Effect of the Stress State on the Adhesive Strength of an Epoxy-Bonded Assembly

The paper studies the adhesive strength of aluminum alloy specimens bonded with the use of an epoxy adhesive, under the tensile-shear stress state, depending on the testing temperature. Tension of modified Arcan specimens with load angles of 0, 22.5, 45, 67.5, and 90° with respect to the plane of adhesion is chosen as the experimental method. Experiments were performed at temperatures of −50, +23, and +50 °С. The analysis of the obtained results yields a linear fracture criterion and a fracture locus for the adhesive failure strain energy density, which takes into account the ratio of the elastic properties of the adhesive to those of the substrate. The region bounded by the fracture loci of adhesive strength and ultimate strain energy density determines the conditions for the safe loading of the bonded assembly in terms of the energy and force criteria of adhesive failure. The proposed fracture loci can be used, preferably simultaneously, to estimate the in-service strength and reliability of adhesively bonded assemblies.

Role of Grain Size and Shape in Superplasticity of Metals

Superplasticity is characterized by an elongation to failure of >300% and a measured strain rate sensitivity (SRS), close to 0.5. The superplastic flow is controlled by diffusion processes; it requires the testing temperature of 0.5Tm or greater where Tm is the absolute melting temperature of metals. It is well established that a reduction in grain size improves the optimum superplastic response by lowering the deformation temperature and/or raising the strain rate. The low-temperature superplasticity (LTSP) is attractive for commercial superplastic forming, in view of lowering energy requirement, increasing life for conventional or cheaper forming dies, improving the surface quality of structural components, inhibiting quick grain growth and solute-loss from the surface layers, thus resulting in better post-forming mechanical properties. This paper will summarize the dependence of superplasticity on grain size and shape in various metallic materials, including ferrous and non-ferrous alloys, which has been considered as an effective strategy to enable the LTSP.

Reliability of one-shot device with generalized gamma lifetime under cyclic accelerated life-test

One-shot devices are products or equipments that can be used only once. A nature characteristic of one-shot devices is that they get destroyed immediately after their use, and therefore their actual lifetimes are never observable. The only information observed is the condition whether they worked or not at the time they are used. These days the quality of products are significantly improved, so that the information obtained under a normal test during a short time is quite limited. A typical test to induce more failures is the accelerated life-test, which is developed by increasing the stress levels under test. In this paper, we will investigate the reliability of one-shot devices with generalized gamma fatigue life under accelerated life-tests with various cyclic temperature fluctuations by assuming a Norris-Landzberg model. Generalized gamma involves many common lifetime distributions, such as gamma, Weibull, lognormal, and positive stable distributions, as special cases. Norris-Landzberg model takes not only temperature change, highest testing temperature, but also the cycling frequency into account when modeling the number of cycles-to-failure, resulting a generalized model with the well-known Coffin-Manson model and Coffin-Manson-Arrhenius model as special cases. Associated inferences are developed. The performance of the proposed model and inferential methods will be evaluated with simulation study and model discrimination. Finally, the chip-scale package solder joints data is analyzed to illustrate the considered model and inferential methods developed in this paper.

Impact of Compaction Mode on Strength Properties of Sustainable Asphalt Concrete

Various modes of compacting the asphalt concrete mixture can createmechanically different behaviour of the prepared specimens and can alterits sustainability. An attempt has been made in the present assessment toprepare asphalt concrete specimens by implementation of three modesof compaction, the gyratory, the roller, and the Marshall hammer. Thespecimens were prepared at the target bulk density of Marshall methodat optimum asphalt content. Extra specimens were prepared at 0.5 %asphalt below and above the optimum. Core specimens have been obtainedfrom the roller compacted slab samples. The specimens were tested forthe Marshall stiffness, tensile, and shear strength. It was observed that atoptimum asphalt content, the indirect tensile strength declines by (18.8and 70.5) % for gyratory and roller compacted specimens respectively ascompared with hammer compacted specimens. At optimum asphalt content,the shear strength declines by (70.5 and 82.2) % while Marshall stiffnessdeclines by (10.2 and 44.8) % for hammer and roller compacted specimensas compared with that of gyratory compacted specimen. Specimensprepared by gyratory compaction are less susceptible to the change inthe testing temperature as compared with other modes of compaction. Itis recommended to consider the mode of compaction to suit the requireddesign property of sustainable asphalt concrete mixture.

