Constitutive models based on flow plasticity theory are widely used to describe the mechanical behavior of thermoplastic composites under static and cyclic loading. These models introduce a yield surface that separates elastic and inelastic deformation regions. However, experimental studies show that plastic behavior appears at the initial stages of loading, which makes the identification of the yield surface position challenging. In the present work, the endochronic plasticity theory was employed to model the plastic behavior of composites reinforced with short glass fibers and based on polyphenylene sulfide and polypropylene matrices. The constitutive relations were presented in a general form with an extended set of parameters, enabling accurate modeling of composite behavior without invoking a yield surface. The model parameters were identified by minimizing the deviation function between calculated and experimental curves under uniaxial, off-axis, and shear loading. For the numerical implementation of the proposed approach, the method of successive approximations was used. The model was implemented into the ABAQUS solver using a user-defined UMAT subroutine. To validate the model, three-point bending simulations and fatigue modelling were performed. The overall simulation results demonstrated good agreement with the experimental data. However, several discrepancies remain, which may be attributed to the scatter in the mechanical properties of the thermoplastics and to the parameter identification process.

In this study, the micromechanical response of a representative volume element (RVE) under cyclic loading was simulated using the crystal plasticity finite element method (CPFEM) to obtain the local stress-strain response and accumulated plastic strain. Based on the high-fidelity data generated by CPFEM, an incremental neural network (INN) model was constructed. The INN model takes the load ratio and the current accumulated plastic strain as inputs to predict the corresponding accumulated plastic strain increment for a given number of cycles. Compared with traditional fatigue prediction models, this model does not require presetting empirical equations. The results demonstrate that this incremental learning approach can effectively capture the nonlinear evolution of plastic strain with the number of cycles. The developed single-hidden-layer INN model accurately predicts the plastic strain accumulation process in laser powder bed fusion (LPBF) GH4169 (Inconel 718) under cyclic loading and achieves the highest prediction accuracy.

Pultruded glass-fiber composites have been widely used in road infrastructures and in construction but their strength characteristics under complex stress state at different temperatures have not been determined, which complicates their further use. This work is devoted to the determination of static strength and stiffness of pultruded glass-fiber composites under normal and shear stresses (induced by simultaneous axial loading and torsion of specimens) for three different temperatures. The paper reveals the methodological aspects of testing under multiaxial loading; in particular, it analyzes different variants of specimen tabs. The experimental part included uniaxial tensile, torsion, compression tests, as well as multiaxial combined tensile and compression tests with torsion at normal, elevated, and low temperatures. Sensitivity of pultruded glass-fiber composites to complex stress state is revealed. Failure envelopes were built after axial and multiaxial testing at various temperatures. A failure criterion of the fourth order is proposed. The displacement and strain fields of the tubular specimens were analyzed using the VIC 3D at the chosen loadings and temperatures. The displacement fields were similar on the surfaces of the specimens, which indicates correct and uniform loading. The characteristic fractures are presented. The cracks on the specimens were different and correspond to the respective loading parameters and temperatures. This data allows us to predict the failure of the pultruded glass-fiber composites under complex loadings and different temperatures, which is necessary for the use of such materials.

Fiber Bragg Grating (FBG) sensors are extensively employed in aerospace, energy, and biomedical applications for temperature monitoring, due to their compact form factor, high sensitivity, and multiplexing capability. Conventional lumped thermal models offer acceptable accuracy only under conditions characterized by low Biot numbers (Bi < 0.1), where internal temperature gradients are negligible. However, their predictive accuracy deteriorates in high convective heat transfer environments, such as boiling fluids, molten polymers, or sheathed fibers with large diameters, due to the neglect of radial temperature gradients and nonlinearities arising from temperature-dependent material properties. To overcome these limitations, this study critically examines classical lumped models and proposes equivalent formulations that incorporate nonlinear thermal behavior. Numerical simulations demonstrate that for Biot numbers exceeding 10, conventional models can underpredict core temperature response by as much as 35 % relative to distributed models, whereas the proposed models maintain deviations below 5 %. Notable improvements include the integration of environmental temperature change rates, explicit dependence on the Biot number, and incorporation of temperature-dependent thermal properties. These enhancements reduce core temperature prediction errors by up to 30 % and extend the model's validity across a wide temperature range (20 degrees C to 800 degrees C). Consequently, the developed equivalent lumped models offer a computationally efficient and accurate alternative to fully distributed models, facilitating reliable FBG sensor design and improved prediction of thermal response dynamics.

