Institut Clément Ader

Polyamide 6,6 (PA66) is a key engineering polymer, whose unique mechanical properties arise from strong interchain hydrogen bonding. However, its hygroscopic nature makes it highly sensitive to water uptake, which markedly alters its thermomechanical behavior. Contrary to traditional experimental approaches, this study uses atomistic molecular dynamics (MD) simulations to investigate the role of water in modifying the glass transition temperature (Tg) and the viscoelastic response of amorphous PA66. Simulations capture a nonmonotonic dependence of Tg on water content. At low water concentrations, isolated water molecules bind to amide groups and restrict chain mobility, while beyond ∼2.5 wt %, water clustering disrupts the hydrogen-bond network and causes a pronounced Tg depression. Analysis of amide-group fluctuations reveals a master correlation between local segmental dynamics and bulk density, verifying the known temperature-humidity equivalence in terms of density variation. The computed Young's modulus exhibits systematic softening with increasing temperature and water content, consistent with experimental trends, albeit a more pronounced impact of water at low temperatures. Time-temperature superposition behavior is observed for both dry and hydrated systems. This work provides molecular-scale information on the hygromechanical coupling in PA66 and demonstrates the ability of MD simulations to predict water-induced transitions that govern the macroscopic behavior of polyamides.

ABSTRACT This study investigates the mechanical, thermal, and dynamic behavior of polylactic acid (PLA)/poly(butylene succinate) (PBS) biopolymer blend processed by injection molding (IM) and fused granular fabrication (FGF 3D printing), with and without 20 wt% short flax fibers. Morphological analysis, fiber length distribution, differential scanning calorimetry, and dynamic mechanical analysis were conducted to highlight how processing methods affect fiber integrity and the resulting biocomposite properties. The results show that the incorporation of 20 wt% flax fibers significantly increases the melting enthalpy from 30.75 to 45.30 J/g and the degree of crystallinity from 41.33% to 60.92%. In addition, PLA‐PBS pellets containing 20 wt% short flax fibers were successfully processed and printed via FGF. The average fibers length was higher in injection‐molded samples (930 μm) than in 3D‐printed ones (659 μm), indicating that injection molding better preserves fiber integrity and thus provides more efficient reinforcement. IM‐FF/PLA‐PBS retained a higher proportion of long fibers, with 30% exceeding 1 mm, compared to only 16% in 3D‐FF/PLA‐PBS samples. Consequently, IM‐FF/PLA‐PBS specimens showed enhanced mechanical performance, with higher stiffness ( E = 4.90 GPa) and tensile strength ( σ = 42.4 MPa) than their 3D‐FF/PLA‐PBS counterparts ( E = 2.22 GPa, σ = 11.5 MPa). Furthermore, DMA results revealed higher storage moduli for IM‐FF/PLA‐PBS (3620 MPa) compared to 3D‐FF/PLA‐PBS (800 MPa), confirming increased stiffness and improved fiber–matrix adhesion. However, 3D‐FF/PLA‐PBS and 3D‐PLA‐PBS yielded lighter components, advantageous for weight‐critical applications. Overall, these findings highlight the significant influence of processing techniques on biocomposite performance.

The thermal response of Carbon Fiber Reinforced Thermoplastic (CFRTP) tapes under short-term localized heating is critical for automated manufacturing processes. Conventional homogenized models often overlook microstructural heterogeneities that can promote non-uniform heating and affect the quality of the consolidated part. In this work, we combine insights from infrared thermography with finite element simulations at the fiber scale built on micrographs extracted from real tapes to quantify the effect of individual heterogeneities—including surface roughness, thickness variation, fiber agglomeration, and porosity—on thermal propagation. Three modeling configurations were compared under identical conditions: a full microstructure model; a simplified geometry-aware model (where the real geometry is taken into the account, including the surface roughness and thickness variability, but the properties of the domain are considered as a homogeneous-equivalent material); and a homogeneous-equivalent baseline with flat borders and uniform thickness. Results show that porosity effects depend strongly on location and orientation: large, horizontally aligned pores near the heated surface produce the highest gradients. Surface roughness, on the other hand, exerts dominant effects on surface temperature non-uniformity with respect to thickness variation and fiber distribution. These findings demonstrate that accounting for microscale heterogeneities is essential to achieve more accurate, optimized, and application-tailored analyses of CFRTP tapes in advanced manufacturing.

Abstract Structural optimization may imply in many instances the presence of categorical, discrete, and continuous variables. This is particularly the case for composite structures due to the stacking sequence definition which involves not only structural considerations, but also a set of complex manufacturing- and cost-related constraints. While approaches ranging from relaxation of all variables to evolutionary methods have been proposed in the literature, it is still challenging to deal consistently with all manufacturing and structural constraints, especially in high-dimensional problems (hundred or more design variables). This work proposes a mixed-variables optimization formulation that can integrate a large variety of manufacturing and structural constraints in a monolithic approach based on a modified Bi-level Outer Approximation algorithm. The proposed framework allowed the treatment of composite optimization problems accounting for catalog selection of stacking sequences, manufacturing compatibility of adjacent elements, as well as variability control schemes. Various optimization formulations have been proposed tailored both for conventional laminates and for double–double laminates. The proposed approaches were tested on both an academic use-case, a multi-cell panel and, on an industrial use-case, a vertical tail plane front spar.

This paper presents a framework and one pedagogical application of motion tracking algorithms applied to structural dynamics. The aim of this work is to show the ability of high speed camera to study the dynamic characteristics of simple mechanical systems using a marker less and simultaneous Single Input Multiple Output (SIMO) broadband analysis. KLT (Kanade-Lucas-Tomasi) trackers are used as virtual sensors on mechanical systems video. First we introduce the paradigm of virtual sensors in the field of modal analysis using video processing. Then we present a pedagogical example of flexible beam (Fishing rod) video. From KLT tracking we extracted displacements data (virtual sensors) which are then enhanced using filtering and smoothing and then we can identify natural frequency and damping ratio from classical modal analysis. The experimental results (mode shapes) are compared to an analytical flexible beam model showing high correlation but also showing the limitation of linear analysis. The main interest of this paper is that displacements are simply measured using only video at FPS (Frame Per Second) that respects the Nyquist frequency. There is no target needed on the structure only few critical pixels that are good features to track and which become virtual sensors.