Mechanical Properties Of Materials Research Articles

Mechanical and Microstructural Properties of Cement-Stabilized Soft Clay Improved by Sand Replacement and Biochar Additive for Subgrade Applications

Biochar (BC) is an eco-friendly material produced through coal pyrolysis and can improve the mechanical properties of cement-based construction and building materials. This research study explored the effects of BC and natural sand (Sand) replacement on the improved static and cyclic response of blended hydraulic cement (BHC) stabilized soft clay (SC) as a greener subgrade material. Unconfined compressive strength (UCS), indirect tensile stress (ITS), and indirect tensile fatigue life (ITFL) of the BHC-stabilized SC-BC-Sand samples were examined. Adding 10% BC to the BHC-stabilized samples was found to enhance cementitious products due to its porous structure and high water absorbability. The UCS, ITS and ITFL at this optimum ingredient were improved up to 315%, 347% and 862%, respectively, compared to the BHC-stabilized SC. Fourier transform infrared spectrometer, thermogravimetry differential thermal analysis and a scanning electron microscope with energy-dispersive -ray spectroscopy analyses the BHC-stabilized sample at the optimum ingredient showed the highest C-S-H and Ca(OH)2 in the pores. This investigation will encourage the utilization of BC to create both environmentally friendly and durable stabilized subgrade material.

Photoactivated rose bengal-doped TiO2 nanoparticles modified fifth-generation adhesive on the survival rate of Streptococcus mutants and mechanical properties of tooth-colored restorative material to carious dentin.

Assessment of the antimicrobial, micro tensile bond strength (μTBS), and degree of conversion (DC) of fifth-generation adhesive modified using photoactivated 0.5% rose bengal (RB) and photoactivated RB-doped titanium dioxide nanoparticles (TiO2NPs) in different concentrations (2% and 5%) as compared with the unmodified adhesive bonded to the carious affected dentin (CAD). Forty mandibular molars with caries progression up to the middle third of the dentin, as per the International Caries Detection and Assessment System (ICDAS) score of 4 and 5 were included. Specimens were divided into four groups based on etch and rinse adhesive (ERA) modification group 1: unmodified ERA, group 2: photoactivated 0.5% RB photosensitizer (PS) modified ERA, group 3: photoactivated RB-doped 2 wt% TiO2NPs adhesive, group 4: photoactivated RB-doped 5 wt% TiO2NPs adhesive. Followed by adhesive and composite restoration on the CAD surface. All the specimens were thermocycled and an assessment of μTBS and failure pattern analysis was performed. The antibacterial potency of RB and RB-doped TiO2NPs (2% and 5%) followed by their activation using visible light against Streptococcus mutans (S.mutans) were tested. The survival rate of S.mutans was assessed using the Kruskal-Wallis test. The analysis of μTBS involved the use of ANOVA, followed by a post-hoc Tukey honestly significant difference (HSD) multiple comparisons test. Group 1 (Unmodified ERA) (0.52 ± 0.31 CFU/mL) treated samples unveiled the highest means of bacterial survival and lowest μTBS (11.32 ± 0.63 MPa). Nevertheless, group 4: photoactivated RB-doped 5 wt% TiO2NPs adhesive displayed the lowest outcomes of S.mutans survival (0.11 ± 0.02 CFU/mL) and highest bond strength (18.76 ± 1.45 MPa). The photoactivated RB-doped 2 wt% TiO2NPs in adhesive demonstrated promising enhancements in both μTBS and antibacterial efficacy against S.mutans. However, it is noteworthy that this modification led to a decrease in the DC of the adhesive. RESEARCH HIGHLIGHTS: Unmodified ERA-treated samples unveiled the highest bacterial survival and the lowest μTBS. Photoactivated RB-doped 5 wt% TiO2NPs adhesive displayed the lowest S.mutans survival rate and highest bond strength. DC decreased with an increase in concentration of TiO2.

