Structural fatigue is the major obstacle that prevents practical applications of the elastocaloric effect (eCE) in cooling or heat-pumping devices. Here, the eCE and fatigue behaviour of Ni-Ti plates ...are systematically investigated in order to define the fatigue strain limit and the associated eCE. Initially, the eCE was evaluated by measuring adiabatic temperature changes at different strain amplitudes and different mean strains along the loading and unloading transformation plateaus. By comparing the eCE with and without pre-strain conditions, the advantages of cycling an elastocaloric material at the mean strain around the middle of the transformation plateau were demonstrated. In the second part of this work, we evaluated the fatigue life at the mean strain of 2.25% at the loading plateau and at the unloading plateau after initial pre-straining up to 6% and 10%, respectively. It is shown that on polished samples, durable operation of 105 cycles can be reached with a strain amplitude of 0.50% at the loading plateau, which corresponds to adiabatic temperature changes of approximately 5 K. At the unloading plateau (after initial pre-strain of 10%), durable operation was reached at a strain amplitude of 1.00%, corresponding to adiabatic temperature changes of approximately 8 K. The functional fatigue was analysed after the cycling and it is shown that once the sample has been stabilized there is no further degradation of the eCE, even after 105 cycles. These results present guidelines for the design and operation of efficient and durable elastocaloric devices in the future.
Display omitted
The aim has been to determine the effect of thermo-mechanical cycling on shear-bond-strength (SBS) of dental porcelain to Co–Cr and Ti-based alloys fabricated by casting, ...computer-numerical-controlled milling, and selective-laser-melting (SLM).
Seven groups (n=22/group) of metal cylinders were fabricated by casting (Co–Cr and commercially pure-cpTi), milling (Co–Cr, cpTi, Ti-6Al-4V) or by SLM (Co–Cr and Ti-6Al-4V) and abraded with airborne-particles. The average surface roughness (Ra) was determined for each group. Dental porcelain was applied and each metal–ceramic combination was divided into two subgroups — stored in deionized water (24-h, 37°C), or subjected to both thermal (6000-cycles, between 5 and 60°C) and mechanical cycling (105-cycles, 60N-load). SBS test-values and failure modes were recorded. Metal–ceramic interfaces were analyzed with a focused-ion-beam/scanning-electron-microscope (FIB/SEM) and energy-dispersive-spectroscopy (EDS). The elastic properties of the respective metal and ceramic materials were evaluated by instrumented-indentation-testing. The oxide thickness on intact Ti-based substrates was measured with Auger-electron-spectroscopy (AES). Data were analyzed using ANOVA, Tukey’s HSD and t-tests (α=0.05).
The SBS-means differed according to the metal–ceramic combination (p<0.0005) and to the fatigue conditions (p<0.0005). The failure modes and interface analyses suggest better porcelain adherence to Co–Cr than to Ti-based alloys. Values of Ra were dependent on the metal substrate (p<0.0005). Ti-based substrates were not covered with thick oxide layers following digital fabrication.
Ti-based alloys are more susceptible than Co–Cr to reduction of porcelain bond strength following thermo-mechanical cycling. The porcelain bond strength to Ti-based alloys is affected by the applied metal processing technology.
Adhesive bonding has proven to be a reliable method of joining materials, and the development of new adhesives has made it possible to use bonding in a variety of applications. This article addresses ...the challenges of bonding metals such as the aluminum alloy EN AW-5754 and the stainless steel X5CrNi18-10. In this study, the effects of laser cleaning and texturing on the surface properties and strength of two bonded joints were investigated and compared with mechanical preparation (hand sanding with Scotch-Brite and P180 sandpaper). The bonded joints were tested with three different epoxy adhesives. During the tests, the adhesion properties of the bonded surface were determined by measuring the contact angle and assessing the wettability, the surface roughness parameters for the different surface preparations, and the mechanical properties (tensile lap-shear strength). Based on the strength test results, it was found that bonded joints made of stainless steel had 16% to 40% higher strength than aluminum alloys when using the same adhesive and surface preparation. Laser cleaning resulted in maximum shear strength of the aluminum alloy bond, while the most suitable surface preparation for both materials was preparation with P180 sandpaper for all adhesives.
When extruding the casted rods from EN AW 2011 aluminium alloys, not only their homogenized structure, but also their extrudable properties were significantly influenced by the hardness of the alloy. ...In this study, the object of investigations was the EN AW 2011 aluminium alloy, and the effect of homogenisation time on hardness was investigated. First, homogenisation was carried out at 520 °C for different times, imitating industrial conditions. After homogenisation, the samples were analysed by hardness measurements and further characterised by microscopy and image analysis to verify the influence of homogenisation on the resulting microstructural constituents. In addition, non-equilibrium solidification was simulated using the program Thermo-Calc and phase formation during solidification was investigated. The homogenisation process enabled more rounded shape of the Al2Cu eutectic phase, equilibrium formation of the phases, and the precipitation in the matrix, leading to a significant increase in the hardness of the EN AW 2011 aluminium alloy. The experimental data revealed a suitable homogenisation time of 4–6 h at a temperature of 520 °C, enabling optimal extrusion properties.
