Structural Stress Concentration Research Articles

Dislocation interaction and fracture of Cu/Ta bilayer interfaces

In this article, the mechanical properties and dynamic responses of Cu/Ta nanofilms with a hole defect under tension process were investigated using molecular dynamics simulation. The effects of temperature, the diameter of hole defect, strain rate and growing orientation were evaluated in terms of stress–strain relationship, dislocation mechanism, structural phase transformations, interface response, and local stress concentration. The results show that the phase transformations from fcc into HCP structures, and 〈112〉, 〈110〉 dislocations were found in Cu sections. The tensile strength reduced with the increasing temperature and increasing the diameter of the hole defect. In contrast, the tensile strength increased under the strain rate increased. The greater stress concentration factor at the higher temperature, however, the smaller stress concentration factor with the larger diameter of hole defect. The void growth expanded stronger at smaller hole defect. Between the nanofilms with different growing orientations, the Cu [100]/Ta [111] nanofilms exhibited the most excellent mechanical characteristics.

Low-cycle fatigue of welded structures made from domestic and imported materials

In various branches of modern engineering, corrosion-resistant steels and titanium alloys are widely used as structural materials. At the same time, it is possible to connect parts made from domestic and imported alloys using automatic argon-arc electric welding, which leads to the formation of a material with unexplored properties in the weld. Welded joints are stress concentrators and currently there is no information about low-cycle fatigue of welded joints obtained by fusing domestic and imported materials. In the course of the research, the modes of welding and heat treatment of butt welded joints obtained from sheet titanium alloys and corrosion-resistant steel of domestic and foreign production have been developed. Resource tests for low-cycle fatigue of samples of welded joints were carried out. Tests on low-cycle fatigue were carried out on the upgraded testing machine UMM-10 with repeated static stretching with an asymmetry factor of +0.1 and at a frequency of 0.6–0.8 Hz. The maximum tensile stress was 80% of the temporary tensile strength of the weakest alloy in the pair. The main stress axis from external loading in all cases was perpendicular to the weld. The tests were carried out until the destruction of the sample. As a result of research, it was established that all welded joints were destroyed along the fusion line, which is explained by the simultaneous action of geometric and structural stress concentrators. In this case, the destruction of the samples, as a rule, began near the seam from the side of the weakest alloy in the pair. It was also established that the use of temperatures of incomplete annealing in comparison with the full one allows to increase the cyclic durability for welded joints of titanium alloys by 1.3–2 times. From the results of comparative tests of samples of corrosion-resistant steels, it follows that domestic and imported steels, as well as their welded joints, have similar properties, both in strength and in re-static durability.

Computational analysis of thermally induced stress concentration in structures with geometric constraints

Fracture and fatigue life of an engineering component is highly sensitive to stress concentrations that can arise at sharp corners. Stress concentration can be caused by impact, pressure, and high temperatures, among other factors. High operating temperatures are common in internal combustion engines, batteries, computers, and additive manufacturing. In this work, we aim to provide systematic notch design guidelines for thin metallic plates subjected to strict geometrical boundary conditions under high temperatures. Computational analysis is used to investigate how geometrical configurations, near critical areas, affect the magnitude of thermally-induced stress and strain under the influence of uniform heating conditions. Results show that tuning the curvature of sharp corners can allow for the mitigation of potentially destructive thermally-induced stresses. Additionally, this study shows that the use of functionally graded materials (with varying values of coefficient of thermal expansion and modulus) can redistribute thermally-induced stresses, improving the structural performance of metallic materials in the presence of high temperature environments.