Optimizing Carbonation Efficiency and Mechanical Properties of Low-Calcium Cementitious Materials with MgSO4 Addition

This study investigates the effects of magnesium sulfate (MgSO4) addition on the carbonation efficiency and mechanical properties of low-calcium CO2-sequestering cementitious material (LCC), which is prepared by calcining a mixture of 78.2% limestone and 21.8% sandstone at 1275 °C for 2h. LCC samples were prepared with varying concentrations of MgSO4 solution (0, 0.5, 1, 2, and 3mol/L) and subjected to CO2 curing for 24h. The carbonation behavior, compressive strength, and microstructural characteristics were examined using XRD, TGA, FT-IR, SEM, and LF-NMR techniques in combination. Results demonstrate that the addition of MgSO4 significantly influences the carbonation process and mechanical performance of LCC. Optimum performance was achieved after subjecting the paste prepared with LCC at a concentration of 0.5mol/L MgSO4 to a carbonation period lasting 24h. This resulted in a notable increase in compressive strength by approximately 28% (145MPa) compared to control samples along with an observed enhancement in CO2 uptake by around 4%. Microstructural analysis reveals that the inclusion of MgSO4 promoted the formation of more stable carbonate phases such as Mg-calcite and vaterite while also enhancing silica gel polymerization within the matrix structure of LCC materials. Additionally, it was found excessive concentrations (>1mol/L) of MgSO4 led to decreased carbonation efficiency and reduced strength due to gypsum formation as well as limited pore water availability. This study provides valuable insights into optimizing the carbonation process of LCC materials while demonstrating the potential efficacy of MgSO4 as an effective additive for enhancing the performance of low-carbon CO2-sequestering cementitious materials.

Application of big roll and heavy reduction in continuous casting of thick slabs

ABSTRACT Porosity and segregation are common internal defects in thick slabs, which negatively affect the flaw detection qualification rate and mechanical properties of heavy plate materials. This study presents a novel big roll and heavy reduction (BRHR) technology to mitigate these internal defects. The Q345 slab with a thickness of 400 mm and a width of 2400 mm was selected as the specific research object. The cooling strategy was investigated to optimise the solidification front during the BRHR process. Achieving uniform solidification within the slab width significantly reduced porosity during the BRHR process. As the BRHR amount increased from 0 to 20 mm, porosity spots in the center of the thick slab significantly decreased. Additionally, carbon segregation decreased in slabs with BRHR amounts of 10 and 20 mm compared with 0 mm BRHR slabs. Statistical measurements revealed that as the BRHR amount increased from 0 to 20 mm, the ratio of porosity spot area decreased from 0.244% to 0.014%, while the maximum carbon segregation degree decreased from 1.17 to 1.09. Optimal control of uniform solidification across width direction and the total BRHR amount contributed to improve solidification porosity and segregation.

Optimization of microstructure and mechanical properties of 7075 aluminum alloy by equal channel angular pressing technology

Abstract The equal channel angular pressing technology, as an effective technique for modifying the surface of a material, has had a significant impact on the microstructure and mechanical properties of the 7075 aluminum alloy. The study achieved the refinement of 7075 aluminum alloy grains and the homogenization of the second phase particle distribution through equal channel angular pressing technology, thereby significantly improving the strength and plasticity of the material. Through orthogonal experimental design, the effects of extrusion angle, extrusion speed, and remelting temperature on material properties were studied. The results showed that appropriate ECAP parameters could effectively regulate the microstructure of 7075 aluminum alloy, thereby optimizing its mechanical properties. Especially in the analysis of equivalent strain rate, when the extrusion angle was 50°, the position of the maximum equivalent strain rate at the forefront point remained unchanged, but the equivalent strain rate decreased to 0.107-1. In the aging hardness analysis, when the number of equal channel angular pressing passes was 3, the aging hardness of G group 7075 aluminum alloy increased to 195HV in the first 5 minutes. The equal channel angular pressing technology can optimize the internal structure of 7075 aluminum alloy and improve the material's plasticity and mechanical properties. The research provides important technical support for the application of 7075 aluminum alloy in high-end equipment manufacturing.

