In the paper it is studied how the modes of thermal treatment of low-carbon steels alloyed with manganese (09G2S) and with manganese, vanadium, and niobium (10G2FBYu) affect their hierarchical ...structure and toughness in the low-temperature region from +20 down to −70 °C. A fine-grained structure, quasi-homogeneous lattice curvature, and nanoscale mesoscopic structural states arising due to radial-shear rolling at 850 °C are shown to be responsible for the formation of a nonequilibrium nanoscale bainitic structure, being a highly effective damping factor in a deformed material. The pearlitic steel structure is equilibrium and is formed within a translation-invariant crystal lattice, while the bainitic structure results from nanoscale mesoscopic structural states in interstices of lattice curvature zones. A spatial change in the lattice curvature is accompanied by a synchronous transformation of the nanoscale mesoscopic structural states and structural geometry of the bainitic phase. That is why the toughness of the steel with bainitic structure is very high down to the temperature −70 °C. Scratch testing is used to estimate the possibility of elastic recovery of bainitic phases in deformed steels.
•Impact toughness of as-built Ti6Al4V wire-feed EBAM parts is much higher than that for as-built SLM and EBM ones.•Dislocation glide does not provide accommodation of shear strain in SLM and EBM ...samples.•Cooling rate controls the microstructure and fracture behavior of AM fabricated samples.
Comparison of the microstructure and the phase composition of the Ti-6Al-4V alloy parts built by various additive manufacturing technologies was carried out by optical and transmission scanning electron microscopy, as well as X-ray diffraction analysis. It was shown that the martensitic αʹ phase, formed due to the melt pool fast cooling rate, determined the accommodation mechanisms of shear deformation of the as-built Ti-6Al-4V AM fabricated samples under uniaxial quasi-static tension and impact bending (and, accordingly, their mechanical properties). The effect of microstructure on impact toughness, ultimate impact strength, as well as the crack initiation and propagation energy was investigated by the impact bending tests with recording impact load diagram (force–displacement graph). It was concluded that impact toughness of the Ti-6Al-4V samples built by wire-feed electron beam additive manufacturing was significantly higher than that of ones manufactured by selective laser melting and electron beam melting of a powder. Matching the mechanical properties and SEM micrographs of the fracture surfaces of the as-built Ti-6Al-4V AM fabricated samples enabled to reveal the crack initiation and propagation mechanisms during their quasi-static and dynamic loading. Discussion of the results was carried out using the concept of scale levels of plastic deformation.
The optimal mode for ultrasonic welding (USW) of the "PEEK-ED (PEEK)-prepreg (PEI impregnated CF fabric)-ED (PEEK)-PEEK" lap joint was determined by artificial neural network (ANN) simulation, based ...on the sample of the experimental data expanded with the expert data set. The experimental verification of the simulation results showed that mode 10 (t = 900 ms, P = 1.7 atm, τ = 2000 ms) ensured the high strength properties and preservation of the structural integrity of the carbon fiber fabric (CFF). Additionally, it showed that the "PEEK-CFF prepreg-PEEK" USW lap joint could be fabricated by the "multi-spot" USW method with the optimal mode 10, which can resist the load per cycle of 50 MPa (the bottom HCF level). The USW mode, determined by ANN simulation for the neat PEEK adherends, did not provide joining both particulate and laminated composite adherends with the CFF prepreg reinforcement. The USW lap joints could be formed when the USW durations (
) were significantly increased up to 1200 and 1600 ms, respectively. In this case, the elastic energy is transferred more efficiently to the welding zone through the upper adherend.
This research addresses the development of a formalized approach to dental material selection (DMS) in manufacturing removable complete dentures (RDC). Three types of commercially available ...polymethyl methacrylate (PMMA) grades, processed by an identical Digital Light Processing (DLP) 3D printer, were compared. In this way, a combination of mechanical, tribological, technological, microbiological, and economic factors was assessed. The material indices were calculated to compare dental materials for a set of functional parameters related to feedstock cost. However, this did not solve the problem of simultaneous consideration of all the material indices, including their significance. The developed DMS procedure employs the extended VIKOR method, based on the analysis of interval quantitative estimations, which allowed the carrying out of a fully fledged analysis of alternatives. The proposed approach has the potential to enhance the efficiency of prosthetic treatment by optimizing the DMS procedure, taking into consideration the prosthesis design and its production route.
The Physical Nature of Plasticity Egorushkin, V. E.; Panin, V. E.; Panin, A. V.
Physical mesomechanics,
2021/1, Letnik:
24, Številka:
1
Journal Article
Recenzirano
The paper analyzes the plasticity of deformed solids as a hierarchical translational-rotational system. The analysis shows that accounting for plastic rotation in addition to plastic translation, ...which is conventionally related to dislocation motion, radically changes the description of plasticity as a displacement gauge potential. With both deformation modes, and hence, with two-component displacements, introducing a lattice curvature and a Berry–Ishlinsky phase allows one to identify all plastic mechanisms associated with slip, twinning, lattice bending, shear and torsion, generation of nanoscale mesoscopic structural states at lattice curvature interstices and motion of point defects in their zones. Experimental evidence is presented for plastic dynamics via point-defect motion in interstitial curvature zones.
