Minimum Plastic Strain Research Articles

Prediction of grain size and dislocation density in the cold spraying process using a dislocation-based model

The cold spray particle deposition process was simulated by modeling the high-velocity impacts of spherical particles onto a flat substrate at different velocities. In addition to simulating the single particle, this study also employed a unique simulation technique to predict the evolution of the material's microstructure under real process conditions. A dislocation-based model was implemented as a VUMAT subroutine to model the microstructure. Additionally, Python scripting was also utilized in the multi-particle impact simulation to create particles and assign velocity and interaction randomly. The relationships between stress-strain values and material microstructure are linked together. By using this model, calculations were done for the equivalent plastic strain, dislocation density, and grain size in the particle, substrate, and interface. At the interface, there were high levels of plastic strain and strain rates reaching up to, which formed a bond between the particles and the substrate. It was found that a minimum plastic strain of approximately 1 is needed to create this bond. At velocities exceeding a critical velocity, a jet was produced. The particle grain size in regions distant from the interface measured between 200 and 500 nm, which is in good agreement with the experimental results. The smallest grain size was observed at the interface, measuring 140 nm. The validation outcomes demonstrated that the suggested model effectively replicates the cold spraying process. As a result, the suggested model has the capability to predict and evaluate the changes in the material's microstructure during the cold spraying process using various factors.

Verification of the Simulated Carburizing Process in Different Bore Sizes

Carburizing is one of the leading surface treatments in the industry. For this study, 20MnCr5 steel was gas carburized and quenched in real circumstances and simulated with Simufact software. The research investigated the dimensions and types of bores. A through and blind bore was used in this study to analyze how the geometry affects the created layer and, additionally, it takes into account the placement in the heat treatment furnace. An optical microscope and Vickers hardness tester were used to detect the changes in microstructure and measure the layer thickness. After the experiments, a simulation calculated the same variables to compare and validate the results to each other. It can be stated that the placement in the chamber did not influence the form of the high carbon content layer. The simulation and the measured results were in good agreement. The maximum hardness difference was 17%, but the calculated layer thicknesses were always between the measured data. For example, in the case of a small blind bore, the calculated layer thickness was 1.68 ± 0.18 mm, while the measured value was 1.54 ± 0.37 mm. Additionally, the hardness change in depth was similar in both cases. After this validation process, the residual stresses and plastic strains were determined. The maximum residual stresses were similar for every case, namely around 1900 MPa, while the maximum plastic strain was measured in a small blind bore with a value of 0.18. The minimum plastic strain was 0.04 in the through bore.

Experimental and analytical investigation of drilled flange connections (DFCs) with radial drilling patterns

The capability of reduced beam sections (RBS) to improve the seismic behavior of rigid connections leads to their notable application in steel moment-resisting frames. However, they may encounter shortcomings such as local buckling and lateral-torsional buckling. To resolve such problems, the idea of utilizing perforated beam flanges instead of cut-out flanges as reduced beam sections was presented. These type of connections can reduce the potential for local and lateral-torsional buckling of conventional RBS connections and improve their seismic performance. Achieving such advantages requires special care to geometrical specifications and drilling patterns (DPs). Appropriate arrangement of holes reduces the stress concentration around the beam flange-to-column Complete Joint Penetration (CJP) groove weld and, consequently, prevents the groove weld and inter-hole area from tearing and fracturing in early cycles of loading. The present study investigated the hysteretic performance of the drilled flange connections (DFCs) of various drilling arrangements through an experimental program.It should be noted that the box-shaped columns were considered in this study to target the 3D moment resisting frames (MRFs) that all their axes participate in the lateral force-resisting system. The most appropriate drilling patterns were selected using remarkable numbers of numerical analyses. Four laboratory specimens with different drilling arrangements of top and bottom flanges were fabricated based on preliminary numerical analyses. The laboratory specimens were subjected to cyclic loading. To evaluate the specimens' seismic performance, moment-rotation hysteresis curves were compared, and the stress distribution at the connection CJP groove welds. The results of the experimental tests revealed that those DFCs with combined DP with notches and holes (DFC1), radial DP (DFC2), semi radial DP with notches (DFC3), and semi radial DP (DFC4) could increase the rotational capacity of the DFCs up to 50% of the required rotation of the special moment-resisting connections. Moreover, semi radial DP (DFC4) may increase the connection's rotational capacity up to 75% of the special moment-resisting connections. Also, Based on the analytical results, conventional RBS connections may increase the 0.04 radians, needed rotational capacity up to 37.5% for a pre-qualified moment-resisting connection. It was also shown that no lateral-torsional buckling and smaller degradation in stiffness and strength took place in plastic hinge zones at the relative deformation of 0.04 radians. The plastic strain at the center of the CJP groove weld line in the beam longitudinal direction was more significant than those corners of the groove weld line in the same direction. In all specimens, the minimum plastic strain of the CJP groove weld took place in central parts of the weld in the transverse direction. In DFC1, DFC2, DFC3, and DFC4 specimens, the contribution of panel zone rotation in the overall plastic rotation of the connection is negligible.

The effect of processing parameters on rapid-heating β recrystallization in inter-pass deformed Ti-6Al-4V wire-arc additive manufacturing

Relatively low levels of inter-pass deformation have been found to be very effective in refining the coarse columnar grain structures normally seen in Ti-6Al-4V components, built using wire-fed high-deposition-rate additive manufacturing processes. The most important process parameters that control the level of β recrystallization – the final grain size and micro-texture – were systematically investigated by simulating the deformation and high heating rate conditions in controlled samples, to develop the process knowledge required to optimise inter-pass deformation and obtain predictable grain sizes. Overall, it was found that the level of β-grain refinement achieved by inter-pass deformation was surprisingly insensitive to the ranges of deformation temperatures, deformation speeds, and changes to the as-deposited α + β microstructure, expected within the WAAM process window, provided a minimum plastic strain of only 14% was achieved in each added layer. Conversely, the final component grain size was shown to be strongly affected by rapid grain growth on re-heating above the β transus. The texture results obtained were consistent with previous work which suggested that, with fine AM transformation microstructures, new β-grain orientations may be produced during the α → β transformation from the development of twinning faults, induced by the prior deformation and rapid heating. In contrast, greatly increasing the starting α lamellar spacing – to be more similar to that found in a wrought material – greatly reduced the level of recrystallization and also appeared to change the recrystallization mechanism to favour new β orientations produced largely by local lattice rotation.

Controlling chloride induced stress corrosion cracking of AISI 316L stainless steel by application of buffing

Abstract This paper presents the effect of buffing on the stress corrosion cracking (SCC) resistance of machined surfaces of AISI 316L stainless steel (SS). