Szymon Duda

Composite materials have been extensively used across numerous industries due to their exceptional specific strength and corrosive resistance. However, ensuring their mechanical performance and structural integrity remains a critical challenge. This study provides an in-depth investigation into the damage mechanisms occurring in composite rebars manufactured via a modified pultrusion process, with a special emphasis on carbon, glass, and hybrid continuous fiber-reinforced polymers with epoxy resin matrix subjected to static tensile loading. To reveal the damage development, the acoustic emission (AE) technique was employed. Given the inherent complexity of composite microstructures, multiple failure modes can occur simultaneously, often masked by background noise and attenuation effects. Therefore, the core objective of this research is to evaluate and quantify the influence of acoustic attenuation on damage assessment in composite materials. This study introduces an optimization approach to minimize discrepancies between signals captured by different sensors, thereby enhancing the reliability of AE data interpretation. Results reveal that attenuation is strongly dependent on signal travel distance, frequency spectrum, and sensor type. Importantly, a data correction methodology is proposed to mitigate these effects, improving the accuracy of damage detection. Among the analyzed AE parameters, the initial frequency emerged as the most reliable feature for identifying the origin of acoustic events within hybrid composite structures. This finding represents a significant step toward more precise, attenuation-compensated acoustic emission monitoring, offering improved insight into failure mechanisms and contributing to the development of smarter diagnostic tools for composite materials.

The fatigue behavior of continuous fiber-reinforced composite materials is still not fully understood, particularly under multiaxial out-of-phase loading conditions. This study assesses the multiaxial fatigue behavior of thin-walled carbon fiber-reinforced polymer (CFRP) tubular specimens fabricated by filament winding (FW). A comprehensive experimental study is presented, investigating axial-torsion loads, phase shifts (0°, 45°, and 90°), and load ratios (−1, 0.05, and 0.5). Simultaneously, the acoustic emission (AE) method provides supplementary data for assessing fatigue damage accumulation. Consequently, a shear nonlinear material model and progressive damage in a shell-based finite element model were applied for stress analysis. The experimental results demonstrate the negative influence of a 90° out-of-phase load and the detrimental effect of mean stress for investigated positive load ratios. These findings offer valuable insights into the impact of phase shift (δ) and load ratio (R) in filament-wound carbon composites. These are essential for accurately modeling the fatigue behavior of composite materials under complex multiaxial loading.

This paper presents extension of the application of a damage parameter recently shown to successfully model multiaxial fatigue behavior of short fiber-reinforced composites to continuous fiber composite materials. The authors conducted an experimental campaign on the fatigue life of carbon fiber-reinforced polymer composite (CFRP), taking into consideration several load ratios (R) as well as in-phase and out-of-phase loadings. The study focused on modeling fatigue damage initiation by incorporating the shear stress range and maximum normal stress. The analysis of stiffness degradation confirmed that most of the fatigue life was governed by damage onset and the propagation of small cracks. The observed damage was primarily associated with matrix-fiber debonding. This approach was verified with the literature's experimental data for glass fiber-reinforced polymer (GFRP) and the authors' experimental data for CFRP. Although those materials exhibit a similar fracture nature, they differ significantly in many aspects, i.e., fiber orientation, layup configuration, constituent materials and stiffness properties. The correlation of the stress-based damage parameter shows good agreement with both the authors’ and the literature experimental data, with coefficient of correlation (r) 0.84 and 0.89, respectively. The critical plane concept was also applied to estimate fatigue life, revealing that the long-life fatigue strength corresponds to approximately 25% of the ultimate tensile strength (UTS) for cylindrical specimens. Based on this relationship, the S–N curve for fatigue prediction can be constructed using only the UTS. Highlights: Modeling of fatigue failure based on the critical plane approach using multiaxial experimental data for CFRP and GFRP Estimation of fatigue performance from static tension test

Thermoplastic fiber metal laminates (FMLs) are hybrid material systems that consist of a thin aluminum alloy sheet bonded to plies of fiber-reinforced adhesive. They provide excellent properties like fatigue strength, damage-tolerant properties, and inherent resistance to corrosion. However, they are still challenging materials in terms of the metal–composite interface, which is the weakest link in this material system. In this paper, an experimental–numerical method for the determination of the fracture stress and energy for metal–composite interlayer is presented and verified. The proposed method utilizes four different experimental tests: DCB test (interface opening—mode I), ENF test (interface shearing—mode II), MMB test (mixed-mode I+II—opening with the shearing of the interface) and three-point bending test (3PB). For each test, digital twin in the form of a numerical model is prepared. The established numerical models for DCB and ENF allowed us to determine fracture stress and energy for mode I and mode II, respectively. On the basis of the numerical and experimental (from the MMB test) data, the B-K exponent is determined. Finally, the developed material model is verified in a three-point bending test, which results in mixed-mode conditions. The research is conducted on the thermoplastic FML made of aluminum alloy sheet and glass fiber reinforced polyamide 6. The research presented is complemented by fundamental mechanical tests, image processing and Scanning Electron Microscopy (SEM) analysis. As an effect, for the tested material, fracture parameters are determined using the described method.

This study employs axial compression testing to analyze the impact of mosaic patterns on the mechanical properties, specifically the strength and stiffness, of composite tubes. Three mosaic patterns (1/1, 2/1, and 3/1) and three winding angles (45°, 55°, and 75°) were selected for evaluation. Additionally, the acoustic emission (AE) technique was utilized to investigate the damage mechanisms occurring in the composite structures. The results showed that the higher the mosaic pattern the higher the stiffness and strength of the composite tubes under axial compression loading.