Increase In Internal Stress Research Articles

Anomalous temperature-dependent strength of copper alloy manufactured by laser-beam powder bed fusion

This study reports an anomalous temperature-dependent tensile behavior of laser-beam powder bed fusion (PBF-LB) processed Cu–Cr–Zr alloy. The yield strength of the alloy initially decreases as the temperature increases to 200±5 MPa and then increases to 350±11 MPa at 500°C before reducing to 234±6 MPa at 600°C. The microstructure consists of elongated Cu grains with a high concentration of Cr solute (∼1 mass%), resulting from rapid solidification during the PBF-LB process. Transmission electron microscopy for the specimens deformed at 500°C revealed the presence of numerous nanoscale Cr-rich particles embedded inside the supersaturated solid solution of the Cu matrix. Nanoscale particles can act as barriers to dislocation motion, leading to an increase in internal stress during plastic deformation at elevated temperatures. This work provides the high potential of post heat treatments for achieving superior mechanical performance using high solute supersaturation formed by the PBF-LB process.

Influence of Long Milling Time on the Electrical Resistivity of Nanocrystalline Ni2MnSn Heusler Alloy Obtained by Mechanosynthesis.

A Ni2MnSn Heusler alloy was obtained as a single B2 phase after 12 h of mechanical milling. The influence of prolonged milling on the phase stability was analysed for milling times up to 50 h, related to mean crystallite size, lattice strain, and electrical resistivity. The nature of the powders in the milled range was found to be nanocrystalline, with a mean crystallite size of about 33 ± 2 nm. An evaluation of the internal stresses induced by milling was performed, a linear behaviour was found, and a coefficient of the internal stress increase with milling time was proposed. Particle size distributions of milled samples were analysed, and the morphology of the powders was visualised by scanning electron microscopy. The elemental distribution of milled samples was quantified by energy-dispersive X-ray spectroscopy. Electrical resistivity measurements were performed on compacted samples, and their behaviour with milling time was analysed.

Internal stress and electrical properties of the CBT-BiT ceramics with intergrowth structure

Intergrowth ceramic series (1-x) CaBi4Ti4O15 - x Bi4Ti3O12, x = 0, 0.2, 0.4, 0.6, 0.8, 1, were prepared by traditional solid-state reaction process. (1-x) CBT – x BiT ceramics were investigated systematically in the evolution of structural and electrical properties, especial focus on the role of conductivity and spontaneous polarization by the construction of intergrowth structure. On the one hand, an increase in internal stress of bismuth layers (Bi2O2)2+ in CBT-BiT intergrowth structure with a long-range disorder array, inducing a raising in spontaneous polarization. On the other hand, a decrease in the aspect ratio (length/thickness) of grains and the conductivity for CBT-BiT ceramics by intergrowth structured construction, which is beneficial for domain deflection and piezoelectricity. Moreover, the relaxation mechanism of all ceramics is dominated by the dipole conduction mechanism containing oxygen vacancies. The optimal piezoelectric performance for (1-x) CBT – x BiT ceramics was obtained with x = 0.8 (d33 ∼ 19 pC/N), exhibiting higher Curie temperature of 696 °C.

The effect of SiC on the growth habit of Fe-Ni-C system Ib gem grade diamond crystals

This article investigates the influence of different silicon carbide contents on the morphology and quality of diamond single crystals synthesized in the Fe-Ni-C-Si system under high temperature and pressure environments. It was found that silicon successfully entered the diamond lattice through bonding. As the doping concentration increases, the growth rate of diamond crystals decreases and defects appear on the crystal surface. The increase in internal stress in the lattice reduces the quality of diamond, as evidenced by the shift in Raman peak and half peak width of the diamond. Moreover, the infrared characterization results showed that the nitrogen element in the crystal exists as a single atom (C-center) in the diamond crystal, and the nitrogen content decreases with the increase of SiC content. This work provides insights for understanding the growth mechanism of natural diamonds and enriching the application of silicon doped diamond single crystals.