High residual stresses (RS) in laser powder bed fusion (PBF-LB) components present significant structural challenges, yet traditional characterization techniques are cost- and time-intensive. This study introduces three rapid mechanical testing methodologies to estimate longitudinal RS in PBF-LB 316L stainless steel by using the premature micro-yielding of pre-tensioned near-surface regions in cylindrical geometries. The proposed methods evaluate RS through: (I) plastic work density calculated from cyclic loading-unloading, (II) virtual unloading mathematically projected from standard monotonic tensile data, and (III) the detection of a macroscopic micro-yielding threshold (0.01% strain). Estimations were quantitatively validated against neutron diffraction (ND) measurements across multiple heat treatment conditions (as-built to 1000 °C). The energy-based approaches (Proposed Methods I and II) demonstrated linear proportionality with relative ND stress reductions (R2= 0.82 and 0.77, respectively). Furthermore, by subtracting the empirical micro-yielding threshold from condition-specific macroscopic yield strengths, Proposed Method III tracked absolute RS relaxation. Electron backscatter diffraction (EBSD) analysis physically validated these techniques, confirming that the initial non-linear macroscopic mechanical response is governed by localized geometrically necessary dislocation (GND) accumulation prior to bulk yielding. These findings establish the proposed mechanical methods as highly accessible alternatives for evaluating stress mitigation strategies in additive manufacturing.

Today, composite profiles of constant cross section are widely used in advanced engineering structures. The use of composite profiles in window and door structures can reduce thermal bridging and reduce energy consumption for heating and cooling. This article focuses on the production of new, thermoplastic-based structural pultruded profiles and their application in a PVC (polyvinylchloride) window structure as a reinforcement. The heat transfer model was developed to determine die temperature and pulling speed for pultrusion of the 30 × 20 × 3.5 mm tube and a 31.5 × 25.0 × 3.5 mm channel from tapes. The microscopy results demonstrated full consolidation of all tapes in the material, thus confirming proper selection of pultrusion parameters. The mechanical tests results of the welded angle joint show that the window structure with composite reinforcement can be twice as strong as the steel reinforced one. This was achieved by welding the composite reinforcement simultaneously with welding of the PVC frame on a butt welding machine. The results of the hot box test show that the U-value of the window sash and frame with the composite reinforcement is 12% lower than that of a window with a steel reinforcement. The U-value of the window with composite reinforcement is 1.47 W/(m2·K), and that of the steel reinforced window is 1.55 W/(m2·K). Thus, the windows with composite reinforcement have low thermal transmittance complying with building regulations in various countries, and their use is permitted in northern climatic zones.

This study investigates the impact of artificially embedded defects on the very high cycle fatigue (VHCF) of 316L stainless steel produced by laser powder bed fusion (PBF-LB) technology. Each specimen contains a single embedded artificial defect of 180 mu m or 350 mu m in diameter, positioned at specific distances from the surface (ranging from 350 mu m to 1200 mu m). The defect position and size are quantified by X-ray computed tomography. Following VHCF testing, the fracture surfaces of the specimens are examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) with electron diffraction analysis. The results demonstrate that the fracture surfaces predominantly display a fish-eye fracture type, with a fine granular area (FGA) surrounding the internal defects, typical for VHCF. The FGAs dimensions including thickness are estimated by means of microscopy analysis. However, despite the presence of relatively large artificial defects, cracks sometimes initiate from much smaller process-induced defects closer to the surface than the artificial ones. In conclusion, our study quantifies the relationship between the probability of crack initiation from artificial defects and the distance of the artificial defect from the specimen's surface.