Unraveling the Synergistic Role of Kinks in Zig-Zag Ag2Se Nanorod Arrays for High Room-Temperature zT and Improved Mechanical Properties: Experimental and First-Principles Studies.

Flexible thermoelectric materials are usually fabricated by incorporating conducting or organic polymers; however, it remains a formidable task to achieve high thermoelectric properties comparable to those of their inorganic counterparts. Here, we present a high zT value of 1.29 ± 0.31 at room temperature in the hierarchical zig-zag Ag2Se nanorod arrays fabricated using the glancing angle deposition (GLAD) technique followed by a facile selenization process. The high zT value at 300 K is ascribed to the ultrahigh power factor of 3101 ± 252 μW/m-K2 and the reduced thermal conductivity of 0.72 ± 0.01 W/mK. Based on ab initio computational and experimental evidence, we reveal that kinked Ag2Se nanorod arrays consisting of rough interfaces modulate the lattice thermal conductivity up to 48.5% at room temperature. The modulation results from interchanging of phonon modes at kink points and enhanced scattering from a large number of rough interfaces. Further, benefiting from kinked hierarchy, a notable improvement in the mechanical performance is observed for zig-zag Ag2Se nanorods which is confirmed by nanoindentation measurements. The synergic improvement in thermoelectric and mechanical performance not only unravels a paradigm to harness thermoelectric heat but also offers deeper insights into tuning the mechanical properties of inorganic thermoelectric materials.

Study on the mechanical properties of layered gradient structure PBT-based flame retardant composite materials

Abstract To enhance the flame retardancy of Polybutylene Terephthalate (PBT) based composite materials and overcome the decrease in mechanical properties due to the high content of nano-antimony trioxide (nano-Sb2O3), this study initially applies a composite modification method to treat the surface of nano-Sb2O3. Subsequently, composite materials with a gradient distribution of nano-Sb2O3 are prepared through lamination hot-pressing technology, and their mechanical and flame retardant properties are investigated. The results show that the modifiers CTAB and KH560 are successfully grafted onto the surface of nano-Sb2O3, altering its surface properties. Through lamination hot-pressing technology, a gradient distribution from the surface to the core is achieved. Compared to the homogeneous distribution P666, G828 exhibits a 6% increase in Limiting Oxygen Index (LOI), improving the flame retardancy of the composite material. As nano-Sb2O3 enriches on the composite material's surface, the amount of char residue increases. The tensile strength, bending strength, and impact strength of G828 composite material increase by 19.7%, 37.1%, and 103%, respectively, compared to the homogeneous composite material P666, proving the significant effect of G828's gradient distribution in enhancing the mechanical properties of the composite material.

A Numerical Study of Dynamic Behaviors of Graphene-Platelet-Reinforced ETFE Tensile Membrane Structures Subjected to Harmonic Excitation

This study presents a numerical investigation of the dynamic behavior of graphene platelet (GPL)-reinforced ethylene tetrafluoroethylene (ETFE) tensile membrane structures subjected to harmonic excitation. Modal and harmonic response analyses were performed to assess both the natural frequencies and the dynamic responses of the ETFE membrane. GPLs were employed as the reinforcements to enhance the mechanical properties of the membrane materials, whose Young’s modulus was predicted through the effective medium theory (EMT). Parametric studies were conducted to examine the impact of pre-strain and the attributes of the GPL reinforcements, including weight fraction and aspect ratio, on the natural frequencies and amplitude–frequency response curves of the membrane structure. The first natural frequency substantially increased from 5.46 Hz without initial strain to 31.0 Hz with the application of 0.1% initial strain, resulting in a frequency shift that moved the natural frequency out of the range of typical wind-induced pulsations. Embedding GPL fillers into ETFE membrane was another potential solution to enhance the dynamic stability of the membrane structure, with a 1% addition of GPLs resulting in a 48.6% increase in the natural frequency and a 45.1% reduction in resonance amplitude. GPLs with larger aspect ratios provided better reinforcement, offering a means to fine-tune the membrane’s dynamic response. These results underscore that by strategically adjusting both pre-strain levels and GPL characteristics, the membrane’s dynamic behavior can be optimized, offering a promising approach for improving the stability of structures subjected to wind-induced loads.

