Theodore Matikas

The residual life of metallic structures is dictated by microstructural plastic damage, which conventional linear ultrasonics often fail to detect early. This study investigates the continuous, in-situ evolution of the second-order acoustic nonlinearity parameter (β₂) throughout the monotonic tensile response of S235 structural steel, from the elastic regime to fracture. Bulk longitudinal waves were excited by a contact piezoelectric transducer and detected through-thickness using a non-contact Laser Doppler Vibrometer on a notched specimen. This allowed the normalized acoustic parameters to be mapped directly onto the macroscopic stress-strain curve. Three distinct acoustic regimes were identified. Up to the onset of the yield plateau, the nonlinearity parameter and the linear attenuation remained at unity within the specimen-to-specimen scatter. Beyond the yield point, β₂/β₂₍₀₎ rose steeply to approximately 4.5, whereas the normalised attenuation increased by at most approximately 20%. The rise extended over the yield plateau of the stress–strain curves and the parameter saturated when work hardening resumed (nominal strain ≈ 0.031–0.033), ahead of the maximum load (≈0.043); a lower value was recorded only at the last acquisition, at which the signal quality was degraded. The departure of β₂ from unity, i.e., the first increase of the second-harmonic amplitude, occurred between nominal strains of 0.015 and 0.021, coinciding with the onset of the yield plateau (≈0.017) and slightly above the 0.2% offset yield strain (0.0128 ± 0.0004), as expected for a notched specimen in which plasticity reaches the interrogated volume once it spreads across the ligament; this supports its potential as a marker of yield onset. The increasing trend provides a continuously acquired ferritic-pearlitic reference data set and is consistent with, although not by itself proof of, a governing role of the initial dislocation substructure in harmonic generation.

This study investigates the design, microstructure, and performance profile of a novel composite coating combining a high-entropy alloy (HEA) matrix with cermet reinforcement. A 50wt.%CoCrFeMnNi_0.8V–50wt.% Cr₃C₂-Ni₈₀Cr₂₀ powder mixture was successfully deposited onto steel substrates via high-velocity oxy-fuel (HVOF) thermal spraying. Microstructural analysis revealed a highly dense, well-bonded coating architecture (450–500 μm thick) where partially melted, spherical HEA splats were uniformly surrounded by the Cr₃C₂-Ni₈₀Cr₂₀ phase. X-ray diffraction confirmed a complex multiphase evolution consisting of FCC, BCC, and σ-NiCr phases driven by the rapid solidification inherent to the HVOF process. Tribological evaluations via ball-on-disc testing demonstrated that incorporating the Cr₃C₂-Ni₈₀Cr₂₀ reinforcement significantly improves wear resistance compared to the monolithic HEA coating. The composite’s wear behavior is governed by a synergistic mechanism: the ductile HEA matrix accommodates plastic deformation, while the harder carbide particles enhance load-bearing capacity, transitioning from adhesive wear to mild third-body abrasion and protective tribo-oxidation. Conversely, electrochemical testing in a 3.5 wt.% NaCl solution showed that the composite coating exhibits higher corrosion current densities (10.53 × 10⁻⁶ A/cm²) and more active corrosion potentials than the pure HEA matrix. This behavior is attributed to localized micro-galvanic cells forming at the heterogeneous interfaces between the different phases, alongside chloride-induced destabilization of the surface oxide film. Overall, the novel composite coating offers a compelling, sustainable alternative for surface engineering applications requiring a balanced trade-off between mechanical toughness and acceptable environmental durability. This behavior is also verified by the comparison with previous results dealing with monolithic CoCrFeMnNi_0.8V and 75wt.%CoCrFeMnNi_0.8V–25wt.% Cr₃C₂-Ni₈₀Cr₂₀ thermal sprayed coatings, where it is evident that the increase of the reinforcing phase leads to an optimum combination of properties.