Side surface quality is a critical concern in the fatigue performance of laser powder bed fused (PBF-LB) components. Increasing contour passes with customized parameters along sample edge can tune side surface/subsurface quality and thereby enhance fatigue resistance. This study critically evaluates the surface and subsurface characteristics resulting from varying contour parameters and their impact on the fatigue performance of PBF-LB 304L steel through multiple detailed characterizations. The fatigue damage mechanisms for varying contour parameters are investigated through fatigue fractography, temperature field analysis, and microstructural evolution. Results indicate that optimal contour parameters differed from infill parameters due to the energy absorption from powder fusion and solidification remelting. The contour defects, including spherical vapor cavities and irregular lack-of-fusion (LoF) defects resulting from inappropriate parameters, significantly degrade fatigue lifetime due to their high-stress concentration factors. Appropriate contour parameters (approximately 300 J/ mm3 in energy density) can minimize defect content while simultaneously enhancing microstructural heterogeneity in the contour region. The identified physical mechanisms of defect formation and fatigue damage will assist in designing and optimizing contour process for enhancing fatigue performance.

Low plastic straining combined with aging leads to critical changes in the mechanical properties of steels used in submarine pipelines constructed using the pipe reeling method. This includes alterations in the mechanical behavior of steel at the initial stages of plastic strain, which can be critical for the long-term operation of pipelines due to localized plastic deformation in the form of buckling. In this study, the effects of plastic preliminary strain of 3% through monotonic tension and aging at 250 degrees C for 1 h on the mechanical behavior of lowcarbon steel in various microstructural states were investigated. These states differed in terms of the volumetric fraction of structurally free ferrite after quenching or normalizing. It was shown that the overall yield strength increase, as well as the contributions of strain hardening and bake hardening, depend on the volumetric fraction of ferrite and the heat treatment method. A direct proportional relationship between uniform elongation and the strain hardening exponent, both before and after strain aging, was confirmed. The results have been used for numerical modeling of the plastic strain of steel, aiming to predict and prevent the buckling of deep-water pipelines, as presented in Part II of the article.

ABSTRACT In situ fatigue crack propagation experiment was conducted on laser cladding with coaxial powder feeding (LCPF) K477 under various stress ratios and temperatures. Multiple crack initiation sites were observed by using in situ scanning electron microscopy (SEM). The fatigue short crack growth rate was measured, and the impacts of temperature and stress ratio on this growth rate were analyzed. Based on these experiments, the experimental data were expanded, and three ensemble learning algorithms, that is, random forest (RF), extreme gradient boosting (XGBoost), and light gradient boosting machine (LightGBM), were employed to establish a fatigue short crack growth rate model controlled by multiple parameters. It is indicated that the RF model performs the best, achieving a coefficient of determination ( R 2 ) of up to 0.88. The fatigue life predicted by the machine learning (ML) method agrees well with the experimental one.

Using high-spatial-resolution ( 0.2 mm) neutron diffraction, we examined the residual stresses in structural alloyed steel (Cr, Si, Mn) plates, 5 mm in thickness, following shot peening. The analysis revealed that residual stresses form not only near the treated surface but throughout the entire thickness of the plate. Compressive stress zones appear near both treated and untreated surfaces, while tensile stress zones emerge in the middle region. The intensity of the shot peening affects the width of these zones and the magnitude of the maximum stresses. Neutron experiments were conducted to measure stresses near the treated surface of the plates, employing the sin2ψ method. Results obtained via the sin2ψ neutron method were consistent with those from traditional three-component strain measurement techniques. The sin2ψ neutron method proves to be advantageous for measuring stresses near the surfaces of thick samples, since it lacks the limitations of traditional measurement techniques on the thickness of the sample.

