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EN
This paper presents an experimental investigation of the hot deformation behaviour of 15% B₄C particle reinforced AA6061 matrix composites and the establishment of a novel corresponding unified and physically-based visco-plastic material model. The feasibility of hot forming of a metal matrix composite (MMC) with a low volume fraction reinforcement has been assessed by performing hot compression tests at different temperatures and strain rates. Examination of the obtained stress-strain relationships revealed the correlation between temperature and strain hardening extent. Forming at elevated temperatures enables obvious strain rate hardening and reasonably high ductility of the MMC. The developed unified material model includes evolution of dislocations resulting from plastic deformation, recovery and punching effect due to differential thermal expansion between matrix and reinforcement particles during non-steady state heating and plastic straining. Good agreement has been obtained between experimental and computed results. The proposed material model contributes greatly to a more thorough understanding of flow stress behaviour and microstructural evolution during the hot forming of MMCs.
EN
The aim of this research was to investigate the effects of contact pressure and lubrication on the interfacial heat transfer coefficient (IHTC) between AA7075 sheet and H13 tool steel and the in-die quenching performance in hot stamping. Firstly, a series of designed in-die quenching experiments were performed using different contact pressures, 0.05–30 MPa and lubrication conditions to determine the IHTC values using an efficient methodology. Secondly, temperature evolution of the tools and blank during the in-die quenching was investigated. Mechanical properties of material in-die quenched under different process conditions, were measured to determine their relation to quench conditions. The results have shown that IHTC values increased with increasing contact pressure and use of lubricant. A strength level of T6 condition could be obtained using a contact pressure greater than 5 MPa in the lubricated condition.
EN
Broadband seismic networks are becoming more intensive, generating a large amount of data in the long-term collection process. When processing the data, the researchers rely almost on instrument response files to understand the information related to the instrument. Aiming at the process of instrument response recording and instrument response correction, we identify several sources of the instrument response phase error, including pole–zero change, the causality difference in instrument correction method, and the problem of filter coefficient recording. The data time offset range from the instrument response phase error is calculated from one sample point to several seconds using the ambient noise data recorded by multiple seismic stations. With different data delays, the time offset of the noise correlation function is estimated to be 74% to 99% of the data delay time. In addition, the influence of instrument response phase error on the measurement of seismic velocity change is analyzed by using ambient noise data with pole–zero change, and the results show that the abnormal wave velocity with exceeding the standard value is exactly in the time period of the instrument response error, which indicates that the instrument response error affects the study of seismology.
EN
A new seismic interpolation and denoising method with a curvelet transform matching filter, employing the fast iterative shrinkage thresholding algorithm (FISTA), is proposed. The approach treats the matching filter, seismic interpolation, and denoising all as the same inverse problem using an inversion iteration algorithm. The curvelet transform has a high sparseness and is useful for separating signal from noise, meaning that it can accurately solve the matching problem using FISTA. When applying the new method to a synthetic noisy data sets and a data sets with missing traces, the optimum matching result is obtained, noise is greatly suppressed, missing seismic data are filled by interpolation, and the waveform is highly consistent. We then verified the method by applying it to real data, yielding satisfactory results. The results show that the method can reconstruct missing traces in the case of low SNR (signal-to-noise ratio). The above three problems can be simultaneously solved via FISTA algorithm, and it will not only increase the processing efficiency but also improve SNR of the seismic data.
5
Content available remote Evaluation of the biocompatibility of a hydroxyapatite-CaTiO3 coating in vivo
EN
Objective: This study was designed to evaluate the biocompatibility and osteointegrative activity of hydroxyapatite (HA)-CaTiO3, titanium substrate, traditional HA coating and CaTiO3 coating via an animal experiment. Method: Four types of screws (type 1: coated with HA; type 2: coated with CaTiO3; type 3: coated with HA-CaTiO3; type 4: untreated titanium screws) were implanted into femur bone of 48 New Zealand rabbits. Histological and mechanical investigations were employed at the end of 2, 4, 8 and 12 weeks to evaluate the material osteointegration. Results: (1) All of the experimental rabbits were healthy during the experiment process. (2) Histological investigation showed fully regenerated and well integrated bone tissue surrounding the screws coated with HA, HA-CaTiO3 and CaTiO3. (3) Mechanical investigation showed that the bonding strength of HA-CaTiO3 coating was significantly higher than that of CaTiO3 coating or titanium materials without coating, but was lower than those coated with HA. Conclusion: HA-CaTiO3 coating possesses similar admirable biocompatibility and osteointegration activity with HA coating, indicating a promising coating material for implants in orthopedics.
