## 1 Introduction  
The aim of the HYPROB Program is to design, manufacture, and test a ground demonstrator of a liquid rocket engine employing liquid methane and liquid oxygen. Such a propulsion technology may cover a wide range of aerospace propulsion systems, from launcher main stages up to small thrusters. When the requested levels of thrust are relatively high, heavy heat fluxes and high chamber pressures are expected in the combustion chamber. Therefore, active cooling techniques can be used to keep the selected materials at acceptably low temperature values. The thrust chamber considered in the current investigation comprised an inner structure made of a CuCrZr alloy, and an external cold structure made of electrodeposited nickel. Methane, before being injected as fuel into the combustion chamber, also acts as a coolant fluid flowing through axial cooling channels, so as to partially remove heat from the inner structures, see Figure 1. Even though temperature levels are lowered with such a cooling process, very high thermal gradients occur within the structure, separating the hot gas from the cooling channels, where the fuel temperature is considerably lower. Several works can be found in the literature regarding the thermal-mechanical behavior of liquid rocket engines thrust chambers under cyclic loading conditions. These have demonstrated that plasticity effects should be modeled by means of both isotropic and kinematic hardening models. Additionally, a nonlinear kinematic hardening model should be adopted to simulate shakedown and ratcheting phenomena [1; 2; 3]. In particular, when employing the Chaboche kinematic hardening model [4], at least three nonlinear hardening models should be considered to simulate ratcheting behavior properly, i.e., the progressive plastic strain accumulation during cycling [5]. Other researchers also demonstrated that low-cycle fatigue (LCF) and creep can be the main possible causes of failure, in turn, highlighting the need to accurately investigate these phenomena [6; 7; 8]. Creep effects can be dominant when temperatures and stresses reach significantly high values [9; 10; 11; 12]. Even if [PERSON] secondary creep law [13] has been usually considered for these applications in  
Figure 1: Rocket engine thrust chamber (**a**) and related functional scheme (**b**). literature, the authors have chosen to adopt a more complete creep model taking account of both primary and secondary creep effects through combined time hardening rules. With regard to the LCF phenomenon, uniaxial fatigue criteria are usually adopted for these kind of applications, since circumferential plastic strains are the dominant contributors with respect to the radial and axial ones. However, authors have adopted a critical plane approach, namely, the Wang-Brown criterion [14], to consider the effects of the multi-axiality of the stress-strain fields and non-proportionality of the loading conditions, because the influence of multiaxial and non-proportional loading effects is not negligible when evaluating the service life of these kind of structures. The latest advances of numerical methods, such as the finite element method (FEM), allowed us to study these complex structures and loading conditions, reducing the burden of expensive experimental tests. The aim of the present work was to conduct a numerical investigation on the 3D thermal-mechanical behavior of a cooling channel of the thrust chamber by employing state-of-the-art numerical FEM models. This allowed us to take account of plasticity, creep, and LCF phenomena simultaneously. These analyses were conducted by means of the FEM code ANSYS. A preliminary work involving the same case study and concerning the adoption of global/local approaches can be found in [15]. As a continuation of [15], in the present work, plasticity effects were studied adopting the von [PERSON] yield criterion, Prandtl-Reuss flow rule, and both isotropic and non-linear Chaboche kinematic hardening models.