Thermal plasticity deformation buckling analysis on the carbon fiber-reinforced aluminum matrix composite material

As light weight and durability have become the important design objectives for the modern aircraft structures, the carbon fiber-reinforced resin matrix composites are widely applied to the aircraft structures for it can reduce the airframe weight effectively at the same time of guaranteeing the load-carrying capacity. In aircraft structures, the mechanical connection remains the important connection pattern for the composite material and the laminated structures; however, existence of the connecting hole has resulted in the stress concentration of structures while the borehole fatigue breakdown of parts is the main failure mode of aircraft structures. According to the low fatigue life of the aircraft connection structures, research on the hole extrusion and reinforcement technology on the carbon fiber-reinforced composite material and the laminated structure is made in this paper.

New proposal to express notch stress approach results by equivalent SCFs

For the fatigue design of welded structures, such as jacket substructures for offshore wind energy turbines, the structural stress approach is state of the art. Standards and guidelines also allow the application of the notch stress approach for complex welded components, weld roots, or to consider the advantages of the real weld geometry if measured or after grinding within the fatigue design. Since the theoretical background as well as the application of both approaches differs widely, it is not possible to directly integrate the fatigue notch factor Kf according to the notch stress approach into the design formalism of the welded structures based on the structural stress approach. Therefore, this paper derives a methodology to transform the elastic fatigue notch factor Kf into an equivalent stress concentration factor. By applying the introduced concept design engineers are able to selectively combine the advantages of both approaches without adapting the approved software tools to perform the fatigue design of complete welded structures.Furthermore, the presented concept of the equivalent stress concentration factor is an effective tool to perform a direct comparison of the relative fatigue resistance according to both approaches – structural stress approach and notch stress approach – on structural stress concentration level rather than on the level of damages or lifetime. This opportunity of comparison is applied on a tubular X-joint of a jacket substructure. The computed stress concentration factors are in good accordance between both approaches which is similar to the lifetime based results given in literature for application of both approaches on tubular K- and Y-joints.

Analysis of stress intensity factors obtained with the fem for surface semielliptical cracks in the zones of structural stress concentrators

Проблемними питаннями при оцінюванні живучості конструкції з поверхневими тріщинами є складності обчислення коефіцієнтів інтенсивності напружень (КІН) для тріщин, які розвиваються у зонах концентраторів напружень. Ці труднощі пов’язані з особливостями зміни форми контуру поверхневої тріщини в процесі циклічного навантаження, який залежить від рівня неоднорідності напружено-деформованого стану в області концентраторів. Проведено аналіз існуючих методів визначення коефіцієнтів інтенсивності напружень у зонах концентраторів: методу вагових функцій, чисельного і гібридного методів. Моделювання зварних з’єднань різних типів з тріщинами проводили у рамках представленої моделі як геометричного фактора – підсилення зварного шва, що дозволило методом скінчених елементів оцінити особливості впливу концентратора напружень на зміну КІН по контуру поверхневої тріщини. При цьому розглянуто особливості побудови сітки скінчених елементів, вплив розмірів сітки на отримані розрахункові значення КІН, порівняння отриманих результатів з відомими рішеннями. При циклічних випробуваннях розтягом отримано кінетику форми втомних поверхневих півеліптичних тріщин у зонах досліджуваних концентраторів напружень та, для порівняння, в однорідному полі напружень. Проаналізовано отримані форми тріщин з результатами модельних досліджень.

Failure analysis of a truck diesel engine crankshaft

Abstract A truck engine crankshaft fractured during the service. The fracture occurred on the crankshaft big-end on which the timing-gear and the flywheel flange were coupled and the fracture location was just situated at the assembling gap between them. The short cracks inclined at about 45° to the shaft axis initiated from the surface of the crankshaft journal at the timing-gear side and mainly extended to the timing-gear side, leading to a zigzag cracking morphology on the journal surface. A complicated ratchet or star-shaped pattern of fracture typical of multiple fatigue cracks occurred on the fracture surface. The journal surface was locally induction-hardened. The surface hardness and the effective case depth on the hardened journal at the flywheel flange side corresponded to the specification. At the timing-gear side the surface hardness on the hardened journal was much lower than the specified lower limit and a low hardness region of 0.4 mm occurred on the most-surface of the hardened journal within which the hardness values were lower than the specified lower limit. The low surface hardness on the induction-hardened journal made fatigue resistance of the crankshaft decrease to lead to initiation and propagation of fatigue cracks in the weaker region. The assembling gap at which the fracture occurred was structure stress concentration site of the assembly constituted of the crankshaft, the timing-gear and the flywheel flange, equivalent to the deep notch. The excessive tightening of the timing-gear on the journal surface also contributed for the increasing of stress concentration. The fatigue crack origins were easy to initiate due to large stress concentration.

