Maximum Internal Stress Research Articles

Structure optimization and contact law study for transmission linkage of manipulator grasping fragile parts

According to the grasping requirements for fragile workpieces with cylindrical inner walls, a manipulator with internal bracing has been designed. The manipulator utilizes a constant-speed push cylinder to drive the linkage mechanism and accomplish the related works. Considering the fragile nature of the workpieces, reducing the impact velocity between the manipulator finger and the workpiece is a primary objective in the mechanism design. To achieve this, an optimization design is carried out, which maintains the overall average speed of the manipulator, while reducing the average speed only during the contact between the finger and the workpiece. The goal of optimization is to minimize the impact velocity at the contact point, using the linkage structure parameters as variables. The genetic algorithm (GA) is employed for the optimization process. The results show that optimization reduces the impact velocity at the contact point by 44.80%. To gain further insight into the influence of the process, a finite element model of a finger grasping a fragile object was created using joint modeling with the SolidWorks, Hyper Mesh, and LS-DYNA software. Simulations reveal that, after optimization, the maximum internal stress of three workpieces with thicknesses of 2.00 mm, 1.00 mm, and 0.50 mm was reduced by 47.42%, 41.53%, and 44.20%, respectively. To study the general law of the mechanical hand grasping fragile workpieces, the relationship between the cylinder driving speed and the impact speed is established. Based on the simulation results of maximum contact impact velocity, workpiece wall thickness, and maximum contact force, a prediction model of workpiece internal force is established using the Kriging prediction model theory. Additionally, the feasible range of grasping parameters under different wall thickness conditions is determined and discussed. The reliability of the designed structure is experimentally verified, providing valuable insights for ensuring the lifting work reliability of manipulators handling fragile workpieces.

Anticorrosive barrier coatings modified by core-shell rubber particles: effects on the property transients and premature crack initiation susceptibility of particle type and concentration

AbstractProtective epoxy coatings, as a result of their inherent brittleness, show insufficient resistance towards initiation and propagation of cracks, which can occur as early as during the curing process. To improve premature crack initiation resistance, it is essential to understand the underlying mechanisms. In this work, a solvent-based novolac epoxy, cured with a cycloaliphatic amine, was reinforced with either an epoxypropoxypropyl-terminated polydimethylsiloxane (PDMS), nanoparticles of strontium titanate (SrTiO3), or core-shell rubber (CSR) nanoparticles. The effects on coating property transients, curing-induced internal stress, and premature crack initiation susceptibility of the modifier types and CSR (MX 217 and MX 267) concentrations were investigated. In addition, using a digital microscope, the defect and crack morphology in coatings applied to rigid, flat substrates and inner 90-degree angles were characterized. Finally, to evaluate the anticorrosive barrier performance of the reinforced coatings, an electrochemical impedance spectroscopy analysis was employed. Despite a slightly reduced crack initiation susceptibility, the flexible PDMS chains, due to phase separation, resulted in a deteriorated barrier performance. The inclusion of SrTiO3 nanoparticles also led to a reduced anticorrosion performance, relative to a neat epoxy coating, with a slightly lower crack initiation susceptibility and a minor increase (around 0.2 MPa) in the average internal stress. For 5 wt% MX 217 and MX 267 CSR toughened coatings, the maximum internal stress and crack initiation susceptibility in the series, as well as an associated reduced corrosion resistance, were seen. In spite of a reduction in the elastic modulus, an improved barrier performance and a reduced internal stress and crack initiation susceptibility were observed for 25 wt% MX 217 and 37 wt% MX 267 CSR toughened coatings. To improve barrier properties and avoid premature crack initiation of epoxy coatings, guidelines on modifier selection are provided.

Biomechanical stability of oblique lateral interbody fusion combined with four types of internal fixations: finite element analysis.