Optimizing scaffold pore size for tissue engineering: insights across various tissue types

Scaffold porosity is a critical factor in replicating the complex in vivo microenvironment, directly influencing cellular interactions, migration, nutrient transfer, vascularization, and the formation of functional tissues. For optimal tissue formation, scaffold design must account for various parameters, including material composition, morphology, mechanical properties, and cellular compatibility. This review highlights the importance of interconnected porosity and pore size, emphasizing their impact on cellular behavior and tissue formation across several tissue engineering domains, such as skin, bone, cardiovascular, and lung tissues. Specific pore size ranges enhance scaffold functionality for different tissues: small pores (∼1–2 µm) aid epidermal cell attachment in skin regeneration, moderate pores (∼2–12 µm) support dermal migration, and larger pores (∼40–100 µm) facilitate vascular structures. For bone tissue engineering, multi-layered scaffolds with smaller pores (50–100 µm) foster cell attachment, while larger pores (200–400 µm) enhance nutrient diffusion and angiogenesis. Cardiovascular and lung tissues benefit from moderate pore sizes (∼25–60 µm) to balance cell integration and nutrient diffusion. By addressing critical design challenges and optimizing pore size distributions, this review provides insights into scaffold innovations, ultimately advancing tissue regeneration strategies.

Characterization of microstructural evolution and mechanical properties of Cu-Ni-Cr alloys deformed by ECAP

In recent years, Cu-Ni-Cr alloy materials have been broadly applied in the marine industry. However, owing to its dendritic microstructure, this material is an obstacle to the manufacturing process, particularly in the metal-forming process. In this study, an equal-channel angular pressure (ECAP) process is proposed to improve the material microstructure and mechanical properties. A single pass of the ECAP was performed at room temperature, and the microstructural evolution and mechanical properties of the material were characterized. The ECAP compressed and tensioned the microstructures on the top and bottom surfaces, respectively. The microstructures on the front and back surfaces were similar because they were sheared. This also caused a tear-off of the dendritic microstructure. Based on the changes in the characterization of the microstructural evolution, the ultimate tensile strength increased by 32% to approximately 680 MPa, and the elongation decreased by 200% to approximately 8%. The microhardness distribution was irregular on the front and back surfaces, causing nonuniform deformation. However, the microhardness increased by an average of 35% to approximately 270 HV. Consequently, the nonuniform microstructural evolutions and mechanical properties of each surface must be considered when applying the ECAP process to improve the mechanical properties of Cu-Ni-Cr alloy materials.

Biodegradable and Implantable Triboelectric Nanogenerator Improved by β-Lactoglobulin Fibrils-Assisted Flexible PVA Porous Film.

Triboelectric nanogenerators (TENGs) are highly promising as implantable, degradable energy sources and self-powered sensors. However, the degradable triboelectric materials are often limited in terms of contact electrification and mechanical properties. Here, a bio-macromolecule-assisted toughening strategy for PVA aerogel-based triboelectric materials is proposed. By introducing β-lactoglobulin fibrils (BF) into the PVA aerogel network, the material's mechanical properties while preserving its swelling resistance is significantly enhanced. Compared to pure PVA porous film, the BF-PVA porous film exhibits an eightfold increase in fracture strength (from 1.92 to 15.48J) and a fourfold increase in flexibility (from 10.956 to 39.36MPa). Additionally, the electrical output of BF-PVA in triboelectric performance tests increased nearly fivefold (from 45 to 203V). Leveraging these enhanced properties, a biodegradable TENG (bi-TENG) for implantable muscle activity sensing is developed, achieving real-time monitoring of neuromuscular processes. This innovation holds significant potential for advancing implantable medical devices and promoting new applications in bio-integrated electronics.

