Applied Mechanics

A mid-story pin system to avoid moment-resisting frame column failure during seismic action was proposed recently. The solution consists of a reinforced concrete (RC) pier protruding from the foundations, the steel column connected with the superstructure, and plates and the anchor bolt working as a pinned connection in between. This paper utilizes shell finite element analysis (FEA) models to examine the demanded column-to-beam strength ratio to keep the column elastic and maximize the story drift at the moment of beam buckling of the frame. The method of calculating post-seismic residual strength based on maximal buckling deformation of the beam is also proposed. Full article

In this research, an investigation related to the tensile testing of 3D-printed specimens, under different fabrication parameters, is presented. The control samples were fabricated using Recycled-PETG: EVO (NEEMA3D™, Athens, Greece). It consists of recycled polyethylene terephthalate glycol (PETG) raw material, already used in industry, modified so that it becomes filament and can be printed again. More specifically, the parameters set to be studied are the percentage of infill, the speed and the type of infill. Both infill density and printing speed have three value levels, whereas for the infill pattern, two types were selected. Two sets of 18 specimens each were fabricated, with respect to the different parameter combinations. Through the results of the tests, the maximum tension of each specimen was obtained separately. Of the three parameters defined, it was found that the most important are the type of infill (44.77%) and the percentage of infill (24.67%). Speed (13.22%) did not strongly affect the strength of the specimens. In conclusion, the empirical model developed was considered reliable in terms of the value of the squared error, R-sq(pred) (97.72%), but also of the rest of the resulting analysis residual graphs (through the full factorial design). Full article

Damage refers to the degradation of a material subjected to an external condition such as loading, temperature, and environment. Several investigations have been undertaken to understand the damage of materials like steel, aluminum alloy, titanium alloy, and other materials. However, a comprehensive study on the range of damage values for various materials is scarce. Therefore, an attempt has been made in the current study to investigate the range of damage values of 32 aluminum alloys because of their widespread applications in the aerospace, railway, automotive, and marine industries. The damage value of materials is determined by incorporating the Continuum Damage Mechanics (CDM)-based Bhattacharya and Ellingwood model. This model demands the monotonic properties of materials as inputs, and these are obtained from the literature. The critical damage values of the alloys were determined, and their values vary in the range of 0.1 to 0.9. It was observed that damage value is primarily influenced by plastic strain. The variation in the damage value of aluminum alloys is also analyzed under different plastic strain conditions. The comprehensive results of critical damage value and the variation in the damage value of the aluminum alloys obtained helps in selecting an appropriate aluminum alloy for applications where damage criteria play a significant role. Full article

Related to microelectronics’ reliability, lifetime estimation methods have gained importance, especially for surface-mounted devices. The virtual testing of electronic assemblies necessitates the geometry modeling and finite element analysis of the solder joint. The effect of the simplification of the solder geometry on the predicted lifetime is an open question. Furthermore, there is still not yet straightforward guidance for the choice of the material model and fatigue lifetime model. In this study, the impact of the geometry input method, the material model and the lifetime model choice is investigated on two different surface-mounted capacitors in a simulation-based benchmark analysis under thermal cyclic loading. Four different types of solder geometry modeling approaches are compared, among which one is a physics-based approach. Ten different fatigue models founded on plastic and viscoplastic material models are benchmarked. The results show that the component standoff height and the solder volume have a positive effect on the lifetime, while the capacitor size has a slightly negative effect on the lifetime. The results also suggest that approximate geometries can be used to replace the physics-based model with a restriction for the minimum standoff height. Full article

Monumental artifacts belong to our cultural heritage, and there is a great need to protect them from earthquake damage. This study experimentally investigates the behavior of replicas of ancient vessels under seismic excitations. Each vessel was placed on a wooden base, which was attached to a shake table and was excited by earthquake signals. The effect of the amplitude of the excitation and the friction coefficient between the object and the base of support was examined. The dynamic response of the vessels included sliding and rocking, which, at high excitation levels, could involve rotation about their vertical axis and translation motion. High levels of excitation could cause the vessels to overturn but this did not always occur at the same level of excitation. The coefficient of friction is a key parameter of their behavior. If it is high, sliding motion is reduced while rocking parallel to the direction of excitation increases, starting at low excitation levels. This could lead to an early overturning of the object. The geometric characteristics and irregularities of the vessel can play an important role in its dynamic response. Full article

