain amplitude has a highly non-linear influence on the fatigue life. The experimental results can be understood in terms of a fracture mechanics based damage mechanism: Cracks quickly initiate due to the TMF loading and the growth of the cracks up to a few mm controls the fatigue life. If HCF-loading cycles are superposed, cyclic crack propagation dramatically accelerates after a threshold has been reached. This threshold is regarded as controlling the fatigue life reduction. The previous ideas have been expressed in a model that can be very simply applied to estimate the fatigue life reduction ratio due to the superposed HCF cycles. It only contains two adjustable parameters and can be combined with any TMF life assessment model.
TMF tests were carried out on EN-GJSA-XNiSiCr35-5-2 at constant minimum temperature (400 °C) and varying maximum temperatures (Tmax = 700 °C, 800 °C, 900 °C) with hold times of 180 s at Tmax and two phase angles (in-phase (IP), 180° out-of-phase (OP)). The results showed a comparable strength under OP- and IP-TMF loading. At Tmax = 700 °C and 900 °C, the lifetime in IP-tests was slightly longer than that of OP-tests, while it is vice versa at Tmax = 800 °C. The IP-tests at Tmax = 900 °C showed a similar lifetime as OP-tests at Tmax = 700 °C and 800 °C, which was unexpected for such a high testing temperature. All IP-tests at Tmax = 900 °C showed a continuous cyclic softening from the beginning on, which was different from all other testing conditions. Complementary metallographic investigations indicated that under this test condition, intergranular creep damage is present in the volume of the test pieces.