Fatigue characteristics of the pultruded glass-fiber composites were extensively investigated experimentally. Tubular specimens with three-layer structure (mat, roving, mat) were subjected to simultaneous cyclic axial and torsional loads to initiate multiaxial complex stress state. The main aim was to determine the influence of additional shear stress on the fatigue performance of the material. A fourth order failure criterion was proposed after the axial and multiaxial static tests, outperforming conventional quadratic criteria. Tension-tension, compression-compression and tension-compression axial fatigue tests were done to obtain the corresponding S-N curves. The multiaxial fatigue tests (under the combined loadings) were conducted in tension-compression mode to determine multiaxial S-N curves. Multiaxial fatigue tests indicated that additional shear stress of more than 70% from the ultimate shear stress significantly reduce the fatigue life. This could be associated with the contribution of mat layers in the composite to shear resistance.

Here, a novel technique utilizing carbon nanotube fibers (CNTFs) for the one-step, dual-stage, non-destructive monitoring of multifunctional nanocomposites is proposed. Nanocomposites with single- and multi-walled carbon nanotubes with concentrations of 0.25 and 0.75 % wt. were evaluated. CNTFs were embedded into the nanocomposites during their manufacturing to monitor changes (1st stage), and were left embedded to also monitor their lifecycle (2nd stage). Monitoring sensitivity and reliability dependence on CNTF diameters (similar to 40-700 mu m), electrical conductivity (similar to 10(2)-10(4) S/m), and the choice of measurement technique (2- and 4-point) were investigated. Promising sensitivity to CNT type and concentration was seen during the 1st stage, with measurements being independent of CNTF diameter, contact resistance, and displaying low noise. For the 2nd stage, nanocomposite electrical and mechanical (tensile and cyclic) properties were determined in both static and dynamic conditions. The CNTFs did not cause any reduction in mechanical performance, unlike the losses observed for metallic electrodes (up to 60 % reduction in ultimate tensile strength). CNTF-based evaluation of the electrical resistivity (between 10(2) - 10(6) Ohm center dot cm) and dynamic electrical response (gauge factors between similar to 3 and 13) matched values from a standard electrode material. Microstructural analysis and molecular dynamics simulations proved that this unique performance was due to the synergistic interfacial interaction of the dispersed CNTs with the CNTFs. These findings show that CNTFs may be used to accurately monitor nanocomposite multifunctional properties both during manufacturing and application using one-step integration, regardless of size and manufacturing technology.

We examine how the physical features of single-walled carbon nanotube (SWCNT) thin films can affect monitoring thermoset polymers during their manufacturing and application. Film thickness (23, 37 and 53 nm), electrode materials and configuration, and embedding within or surface application on a polymer were all investigated. During manufacturing monitoring, thicker films provided higher sensitivity than thinner ones (ΔR/R0 = 111% (53 nm) vs. 74% (23 nm)). Electrode material and configuration showed that sputtered gold contacts allow higher sensitivity and reliability compared with conductive silver adhesive, and that parallel electrode placement provides more distinct and discreet stage measurements compared to diagonal one. The opposite trend was noted for mechanical loading, where thinner films provide a higher piezoresistive response and embedded films outperform surface applied sensors (gauge factors between 23-86 and 4-10, respectively). All thin films accurately measured curing stages as well as mechanical changes and are shown to be self-adjusting when the force increases during mechanical loading. We show that manufacturing monitoring should rely on uniformly deposited electrodes and thicker films for identification of polymerization stages, whereas embedding thinner films in the matrix is recommended for highly sensitive deformation monitoring. This work outlines how simple, overlooked parameters can easily be varied to optimize the dual-stage monitoring performance of CNT-based sensors and helps to identify what parameters should be chosen for specific applications.

We experimentally determined the decrease of residual compressive strength of the pultruded glass-fiber laminate after tension-compression cyclic loading. The adapted Arcan rig was used for tension-compression fatigue tests. The cyclic load was applied with the critical stress ratio R=−0.87. The residual compressive strength was determined after applying the predefined number of loading cycles with the stress amplitudes of 242 MPa and 173 MPa. The results indicated that the residual compressive strength was reduced about 20% at 77% of fatigue life under the stress amplitude of 242 MPa and at 83% of fatigue life under the stress amplitude of 173 MPa. The microstructural analysis showed that the crack growth path and failure mode depend on the stress amplitude.