Objective: Using finite element analysis to identify the optimal internal fixation method for oblique lateral lumbar interbody fusion (OLIF), providing guidance for clinical practice. Methods: A finite element model of the L4 - L5 segment was created. Five types of internal fixations were simulated in the generated L4-L5 finite element (FE) model. Then, six loading scenarios, i.e., flexion, extension, left-leaning, right-leaning, rotate left, and rotate right, were simulated in the FE models with different types of fixations. The biomechanical stability of the spinal segment after different fixations was investigated. Results: Regarding the range of motion (ROM) of the fused segment, OLIF + Bilateral Pedicle Screws (BPS) has a maximum ROM of 1.82° during backward bending and the smallest ROM in all directions of motion compared with other models. In terms of the von Mises stress distribution on the cage, the average stress on every motion direction of OLIF + BPS is about 17.08MPa, and of OLIF + Unilateral Vertebral Screw - Pedicle Screw (UVS-PS) is about 19.29MPa. As for the von Mises stress distribution on the internal fixation, OLIF + BPS has the maximum internal fixator stress in left rotation (31.85MPa) and OLIF + Unilateral Pedicle Screw (UPS) has the maximum internal fixator stress in posterior extension (76.59MPa). The data of these two models were smaller than those of other models. Conclusion: OLIF + BPS provides the greatest biomechanical stability, OLIF + UPS has adequate biomechanical stability, OLIF + UVS-PS is inferior to OLIF + UPS synthetically, and OLIF + Double row vertical screw (DRVS) and Individual OLIF (IO) do not present significant obvious advantages.

Internal stress development within wood during drying: A master curve concept and its application on drying stress evaluation

The generation of internal stress (IS) in flat-sawn and quarter-sawn rubberwood boards during drying has been investigated using an online restoring force (RF) technique, which restrained a half-split rectangular wood specimen. Particular attention was given to the longest IS reversal regime. The IS development proceeds faster in the flat-sawn specimens than that in the quarter-sawn specimens while the maximum IS magnitudes in both specimens are rather similar. By normalizing the IS reversal period, a master IS profile, the derivative of the measured RF to the IS reversal time ratio versus the IS reversal time ratio, is proposed. This master curve, exhibiting some degree of independence from wood orientation and drying temperature, shows variations correlated with the free water content in the wet zone and the dry/wet zone fractions. The process of IS reversal, unaffected by temporary unrestraint, advances as the dry zone expands inwards and ends when the wet zone disappears. Assuming mechanical equilibrium between the dry and wet zones, the IS in both zones can be estimated from the RF data. The maximum tensile IS in the dry zone, indicating a risk of surface checks, evolves at slightly lower magnitudes at higher drying temperatures and is lower in the quarter-sawn specimens. The IS relaxation in the dry zone, still largely taking place in the absence of the applied RF, highlights the main contribution of the dry/wet zone fractions, continuing to proceed without restraining, to the IS development. These findings, emphasizing the significance of dry and wet zones, should pave the way for a better understanding of the IS development within wood during drying.

Quenched Residual Stress Reduction in Pentagon-Curved Aluminum Alloy Forgings Using the Bulging Process.

Quenched residual stress in pentagon-curved forgings (PCGs) often leads to severe deformation during subsequent machining operations. This study aims to mitigate the quenched residual stress in PCGs through the implementation of the bulging method. The edge distance ratio (e/D), a geometric characteristic of PCGs, is defined and considered in the established thermo-mechanical model, which incorporates the effects of quenched residual stress. Increasing e/D resulted in amplified maximum internal stresses and surface stresses. To address this issue, a bulging finite element (FE) model was developed to effectively alleviate the quenched residual stress. The stress reduction in surface stress and internal stress was qualified using average stress reduction (Ra) and peak stress reduction (Rp), respectively. Notably, stress reduction exhibited an inverse relationship with e/D, indicating that decreasing e/D yields greater stress reduction. Furthermore, an overall stress reduction assessment was conducted for different bulging ratios, revealing that the stress reduction increased as the bulging ratio increased. A comprehensive comparison of different bulging ratios highlighted 2% as the most optimal bulging ratio for stress reduction in PCGs. X-ray diffraction measurement and the contour method were employed to determine surface stress and internal stress, respectively. The experimental results were in agreement with the simulation outcomes, validating the high accuracy of the FE model.