PL
Resurs kół pociągu może być znacząco różny w zależności od ich miejsca zamontowania, warunków pracy, charakterystyk związanych z reprofilacją, itp. W artykule, porównano koła dwóch wybranych lokomotyw kursujących na Linii Rud Żelaza w północnej Szwecji, aby zbadać niektóre ze wspomnianych różnic. Zaproponowano możliwość łączenia danych pochodzących z oceny niezawodności z danymi degradacyjnymi oraz danymi z reprofilacji. Przeprowadzone badania pozwalają wyciągnąć następujące wnioski. Po pierwsze, krzywa wykładnicza degradacji oraz zadane warunki pracy można wykorzystać w celu przeprowadzenia badań niezawodności z użyciem modelu Weibulla z efektami losowymi (tzw. "frailty model"); po drugie, główną przyczyną zlecania reprofilacji kół jest zmęczenie toczne (RCF); po trzecie, analiza parametrów reprofilacji pozwala na monitorowanie i badanie zarówno szybkości zużycia kół, jak i ubytku materiału podczas reprofilacji, co może mieć zastosowanie w optymalizacji czynności obsługowych.
EN
The service life of railway wheels can differ significantly depending on their installed position, operating conditions, re-profiling characteristics, etc. This paper compares the wheels on two selected locomotives on the Iron Ore Line in northern Sweden to explore some of these differences. It proposes integrating reliability assessment data with both degradation data and re-profiling performance data. The following conclusions are drawn. First, by considering an exponential degradation path and given operation condition, the Weibull frailty model can be used to undertake reliability studies; second, among re-profiling work orders, rolling contact fatigue (RCF) is the principal reason; and third, by analysing re-profiling parameters, both the wear rate and the re-profiling loss can be monitored and investigated, a finding which could be applied in optimisation of maintenance activities.
EN
The process parameters of aluminum alloy hot stamping are essential for product forming quality. In the case of an anti-collision side beam inside car doors, the finite-element model of aluminum alloy hot stamping is set up, and the forming quality is investigated under an ordinary process condition. The blank hold force (BHF) has a significant impact on the forming quality in hot stamping. Using the Latin hypercube method to sample the simulation data points and the finite-element (FE) model to calculate the forming quality indices of the data points according to the response value of the indices, the quadratic response surfaces between the process parameter inputs and the forming quality indices are initialized. Using the multi-objective genetic algorithm NSGA-II (non-dominated sorting genetic algorithm) to optimize the responses of the process parameters, the Pareto solutions corresponding to combinations of the blank hold force and stamping speed are obtained. Finally, based on the optimal process parameters, stamping tests are carried out. Compared with the results of the stamping trial and numerical simulation, it is demonstrated that the finite-element model can predict forming defects and be consistent with the actual condition and that the optimization procedure proposed in the paper is feasible.
EN
In this paper we study the complexity of some size constrained clustering problems with norm Lp. We obtain the following results: (i) A separation property for the constrained 2-clustering problem. This implies that the optimal solutions in the 1-dimensional case verify the so-called "String Property"; (ii) The NP-hardness of the constrained 2-clustering problem for every norm Lp (p > 1); (iii) A polynomial time algorithmfor the constrained 2-clustering problemin dimension 1 for every norm Lp with integer p. We also give evidence that this result cannot be extended to norm Lp with rational non-integer p; (iv) The NP-hardness of the constrained clustering problem in dimension 1 for every norm Lp (p ≥1).
9
Content available remote 2D FE Micromechanics Modelling of Honeycomb Core Sandwich Panels
EN
A repeating unit cell 2D ?nite element modelling procedure has been established to model the mechanical behaviour of honeycomb core sandwich panels (e.g. Young’s modulus, energy absorbed, etc.). Periodic boundary conditions have been implemented within the model to simulate an in?nitely long sandwich panel. An analytical solution using Timoshenko beam theory has been developed to predict the Young’s modulus of the honeycomb core, and this has been compared with the FE model results; it is found that there is good agreement between the two values. The FE model can shed light on the mechanics of more complex 3D metal foams.