This study investigates the microstructural evolution of the CoCrFeNi system after incorporating Gallium (Ga) at varying concentrations (0, 15, and 20 at.%). The systems were synthesized by Vacuum Arc Melting (VAM) and characterized through X-ray Diffraction diffraction (XRD) and Scanning Electron Microscopy (SEM/EDS). Findings showed that the CoCrFeNi medium medium-entropy alloy stabilizes in a single-phase Face-Centered Cubic (FCC) structure. Upon the addition of 15 at.% Ga a dendritic morphology with a transition towards a duplex FCC + BCC microstructure was induced, a trend which was further solified in the equiatomic FeCoNiCrGa system. In this case the proportion of the Ga-rich BCC phase was increased from 18–22% to 31–34% for the Ga15 and Ga20 systems respectively. A combined approach of Electrochemical Frequency Modulation (EFM), Cyclic Potentiodynamic Polarization (CPP), and Electrochemical Impedance Spectroscopy (EIS) was selected for studying the electrochemical corrosion behavior of the produced systems. EFM results indicated a progressive deterioration of corrosion resistance when increasing Ga concentration (Icorr: 4.142, 5.619 and 10.01 μA/cm², and Rp: 12,035, 10,736 and 7254 Ω for the Ga0, Ga15 and Ga20 alloys respectively). Surface inhomogeneity, rapid passivation, and diffusion-controlled processes caused deviations from the ideal causality factors’ values. CPP measurements revealed increasing corrosion current densities with Ga addition within the Tafel region (2.81 × 10⁻⁷, 3.72 × 10⁻⁷ and 5.11 × 10⁻⁷A/cm² for the Ga0, Ga15 and Ga20 alloys respectively). All alloys showed positive hysteresis loops and an absence of repassivation, indicating susceptibility to pitting corrosion. Nevertheless, detailed analysis of the forward polarization region highlighted a more complex aspect. Reverse polarization scans confirmed stable pit growth in all alloys, with the absence of a repassivation tendency. EIS tests, performed after the completion of CPP measurements, further clarified the corrosion mechanisms. Equivalent circuit modeling revealed that although Ga-containing alloys exhibited relatively improved film characteristics in the forward polarization stage, the charge transfer resistance (Rct) was highest for the CoCrFeNi alloy, followed by Ga15 and Ga20 (22,620, 11,380, 10,060 Ω respectively). The overall impedance ranking (Ga0 > Ga15 > Ga20, i.e., 27,139 > 20,279.5 > 16,341 ohms respectively) showed that, despite microstructural and entropic effects enhancing certain passivation aspects, the reduced Cr content highly impacted long-term corrosion resistance. This holistic electrochemical approach showcases the complex interactions between compositional alterations, phase structure, grain refinement, passive film chemistry, and diffusion trends in establishing the corrosion performance of Ga-modified CoCrFeNi HEAs.

This paper presents a fully automated platform for real-time monitoring of fatigue-induced microstructural changes in metallic materials, using Rayleigh surface waves and Laser Doppler Vibrometry (LDV). The system integrates ultrasonic excitation, non-contact optical sensing, and high-speed signal processing in a unified LabVIEW environment. Rayleigh waves are generated via a contact transducer, while LDV captures surface vibrations with sub-nanometric velocity resolution, ensuring repeatability and eliminating coupling variability. The software automates synchronization, deterministic data acquisition, filtering, FFT analysis, and extraction of nonlinear coefficients (β₂, β₃) at high execution rates without the need for post-processing. Experimental validation under cyclic loading revealed a clear sensitivity hierarchy: the Rayleigh wave velocity remained invariant, the acoustic attenuation responded gradually, while the nonlinear parameters exhibited the earliest and steepest response to fatigue damage, confirming their superiority as early-stage indicators. The system offers low-latency timing, long-term stability, and modular design, establishing a robust data-streaming foundation that can support future integration with digital twin frameworks and machine learning models. Furthermore, the acoustic findings were successfully cross-validated using Infrared Thermography, which confirmed the critical damage transition phase. This work bridges nonlinear acoustics and software automation, providing a scalable diagnostic solution for predictive maintenance within structural health monitoring systems.

Recent advances in nanotechnology have highlighted the transformative potential of carbon-based nanomaterials, such as carbon nanofibers, carbon nanotubes, and graphene, in cementitious systems. These materials have shown a remarkable ability to enhance the mechanical strength, fracture toughness, and overall functional performance of cementitious composites. Their nanoscale dimensions and exceptional intrinsic properties allow for effective stress bridging, crack arrest, and matrix densification. Despite these promising features, the current understanding remains limited, particularly regarding their application to concrete. Furthermore, literature lacks systematic, parallel evaluations of their respective effectiveness in improving both mechanical performance and long-term durability, as well as their potential to impart true multifunctionality to concrete structures. It is worth noting that significant and statistically significant improvements in fracture behavior were observed at specific nanofiller concentrations, suggesting strong potential for the material system in next-generation innovative infrastructure applications. Experimental results demonstrated that both CNTs and GNPs significantly enhanced the mechanical performance of concrete, with flexural strength increases of approximately 49% and 38%, and compressive strength improvements of 22% and 47%, respectively, at optimum contents of 0.6 wt.% CNTs and 0.8 wt.% GNPs. SEM analyses confirmed improved matrix densification and interfacial bonding at these concentrations, while higher dosages led to agglomeration and reduced performance. This gap highlights the need for targeted experimental studies to elucidate the structure-property relationships governing these advanced materials.

This study investigates an alternative methodology for incorporating polymeric and steel fibers into concrete. Conventional reinforcement approaches often require complex application techniques and face industrial limitations. In contrast, the present work evaluates the use of short, discontinuous fibers—commercial polypropylene fibers (PFRC), polypropylene fiber braid (PFBRC) and steel fibers (SFRC)—which enable improved dispersion, ease of mixing and potential mechanical benefits. The fibers were randomly oriented and evenly distributed within the cementitious matrix. Mechanical performance was assessed through four-point bending tests combined with displacement measurements, acoustic emission analysis and uniaxial compression tests, while scanning electron microscopy (SEM) confirmed fiber–matrix interaction and fragment retention. The results demonstrated significant improvements, with compressive strength exceeding that of unreinforced concrete, while hybrid fiber systems provided enhanced crack resistance and post-cracking stability. Overall, the findings highlight that the integration of discontinuous fibers may provide tangible mechanical advantages, potentially outweighing the structural benefits of continuous reinforcing bars in applications requiring high strength and reliable mechanical performance.