Creep is defined as the permanent deformation of materials under the effect of sustained stress and elevated temperature for long periods of time, which can essentially lead to fracture. Due to very ...time-consuming and expensive testing requirements, existing experimental creep data are often analyzed using derived engineering parameters and models to predict and find the correlations between creep life (time to rupture), temperature and stress. The objective of this study was to analyze and compare different numerical algorithms by using the Larson–Miller parameter (LMP) extrapolation model. Calculations were performed using the classical LMP equation where different values of parameter C were selected, as well as using a modified LMP equation in which parameter C was stress dependent C(σ). The impact of two different approaches of extrapolation and correlation functions (linear and polynomial) applied to fit the LMP model was also investigated. A detailed analysis was performed to choose the best extrapolation fit function and error tolerance. The numerical algorithm implemented was validated through creep rupture testing performed on 10CrMo9–10 steel at 600 °C (873 K) and 80 MPa. Creep model behavior analysis proved that different values of C can significantly change the estimated time to rupture. An excellent response of the LMP model was obtained by considering polynomial dependency when parameter C was assumed to be 18, especially for the temperature range from 773 to 873 K. Promising results were also achieved when parameter C was taken as stress-dependent, but only for linear fitting, which requires further analysis. However, at validation stage it turned out that only the linear extrapolation function and C taken as a constant value provided adequate time-to-rupture prediction. In the case of C = 18, estimated time was slightly overestimated (~8%) and for C = 20 it was underestimated by 27%. In all other cases error largely exceeded 50%.
In order to properly use materials in design, a complete understanding of and information on their mechanical properties, such as yield and ultimate tensile strength must be obtained. Furthermore, as ...the design of automotive parts is constantly pushed toward higher limits, excessive measuring uncertainty can lead to unexpected premature failure of the component, thus requiring reliable determination of material properties with low uncertainty.
The aim of the present work was to evaluate the effect of different metrology factors, including the number of tested samples, specimens machining and surface quality, specimens input diameter, type of testing and human error on the tensile test results and measurement uncertainty when performed on 2xxx series Al alloy. Results show that the most significant contribution to measurement uncertainty comes from the number of samples tested, which can even exceed 1 %. Furthermore, moving from experimental laboratory conditions to very intense industrial environment further amplifies measurement uncertainty, where even if using automated systems human error cannot be neglected.
This study investigates the influence of heat treatment parameters on effectiveness of deep cryogenic treatment (DCT) and on properties of three high-speed steels (HSS) (AISI M2, AISI M3:2 and AISI ...M35) with different chemical composition. Within this research the same preselected soaking temperature, time, cooling/warming rate and placement of DCT, being found in the literature as the most effective, were applied to the selected HSS grades. The fatigue resistance, hardness, impact and fracture toughness, compressive strength and strain-hardening exponent were selected as the evaluators of DCT effectiveness. The results indicate that the influence and effectiveness of DCT is strongly subjected by the type (wrought or PM steel), chemical composition of the particular steel grade and heat treatment parameters, resulting in either degradation or improvement of the steel properties. Furthermore, the study elucidates the effect of DCT on different HSS grades based on the selected heat treatment temperatures (austenitization and tempering). The research confirms that DCT generally improves the hardness, fatigue properties, fracture and impact toughness, compressive strength and strain-hardening exponent for HSS grades. However, it may also have negative impact on HSS properties. For all HSS investigated, the level of DCT effect depended also on the heat treatment conditions, i.e. austenitization and tempering temperature. The lower austenitization and higher tempering temperature in general resulted in improvement of steel properties, as compared to higher austenitization and lower tempering temperature with predominantly negative effect.
•Systematic research of DCT effect on mechanical and fatigue properties of HSS.•DCT samples have more homogeneous matrix with MC, M6C, M2C and M3C carbides.•Heat treatment (Ta and Tt) defines the steels' properties modification with DCT.•DCT with lower Ta and higher Tt generally improves HSS properties.•Cr, V, Co influence the effectiveness of DCT in terms of the steels properties.
Despite struggling to achieve the requirements necessary in many industrial applications, the additive manufacturing approach excels in complex designs often encountered in wearable technologies, ...prosthetics, implants, airfoils, and reverse engineering, where top-down machining can get quite expensive. Since fiber-reinforced composites are rarely used as unidirectional in engineering applications, analyzing the behavior of multilayered additively manufactured composites is critical, yet seldom found in the literature. Therefore, a progressive damage model based on Puck-Schurmann failure criteria has been proposed in this study. The continuous carbon fiber reinforced lamina properties have been acquired based on the available data in the literature and performed uniaxial experiments. The adopted properties were integrated within the damage model subroutine and used in finite element analysis software, while model parameters were calibrated using the response surface algorithms in the design of experiments according to the behavior of a distinctive multi-directionally reinforced test case and validated experimentally.
PDF