10
Content available remote Fabrication and performance of carbon coated copper nanoparticles
EN
Carbon coated copper nanoparticles were synthesized by the carbon arc discharge method using a powder mixture containing copper and carbon at the ratio different than in the raw material. The structure, topography, size distribution, phase composition and anti-oxidation property of the nanoparticles were characterized by X-ray diffraction, transmission electron microscopy, thermogravimetry and differential scanning calorimetry experiments. The results indicated that carbon coated copper nanoparticles had a clear core-shell structure, the core of the particles being copper single crystal, and the shell of the particles - a carbon layer with a graphite-like structure. Carbon coated copper nanoparticles had diameters of ca. 20-60 nm. As the copper content increased, the inner copper core became more crystallized. Copper promoted catalysis to the external carbon layers, the graphitization degree became more obvious as the content of copper increased. The outer graphitic carbon layers effectively prevented oxidation of the copper core inside. The oxidation resistance of carbon coated copper nanoparticles was superior to that of pure copper powder.
11
Content available remote Crystallization of TeO2-Nb2O5 glasses and their network structural evolution
EN
TeO2-Nb2O5 glass is a kind of heavy metal oxide glass with a chain-like network structure, in which Nb5+ ions connect the Te-O chains and adjust the types of [TeOx] (x = 3, 4) coordination polyhedra to stabilize the glass network. (100 - y)TeO2-yNb2O5 (y = 3-20 mol %) glasses were prepared, the crystallization behaviour and their network structural evolution were studied by means of differential thermal analysis (DTA), X-ray diffraction (XRD) and Fourier transform infrared spectra (FT-IR). The results show that the stabilization of the TeO2-Nb2O5 glass network is greatly influenced by the constitution of Te-O chains and their linkage. The glass structure with lower Nb5+ content is inhomogeneous, it is composed of edge sharing Te-O chains, partly edge sharing chains connected by Nb5+ ions, apical sharing chains and apical sharing chains connected by Nb5+ ions. Crystalline phases of ß-TeO2, Nb2Te4O13, .beta.-TeO2 and Te3Nb2O11 will be formed in turn when the treatment temperature of the glass is increased. When the concentration of Nb5+ ions is sufficient to well connect Te-O chains, the glass network will tend to homogenize, only one crystalline phase, Nb2Te4O13, will be formed. A suitable preheat treatment will also help to homogenize the glass structure and make the glass more stable.
12
Content available remote On micro-damage in hot metal working. P. 2 Constitutive modelling
EN
Damage constitutive equations are formulated to model the evolution of grain boundary and plasticity-induced damage for free-cutting steels under hot forming conditions. During high temperature, high strain rate deformation, material degradation has characteristics of both creep damage at grain boundaries, and ductile damage surrounding hard inclusions. This has been experimentally observed and is reported in the companion paper. This paper describes the development of unified viscoplastic-damage constitutive equations, in which the nucleation and growth of both damage types are considered independently. The effects of deformation rate, temperature, and material microstructure on damage evolution are modelled. The proposed damage evolution equations are combined with a viscoplastic constitutive equation set, enabling the evolution of dislocation hardening, recovery, recrystallisation, grain size, and damage to be modelled. This set of unified, mechanism-based, viscoplastic damage constitutive equations is determined from experimental data of a freemachining steel for the temperature range 1173- 1373 K. The fitted model is then used to predict damage and failure features of the same material tested using a set of interrupted constant strain rate tests. Close agreement between the predicted and experimental results is obtained for all the cases studied.
EN
Experiments have been described in which copper components have been subjected to combined cyclic thermal and constant mechanical loading. Two thermal cycles were employed leading to predominantly cyclic plasticity damage and balanced creep - cyclic plasticity damage loading cycles. The combined loading led to component ratchetting and ultimately to failure. Continuum damage-based finite element techniques have been developed for combined cyclic plasticity, creep and ratchetting in components subjected to thermo-mechanical loading. Cycle jumping techniques have been employed within the finite element formulation to minimise computer CPU times. The finite element methods have been used to predict the behaviour of the copper components tested experimentally and the results compared. Steady-state ratchet rates were found to be well predicted by the models. Modes of failure and component lifetimes were also found to be reasonably well predicted. The experimental results demonstrate the importance of isotropic cyclic hardening on the initial component ratchetting rates.
