This thesis investigates the constitutive modeling of GrCop-42 Low Cycle Fatigue (LCF) behavior and its application to the combustion chamber of a liquid rocket engine developed by Pangea Propulsion. The main objective is to develop a predictive framework for fatigue life estimation under severe thermomechanical loads, which induce cyclic viscoplasticity and progressive damage, ultimately leading to the prediction of the chamber failure mode known as the Doghouse effect. The Chaboche model was adopted as the baseline for cyclic plasticity and extended to account for viscoplastic and damage effects. Viscous behavior was described using
the Norton–Bailey law. Damage was accounted for with two different models: as a reduction in cross-sectional area, affecting the material’s mechanical response only in tension, and as a reduction in stiffness, influencing the response under both tensile and compressive loading. In both cases, damage is represented as a second-order tensor, making the resulting damage state anisotropic. Additionally, three damage evolution laws — linear, multilinear, and nonlinear (exponential), with and without isotropic softening — were evaluated and compared
with experimental observations. Due to the lack of available LCF data for GrCop-42, a dedicated test campaign was designed by Pangea Propulsion and performed by an external laboratory to provide reference data for model calibration and validation. The implementation began with the development of a fitting tool based on the residual sum of squares method, enabling the extraction of Chaboche parameters from the experimental
dataset. This tool was first implemented in MATLAB and used to simulate the stress–strain response of a LCF test, which was then compared and verified against data available in the literature. Building on this step, the Chaboche plasticity model was implemented in an Abaqus User Material (UMAT) subroutine, where the LCF test conditions were replicated on a dogbone specimen and verified against published results. The UMAT was subsequently extended to include viscous effects and verified using the same methodology. Finally, the damage model was incorporated into the UMAT and verified through both analytical computations and comparisons with literature. With all tools successfully verified, the validation phase was conducted against the experimental data obtained from the test campaign. The results reveal that at elevated temperatures, GrCop-42 exhibits a three-stage fatigue response: initial isotropic softening, stabilization, and final softening leading to failure. This behavior was best captured using a nonlinear (exponential) damage evolution law combined with isotropic
softening and kinematic hardening, with the damage modeled as a reduction in stiffness.. The application of the non-linear model to the combustion chamber demonstrated that the UMAT successfully predicted mid-channel crack formation and the Doghouse effect, with damage accumulation occurring primarily during shut-off phases. Specifically, two firing scenarios—10 seconds and 300 seconds—were analyzed, highlighting that once thermal steady state is achieved, viscous effects become negligible for the mission durations studied. In conclusion, this thesis provides a validated modeling approach for predicting low-cycle fatigue-driven failure in GrCop-42 and proves its applicability in the framework of the combustion chamber.