Gleeble thermomechanical simulators are widely utilized tools for the investigation of high-temperature deformation behavior in materials. However, temperature gradients that develop within the specimen during Gleeble compression tests have the potential to result in non-uniform deformation, which may subsequently impact the accuracy of the measured mechanical properties. This study presents an experimental and numerical investigation of the temperature fields in 2017-T4 aluminum alloy specimens prior to Gleeble compression tests at temperatures ranging from $300 °\mathrm{C} \mathrm{to} 500 °\mathrm{C}$ utilizing uniform temperature distribution (ISO-T) tungsten carbide anvils. The use of multiple thermocouples, welded to both the specimen and anvils, offers valuable insights into the temperature gradients and their evolutions. A coupled thermal–electrical finite-element model was developed in Abaqus for the purpose of simulating the resistive heating process. A user amplitude subroutine (UAMP) is implemented to regulate the heating based on a proportional–integral–derivative (PID) algorithm that modulates the current density to follow the specified temperature profile. The numerical results demonstrate that the temperature gradients within the specimen at the end of the heating process, reaching a temperature of $400 °\mathrm{C}$, are minimal, with values below $1.9 °\mathrm{C}$. This is in accordance with the experimental observations. The addition of graphite foils between the specimen and anvils has been shown to effectively reduce the gradients. The use of the measured anvil temperature as a boundary condition, rather than a constant value of $20 °\mathrm{C}$, has been demonstrated to improve the agreement between the simulated and experimental cooling curves. The modeling approach provides a framework for quantifying temperature gradients in Gleeble compression specimens and for assessing their impact on the measured constitutive response of materials at elevated temperatures. Full article

Stockbridge dampers are widely used to mitigate the vibrations of cable-stayed bridges and of many other cable-suspended or cable structures exposed to the action of pedestrians, traffic or wind load. Within the current research work, one of the most effective and likely used damper types, the Stockbridge damper, was investigated to support its design and application within the daily engineering praxis. The Stockbridge damper has a relatively simple structural layout, which ensures its modular design allows it to easily adapt the damper to cables having different dynamic properties (eigenfrequencies, mass, etc.). This paper focuses on two main research areas: (i) to understand the static and dynamic behaviour of the damper and the stay cable interaction to investigate the effectiveness of its damping; (ii) to study the sensitivity of the natural frequencies of the damper to the design parameters. The final aim of the research is to develop a simple design method that is easy to apply in engineering practice and allows the efficient adaptation of the Stockbridge damper to different cable-stayed bridges. Key findings include the recommendation to position the damper at approximately 20% of the cable length for optimal attenuation, the importance of detuning to maintain effectiveness under varying cable forces, and the observation that increasing the damper mass improves efficiency, particularly for detuned elements. Full article

Recently, the growth of the recyclability of carbon fiber reinforced polymer (CFRP) composites has been driven by environmental and circular economic aspects. The main aim of this research work is to investigate the strength retention of a bio-based vitrimer composite reinforced with carbon fibers, which offers both recyclability and material reusability. The composite formulation consisted of an epoxy resin composed of diglycidyl ether of bioshpenol A (DGEBA) combined with tricarboxylic acid (citric acid, CA) and cardanol, which was then reinforced with carbon fibers to enhance its performance. Differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) spectroscopy were performed to analyze the chemical composition and curing behavior of the vitrimer. Mechanical testing under tensile loading at room temperature was carried out on epoxy, vitrimer, and associated carbon fiber reinforced composite materials. The results demonstrated that the DGEBA/CA/cardanol vitrimer exhibited thermomechanical properties comparable to those of an epoxy cured with petroleum-based curing agents. It was observed that the maximum tensile strength of vitrimer is about 50 MPa, which is very close to the range of epoxy resins cured with petroleum-based curing agents. Notably, the ability of the vitrimer composite to be effectively dissolved in a dimethylformamide (DMF) solvent is a significant advantage, as it enables the recovery of the fibers. The recovered carbon fiber retained comparable tensile strength to that of the fresh carbon composites. More than 95% strength was retained after the first recovery, which confirms the use of fibers for primary and secondary applications. These research results open up new avenues for efficient recycling and contribute to the overall sustainability of the composite material at an economic level. Full article