Understanding environmental stress cracking in the Bell Test at the notch region

AbstractThe Bell Test (ASTM‐D1693) is commonly used to accelerate part failure in order to evaluate resin selection for applications like piping and tanks where environmental stress cracking is possible, even though the method tends to offer less than desirable repeatability. The working hypothesis for this new study with polyethylenes was that the variability seen in failure times may be attributed to a higher sensitivity of the phase morphology primarily at or near the surfaces of a notch than more commonly referenced bulk descriptors of a polymer, affecting stresses and the localized absorption of the preferred stress cracking agent. Using an organic dye tracer to monitor penetration of stress cracking agent into the polymer along with image analysis for the notch dimensions and a load cell, the local aspects of the mechanism were evaluated for up to four grades of polyethylene of varying crystallinity. By this detailed analysis, it was revealed that for failure to occur by ESC, influences in the polymer by absorption of the stress cracking agent preceded the changes in stresses in the specimen, contrary to original expectation. There was an imminent rate increase in both stresses and notch expansion once absorption of the surfactant reached a critical concentration. This critical nature was shown to be dependent on the crystallinity of the polymer under observation, considering the size/arrangement of crystals present and entanglements formed through the amorphous contents to dictate the rate by which IGEPAL can be absorbed into a polyethylene specimen. The results of this study validate the need for greater focus on polymer properties at the notch rather than the bulk of the specimen, though more techniques for providing analysis of these properties at the notch surfaces are needed for future improvements to the mechanism.Highlights Bromocresol dye acts as a novel visualization aid in tracking IGEPAL CO‐630 sorption Crystallinity was related to a plasticization threshold for absorption of IGEPAL Accelerated increase in internal stresses follow absorption. Variability of the Bell Test was related to crystalline properties at the notch

Temperature gradient enhances the solidification process and properties of a CoCrFeNi high-entropy alloy: Atomic insights from molecular dynamics simulations

Material properties are substantially affected by the process during fabrication. To what extent for high-entropy alloys (HEAs), however, is still an open question. Herein, we investigated the effect of a temperature gradient on the solidification of a CoCrFeNi HEA using molecular dynamic simulations. The nucleation and crystal growth under gradient temperatures significantly differ from those under homogeneous temperatures. The HEA solidified by a temperature gradient forms a single and uniform face-centered cubic (FCC) crystalline phase, and the residual stresses in the solidified tissue are optimized. During the homogeneous solidification process, in addition to the FCC phase, the hexagonal close-packed (HCP) phase and a small amount of body-centered cubic (BCC) phase were also formed. When the temperature gradient is 100 K, the stress distribution in the solidification microstructure is relatively low. Increasing the temperature gradient can enhance the crystallinity of the solidification microstructure. While, an increase in cooling rate will lead to a reduction in the crystallinity of the solidification microstructure and an increase in internal stresses within the solidification microstructure. A slight short-range order (SRO) phenomenon present in both solidified structures. Our atomistic insights might be helpful in the fundamental understanding and material design of HEAs.

Creep deformation and constitutive modelling of 316LN SS with varying nitrogen content

In this investigation, two different constitutive models were employed to examine the creep deformation and damage behaviour of nuclear grade austenitic 316LN SS with four different nitrogen contents (0.07–0.22 wt%) at 923 K. Creep deformation resistance of the alloys investigated by “internal-stress based kinetic creep law” for primary and secondary creep stages revealed an increase in internal stress and decrease in obstacle spacing accompanied with lower dynamic recovery with increasing nitrogen. The aforementioned outcomes substantiated the experimentally observed beneficial role of nitrogen on lowering of creep strain rate with increasing nitrogen level. In contrast to the above observations, the analysis of coupled creep deformation and damage using the Kachanov-Rabotnov model and the subsequent development of iso-damage contours indicated evolution of higher creep damage at lower strains with increasing nitrogen content. With increasing % nitrogen, damage rate dominated over the strain rate thus justifying the observed significant intergranular damage at and above 0.14% N. Based on the above results and in-house studies on low cycle fatigue and creep–fatigue interaction, optimum nitrogen content is suggested between 0.07 and 0.14% N in 316LN SS that combines the benefits of low nitrogen content and strengthening effects of high nitrogen content.

Development of prediction models for evaluation of alkali-silica reaction in concrete