PL
W pracy przedstawiono doświadczenia, w których miedziane elementy poddawano złożonym obciążeniom zawierającym cykle termiczne i obciążenia stałe. Zastosowano dwa cykle termiczne prowadzące do uszkodzenia z przewagą cyklicznej plastyczności i zrównoważonego pełzania oraz cykle obciążeń wywołujących uszkodzenie typowe dla cyklicznej plastyczności. Obciążenia złożone wywoływały ratcheting badanych elementów, a w efekcie końcowym zniszczenie. Techniki metody elementów skończonych kontynualnej mechaniki uszkodzeń zostały rozwinięte dla cyklicznej plastyczności w złożonym stanie naprężenia, pełzania oraz ratchetingu w elementach poddawanych obciążeniom termo-mechanicznym. W sformułowaniu metody elementów skończonych, aby zminimalizować czasy centralnego procesora (CPU) komputera, zastosowano techniki skoków cyklicznych. Metody elementów skończonych zastosowano w przewidywaniu zachowania się elementów miedzianych wcześniej badanych doświadczalnie, a otrzymane wyniki porównano. Zaproponowane modele w sposób satysfakcjonujący pozwalają przewidywać prędkości stanu ustalonego ratchetingu. Także przewidywania dotyczące sposobów zniszczenia oraz żywotności badanych elementów są zadowalające. Wyniki doświadczalne pokazują istotny wpływ izotropowego wzmocnienia cyklicznego na początkowe składowe prędkości ratchetingu.
14
Content available remote On micro-damage in hot metal working. Part 1: Experimental investigation
EN
An experimental programme was defined and performed to investigate the characteristics of micro-damage for a plain CMn and a free machining steel under hot forming conditions. To investigate damage locations - at grain boundaries and around second phase inclusions - a series of constant strain rate tests were carried out on the free machining steel, which contained manganese sulphide inclusions. Specimens from both materials were strained to failure under tension using a Gleeble material simulator at a constant temperature of 1273 K, with strain rates = 0.01-10 s-1. The damage characteristics of the two different steel microstructures was analysed through microstructural examinations of the tested specimens. Particular attention is focussed on damage locations and features. To investigate the recovery of materials between the intervals of hot deformation, a series of two-step tensile tests were carried out at 1273 K and 10 s-1. The two-step specimens were initially deformed to a strain varying from 0.3-0.7, held for varying recovery periods of 0.3-10 s, then stretched to failure. Flow stress features and strains to failure during the second stage of deformation were analysed with respect to different recovery periods and strain levels at the first stage of deformation. The damage features discovered from the experimentation and microstructural examination provide theoretical evidence to form unified viscoplastic damage constitutive equations for hot forming of free machining steels, which are described in the companion paper.
EN
Measurement of oxidation kinetics of as-received Zr-2.5Nb samples, texture modified Zr-2.5Nb samples, and pure zirconium were carried out by using thermo gravimetric analysis (TGA). A role of substrate texture, substrate microstructure on oxidation kinetics and mechanism of oxidation processes was discussed. The substrate exhibits better corrosion resistance after heat treatment at 560C, while the corrosion resistance becomes worse after heat treatment at 855C. The improvement of oxidation resistance of the substrate is due to the decomposition of Beta-Zr phase, rather than the minor change of texture of alfa-Zr phase induced by the heat treatment. Among the samples tested, Zr-2.5Nb substrate after heat treatment at 560C shows the best oxidation resistance, followed by as-received Zr-2.5Nb sample and the Zr-2.5Nb samples with heat treatment at 855C.
PL
Analiza termograwimetryczna (TGA) była uzyta do pomiaru kinetyki utleniania technicznego stopu Zr-2,5Nb wyjściowego, jak również po zmodyfikowaniu jego tekstury i dla czystego cyrkonu. Rozważana jest rola tekstury i mikrostruktury podkładu na kinetykę oraz mechanizm procesu utleniania. Podkład staje sie lepiej odporny na korozje po obróbce cieplnej w temperaturze 560C, a najmniej odporny po obróbce cieplnej w temperaturze 855C. Polepszenie się odporności na utlenianie podkładu wynika raczej z rozkładu fazy beta-Zr niż z nikłej zmiany tekstury fazy alfa-Zr wynikającej z obróbki cieplnej. Wśród badanych próbek podkład Zr-2,5Nb po obróbce cieplnej w temperaturze 560C wykazuje najlepszą odporność na utlenianie. Gorsze własności wykazują próbki dostarczonego technicznego stopu Zr-2,5Nb oraz Zr-2,5Nb po obróbce cieplnej w temperaturze 855C.
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