This study comprehensively investigates the frequency- and temperature-dependent viscoelastic properties of two elastomer materials, focusing on the comparison between experimental results and theoretical models derived from Prony series coefficients. Dynamic Mechanical Analysis (DMA) was performed across a broad temperature range of 0–100 °C and frequency range of 0.1–100 Hz to generate storage modulus and relaxation modulus data for both materials. Relaxation tests were conducted at 25 °C to further characterize the time-dependent behavior. Time–Temperature Superposition (TTS) was applied to the resultant shift factors used to fit both Williams–Landel–Ferry (WLF) and Arrhenius equations. Additionally, sinusoidal sweep tests were carried out at 0 °C, 25 °C, 50 °C, and 80 °C, with frequencies ranging from 1 Hz to 1000 Hz, to experimentally determine the natural frequencies of the elastomers. The findings demonstrate that Prony series coefficients derived from storage modulus data offer a more accurate prediction of the viscoelastic response and natural frequencies compared to those derived from relaxation modulus data. The storage modulus data closely match the experimentally observed natural frequencies, while the relaxation modulus data exhibit larger deviations, particularly at higher temperatures. The study also reveals temperature-dependent behavior, where increasing temperature reduces the stiffness of the materials, leading to lower natural frequencies. This comprehensive analysis highlights the importance of selecting appropriate modeling techniques and data sources, particularly when predicting dynamic responses under varying temperature and frequency conditions. Full article

This paper investigates the tensile properties of jute-reinforced composites fabricated using stereolithography (SLA) 3D printing. Tensile tests were conducted using dog-bone tensile specimens following ASTM D638 Type IV specifications. Additionally, the study explores the effect of layer thickness on the tensile properties of the 3D-printed composite material, examining four different layer thicknesses: 0.025 mm, 0.05 mm, 0.075 mm, and 0.1 mm. The findings revealed that the tensile strength of the 3D-printed jute-reinforced composites increased with the printing layer thickness, reaching its maximum at a layer thickness of 0.1 mm. This represents an enhancement of approximately 84% compared to pure resin. Examination of the fiber–matrix interface under an optical microscope revealed a wavy pattern, suggesting that the interface may act as a mechanical interlock under tensile loads, thereby significantly enhancing tensile strength. The strength of the 3D-printed jute-reinforced composites was found to be comparable to that of glass fiber mat epoxy composites. This demonstrates that 3D SLA-printed jute-reinforced composites offer a promising avenue for producing next-generation composites that are typically challenging to manufacture using traditional fabrication techniques. Full article

This paper mainly studies the fatigue cracks growth of fillet weld specimens in a fashion that is consistent with that used to assess the fatigue performance of complex aerospace structures under operational flight loads. The fatigue test loads were determined using the overall finite element analysis results of the hopper wagon. The actual applied test loads were monitored using strain gauges. The residual stress in the critical region was determined by combining the stress field of the welded specimen obtained by a thermal imager under cyclic loading with the results of the three-dimensional finite element analysis of the specimen. During the fatigue test, a digital camera (with microscope lens) was used in conjunction with infrared measurement technology to obtain the crack growth information. As in prior studies, the three dimensional finite element alternating technique was used to calculate the stress intensity factor in the critical area of the crack in the fillet weld specimen. The Hartman–Schijve crack growth equation was then used, in conjunction with the calculated stress intensity factor solutions, to compute the crack growth history in a fatigue test of a critical welded component in a hopper wagon. The resultant computed crack growth histories are relatively consistent with the test results. Full article

In this study, the effect of certain 3D printing conditions on the tensile strength of 3D-printed specimens was investigated. The printing material was CARBON: PLUS (NEEMA3D™, Athens, Greece), which consists of Polyethylene Terephthalate Glycol (PET-G) reinforced with 20% carbon fiber. All samples were printed with a closed-type, large-format Fused Filament Fabrication (FFF) 3D printer. Before printing the samples, three parameters related to the 3D printing settings were selected in order to vary their values (flow = the flow of the material, wall = the total thickness of the wall, and layer = the thickness of the print layer). Each parameter was given three different values for experimentation. In this study, all 27 possible combinations of variable parameters were fabricated. Each experiment was repeated twice, and from the test results, the maximum tensile strength was obtained for each specimen separately. From the results of the measurements, the most critical parameter appeared to be the height of the layer. The other two variable parameters, the flow and wall, locally affected the strength of the specimens. Later, an empirical model was developed according to the full factorial design for each combination of values. Finally, the R-sq (pred) value achieved was equal to 97.02%, and together with the residual analysis performed, the accuracy of the proposed maximum tensile strength mathematical model was proven. Full article