Improvement of the Thermocouple Method for Testing Energy-Intensive Emulsions for Compatibility with the Sulfide Ores

The results are presented concerning improving the thermostatic method for studying the chemical compatibility of modern industrial emulsion explosives based on the ammonium nitrate with surrounding materials, the increased reactivity of which can lead to spontaneous ignition and even explosion. An assessment of the compatibility of emulsion explosives with sulphide ores was conducted using an original thermocouple methodology developed at the D. Mendeleyev University of Chemical Technology of Russia, fixation of the thermal effects of the interaction of shell-free explosives based on the ammonium nitrate with sulfide minerals. Improved thermocouple method allows to determine chemical compatibility of the industrial explosives with the reactive rocks. It is distinguished by the possibility of continuous recording of the thermocouple measurements during tests using an oscilloscope and combines the reliability of the results with small laboratory weights of the test samples (no more than 20 g, which ensures safety testing). Temperature measurement accuracy is ± 2 °С. It is concluded that the method used is able to identify the cases of the most dangerous interaction from the practice point of view using the emulsion explosives when the pyrite content in the ore exceeds 85 %. The results of experiments on the applicability of thermocouple measurements to testing low-activity rocks, highly reactive pyrite ores, mixed sulfide ores of medium activity, as well as on the identification of the main regularities of heat release during the interaction of emulsion explosives with the sulfide ores are considered.

Chemical Strain of Graphite-Based Anode during Lithiation and Delithiation at Various Temperatures

Electrochemical lithiation/delithiation of electrodes induces chemical strain cycling that causes fatigue and other harmful influences on lithium-ion batteries. In this work, a homemade in situ measurement device was used to characterize simultaneously chemical strain and nominal state of charge, especially residual chemical strain and residual nominal state of charge, in graphite-based electrodes at various temperatures. The measurements indicate that raising the testing temperature from 20°C to 60°C decreases the chemical strain at the same nominal state of charge during cycling, while residual chemical strain and residual nominal state of charge increase with the increase of temperature. Furthermore, a novel electrochemical-mechanical model is developed to evaluate quantitatively the chemical strain caused by a solid electrolyte interface (SEI) and the partial molar volume of Li in the SEI at different temperatures. The present study will definitely stimulate future investigations on the electro-chemo-mechanics coupling behaviors in lithium-ion batteries.

Experimental and numerical investigation of the thermomechanical load on a turbine housing in a radial turbocharger

The fatigue life of turbine housing is an important index to measure the reliability of a radial turbocharger. The increase in turbine inlet temperatures in the last few years has resulted in a decrease in the fatigue life of turbine housing. A simulation method and experimental verification are required to predict the life of a turbine housing in the early design and development process precisely. The temperature field distribution of the turbine housing is calculated using the steady-state bidirectional coupled conjugate heat transfer method. Next, the temperature field results are considered as the boundary for calculating the turbine housing temperature and thermomechanical strain, and then, the thermomechanical strain of the turbine housing is determined. Infrared and digital image correlations are used to measure the turbine housing surface temperature and total thermomechanical strain. Compared to the numerical solution, the maximum temperature RMS (Root Mean Square) error of the monitoring point in the monitoring area is only 3.5%; the maximum strain RMS error reached 11%. Experimental results of temperature field test and strain measurement test show that the testing temperature and total strain results are approximately equal to the solution of the numerical simulation. Based on the comparison between the numerical calculation and experimental results, the numerical simulation and test results were found to be in good agreement. The experimental and simulation results of this method can be used as the temperature and strain (stress) boundaries for subsequent thermomechanical fatigue (TMF) simulation analysis of the turbine housing.