3D printed sodium alginate/gelatin/tannic acid/calcium chloride scaffolds laden bone marrow mesenchymal stem cells to repair defective thyroid cartilage plate.

Due to the absence of blood vessels, cartilage exhibits extremely limited self-repair capacity. Currently, repairing laryngeal cartilage defects, resulting from conditions such as laryngeal tumors, injury, and congenital structural abnormalities, remains a significant challenge in the Department of Otolaryngology, Head and Neck Surgery. Previous research has often focused on enhancing the mechanical properties of synthetic materials. However, their low biological activity and weak cell adhesion necessitate compensatory measures. This study aims to capitalize on the advantages of natural materials in cartilage tissue engineering. Sodium alginate, gelatin, tannic acid, and calcium chloride were utilized to prepare bioinks through cross-linking for application in 3D printing cartilage scaffolds. Bone marrow mesenchymal stem cells with multidirectional differentiation potential were chosen as seed cells, with appropriate growth factors incorporated to promote their differentiation into cartilage during in vitro culture. The scaffold laden cells was subsequently implanted into rabbit thyroid cartilage plate defects at the appropriate time. HE staining, toluidine blue staining, Masson staining, and collagen type II staining were employed to assess cartilage defect repair at 4, 8, and 12 weeks, respectively. Results demonstrated that scaffolds made from natural materials could emulate the mechanical properties of fresh cartilage with commendable biocompatibility. Stained sections further confirmed the efficacy of the composite hydrogel scaffolds identified in this study in promoting rabbit thyroid cartilage plate restoration. In summary, this study successfully fabricated a natural material scaffold for rabbit laryngeal cartilage tissue engineering, thereby furnishing a new idea and experience for the clinical application of laryngeal cartilage defect reconstruction.

Use of pneumatic waste in thermal insulation systems in arid áreas

Recent decades have seen a major global concern to protect the environment from non-biodegradable waste as little as possible. On the other hand, the problem of energy saving has emerged. Builders in arid areas and particularly in the Algerian Sahara are always faced with a very difficult choice: “build with traditional and local materials promoting a certain thermal comfort or build with modern materials offering more convenience, aesthetics and space but with less or no thermal comfort and very high consumption of air conditioning energy. The appearance of new materials and the innovation of waste in construction have not only improved the mechanical properties of materials but also energy saving through insulation, waterproofing and even the appearance of the construction. In the present work, an investigation was undertaken in order to respond to the problem of thermal comfort of construction while getting rid of tires which, through their long life, occupy large environmental spaces and through their incineration contribute to environmental pollution. Numerous tests have been carried out on reduced or real models representing either the floors or the partition walls which represent the two elements which are the main players in the heat transfer of the building. In comparison with the usual materials used in thermal insulation, the innovation of pneumatic waste has shown technical performances and thermal insulation properties superior to those existing on the market, which has the effect of increasing the energy efficiency of the buildings thus generating great economic interest. It is necessary to point out that only the plastic part of the tires was used while the metal part constituting the structural hoops of the tires has a wide use as a reinforcement system.

Study on withdrawal force capacity of insert nut in wood-based materials used for case furniture

ABSTRACT This study aimed to further investigate optimal interference fits of the insert nut in wood-based materials using the digital image correlation method (DICM). A complete two-factor 3 × 5 factorial experiment was designed to study the withdrawal force capacity (WFC) of the insert nut in wood-based materials considering three material types, particle board (PB), plywood (PL), and block board (BB), and five interference fits, 0.2, 0.7, 0.9, 1.2, and 1.7 mm. Additionally, the mechanical properties of wood-based materials were measured and used to further analyse WFC. The results showed that material type, interference fit and their interaction had significant effects on WFC. The maximum WFC of PL (1117 N) was greater than that of BB (1015 N) followed by PB (690 N). The DICM can visually evaluate and determine the optimal interference fit of the insert nut in wood-based materials. Three-order polynomial equations can be used to predict the WFCs of the insert nut in wood-based materials evaluated relating to interference fit. The maximum WFCs of wood-based materials may relate to the yield strengths of materials perpendicular to plane. The optimal interference fits for the insert nut in PB, PL, and BB are 0.7, 0.9, and 1.2 mm, respectively.