The hydrogen embrittlement (HE) phenomenon occurring in drawn pearlitic steel wires sometimes results in dangerous delayed fracture and has been an important issue for a long time. HE is very sensitive to the amount of plastic deformation applied in the drawing process. Hydrogen (H) atom diffusion is affected by ambient thermal and mechanical conditions such as stress, pressure, and temperature. In addition, the influence of stress gradient (SG) on atomic diffusion is supposed to be crucial but is still unclear. Metallic materials undergoing plastic deformation naturally have SG, such as residual stresses, especially in inhomogeneous regions (e.g., surface or grain boundary). In this study, we performed molecular dynamics (MD) simulation using EAM potentials for Fe and H atoms and investigated the behavior of H atoms diffusing in pure iron (α-Fe) with the SG condition. Two types of SG conditions were investigated: an overall gradient established by a bending deformation of the specimen and an atomic-scale local gradient caused by the grain boundary (GB) structure. A bi-crystal model with H atoms and a GB structure was subjected to bending deformation. For a moderate flexure, bending stress is distributed linearly along the thickness of the specimen. The diffusion coefficient of H atoms in the bulk region increased with an increase in the SG value. In addition, it was clearly observed that the direction of diffusion was affected by the existence of the SG. It was found that diffusivity of the H atom is promoted by the reduction in its cohesive energy. From these MD results, we recognize an exponential relationship between the amount of H atom diffusion and the intensity of the SG in nano-sized bending deformation. Full article

Power transformers play a vital role in adjusting voltage levels during transmission. This study focuses on optimizing the structural design of power transformer tanks, particularly high-voltage (HV) tank walls, to enhance their mechanical robustness, performance, and operational reliability. This research investigates various stiffener designs and their impact on stress distribution and deformation through finite element analysis (FEA). Ten different configurations of stiffeners, including thickness, width, type, and position variations, were evaluated to identify the optimal design that minimizes stress and deflection while considering weight constraints. The results indicate that specific configurations, particularly those incorporating 16 mm thick H beams, significantly enhance structural integrity. Experimental validation through pressure testing corroborated the simulation findings, ensuring the practical applicability of the optimized designs. This study’s findings have implications for enhancing the longevity and reliability of power transformers, ultimately contributing to more efficient and resilient power transmission systems. Full article

The paper presents the progressive collapse analysis of structures, focusing on the impact of the initial conditions (particularly initial velocity) and the damage. It proposes a method that calculates the residual axial load capacity and damage of columns based on their strain profile and considers the effects of multiple blast locations. The methodology involves the conventional design of a three-story moment-resisting frame, selecting blast parameters, calculating blast pressures, and performing structural and progressive collapse analyses. The findings reveal that the Alternate Load Path Method (APM) overestimates the capacity compared to a benchmark blast–structure interaction analysis, especially when unsuitable initial conditions and damage properties are used. To address this limitation, the paper concludes the recommendations for incorporating appropriate initial conditions and damage considerations for a relatively accurate progressive collapse analysis. Full article

While zinc die-casting alloy Zamak is widely used in vehicles and machines, its solidified state has yet to be thoroughly investigated experimentally or mathematically modeled. The material behavior is characterized by temperature and rate sensitivity, aging, and long-term influences under external loads. Thus, we model the thermo-mechanical behavior of Zamak in the solid state for a temperature range from −40 °C to 85 °C, and the aging state up to one year. The finite strain thermo-viscoplasticity model is derived from an extensive experimental campaign. This campaign involved tension, compression, and torsion tests at various temperatures and aging states. Furthermore, the thermo-physical properties of temperature- and aging-dependent heat capacity and heat conductivity are considered. One significant challenge is related to the multiplicative decompositions of the deformation gradient, which affects strain and stress measures relative to different intermediate configurations. The entire model is implemented into an implicit finite element program and validation examples at more complex parts are provided so that the predicability for complex parts is available, which has not been possible so far. Validation experiments using digital image correlation confirm the accuracy of the thermo-mechanically consistent constitutive equations for complex geometrical shapes. Moroever, validation measures are introduced and applied for a complex geometrical shape of a zinc die casting specimen. This provides a measure of the deformation state for complex components under real operating conditions. Full article