Retracted: Breaking Stress Criterion That Changes Everything We Know about Materials Failure

The perennial deficiencies of the failure models in the materials field have profoundly and significantly impacted all associated technical fields that depend on accurate failure predictions. The heart of this study is the presentation of a methodology that identifies a newly derived one-parameter criterion as the only general failure theory for noncompressible, homogeneous, and isotropic materials subjected to multiaxial states of stress and various boundary conditions, providing the solution to this longstanding problem. This theory is the counterpart and companion piece to the theory of elasticity and is in a formalism that is suitable for broad application. Utilizing advanced finite-element analysis, the maximum internal breaking stress corresponding to the maximum applied external force is identified as a unified and universal material failure criterion for determining the structural capacity of any system, regardless of its geometry or architecture. A comparison between the proposed criterion and methodology against design codes reveals that current provisions may underestimate the structural capacity by 2.17 times or overestimate the capacity by 2.096 times. It also shows that existing standards may underestimate the structural capacity by 1.4 times or overestimate the capacity by 2.49 times. The proposed failure criterion and methodology will pave the way for a new era in designing unconventional structural systems composed of unconventional materials.

The cracking behavior of the new Ni-based superalloy GH4151 in the triple melting process

In this paper, the mechanism of crack formation in VIM, ESR, and VAR of GH4151 alloy ingots is discussed. The cracks in VIM ingots are hot tearing, mainly caused by severe segregation of elements such as Nb, Ti, and Al in VIM electrodes, which promotes the precipitation of (γ+γ′) eutectic and Laves phases and thus leads to the formation of hot tearing. On the other hand, cracks in ESR and VAR ingots are cold cracks caused by the uneven distribution of γ′ phases. The temperature range for the precipitation of (γ+γ′) eutectic phases is 1160°C–1148 °C, while that for γ′ phases is 1137°C–1072 °C. At temperatures below 884 °C, the uneven precipitation of γ′ phases results in a maximum internal stress of 1096 MPa at room temperature. Based on computational fluid dynamics (CFD) simulations, to avoid large internal stresses caused by the precipitation of γ′ phase, the ingot should be removed from the water-cooled crystallizer within 57 min. Annealing is an effective measure to improve the quality of electrodes. After annealing treatment, the γ′ phase in the alloy transforms from short rod-shaped to square-shaped, and the decomposition of Laves phase promotes the precipitation of μ phase and σ phase from the matrix, improving the mechanical properties of the alloy.

Analysis of the Failure Area of the Slope Using the Slip Line Method

Most geotechnical engineers focus on ground strength and stability. However, when determining stability by analyzing the exact strength of the ground, the failure surface is essential. Nevertheless, limited studies have been conducted on the methods to determine the exact failure surface of the ground. This study shows that the failure surface and plastic area can be analyzed using the slip line method based on the slip line and lower bound analysis. To improve existing studies limited to single ground, we analyzed the failure area of heterogeneous ground conditions. The results of the slip line method were compared and verified through FELM, a finite element analysis method to which a lower limit analysis was applied. As a result, the failure area and maximum internal stress according to the ground properties and the slope angle are presented. In addition, the points of the slip line method and the finite element limit analysis are summarized. Finally, we propose limitations and solutions when applying the slip line method to the slope.