A Review of Stress Corrosion Cracking of Welded Stainless Steels

This article gives a brief review of scientific researches of high-alloyed chromium-nickel steels stress corrosion cracking (SCC) process of welded joints. Welded joints are one of the most complex objects from the standpoint of the general theory of corrosion fatigue of metals. During their operation a number of problems arise related to the heterogeneity of the material properties, the complexity of the structure, the presence of structural and technological stress concentrators, residual stresses, etc. The characteristic features of corrosion cracking cited in the studies of Russian and foreign scientific schools are noted. The main types of corrosion damage at various sections of the welded joint are examined: corrosion of the weld metal, damage to the zones of thermal influence, knife damage occurring at the boundary between the joint and the base metal. The models of nucleation and development of cracks in the mechanism of Inter-granular corrosion (IGC) were analyzed. The article provides information on the causes of the steel’s propensity to Inter-granular corrosion. The data on the effect of heat treatment on the tendency of the weld metal to Inter-granular corrosion were systematized. The main regularities of corrosion-resistant alloying are noted. Data on the methods for preventing Inter-granular corrosion and reducing the tendency of the weld metal to stress corrosion cracking are given. Methods for estimating the tendency of weld metal to stress corrosion cracking are classified. The analysis of methods for preventing the tendency of welded joints to stress corrosion is given: metallurgical methods related to the effect on the chemical composition of the weld and the structure of the welded joint and technological methods associated with controlling the parameters of the welding and heat treatment regime.

Mechanical Behavior of Metal under Hydrostatic Pressure and Low Temperature

The current work describes the behavior of metal under hydrostatic pressure and low temperature using finite element concept. This study was carried out in three stages: modelling, analysis and post processing. Structural steel and aluminum are tested at 600 bar pressure and 4°C temperature. Solid model of cylindrical shape having internal diameter 420 mm and thickness 25 mm was created and same was investigated for stress analysis using ANSYS 17.2 APDL emphasizing on hydrostatic behavior. Failure criterion is based on maximum stress to determine the onset of damage in material. In post processing phase, the material was checked for deformation and equivalent Von-Mises stresses developed around the material. The objective of this present paper is of two fold. Firstly the investigation of metal under high pressure through stress analysis is carried out followed by metallographic studies. The stress analysis results suggest that metals like steel which has BCC structure shows the remarkable decrease in ductility over the range of temperature. The maximum Von Mises stresses that the material can with stand were 65.14 MPa and 56.38 MPa for structural steel and aluminum respectively and revealed having high factor of safety for desired applications. It can be said that aluminum having FCC crystal structure are stiff and ductile even at low subzero temperature. The theoretical calculations are done and they are in agreement with the experimental results. It can be anticipated that influence of high pressure and low temperature can be criticized by two factors in terms of flow strength and fracture strength of metal. Ductile to Brittle Transition Temperature (DBTT) plays a vital role and can be considered as a measure of behavior of metal under high pressure and low temperature. So, it revels that the crystal structures of metals is the major factor which affects the behavior of metal which eventually affect the ductility and toughness of metal when subjected to high pressure and low temperature operations. However, in thick material the question of structural discontinuity and stress concentration is still regarded as open for further scope of study.