A comparative analysis of fracture mechanisms in high- and very high- cycle fatigue (HCF, VHCF) regimes was carried out based on the results of multifractal analysis of the fracture surfaces of additively manufactured 316L stainless steel samples. In terms of scale invariants, the morphology of fracture surfaces in HCF and VHCF regimes inside and outside the fine granular area is shown. The analysis demonstrated that chaotic patterns of relief formation prevail in the crack initiation zone of VHCF samples. However, there is a self-similar relief with a pronounced correlation in the crack propagation area. The relief of the crack growth areas for HCF and VHCF samples are similar to each other.

The direct impact method provides a higher sample deformation rate in comparison with the classical Split Hopkinson Pressure Bar method. The principal assumption of strain rate calculation for the direct impact method is constancy of velocity of the impacted face of the specimen during the impact. This assumption delivers reliable results for materials with relatively low yield strength and hardening rate. The present study proposes an alternative procedure for calculation of the specimen strain rate in order to improve accuracy of the direct impact method for wide range of metals and alloys. The proposed procedure is based on the assumption that the transmitted pulse can be splitted into parts, one of which corresponds to stress and the other to strain rate. The procedure has been validated by finite element analysis and the semi-analytical modelling of the direct impact tests of pure copper C101 and additively manufactured Inconel 718 alloy. The proposed method qualitatively changes the shape of the stress-strain curve by adding an unloading area. The numerically estimated accuracy of the method is limited by 4.5% for Inconel 718 and 0.6% for copper C101. The proposed alternative analysis of the strain rate for a direct impact has a number of advantages over the classical methods and can be used in the study of various materials in a wide range of strain rate.

The effect of scanning strategies and hatch distances on the porosity and mechanical properties of 316L stainless steel produced by laser powder bed fusion (PBF-LB) is studied. Two scanning strategies were analyzed along with three hatch distances (50 µm, 80 µm, and 110 µm). Using X-ray computed tomography (XCT), the study revealed different defect distributions: 90° alternating hatch scanning strategy resulted in a higher number of lack of fusion defects, while the chessboard strategy produced smaller, more uniformly distributed gas pores. The optimal hatch distance of 80 µm was found to minimize porosity and maximize tensile strength for both strategies. Mechanical testing supported these findings, showing that specimens printed with an 80 µm hatch distance exhibited better mechanical characteristics. This study highlights the critical role of scanning parameters in the quality and reliability of additively manufactured parts.

Here, we propose a novel application of carbon nanotube fibers (CNTFs) for the one-step, dual-stage, non-destructive monitoring of multifunctional conductive nanocomposites. Hierarchical nanocomposites were created by embedding CNTFs into carbon nanotube (CNT) - modified matrices during their manufacturing to assess production variables. CNTFs are then left embedded in the structure for monitoring during nanocomposite application. We investigated the dependence of detection sensitivity and reliability on the CNTF diameter (~ 40–700 µm), electrical conductivity (~ 102-104 S/m), and the choice of measurement technique (2- and 4-point) for single-walled and multiwalled CNT fillers at different concentrations. The sensors showed promising sensitivity to CNT type and concentration, the results were independent of CNTF diameter and contact resistance, and showed low noise. For application monitoring, nanocomposites electrical and mechanical (tensile and cyclic) properties were tested to determine sensitivity to static and dynamic conditions. CNTFs did not cause any reduction in mechanical properties, unlike the losses observed for metallic electrodes (up to 60% reduction in ultimate tensile strength). CNTF-based evaluation of the electrical resistivity (between 102 — 106 Ohm∙cm) and dynamic electrical response (gauge factor between ~ 2 — 12) matched values from a standard electrode material. Microstructural analysis proved that this unique performance was due to the surface and internal volume infiltration of the nanocomposite matrices into the CNTFs, causing interconnection of the CNTs of the matrix and CNTFs. These findings show that CNTFs may be used to accurately monitor nanocomposite multifunctional properties both during manufacturing and application using one-step integration, regardless of the sample size and manufacturing technology.