Modeling and experimental investigation of monocrystalline silicon wafer cut by diamond wire saw

Diamond wire sawing is one of the key technologies in solar cell manufacturing process and semiconductor chip manufacturing process. The thinned of silicon wafer has become a development trend of semiconductor industry, and the reduction of silicon wafer thickness can improve the material utilization and reduce the manufacturing cost. Therefore, the minimum thickness analytical model of diamond wire saw cutting monocrystalline silicon wafer is proposed in this paper to study the processing mechanism of minimum thickness of monocrystalline silicon wafer. Firstly, based on the Kirchhoff’s thin plate theory, the theoretical equation of the maximum internal stress of single silicon wafer and the sawing thickness of the silicon wafer is derived from the sawing force model. Combined with the Mohr’s strength theory, the analytical model of minimum sawing thickness of monocrystalline silicon wafer is established. Then, the analytical model is verified by the finite element simulation. The average error between the simulation results and the calculation results of the analytical model about the minimum sawing thickness of silicon wafer is 9%. Finally, the monocrystalline silicon sawing verification experiments are conducted under some groups of processing conditions. The experimental results show that the minimum sawing thickness of the silicon wafer decreases with the increasing axial speed of the wire saw, increases with the increase of the feed rate of the wire saw, and decreases first and then increases with workpiece speed becoming large. The average error of the experimental results and the calculation results of the analytical model of the wafer minimum sawing thickness is 7.4%, which verifies corrections of the analytical model.

Study on Construction Molding Technology of Long-Span Space Truss Suspended Dome Structure

Typically, the upper part of the roof a gymnasium building is a radial inverted triangular truss structure, and the lower part is a cable structure. They are connected by vertical braces to form a self-balancing structural system. The whole roof is supported by a complex, spatial, prestressed structure comprising tilted Y-shaped laced columns. Such structures rely on the integrity of the form and the application of prestress to achieve the best performance; it is in an extremely unstable state during construction. In order to study the mechanical behavior of the structure in this process, finite element software was used to analyze the cumulative slip of the structure and the construction process of cable tension, and the simulation values were compared to the actual monitoring values. The stress and deformation of the structure in different construction stages were investigated, and a reasonable structural unloading scheme was put forward. The study results showed that the stiffness of the long-span space truss suspended dome structure gradually increased with the structural integrity during construction, and the vertical deformation decreased from 25.4 mm to 19.26 mm with the construction process. The location and magnitude of the structure’s maximum internal force and maximum stress varied greatly compared to the static analysis when considering the construction process effects. Hence, conducting a construction process analysis is necessary. The construction technology of symmetrical rotating cumulative slip proposed in this paper has the advantages of a short construction duration, safe and stable construction process, etc., providing technical references for similar engineering constructions.

Thermal Properties of Hydrated Lime-Modified Asphalt Concrete and Modelling Evaluation for Their Effect on the Constructed Pavements in Service

Flexible pavements are subjected to three main distress types: fatigue crack, thermal crack, and permanent deformation. Under severe climate conditions, thermal cracking particularly contributes largely to a considerable scale of premature deterioration of pavement infrastructure worldwide. This challenge is especially relevant for Europe, as weather conditions vary significantly throughout the year. Hydrated lime (HL) has been recognized as an effective additive to improve the mechanical properties of asphalt concrete for pavement applications. Previous research has found that a replacement of conventional limestone dust filler using hydrated lime at 2.5% of the total weight of aggregates generated an optimum improvement in the mechanical properties of the asphalt concrete mixes used for all three purposed layers (i.e., wearing, levelling, and base) at atmospheric temperatures from mild to relatively high. This paper reports on a continuous experimental test for the thermal properties of the optimized hydrated lime-modified mixes. The experiment together with that conducted before provides the required data to characterize the thermomechanical constitutive relations of the optimized hydrated lime-modified mixes. The obtained thermal and mechanical properties thereafter were implemented in a numerical modelling study for a scenario involving pavement exposed to coupled thermal and traffic service conditions. The study has demonstrated that using HL in mineral filler enhances the thermal properties of asphalt concrete, which, however, showed little influence on the local temperature profiles within the pavement structure. The thermal effect is pronounced under the coupled thermomechanical conditions for a pavement exposed to both traffic and climatic impacts. The HL pavement has about 1.5% less deformation, and 39% less stress level under traffic loading only, but the thermal effect increases the maximum total internal tensile stress level by 26% in the HL pavement in winter season. The modelling analysis has shown that the local maximum tensile stress dominates in the surface region of the HL pavement. It will help to reduce the workload of crack repairing and in long term help on saving costs and efforts of maintenance.