Influence of overlapping pattern of multiple overlapping uncovered stents on the local mechanical environment: A patient-specific parameter study

BackgroundMultiple overlapping uncovered stents (MOUS) system has shown potentials in managing complex aortic aneurysms with side branches involvement. It promotes the development of thrombus by modulating local flow pattern that reduces the wall tension, while maintaining patency of side branches. However the modulation of local hemodynamic parameters depends on various factors that have not been assessed comprehensively. MethodsAneurysm 3D geometry was reconstructed based on CT images. One-way fluid-structure interaction analysis was performed to quantify structural stress concentration in the wall, and changes of blood velocity, wall shear stress (WSS), oscillatory shear index (OSI), relative residence time (RRT) and pressure in the sac due to the stent deployment. ResultsHigh structural stress concentration due to stent deployment was found in the landing zone and it increased linearly with the number of stents deployed. The wall tension in the sac was unaffected by the stent deployment. Stress within the wall was insensitive to the different overlapping pattern. After one stent was deployed, the mean flow velocity in the sac reduced by 36.4%. The deployment of the 2nd stent further reduced the mean sac velocity by 10%. WSS decreased while both OSI and RRT increased after stent deployment, however pressure in the sac remained nearly unchanged. Except for the cases with complete stents struts alignment, different overlapping pattern had little effect on flow parameters. ConclusionsMechanical parameters modulated by the MOUS are insensitive to different overlapping pattern suggesting that endovascular procedure can be performed with less attention to the overlapping pattern.

In Situ Transmission Electron Microscopy Studies on Silicon Anodes in Lithium Ion Batteries

Extensive research for new energy storage materials has created a high demand for experimental techniques that can provide real-time, single-particle-level information on the dynamic electrochemical processes taking place at the electrode materials during battery charge/discharge cycles. In situ transmission electron microscopy (TEM) on lithium ion batteries has been offered exceptional opportunities for monitoring the dynamic processes of electrode materials during electrochemical reaction at both spatial and temporal resolution.In this talk, I will introduce in situ TEM studies on Si anodes which suffers the anomalous volumetric changes and fracture during lithiation process. Previously, the lithiation behaviour of a single Si particle has been explained in detail by simulation data and experimental observation. However, in real batteries, since lithiation occurs simultaneously in clusters of Si in a confined medium, understanding how the individual Si structures interact during lithiation in a closed space is necessary. In this regard, I demonstrated physical and mechanical interactions of swelling Si structures during lithiation using well-defined Si nanopillar pairs. Ex situ SEM and in situ TEM studies reveal that compressive stresses change the reaction kinetics so that preferential lithiation occurs on free surfaces when the pillars are mechanically clamped. Such mechanical interactions enhance the fracture resistance of lithiated Si by lessening the tensile stress concentrations in Si structures. This experiment reveals the surprising effects of nanostructure shape, size, and void space for lithiation and the results will contribute to improved design of Si structures at the electrode level for high-performance Li-ion batteries.

Numerical implementation for fatigue assessment of butt joint improved by high frequency mechanical impact treatment: A structural hot spot stress approach

In the study, three-dimensional finite element modelling of high frequency mechanical impact (HFMI) treatment is presented for butt joints of four strength grade materials (Q235B, AISI 1006, 45 Steel and AISI 2205). The fatigue assessment of butt joints by HFMI is implemented by structural hot spot stress approaches, i.e., linear surface extrapolation (LSE) and through thickness at the weld toe (TTWT). The effective structural hot spot stress concentration factors for HFMI-treated joints are determined, and the dependence of HFMI improvement on material strength and external stress is well captured by TTWT method. Following structural hot spot stress S-N curves for as-welded joints recommended by IIW, the structural hot spot stress S-N curves for HFMI-treated joints are suggested if the slope m of S-N curves for HFMI-treated joints is designated to vary from 5 to 10, inversely proportional to material strength. At final, the characteristic fatigue strength (FAT) has been determined and verified with the available experimental data.

