Near-component testing of materials for cylinder heads to determine thermomechanical fatigue under superimposed high-frequency mechanical loads

Due to higher combustion chamber temperatures and pressures in efficient combustion engines, both the high-cycle and thermomechanical fatigue loads on service life-critical components, such as the cylinder head, are increasing. Material comparisons and analysis of damage behavior are very expensive and time-consuming using component tests. This study therefore develops a test method for cylinder head materials that takes into account the combined loading conditions from the above-mentioned loads and allows realistic temperature transients and gradients on near-component samples. The near-component cylinder head sample represents the failure-critical exhaust valve crosspiece and is tested in a test rig specially designed with the aid of conjugate heat transfer simulations. In the test rig, the sample is subjected to thermal stress by a hot gas burner and to mechanical stress by a high-frequency pulsator. Optical crack detection allows permanent observation of fatigue crack growth and crack closure during the test. Fractographic and metallo-graphic examinations of the fracture areas as well as analyses of the damage patterns show that loads close to engine operation can be set in this way and their influences on the damage can be monitored.

Architected Tunable Structure for Improved Capability in Extreme Environments

A novel patterned-void structure is developed to improve the fatigue life compared to conventional circular cooling holes typically used in gas turbine components exposed to high temperatures. The distinctive S-shape of the voids and their specific arrangement enable manipulation of the structure's macroscopic stiffness and Poisson's ratio. An investigation of the isothermal and thermomechanical fatigue properties of the proposed structure is carried out in strain-controlled conditions. The testing is performed on tubular specimens machined from a Nickel-based superalloy commonly used in gas turbine combustion systems (Haynes 230 ™). The isothermal fatigue tests, performed at 300°C, 600°C and 800°C, demonstrated an increase in crack-initiation life of the proposed structure by a factor of up to 28 compared to the standard circular holes. The thermomechanical fatigue tests, performed across temperature ranges 300°C - 750°C and 300°C - 850°C, and using in-phase and out-of-phase strain ratios, demonstrated an increase in crack-initiation life by a factor of up to 16. The life after crack initiation (crack-propagation mode) was also shown to be longer for the proposed structure, which is attributed to a crack-arresting behavior inherent to the structure.