Microstructure-based crystal plasticity modeling of AA2024-T3 aluminum alloy defined as the α-Al, θ-Al2Cu, and S-Al2CuMg phases based on real metallographic image

The increase in the strength of the Al–Cu–Mg alloys is to a large extent due to the formation and distribution of submicroscopic θ-Al2Cu and S-Al2CuMg phases in the aluminum (α-Al) matrix, which causes these alloys to have unique properties such as high initial hardening rate. This paper aimed to examine the anisotropic plasticity and flow behavior of the AA2024-T3 aluminum alloy by measuring the distinctive properties of each of the θ-Al2Cu, S-Al2CuMg, and α-Al phases at the micro-level and using a microstructure-based crystal plasticity model during single-strand tensile strength. The real microstructure of AA2024-T3 aluminum alloy obtained from scanning electron microscopy (SEM) was utilized in the calculations using the polycrystalline image processing method of the phases as a Representative Volume Element (RVE). Each phase’s crystal orientations and texture were randomly generated, given the lattice parameters and their crystal structure. The computational results were compared with the experimental data of the AA2024-T3 aluminum alloy tensile test, the data of the crystal plasticity model without considering the microstructure, and the Johnson-Cook model. Given the heterogeneity of the microstructure and crystal orientations of the grains, it was shown that the maximum internal stress under tensile loading occurred along with local hardening in the α-Al phase adjacent to the grains of the θ-Al2Cu and S-Al2CuMg phases, which was about 43300 MPa.

Influence of restraint conditions on residual stress and distortion of 2219-T8 aluminum alloy TIG welded joints based on contour method

Residual stress is a severe problem concerning about the service life and the safety of welded structures, which is affected by the assembling and restraint conditions. In this study, the residual stresses of 2219-T8 aluminum alloy butt-welded joints are investigated at different restraint conditions through contour method, which can obtain the 2D map of residual stress on the cross-sectional surface of the weld joints. X-ray diffraction measurement method is used to compare the results of the contour method. The results show that the variation trend and distribution of the residual stresses obtained by contour method are in good agreement with the results tested by X-ray diffraction method. The maximum internal residual stress, which locates in the middle thickness and the HAZ, is higher than that on the surfaces, with a difference over 50 MPa. The restraint conditions that mentioned in this study affect the amplitude of the tensile residual stresses and their distributions. Little influence is found on the amplitude of the internal residual stresses. But the surface residual stress is affected by restraint conditions. Higher vertical pressing force results in larger tensile stress region and larger distance between peak tensile stress and the weld center. Higher horizontal clamping force induces a smaller tensile stress region and a steeper variation rate of tensile stress in the HAZ. With higher vertical pressing force and lower horizontal clamping force, the welded joint obtains a smaller angular distortion. • The 2D map of the longitudinal residual stress is obtained by contour method on the cross-section of the weld joints. • The results obtained by contour method are in good agreement with that of XRD method. • Restraint conditions affect the amplitude of the tensile residual stresses and their distributions. • Vertical pressing force prevents angular deformation. Horizontal clamping force, however, has opposite effect.