Many studies focus on brittle–ductile transition stress in intact rocks; however, in real life, we deal with rock mass which contains many discontinuities. To fill this gap, this research focuses on the brittle–ductile transition stress of rock mass by considering the influence of different Geological Strength Index (GSI) values on the brittle–ductile transition stress of rock mass. In other words, the Hoek–Brown failure criteria for rock mass were reformulated mathematically including the ductility parameter (d), which is defined as the ratio of differential stress to minor stress. Then, the results were analyzed and plotted between ${\displaystyle \frac{{\sigma }_3^{*}}{{\sigma }_c}}$ and GSI, considering different (d) and Hoek–Brown material constant (m_i) values. The brittle–ductile transition stress, σ₃^*, was determined by intersecting the Hoek–Brown failure envelope with Mogi’s line, with ductility parameters d ranging from 3.4 (silicate rocks) to 5.0 (carbonate rocks). Numerical solutions were derived for ${\displaystyle \frac{{\sigma }_3^{*}}{{\sigma }_c}}$ as a function of GSI using Matlab, and the results were fitted with an exponential model. The analysis revealed an exponential relationship between ${\displaystyle \frac{{\sigma }_3^{*}}{{\sigma }_c}}$ and GSI for values above 32, with accuracy better than 3%. Increased ductility reduces rock mass strength, with higher d values leading to lower ${\displaystyle \frac{{\sigma }_3^{*}}{{\sigma }_c}}$. The diminishing returns in confinement strength at higher GSI values suggest that rock masses with higher GSI can sustain more confinement but with reduced effectiveness as GSI increases. These findings provide a framework for predicting brittle–ductile transitions in rock engineering. Full article

Long-term operation of the supporting surfaces of large-sized parts, in particular tubular units of thermal power plants, leads to the destruction of the contact surfaces. Moisture penetrates into the formed discontinuities, and the vibrations present in the equipment in use rapidly increase the gap, reaching values of 10–15 mm. The authors of this article proposed the application of a composite layer of multimetal 1018 material without performing additional preparatory operations, ensuring the mandatory penetration of the material into the body of the supporting surface. This depth provides additional stability by maintaining boundary conditions. To determine the rational thickness of the composite layer, mathematical modeling of static loading of samples with different thicknesses in a wide range of values (from 2 mm to 12 mm) was performed. It was determined that the effective implementation of the developed technology was possible due to an increase in the load-bearing capacity of the composite material by creating additional grooves, or artificially creating grooves by welding, in the body of the part with a depth of 2.5–3 mm. The optimal excess of the composite was 1.0–1.5 mm. The proposed technology increases the stability of the composite layer up to three times and allows restoration without the use of mechanical treatment. The increase in the maximum stress values was 770 MPa, compared to the standard technology of 205 MPa. Full article

The performance of the porous gas bearing depends on the geometric characteristics, material, fluid properties, and the properties of the porous media, which is a restrictor that controls the gas flow. Its application in industrial environments must support higher loads, higher supply pressure, and, consequently, higher pressure in the lubricant fluid film. Because porous media has a relatively low elastic modulus, it is necessary to consider its deformation when designing porous gas bearings. The design of porous gas bearings is a multi-objective problem in engineering because the optimization objectives commonly are to maximize the load capacity or static stiffness coefficient and minimize the airflow; these objectives conflict. This work presents a multi-objective optimization algorithm based on the nature-inspired Flower Pollination Algorithm enhanced with Non-Dominated Sorting Genetic Algorithm II. The algorithm is applied to optimize the design of a porous gas bearing, maximizing the resultant force and the static stiffness coefficient and minimizing the airflow. The results indicate a better performance of the Multi-Objective Flower Pollination Algorithm than the Multi-Objective Cuckoo Search. The results show a relatively short running time of 6 min for iterations and a low number of iterations of 50. Full article