Strength characteristics of frozen coal–rock interface for rock crosscut coal uncovering

During a freezing method for rock crosscut coal uncovering (RCCU), the mechanical properties of the frozen coal–rock interface have a significant impact on coal-body stability. To investigate characteristic and development mechanism of freezing strength of frozen coal–rock interface, a series of direct shear tests were conducted on frozen coal–rock interface under various testing temperatures, moisture contents in coal and normal stresses. The test results showed that the strength of the frozen coal–rock interface was affected by the moisture content in coal. The larger the moisture content was, the greater strength of the interface was. When the testing temperature was −10°C, the freezing strength increased from 75.46 to 267.42 kPa with the moisture content increasing from 3% to 9%. The ice cementing strength at the interface also increased with testing temperature decreasing. It increased from 6.44 to 73.34 kPa with the testing temperature decreasing from −2°C to −10°C when the moisture content was 5% and the normal stress was 200 kPa. With the increase of normal stress, the residual strength of the frozen coal–rock interface increased. When the moisture content in coal was 9% and the testing temperature was −10°C, the residual strength of the interface increased from 40.68 to 132.28 kPa with the normal stress increasing from 100 to 400 kPa. The testing temperature had no obvious influence on the friction coefficient and the cohesion of residual strength. When the moisture content in coal was 5%, the cohesion of residual strength increased from 23.39 to 98.7 kPa and the friction coefficient of residual strength fluctuated between 0.49 and 0.63 with the testing temperature decreasing from −2°C to −10°C. The relationship between the shear strength and the normal stress followed the Mohr–Coulomb law.

Testing Cyclic Fatigue Resistance of Nickel Titanium Rotary Endodontic Instruments: A Validation Study for a Minimum Quality Criterion in a Standardized Environment

Introduction: Cyclic fatigue resistance of rotary endodontic instruments has been extensively studied in the last two decades, yet with no standardization. While new low-cost instruments are now manufactured, a standard is lacking to guarantee a minimum quality. This study aimed to validate a new model for CF testing through a fixture proposed for ISO Specification 3630-1 and to establish a minimum quality standard based on testing conditions and material characteristics.Materials and methods: Size 25/0.06 instruments of ProFile Vortex (PF) and Vortex Blue (VB) were run until failure in curved metallic fixtures (5 or 7.5 mm radius) built according to a proposal for an additional test for the ISO 3630-1 standard. High resolution videos were recorded, number of cycles to failure (NCF) registered and apical fragments measured with a digital caliper. Surface strain was determined from test dimensions and fragment lengths. Mean life, β and η parameters were calculated with Weibull analysis. NCF data were compared using student's t-tests and referenced to a minimum required cycles at fracture (Cmin) based on austenite finish temperatures, testing temperature and deformation.Results: VB instruments were statistically more resistant than PF in both 7.5 mm radius curve (p = 0.001) and 5 mm radius curve (p = 0.002) simulated canals. Weibull probability plots showed higher mean life for VB than PV. NCF for both instruments were higher than Cmin.Conclusions: The NCF results in this study support the findings of previous studies where VB and PF were compared. The novel test design appears a suitable addition to ISO 3630-1.

A Comparison between Static and Repeated Load Test to Predict Asphalt Concrete Rut Depth

Rutting has a significant impact on the pavements' performance. Rutting depth is often used as a parameter to assess the quality of pavements. The Asphalt Institute (AI) design method prescribes a maximum allowable rutting depth of 13mm, whereas the AASHTO design method stipulates a critical serviceability index of 2.5 which is equivalent to an average rutting depth of 15mm. In this research, static and repeated compression tests were performed to evaluate the permanent strain based on (1) the relationship between mix properties (asphalt content and type), and (2) testing temperature. The results indicated that the accumulated plastic strain was higher during the repeated load test than that during the static load tests. Notably, temperature played a major role. The power-law model was used to describe the relationship between the accumulated permanent strain and the number of load repetitions. Furthermore, graphical analysis was performed using VESYS 5W to predict the rut depth for the asphalt concrete layer. The α and µ parameters affected the predicted rut depth significantly. The results show a substantial difference between the two tests, indicating that the repeated load test is more adequate, useful, and accurate when compared with the static load test for the evaluation of the rut depth.