Associations between changes in loading pattern, deformity, and internal stresses at the foot with hammer toe during walking; a finite element approach

Over the past decade, Finite Element (FE) modelling has been used as a method to understand the internal stresses within the diabetic foot. Foot deformities such as hammer toe have been associated with increased risk of foot ulcers in diabetic patients. Hence the aim of this study is to investigate the influence of hammer toe deformity on internal stresses during walking. A 3D finite element model of the human foot was constructed based on capturing Magnetic Resonance Imaging (MRI) of a diabetic neuropathic volunteer exhibiting hammer toe. 3D gait measurements and a multi-body musculoskeletal model for the same participant were used to define muscle forces. FE simulations were run at five different instances during the stance phase of gait. Peak plantar pressure and pressure distribution results calculated from the model showed a good agreement with the experimental measurement having less than 11% errors. Maximum von Mises internal stresses in the forefoot hard tissue were observed at the 3rd and 5th metatarsals and 4th proximal phalanx. Moreover, presence of hammer toe deformity was found to shift the location of maximum internal stresses on the soft tissue to the forefoot by changing the location of centre of pressure with internal stress 1.64 times greater than plantar pressure. Hammer toe deformity also showed to reduce the involvement of the first phalanx in internal/external load-bearing during walking. The findings of this study support the association between changes in loading pattern, deformity, and internal stresses in the soft tissue that lead to foot ulceration.

StressNet - Deep learning to predict stress with fracture propagation in brittle materials

Catastrophic failure in brittle materials is often due to the rapid growth and coalescence of cracks aided by high internal stresses. Hence, accurate prediction of maximum internal stress is critical to predicting time to failure and improving the fracture resistance and reliability of materials. Existing high-fidelity methods, such as the Finite-Discrete Element Model (FDEM), are limited by their high computational cost. Therefore, to reduce computational cost while preserving accuracy, a deep learning model, StressNet, is proposed to predict the entire sequence of maximum internal stress based on fracture propagation and the initial stress data. More specifically, the Temporal Independent Convolutional Neural Network (TI-CNN) is designed to capture the spatial features of fractures like fracture path and spall regions, and the Bidirectional Long Short-term Memory (Bi-LSTM) Network is adapted to capture the temporal features. By fusing these features, the evolution in time of the maximum internal stress can be accurately predicted. Moreover, an adaptive loss function is designed by dynamically integrating the Mean Squared Error (MSE) and the Mean Absolute Percentage Error (MAPE), to reflect the fluctuations in maximum internal stress. After training, the proposed model is able to compute accurate multi-step predictions of maximum internal stress in approximately 20 seconds, as compared to the FDEM run time of 4 h, with an average MAPE of 2% relative to test data.

Applications of Timoshenko beam theory and free edge effect for interpretation of stress in kiln-dried lumber using the restoring force technique

To assess the residual drying stress within industrial kiln-dried lumber, produced with a variety of sizes and shapes of rectangular cross section, interpretation of the restoring force profile obtained from the restrained half-split wood specimen is described. The finite element (FE) model was employed to simulate the restoring force profile of the half-split specimens having different sizes (thickness, 30–80 mm) and different shapes of rectangular cross section (width-to-thickness, w:d, ratio, 2.0–4.0). The analytical model based on the Timoshenko beam theory has been developed to describe the FE-simulated restoring force profiles. An agreement is found at relatively long half-split length of the very wide specimens (w:d ≥ 4). To describe the entire restoring force profile with a reasonable degree of accuracy, the modified stress decay function near free edges has been incorporated and the corresponding parameters have been determined. The size-independent parameters appear to linearly vary with the shape ratio (w:d). The analytical equation and shape-dependent parameters have been successfully employed to directly deduce (without a knowledge of the modulus data) the average maximum internal stress existing within the kiln-dried rubberwood lumber of various sizes and shapes. The derived magnitudes of the average maximum internal stress are in reasonable agreement with those obtained from the conventional McMillen slice technique. This magnitude of the internal stress could be used as a stress indicator for quality control of the commercial kiln-dried lumber in the sawmill industry.

