Modeling and simulation of the initial state of a thermoplastic matrix composite structure manufactured by laser assisted tape placement

In the last decades, the increasing use of composite materials in the automotive and aerospace industries involved modern and automated production techniques like the Laser Assisted Tape Placement (LATP), which assure high production rates but induce residual stresses and strains in the manufactured structures. Because of the complexities and the numerous sources of residual stresses and strains involved in the LATP manufacturing process, the present work focused on the evaluation of the main physical mechanisms at the meso- and the macro-scale underlying the build-up of the initial state within the composite structure caused by the manufacturing process. From the state of the art, the combined effect of the composite part's geometry and anisotropy is an important source of residual stresses and strains that can be found at the macro-scale. The curved geometry couples the in-plane kinematics with the out-of-plane kinematics while, for the anisotropy of the composite, the in-plane behavior is generally different with respect to the out-of-plane behavior. Therefore, the incompatible thermal strains field generated during processing causes residual stresses within the structure. A new viscoelastic constitutive model suited for unidirectional composite is developed to address the effects of the complex thermal history induced by the LATP. A decomposition of the stress and strain tensors is proposed, making it easier to distinguish the contribution of the fibers and the matrix to the constitutive behavior of the composite material. Assuming a viscoelastic deviatoric response of the matrix and elastic fibers, the linear viscoelastic model is applied only on certain terms of the decomposition. For numerical simulations, the viscoelastic model is implemented in a UMAT Abaqus user subroutine. A one-dimensional heat transfer transient equation is used to describe in a simplified way the thermal history induced by the consolidation phase of the LA TP manufacturing process. The mixed boundary conditions consider the heat fluxes with the mandrel and the ambient, while the initial condition is the temperature distribution in the structure thickness generated by the heating phase of the LATP. The analytical solution of the thermal model is developed and implemented in a UTEMP Abaqus user subroutine. Thermo-mechanical simulations are performed on tubes using the commercial finite element software Abaqus to evaluate the effects of the sources of residual stresses and strains accounted in the present work. The additive manufacturing modeling technique is used to achieve a simplified model of the automated lay-up process of the LATP. In this scenario, residual stresses are assessed and then released by simulating a cut along the axial direction of the tubes. The numerical residual strains are compared with the experimental results provided by CETIM.