Alkali-silica reaction (ASR) and expansion induced are one of the major concerns over durability of concrete structures. It causes an increase in internal stresses and hence deteriorating mechanical properties, reducing load capacity, and compromising structures’ serviceability. Factors governing ASR include water-to-cement ratio, alkali content, the content of reactive aggregate, exposure temperature, relative humidity, etc. Previous prediction models for ASR expansion focus on the measurements under particular conditions and these models are suffering from the neglect of effects of some factors. This study aims to propose a back-propagation neural network (BPNN) model to predict ASR by considering both mix compositions and environmental conditions. In this paper, by adopting two different training functions, namely Levenberg-Marquardt function (LM) and Bayesian regularisation backpropagation (BR), two models of LM-BPNN and BR-BPNN were developed and proposed. To achieve this, a database containing 168 pairs of data from 25 studies was established, its data includes water-to-cement ratio, cement content, alkali fraction, content of non-reactive and reactive fine/coarse aggregate, temperature, and relative humidity. Results indicate that both LM-BPNN and BR-BPNN models can produce reasonable estimates for ASR expansion at 180 days with MSE and R of 0.00209%, 0.002753%, 0.926 and 0.912, respectively. Results reveal that fraction of reactive fine/coarse aggregate is dominant on ASR expansion while alkali content and humidity have a significant correlation with expansion. Water-to-cement ratio, cement content, and content of non-reactive fine/coarse aggregate have low impact on ASR expansion based on the results.

Using the method of calculating the total molar energy of cohesion to choose the most effective plasticizer

It is known that products made of polar, crystallized, and high-strength polymers (polyamides, polyesters, cellulose triacetate, polyvinyl chloride, etc.) lose their initial properties that reduce reliability and durability in the process of operation and even during long-term storage due to the increase in internal stresses in them caused by crystallization processes over time. To reduce the impact of such processes on the properties of polymer products, special substances are introduced into their composition, plasticizers, the selection of which is usually carried out empirically, which is not always successful. We proposed in this study to use the ratio of the calculated total molar volumetric energy of cohesion of the compound (the cumulative interaction of interatomic and intermolecular forces in the composition of the compound) to its molecular weight as the specific mass total energy of cohesion, which can be compared with a similar indicator for a plasticizing polymer. The use of such an approach makes it possible to purposefully, even before experimental verification, evaluate the suitability of using this or that compound, oligomer, or polymer as a plasticizer (or modifier), or develop a molecular structure of a new, necessary compound with the inclusion of certain groups in its composition for the purpose of further use as a plasticizer or modifier. In addition, there is an opportunity to monitor the influence of individual structural components of a new compound, oligomer or polymer, its molecular weight, side substituents and their nature on the value of the specific mass total energy of cohesion, as well as to reduce the time for performing research works in this direction and expenses for their implementation.

Phase transformation and electrochemical properties of La0.6R0.15Mg0.25Ni3.5 (R = La, Pr, Nd, Gd) alloys with multiphase structure

As a promising Ni-MH battery electrode material, the La-Mg-Ni-based superlattice hydrogen storage alloys have attracted extensive attention due to their stable crystal structure, high energy density and good electrochemical properties. However, most of the existing studies are based on annealed alloys with simple phase compositions. In the present work, the effects of rare-earth elements and phase composition on the electrochemical properties and structure stability of a series of as-cast La0.6R0.15Mg0.25Ni3.5 (R = La, Pr, Nd, Gd) alloys are studied. The alloys contain (La, Mg)2Ni7, (La, Mg)5Ni19 and (La, Mg)Ni3 superlattice phases as well as LaNi5 and LaMgNi4 non-superlattice phases. As the atomic radius of the replacement element R decreases, the cell parameters of each phase in the alloys from large to small are: La> Pr> Nd> Gd. The activation cycle of alloys is one charge/discharge cycle, which demonstrate excellent activation property due to their multi-phase structure that offers many pathways and tunnels for the transfer of hydrogen atoms. Moreover, it is found that Pr enhances the cycling stability and structural stability as well as reduces the oxidation degree of the alloys owing to its function on suppressing LaNi5 non-superlattice phase which would cause an increase in internal stress and structure damage due to the continuous expansion and contraction of superlattice phase cell volume with the process of charge/discharge. Furthermore, the addition of Nd can accelerate the diffusion of hydrogen and increase the reaction speed of charge transfer at the electrode/electrolyte interface, resulting in an enhanced rate discharge performance.

Изменение прочности армированных пластиков при длительной эксплуатации в открытых климатических условиях

Structural elements made of polymer composite materials (PCM) during operation in climatic conditions lose their strength and ability for trouble-free safe operation. It is shown that the deterioration of the strength characteristics of PCM in arctic conditions is caused by daily and seasonal thermal and moisture cycles, which cause an increase in internal stresses and the formation of microcracks in polymer matrices. To model changes in the strength of reinforced plastics, information is needed on the emerging gradient of indicators over the thickness of the samples.