Strength Design and Analysis of the Grouting while Drilling Equipment’s Sleeve

According to structure and working-condition of the grouting while drilling equipment, sleeve’s compressive strength and torsion strength are calculated with elastic theory, which are simulated and verified with FEA Software. The results indicate that its maximum internal stress exceeds the elastic yield limit at 30 MPa external pressures, and that the sleeve is working with risk in 3000 m underground soft coal.With Prandtl stress function method, the calculated shear strength in the sleeve’s small end is far more than the material’s shear strength due to 1500 N·m torque, the small end is stress concentration area and it can’t meet the requirement. Finally, sleeve’s theoretical work external pressure is calculated as 29.8 MPa and its axial deformation is about 0.2 mm, with radial deformation level at 0.03 mm, which can be referenced to tolerance design in the assembly.

Dynamic Response of Membrane Structures Under Water Impact

In this paper, the dynamic response of ship-borne membrane radome under the impact of water was investigated.Based on smooth particle hydrodynamics (Smooth particle hydrodynamics SPH), a fluid-structure coupling model was established, to simulate the membrane structure impacted by water, and the dynamics of the shipboard radome membrane structure under the impact of the breaking wave water body are simulated by the computer numerical simulation method response.The influence of the initial velocity of water, the angle of incidence, the initial pretension of the membrane structure and the tensile shape of the membrane surface on the maximum impact deformation and maximum impact stress of the membrane surface are analyzed. According to experimental analysis, under the same impact, the film structure with greater pretension has stronger resistance to impact, that is to say, under the same impact of water body, the greater the pretension of the membrane structure, the smaller the maximum internal stress change amplitude when it is impacted. Therefore, a certain optimal prestress is found to make the entire radome membrane structure suffer the same impact. The minimum maximum membrane stress is generated at the time, so as to achieve the purpose of protecting the fabric membrane structure from large stress and deformation; at the same impact speed, the vertical impact angle has the greatest impact on the membrane structure; under the same impact and pretension, the saddle-shaped film surface has stronger resistance to impact than the flat film surface.

A non-Hertzian contact analysis for the ball and raceway of a deep groove ball bearing

Aiming at the premature damage of deep groove ball bearings, considering axial load, elastic-plastic constitutive, coordinated contact and bearing clearance and so on, a non-Hertzian elasto-plastic contact analysis for the bearing ball and the raceway is performed by trans-scale three-dimensional finite element simulation. The results shown that the failure mechanism of the bearing is extremely similar to the L-P fatigue life theory. The effects of load magnitude and direction on contact state were further studied. It indicates that a certain angle of load is beneficial to reduce the maximum internal stress of the bearing, but the angle should not be too large. Besides, increasing the load magnitude results in an increase in the stress value and the depth of maximum stress, but when the load is too heavy, the area that touches the contact surface will be damaged firstly and quickly.

An X-Ray Diffraction Study of Mechanical Activation-Induced Structural Changes in Ni + Al Mixtures Containing Nickel Powders Prepared by Different Techniques

Mechanically activated Ni + Al mixtures containing two types of nickel powder, PNK (carbonyl nickel) and PNE-1 (electrolytic nickel), have been characterized by X-ray diffraction. The behavior of structural parameters of the mixtures containing both nickel powders has been studied as a function of milling time. During milling, the internal stress of the nickel and aluminum in the mixture containing the PNK nickel powder reaches a higher level in comparison with the PNE-1 powder. Moreover, in the case of the PNK powder, synthesis directly in the grinding vial of the mill requires a longer milling time in comparison with the PNE-1 powder (11 and 7 min, respectively). Milling for a time shorter than the critical one increases the density of structural defects in both mixture components (as evidenced by an increase in the full width at half maximum of X-ray diffraction peaks and a reduction in their intensity). Mechanical activation produces different structural changes in the nickel and aluminum: the maximum internal stress in the nickel exceeds that in the aluminum, but the aluminum is comminuted to a higher degree (its crystallite size decreases to a lower level).