Evaluation of opening-hole shapes for rivet connection of a composite plate

The rivet joint is an important mechanical connection for hot structures made of ceramic matrix composites. The development of chemical vapor infiltration technology provides a possible application of novel rivets for these structural components. However, connectors can still be the weakest link for structural integrity due to the severe stress concentration around the rivet joint holes. The geometry parameters of the rivet hole can be key influencing factors to the structural strength. A comparative evaluation is presented to analyze the influence of opening-hole shapes on the structural stress concentration of a composite plate. The opening-hole shapes considered include circle, ellipse, and racetrack. The finite element (FE) models of specimens with the three shapes of rivet holes were constructed. The stress distributions of the specimens under tensile loading were analyzed to compare the stress concentration factor (SCF) of different opening-hole shapes. An optimal shape, i.e. racetrack-shaped, was determined based on the SCF reduction of specimen-level comparison. Then, FE models of a composite plate containing rivet holes were built to verify the outperformance of the racetrack-shaped hole. A stress concentration reduction of the plate with racetrack-shaped holes was observed compared to the one with circular holes.

Recommendations on Alleviating the Saturation of Slab Metal by Hydrogen Based on the Results of Physical Modeling

This article discusses reducing the hydrogen content of low-alloy steels for oil and gas line pipe and shipbuilding and improving these steel’s ductility. In addition to the saturation of the liquid steel with hydrogen, it is also possible for the steel’s hydrogen content to increase after it has solidified. For example, this could occur when slabs are heated in furnaces that are fired with certain types of fuel and have different atmospheres. When a furnace is fired with coke-oven gas and the internal pressure in the furnace is positive, the high hydrogen content of this gas helps saturate the steel with hydrogen. That in turn increases the steel’s contents of discontinuities and nonmetallic inclusions, which leads to the formation of structural stress concentrators and a decrease in ductility. Results are presented from physical modeling of the saturation of pipe steel with hydrogen during the heating of slabs made of the above-indicated types of steel. Aspects of the steel’s saturation in different furnaces atmospheres are examined, the degree of contamination of the steel by hydrogen is determined, and the formation of its structure and hydrogenation are studied. It is definitively established that an increase in the moisture content of the furnace atmosphere increases the degree of saturation of the steel by hydrogen. Recommendations are given on alleviating the hydrogen saturation of steel. It is noted in particular that it is necessary to replace coke-oven gas by natural gas for the slab-heating operation, shorten the heating time to an acceptable extent, and establish the correct values for gas pressure and the gas/air ratio in the furnace.

The dependence of intergrain damageability of casting on the technological treatment route

The study determines the role of heredity varieties in machining and assembling the products. The paper presents an analysis of interdependence between technological heredity, the stages of a product life cycle, and structural features of the surface layer. It emphasizes the importance of taking into account procurement operations in analyzing the impact of technological heredity on the parameters of a product. We have analyzed modern concepts for assessing damageability of materials and products and suggested the method of LM-firmness to analyze and assess the transformation of inhomogeneous surface layers of the cast samples into technological damage during machining. We have experimentally studied the role of technological surface treatment in damage formation and processed the findings. We used the model of functionally-graded structure of grain boundaries to determine the role of the cutting modes in the formation of structural stress concentrators. The devised method for determining the Weibull homogeneity coefficient (m) on the surface of the cast billets allows prognoses of the damage development in the surface layers after machining. The most rational technological treatment route for the casting of AD-type alloys comprises two successive finishing millings, and the Weibull homogeneity coefficient increases by 26-84% compared to the starting workpiece due to a lower damage of the material at the stage of finishing milling.

ランダムな積層構造を有する超細密プレートフィン構造体の応力集中：統計的調査

In this study, the effects of random laminate misalignment on microscopic stress concentration of ultrafine plate-fin structures are statistically investigated using the Monte Carlo simulation. For the stress analysis, the homogenization theory based on unit cell analysis is used. Moreover, the substructure method is introduced into the homogenization theory to deal with random laminate misalignment. The present method is applied to the analysis of microscopic stress distribution and stress concentration of ultrafine plate-fin structures made of Ni-based alloy with random laminate misalignment. Then, the analysis results are statistically organized in histograms. It is shown that the effects of random laminate misalignment on the stress concentration become remarkably strong depending on the loading condition.