(1) Background: this study deals with design of an automated laboratory facility based on a servo-hydraulic testing machine for estimating parameters of mechanical hysteresis loops by means of the ...digital image correlation (DIC) method. (2) Methods: the paper presents a description of the testing facility, describes the grounds for calculating the elastic modulus, the offset yield strength (OYS) and the parameters of the mechanical hysteresis loops by the DIC method. (3) Results: the developed hardware-software facility was tested by studying the fatigue process in neat polyimide (PI) under various amplitude tension-tension loadings. It was found that the damage accumulation was accompanied by the decrease in the loop areas, while failure occurred when it reduced by at least ~5 kJ/m3. (4) Conclusions: it was shown that lowering the loop area along with changing the secant modulus value makes it possible to estimate the level of the scattered damage accumulation (mainly at the stresses above the OYS level). It was revealed that fractography data, namely the pattern and sizes of the fatigue crack initiation and propagation zones, did not correlate well with the dependences of the parameters of the hysteresis loops.
The objective of this research was to predict the fatigue behavior of polyetherimide-based composites loaded with short carbon fibers 200 μm long under cyclic loads. The weight fraction of the filler ...was 10, 20, and 30 wt.%, while the maximum stress in a cycle was 55, 65, and 75 MPa. A modified fatigue model based on the obtained experimental results and Basquin equation was developed. The novelty of the results is related to developing a model on the structure-property relationship, which accounts for both the maximum stress in a cycle and the carbon fiber content in the composites. In addition, an "algorithm" for designing such composites according to the fatigue life criterion was proposed. The approach to determine relationships between the composition, structure, and properties of PCMs described in this study can be applied to further expand the model and to improve its versatility in the use of other thermoplastic matrices and fillers. The results of this study can be applied for the design of composites for structural applications with designated fatigue properties.
Ultrasonic and electron beam treatment of commercial titanium VT1-0 and its alloy VT6 (Ti-6Al-4V) produces a nonequilibrium grain-subgrain hierarchical substructure in the surface layer, which causes ...a multiscale fragmentation of the material and reveals a damping effect. When cooled in the gradient temperature field (during electron beam treatment) and when the β phase of the initial alloy is destroyed by ultrasound, the high-temperature bcc structure of the surface layer undergoes a nonequilibrium phase transition into an hcp α-phase structure. The excess specific volume of the β phase is hierarchically distributed in the a phase through the growth of nonequilibrium α′ and α″ martensite, and in the form of local ω-phase precipitation along the grain boundaries of the α phase. The specific volume of the nonequilibrium phases exceeds the specific volume of the α phase. This eliminates the formation of micropores and causes material fragmentation at the micro- and nanoscale structural levels during the nonequilibrium β → α phase transition. The growing α′ laths cause the fragmentation of the α phase at the microscale level. The α″ laths grow within the nonequilibrium α′ laths; they have a thickness of ∼1.5 nm and fragment the material at the nanoscale level. This process is controlled by the electronic subsystem that creates nanoscale mesoscopic structural states for the formation of nonequilibrium martensite phases. The reversible elastoplastic deformation of the nonequilibrium martensite phases at the nanoscale level governs the damping effect of the surface layer subjected to ultrasonic or electron beam treatment. The generation of nanoscale mesoscopic structural states and the related new mechanism of reversible deformation in the conditions of broken translational invariance of the lattice in a deformable solid has been confirmed experimentally.
The aim of this study is to substantiate the use machine learning methods to optimize a combination of ultrasonic welding (USW) parameters for manufacturing of multilayer lap joints consisting of two ...outer PEEK layers, a middle prepreg of unidirectional carbon fibers (CFs), and two energy directors (EDs) between them. As a result, a mathematical problem associated with determining the optimal combination of technological parameters was formulated for the formation of USW joints possessing improved functional properties. In addition, a methodology was proposed to analyze the mechanical properties of USW joints based on neural network simulation (NNS). Experiments were performed, and threshold values of the optimality conditions for the USW parameters were chosen. Accordingly, NNS was carried out to determine the parameter ranges, showing that the developed optimality condition was insufficient and required correction, taking into account other significant structural characteristics of the formed USW joints. The NNS study enabled specification of an extra area of USW parameters that were not previously considered optimal when designing the experiment. The NNS-predicted USW mode (P = 1.5 atm, t = 800 ms, and τ = 1500 ms) ensured formation of a lap joint with the required mechanical and structural properties (σUTS = 80.5 MPa, ε = 4.2 mm, A = 273 N·m, and Δh = 0.30 mm).
Any deformed solid represents two self-consistent functional subsystems: a 3D crystal subsystem and a 2D planar subsystem (surface layers and all internal interfaces). In the planar subsystem, which ...lacks thermodynamic equilibrium and translation invariance, a primary plastic flow develops as nonlinear waves of structural transformations. At the nanoscale, such planar nonlinear transformations create lattice curvature in the 3D subsystem, resulting in bifurcational interstitial states there. The bifurcational states give rise to a fundamentally new mechanism of plastic deformation and fracture—plastic distortion—which is allowed for neither in continuum mechanics nor in fracture mechanics. The paper substantiates that plastic distortion plays a leading role in dislocation generation and glide, plasticity and superplasticity, plastic strain localization and fracture.