The development of automated systems for real-time material evaluation is becoming increasingly critical for structural engineering applications, infrastructure diagnostics and advanced material research. This work introduces a novel, fully automated nonlinear acoustics monitoring platform that employs Bulk Wave excitation in combination with non-contact Laser Doppler Vibrometry (LDV) detection to continuously assess the microstructural evolution of cement-based composites. Unlike conventional approaches—such as ultrasonic velocity measurements or compressive strength tests—which lack sensitivity to early-stage changes and also require manual operation, the proposed system enables unsupervised, high-precision monitoring of the material by leveraging the second and third harmonic generation (β₂, β₃) as nonlinear indicators of internal material changes. A specialized LabVIEW-based software manages excitation control, signal acquisition, frequency-domain analysis, and real-time feedback. As an initial step, the system’s stability, linearity, and measurement reliability were validated on metallic samples, and verified through long-duration experiments. Subsequently, the system was used to monitor hydration in cement-based specimens with varying water-to-cement and carbon nanotube (CNT) reinforcement ratios, thereby demonstrating its capability to resolve subtle nonlinear responses. The results highlight the system’s enhanced sensitivity, repeatability, and scalability, demonstrating that it as a powerful tool for structural health monitoring, smart infrastructure, and predictive maintenance applications.

The increasing reliance on conventional coatings such as WC-Co raises serious environmental and health concerns due to the toxicity of cobalt and the ecological footprint of these materials. To address this challenge, the present study explores the development of eco-friendly multifunctional coatings via the Plasma Spray (PS) process, using titanium (Ti), silicon carbide (SiC), and tungsten carbide-cobalt (WC-Co) mixtures as alternative feedstocks. Steel substrates were coated under different deposition strategies (powder mixing, layer-by-layer) and current settings (800-900 A). The coatings were characterized by scanning electron microscopy (SEM/EDX), 3D profilometry, sliding wear testing, and potentiodynamic corrosion measurements. Results showed that Ti-WC (mix, 900 A) and Ti-SiC (layer, 900 A) coatings achieved the most favorable performance, combining excellent adhesion, uniform coverage, reduced porosity, and improved resistance to wear and corrosion compared to conventional Cr₂O₃ coatings. Notably, Ti-WC coatings provided surface roughness values comparable to Cr₂O₃, while significantly lowering the environmental impact. These findings demonstrate that PS-based Ti-WC and Ti-SiC systems can serve as sustainable and high-performance alternatives for protective applications in harsh environments, particularly in marine industries, supporting the transition toward coatings with reduced ecological footprint.

This study explores the influence of various nano-inclusions on the electrical and thermal properties of cement-based materials. Specifically, it investigates the incorporation of Multi-Walled Carbon Nanotubes (MWCNTs) and Graphene Nanoplatelets (GNPs) as reinforcement materials in cement composites. These advanced nanomaterials enhance the mechanical strength, durability, and functional properties of cementitious matrices. A series of experimental tests was conducted to evaluate the thermal and electrical behavior of nano-reinforced concrete, employing nondestructive evaluation techniques, such as Infrared Thermography (IRT) and Electrical Resistivity measurements. The results indicate that increasing the concentration of nanomaterials significantly improves both the thermal and electrical conductivity of the composites. Optimum performance was observed at a CNT dosage of 0.6% and a GNP dosage of 1.2% by weight of cement in cement paste, while in concrete, both nanomaterials showed a significant decrease in resistivity beginning at 1.0%, with optimal performance at 1.2%. The study also emphasizes the critical role of proper dispersion techniques, such as ultrasonication, in achieving a homogeneous distribution of nanomaterials within the cement matrix. These findings highlight the potential of carbon nanotubes (CNTs) and GNPs to enhance the multifunctional properties of cement-based materials, paving the way for their application in smart and energy-efficient construction applications.

In this article, a multidisciplinary methodological approach for studying a wooden panel painting is applied. The theoretical framework, within which this research has arisen, is the application of state-of-the-art non-destructive techniques for addressing issues concerning the constituting parts and composing materials of the artwork. Hereby, a post-Byzantine icon was studied, which was dated back to 1836. It is a painting executed on a wooden panel, with a decorated wooden frame attached. The artifact was thoroughly investigated through the application of infrared thermography (IRT), multispectral imaging (MSI), and macroscopic X-ray fluorescence spectrometry (MA-XRF). These analyses provided crucial information about the verso of the painting (i.e., the wooden panel and the frame) and allowed for the revelation of important details of the recto of the painting, which were not visible due to the presence of an old, decayed varnish. Additionally, through the detailed mapping of the distribution of various chemical elements on the recto of the painting and the frame, it was possible to identify the materials used and techniques employed. It is therefore shown that, when combined, the non-destructive methodologies in consideration can provide adequate information referring to the materiality and state of preservation of panel paintings, permitting the conservator to proceed to a tailored conservation treatment.