Kinetics and fracture resistance of lithiated silicon nanostructure pairs controlled by their mechanical interaction.

Following an explosion of studies of silicon as a negative electrode for Li-ion batteries, the anomalous volumetric changes and fracture of lithiated single Si particles have attracted significant attention in various fields, including mechanics. However, in real batteries, lithiation occurs simultaneously in clusters of Si in a confined medium. Hence, understanding how the individual Si structures interact during lithiation in a closed space is necessary. Here, we demonstrate physical and mechanical interactions of swelling Si structures during lithiation using well-defined Si nanopillar pairs. Ex situ SEM and in situ TEM studies reveal that compressive stresses change the reaction kinetics so that preferential lithiation occurs at free surfaces when the pillars are mechanically clamped. Such mechanical interactions enhance the fracture resistance of lithiated Si by lessening the tensile stress concentrations in Si structures. This study will contribute to improved design of Si structures at the electrode level for high-performance Li-ion batteries.

The effect of surface oxidized modification on the mechanical properties of SiC3D/Al

In this study, the effect of surface oxidized modification on the mechanical properties of silicon carbide 3D/aluminum composites (SiC3D/Al) is explained. The basis of this study may be outlined as follows: the shear force of extrusion testing proves the effect of SiO2 layer thickness from the perspective of interface failure mechanism; and the critical pore radius explains the effect of SiO2 layer distribution from the perspective of a biconnected structure. The compression and flexural tests prove that compressive strength is mainly affected by the average SiO2 layer thickness. Bending strength is affected by the critical pore radius, which is the primary contributor to SiO2 layer thickness of the special structural space and stress concentration. Thus, the structural characteristics of SiC3D/Al affect the SiO2 growth mechanism, as indicated by the aforementioned results.

Analysis of the specific features of determining the coefficients of stress concentration in hull structures

Analysis of the specific features of determining the coefficients of stress concentration in hull structures

English

The aim of this study was to analyze the biomechanical behavior of lower premolars regarding the non-carious cervical lesion (NCCL) depth, load type and restoration status, using finite element analysis. Two-dimensional virtual model simulating a healthy lower premolar were created using the CAD software. Based on this image, five models were generated: healthy (H), three types of NCCLs: small lesion (SL – 0.5 mm deep), medium lesion (ML – 1.0 mm), deep lesion (DL – 1.5 mm), and restored lesion (RL). The models were export to a CAE software (ANSYS Finite Element Analysis Software), the areas of all structures were plotted and each model was meshed using a control mesh device. All of the virtual models were subjected to two occlusal load types, (100N each): occlusal load (OL) and buccal load (BL) on buccal cusp. The magnitude and the stress distribution were obtained using the von Mises and maximum principal stress criteria (σ1), in MPa. The quantitative analysis of stress (MPa) was identified at three points of the NCCLs: enamel surface on its upper wall , dentin at the bottom wall and dentin on the lower wall. The results showed a direct relation between sequential removal of cervical structure and higher stress concentration for any groups and for both loads types. For OL the highest value of stress was 8.8 MPa for DL on upper wall of NCCLs.The BL exhibited higher stress values in comparison to the OL for all models.In addtion, the BL was responsible for providing the highest stress accumulation on the bottom wall, 38.2 MPa for DL. The restoration with composite resin was able to restore a stress distribution close to the healthy model, for both load types. In conclusion, the extent of non-carious cervical lesion and loading conditions influenced the stress distribution pattern of lower premolar. The outer load seems to be more critical in affecting the biomechanical behavior of lower premolars, regardless of the lesion size . The restoration of NCCLs with composite resin appears to recover the biomechanical behavior, similar to healthy model.