Flux jumps and mechanical effects in high-temperature superconductors with multi-field coupling model

Mechanical deformation during the magnetization process of high-temperature superconductors can lead to the deterioration in their critical current. A multi-field coupling model involving four physical fields is established to investigate the role of mechanical strain in the flux jump of the superconducting bulk. The electrical, magnetic, thermal, and mechanical coupling behavior is presented and analyzed under different applied fields. The simulation results of the coupled model are compared with experimental data, which showing an agreement. The numerical results indicate that in the superconductors, mechanical deformation significantly affects the electrical, magnetic, and thermal properties owing to a reduction in the critical current density under mechanical strain. The moment and location of magnetic flux jump will change as the mechanical strain is considered. Interestingly, the occurrence of magnetic flux jumps during field-cooling magnetization leads to a decrease in stress in bulk superconductor, suggesting that the fracture may not be caused by flux jumps, which is contrary to common understanding. Moreover, the temperature changes uniformly at different locations within the superconducting bulk. In the extended study, the thermo-magnetic and mechanical properties of the bulk material under ZFCM are presented using the coupled model. The effect of mechanics on the flux jump differs between FCM and ZFCM, which is attributed to the nonlinear changes in the electric field.

A Stretchable Conductive Material with High Fatigue Resistance for Strain Sensors.

Intrinsically stretchable conductive materials based on elastic substrates and conductive components play important roles in biomedical applications, such as exercise rehabilitation monitoring and disease prediction. A persistent challenge is to combine high fatigue resistance with excellent mechanical properties in stretchable conductive materials. Herein, we present a stretchable conductive material with both good fatigue resistance and high tensile properties (∼3170%) based on poly(acrylic acid)-phytic acid-trehalose-polypyrrole (denoted as PPTP). The as-prepared PPTP hydrogel electrode showed no obvious cracking or delamination after 400 loading and unloading cycles and maintained good electrical signal transmission function after 1000 cycles. We further collected stable signals for human motion and handwriting using the stretchable hydrogel electrode as a strain sensor, demonstrating the potential application of the PPTP stretchable hydrogel electrode in biomedicine.

Mandible composition and properties in two selected praying mantises (Insecta, Mantodea).

Insects process their food with their cuticle-based mouthparts. These feeding structures reflect their diversity and can, in some cases, showcase adaptations in material composition, mechanical properties, and shape to suit their specific dietary preferences. To pave the way to deeply understand the interaction between mouthparts and food and to determine potential adaptations of the structures to the food, this study focuses on the mandibles of two praying mantis species. Gongylus gongylodes feeds mainly on Diptera, and Sphodromantis lineola forages on larger prey. Employing scanning electron microscopy, the mandibular morphologies were analyzed. The degree of the cuticle tanning was tested using confocal laser scanning microscopy. Furthermore, the contents of transition and alkaline earth metals in the mandible cuticle were studied using energy-dispersive X-ray spectroscopy and the mechanical properties tested by nanoindentation. We found that S. lineola mandibles show pronounced gradients of Young's modulus and hardness from the basis to the tip, which might be an adaptation against high stresses during biting and chewing. G. gongylodes, in contrast, did not show pronounced gradients, which may indicate that there is less stress involved in feeding-necessary to test in future studies. The mechanical properties of manidibles in both species are related to the degree of cuticle tanning but also positively correlate with the content of magnesium. These findings enrich our understanding of insect cuticle biology but also present new sets of data on praying mantis structures.