Influences of surface state on thermal stability and magnetic properties of Fe-Si-B-C amorphous alloy

Influences of surface state on thermal stability and magnetic properties for Fe82Si1B12C5 completely amorphous alloys and Fe83Si1B12C4 amorphous alloys with partial crystallization at their surface have been investigated by layer polished-removal method. The crystallization enthalpy of α-Fe phase for the two amorphous alloys decreases obviously with polishing removal of surface layer with the thickness of removal, due to the alteration of internal stress state. An increase of only 1.2% in Fe content causes an increase of 8.4% in crystallization enthalpy of α-Fe phase for the alloy system with an initial casting thickness, indicating that the thermal stability is stress-sensitive for Fe-based amorphous alloys with high Fe content close to amorphous forming boundary are easier to crystallize. It was found that in the as-quenched state the coercivity for both of them increases linearly with the thickness of removal, which is caused by an increase in internal stress. And it was confirmed by experiment that adequate annealing can greatly improve the soft magnetic properties and alter thermal stability.

Effect of GTAW on the Tensile Strength and Hardness of Mild Steel

Gas tungsten metal arc welding (GTAW) is used to study the effect of the base metal thickness, welding current and welding speed on the tensile strength and hardness of mild steel welding. The analysis found that base metal thickness had the highest effect and highest means of tensile strength and hardness of the welding. Taguchi’s design (TD) suggested using higher base metal thickness, lower welding current and higher welding speed when welding mild steel in order to obtain maximum tensile strength and hardness. The welding that has higher tensile strength showed higher hardness. However, the hardness increased proportionally with the increased internal stresses of the welding. The welding showed wider heat affected zone (HAZ) with the increase in internal stresses of the welding.

Evaluation of Internal Stresses in Modified Near-Surface Layer of 20Kh13 Steel during Incubation Period of Water Droplet Impact Erosion

This study investigates the change in internal stresses of ion nitrided and Cr–CrC ion-plasma-coated 20Kh13 steel samples during the incubation period of the droplet impact erosion. To obtain internal stress values, a method based on measuring the deformation (curvature) of the sample along the surface profiles before and after its modification/coating, as well as during the subsequent droplet impact has been applied. The results suggest that the initial droplet impact on the uncoated, nitrided and coated material leads to a sharp increase in the initial internal stress values. The rate of increase in internal stress decreases with the surface damage development and accumulation. The proposed method of stress evaluation can be used in selecting a criterion for comparing the erosion resistance of a material and a method for modifying its surface.

The location of adsorbed water in pigmented epoxy-amine coatings

Pigmentation can exacerbate problematic water uptake into protective paints, leading to detrimental effects such delamination, swelling and plasticization. Potential underlying mechanisms include poor adhesion between the pigment and binder, the formation of hygroscopic interphase regions close to the pigment particles, and internal stress. Here, we demonstrate the use of nanoscale chemical mapping using AFM-IR and nanothermal analysis to investigate the contribution of these mechanisms directly for model coatings based on diglycidyl ether of bisphenol-A (DGEBA) cross-linked with triethylenetetramine (TETA). Whilst moisture uptake increases in the presence of a microscale silica matting agent, no chemical interphase could be detected close to the wax-coated particles. Instead, nanothermal analysis reveals attenuated thermal expansion of the polymeric binder and raised Tg values, supporting favourable polymer-pigment interaction and an increase in internal stress in the polymeric network. This is confirmed by mapping water distributions directly using AFM-IR, where no accumulation at the intact pigment-polymer interface/interphase is detected. Finally, further evidence is provided by bulk thermal analysis, where increased plasticization and desorption enthalpies demonstrate that water sorption into the bulk polymer binder increases as a function of added pigment volume concentrations.

EVALUATION OF THE PROBABILITY OF CRACKING OF PAINT AND VARNISH COATINGS UNDER THE INFLUENCE OF CLIMATIC FACTORS

Information is given on the probability of cracking of protective and decorative coatings of cement concrete during operation. The aging process is considered as a random process of changing the cohesive strength of coatings, while the mathematical expectation and standard deviation are presented as a function of some time indicator. The probability of crack formation of coatings in the process of thermal aging and the action of ultraviolet irradiation is estimated. It was found that during the aging process, an exponential decrease in the cohesive strength of coatings and an increase in internal stresses are observed. There is a tendency to increase the probability of coating cracking. Recommendations are given for the selection of paint formulations, coatings based on which are characterized by increased crack resistance.

