Editorial for the Special Issue on Mechanical Properties of Composite Materials and Joints

The papers gathered in this Special Issue present an integrated view of how processing routes, microstructure, and interfacial design govern the mechanical response of modern composites and their joints. They collectively balance polymeric, metallic, and cementitious matrices, combine simulation with experiment, and cover a spectrum from joint design and interfacial engineering to durability under extreme or combined environments. This aligns with the Special Issue’s aim to bridge simulation and experimental studies across stress–strain, fatigue, fracture, impact, and joining phenomena.

Multiple contributions advance reliable joint design for thermoplastic and thermoset systems. A comparative study of ultrasonic welding, bolting, and adhesive bonding for CFRTP under bending quantified stiffness, damage patterns, and predictive accuracy of finite-element models, indicating that ultrasonic welding can reach stiffness comparable to adhesive joints while bolted joints lag due to separation during bending. Bio-inspired interlocking geometries mitigated disengagement by stabilizing the female component against outward bending. Interface tailoring using silica and hydroxyapatite nanofillers in pultruded GF/epoxy demonstrated concurrent gains in tensile, flexural, and interlaminar shear properties via improved dispersion and interfacial bonding. Adhesive-quantity effects in Kevlar-laminate FRLs further underscored the sensitivity of energy absorption and failure modes to resin distribution. Together, these works translate microstructural control into structural metrics for joint integrity.

Several papers interrogated long-term behavior under harsh conditions. Epoxy–nanosilica systems subjected to LEO-like UV-C exposure revealed degradation pathways relevant to space structures. Thermal/operational aging of SBR compounds was quantified via iso-conversional kinetics with implications for activation-energy shifts and conductivity evolution. Thermo-mechanical modeling of ablative protection in hybrid/solid rocket chambers coupled one-dimensional thermal/ablation kinetics with stress analyses to support minimum-mass designs under severe gradients. Collectively, these contributions link degradation science with predictive models for service life in aerospace and energy applications.

The Special Issue highlights low-carbon matrices and circular strategies, including basalt-fiber-reinforced self-compacting geopolymers, lignin-based magnetic composites for rapid Pb(II) removal, and all-polymer composites that upcycle PET fibers from safety belts into high-performance HDPE-based systems. Cement-based composites emerge as multifunctional media: graphene-oxide/MnO₂ formulations combine mechanical strength with thermoelectric response, while epoxy-based nano-cement composite dampers target enhanced damping alongside strength. A comprehensive review on nanoclays for 3D printable concrete organizes rheology–mechanics relations central to pumpability, buildability, and structural performance. These studies connect sustainability with mechanical function and processability.

Mechanistic and computational approaches—including finite-element modeling of joint stiffness/damage, kinetic models for thermal degradation, and ablation/pyrolysis heat transfer—recur throughout the issue. Molecular dynamics of origami graphene/Cu nanocomposites elucidates auxetic behavior at the nanoscale, while graphite-flake-filled epoxies illustrate scalable, lower-cost routes to stiffness and thermal enhancements. Natural-fiber (ramie) reinforcement in rubber demonstrates how fiber length and loading tune tensile/tear strength and hardness, coupling microstructural morphology to macroscale response. A metal-matrix study on Cu-reinforced A356 explores heat treatment and composition on tribology, reinforcing that processing history is inseparable from performance The outcome is a consistent process–microstructure–property narrative across material classes.

Two themes emerge: (i) joints and interphases remain the primary lever for structural reliability, and (ii) durability under realistic, combined exposures needs standardized protocols tied to validated models. Future work should emphasize operando characterization (CT, DIC, and AE in joints), certified accelerated aging linked to Bayesian lifetime inference, and data-centric “digital twins” for composite structures. This Special Issue provides a foundation, spanning experiments, theory, and applications, on which those efforts can build.