Characterization of overmolded thermoplastic composite joints and sizing methods for hybrid structures

The growing demand for lightweight, high-performance composite materials, particularly with thermoplastic matrix, in the transportation industry necessitates the development of efficient and reliable manufacturing processes. In this context, CETIM and its industrial partners have developed an overmolding process that involves injecting discontinuous fiber-reinforced thermoplastic composites over a continuous fiber-reinforced support structure. The mechanical performance of the resulting structures, which feature a critical bonding zone at the interface between the continuous-fiber laminate and the discontinuous-fiber overmolded part, depends on various material, geometric, and process parameters. This PhD thesis aims to develop a comprehensive strategy for the experimental characterization, numerical modeling, and simulation of damage and failure within this bonding zone. The first part of the work focuses on understanding the parameters that influence mechanical performance to optimize design, process definition, and sizing. A Cohesive Zone Model (CZM) is employed to simulate the failure behavior. A dissipation-driven approach is also integrated via an Abaqus User Element to handle instabilities during the failure process, allowing for more accurate predictions of the maximum load. The second part consists in conducting experimental tests to identify parameters that serve as input for the model and validate the numerical approach. For the characterization of the critical energy release rates, Climbing Drum Peel (CDP) and End-Notched Flexure (ENF), are modified to adapt to the asymmetric laminate/overmold case. For the characterization of the maximum stress, as well as for model validation, custom-designed isostatic test fixtures are developed. The used fixture enables control of the boundary conditions by positioning the entire setup on the machine between two ball joints, which renders the assembly/machine chain isostatic. The proposed sizing method, validated through simulation and experimental work, offers a practical approach for the industrial design of structures featuring overmolded joints. The results demonstrate that the methodology effectively captures the critical aspects of the bonding zone's performance, enabling more reliable and optimized designs for thermoplastic composite structures in industrial applications.