RNA-seq analysis of green tea polyphenols modulation of differently expressed genes in Enterococcus faecalis under low pH.

Enterococcus faecasslis (E. faecalis) is a resident bacterium in the host. The increase in internal stress like low pH may affect the biological effects of E. faecalis. The prebiotic-like function of tea polyphenols can enhance the beneficial effects of its tolerance to environmental stress. In this study, RNA-sequence analysis was used to explore the protective effect of green tea polyphenols (GTP) on E. faecalis under low pH stress. A total of 28 genes were found to be responsive to GTP under low pH stress, including 16 up-regulated and 12 down-regulated. GTP intervention can partly relieve some undesired negative influences, such as the down-regulation of the base excision repair gene and amino acid transport and metabolism gene. The significantly changes were associated with selenocompound metabolism and aminoacyl-tRNA biosynthesis after the intervention of GTP. The present study provided new insights into the growth and continuous adaptation of E. faecalis under stress.

Correlation of plastic deformation induced intermittent electromagnetic radiation characteristics with mechanical properties of Cu–Ni alloys

Abstract This paper presents experimental results on intermittent electromagnetic radiation during plastic deformation of Cu – Ni alloys under tension and compression modes of deformation. On the basis of the nature of electromagnetic radiation signals, oscillatory or exponential, results show that the compression increases the viscous coefficient of Cu – Ni alloys during plastic deformation. Increasing the percentage of solute atoms in Cu – Ni alloys makes electromagnetic radiation strength higher under tension. The electromagnetic radiation emission occurs at smaller strains under compression showing early onset of plastic deformation. This is attributed to the role of high core region tensile residual stresses in the rolled Cu – Ni alloy specimens in accordance with the Bauschinger effect. The distance between the apexes of the dead metal cones during compression plays a significant role in electromagnetic radiation parameters. The dissociation of edge dislocations into partials and increase in internal stresses with increase in solute percentage in Cu – Ni alloys under compression considerably influences the electromagnetic radiation frequency.

Estimation of the Probability of Cracking of Facade Coatings

Information on the stress state of protective and decorative coatings during the curing process, in particular on the cohesive state of destruction, is given. The influence of the type of substrate on the change in internal stresses in the coating is considered. It was revealed that the greatest value of shear stresses is observed in coatings on a heavyweight concrete substrate. The subsequent increase in temperature after curing to 50°C leads to an increase in the value of the normal stresses. The probability of cracking of coatings during thermal aging is estimated. It was revealed that during aging there is an exponential decrease in the cohesive strength of coatings and an increase in internal stresses. Aging tends to increase the likelihood of cracking of coatings. The change in stresses in coatings as a result of seasonal fluctuations in air temperature is considered.

Application of analytical model in the prediction of dynamic responses and fatigue damage of flexible risers: Part II – Dynamic analysis of flexible risers in large-scale domain using a direct moment correction method

In recent years, the dynamic responses of flexible risers have been the focus of many researchers. Most flexible risers undergo a substantial level of irregular motion from environmental loadings, which involves a continuous slip of helical wires. The slip of helical wires especially leads to a hysteretic effect by reducing the bending stiffness, making it hard to predict the dynamic responses of flexible risers. The current study, as an extension to Part I, presents a new large-scale dynamic analysis method for flexible risers. The suggested method creates a large-scale model for the dynamic analysis that considers a geometric and bending nonlinearity of flexible risers. The kinematics of each beam element is formulated based on a Green-Lagrangian strain and the interaction with the seabed, providing a realistic analysis of flexible risers. In particular, the current study introduces a direct moment correction method that modifies the internal force vector using an improved analytical model. The improved analytical model is assigned at each node of the large-scale model and estimates an accurate bending hysteresis curve considering the effect of shear deformation and varying tension. The suggested method corrects the bending moment and shear force of all beam elements based on the bending hysteresis curves obtained from the improved analytical model, by which a complex bending behavior of flexible risers is reflected in a large-scale domain. As a result, this study achieves a more accurate prediction of the dynamic responses and fatigue damage of flexible risers. A new dynamic analysis program, called OPFLEX, is developed herein based on the suggested analysis method. Using the developed program, the current study conducts several numerical investigations to identify the effect of the shear deformation and varying tension. Consequently, it is confirmed that the shear deformation of internal layers reduces the fatigue damage of helical wires by delaying the increase of internal stress. It is also identified that the effect of varying tension deteriorates the fatigue life of flexible risers through a continuous change of contact pressure during bending.