Enhancing performance through friction welding of dissimilar aluminium 2014 and 7075 hybrid metal-matrix composite: insights into material characterization, crystallographic texture, and mechanical properties

Abstract Welding high-strength, age-hardenable aluminium alloys from the 2XXX and 7XXX series poses challenges when using fusion-based welding processes. Common issues such as hydrogen porosity, hot cracking, stress corrosion cracking, and solidification cracking can lead to a reduction in mechanical properties due to the loss of strengthening precipitates and alloying elements. Consequently, friction stir welding (FSW) has emerged as a promising solid-state welding technique for joining dissimilar aluminium alloys and composites, particularly in aerospace applications where a combination of strength, corrosion resistance, and other desirable properties is required. This study investigates the FSW of dissimilar aluminium alloys, specifically AA2014 and a hybrid metal-matrix composite of AA7075 reinforced with silicon carbide and graphite particles. The microstructural and mechanical properties of the welded joints were characterized, revealing an average grain size of 2.39 μm ± 1.2 in the stir zone (SZ). A high angle grain boundary volume fraction of 47.25% was observed in the SZ, compared to 2.01% in the AA7075 composite and 15.03% in the AA2014 base metal. The presence of shear textures and the distribution of cube, S, and H textures indicate a homogeneous mixing of the base materials and a high shear strain rate during welding. The hardness of the SZ increased by 13%, and the ultimate tensile strength, yield strength, and elongation of the joint were measured at 251 MPa, 183 MPa, and 6.44%, respectively. The flexural strength of the joints improved significantly, with the CW13 sample showing a 67% increase compared to CW1. The dissimilar joint CW13, welded at 12.5 mm min−1, achieved a maximum joint efficiency of 95%.

Application of Artificial Intelligence Technology for Prompt Diagnosis of Cast Iron Mechanical Properties

Kinetic indentation is widely used to measure physical and mechanical properties of materials as one of the most universal methods for non-destructive testing. This paper uses the latest advances in artificial intelligence and capabilities of the Python programming language libraries allowing to carry out accurate measurements of cast iron hardness based on the data of the material’s micro-impact loading diagram. It has been shown that use of machine learning allows eliminating gross errors and reducing the error of indirect hardness evaluation in several times – down to 10 units according to Brinell HB. It has also been established that formation of additional features for training models (based on traditionally used characteristics: penetration depths, indenter movement speed and contact forces at certain points in time) has a positive effect on the accuracy of measurements, but amount of measurements should also be optimized. Feasibility of effective use of machine learning to evaluate hardness has been demonstrated by comparing of calculated hardness values with data obtained with standard testing methods. Advantage of the developed testing method is the fact that the developed algorithms can be used for prompt diagnostics of cast iron hardness using existing equipment. It is appropriate to extend the proposed approach for determination of other mechanical properties of cast iron: yield strength, strain hardening index, creep, relaxation, determined by indentation methods.

ABOUT THE SINTERING OF HISTORICAL “YELLOW BRICKS” OF SOFIA

The historical “yellow cobblestones” of Sofia are a very well sintered ceramic clinker with practically zero water absorption. However, the high amount of crystal phase formed, which explains the extraordinary mechanical properties of this remarkable material, is in formal contradiction with its low final porosity. In fact, in the modern ceramics clinkers the crystallinity is similar or inferior, but the reached degree of sintering is lower; as a result, the mechanical properties are reduced. The aim of this report is to elucidate some peculiarities of the densification process of “yellow ceramics bricks” because such information is essential for the eventual successful production of replica of this emblematic pavement. The chemical and phase compositions of original “yellow paving bricks” together with those of modern clinker from “Vitosha” boulevard in Sofia were evaluated by XRF and XRD analysis, respectively. The structures of both ceramics were investigated with density measurements and SEM.Their densification behaviour was estimated by re-sintering of milled original samples by Hot Stage Microscopy (HSM) tests. The results elucidate that the sintering temperature of the historical clinker is inferior, and their sintering interval is significantly narrow. This peculiarity is explained by the rapid decreasing of apparent viscosity with temperature rise.Finally, by secondary holding at the sintering temperatures and subsequent fast quenching of a “yellow brick” sample, it is demonstrated that some phase formation occurred during the industrial cooling step. This result explains both the good degree of densification and the better properties of the “yellow cobblestones”.