Experimental and Numerical Fatigue Life Study of Inner Liner Material in Rocket Combustion Chambers

Abstract

The paper on hand presents the experimental and numerical results of a fatigue life study conducted with Cu-HCP being used as the inner liner material for regeneratively cooled cryogenic rocket combustion chambers. This material is suitable for combustion chambers which are prone to moderate levels of hot-gas surface temperature and wall heat flux. The experimental part of this study uses Thermomechanical Fatigue (TMF) panels made of the high-conductivity Cu-HCP that were cyclically tested to failure at the TMF test bench at DLR Lampoldshausen Institute of Space Propulsion. A TMF panel represents a small section of a regeneratively cooled rocket combustion chamber. It typically consists of 7 cooling channels. The coolant being used is supercritical nitrogen instead of hydrogen or methane due to safety and cost concerns. The TMF test bench also incorporates a high power diode laser radiating onto the TMF panel surface. This provides realistic amounts of heat flux and surface temperature. The laser is cyclically powered on and off to represent the multiple load cycles that liquid rocket engines have to endure particularly in reusable rocket engines. The setup of the TMF test bench provides data regarding the pure mechanical behavior of the material without the influences of any combustion or chemicals. Furthermore, heat flux, surface temperature and mass flow rate can be easily determined, hence providing precise input data for a numerical simulation and validation. The test conditions of the TMF panel were a heat flux of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\dot{\mathrm{q}}=24.25$</tex> MW / m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and a maximum surface temperature of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{T}_{\mathrm{s}}=800$</tex> K. For numerical modeling of the experiment, ANSYS Mechanical was used. Herein the nonlinear material models implemented within the software package were used. In particular, the kinematic hardening model according to Chaboche, the isotropic hardening and softening model after Voce, Norton's creep fatigue law and the strain rate dependency model according to Peirce were used. The mechanical material parameters for the numerical model were determined by means of an extensive test series including low cycle fatigue (LCF) and tensile tests with uni-axial specimen covering a range of temperatures starting from cryogenic conditions at 77 K towards 1000 K. The numerical fatigue life is estimated by a post processing tool.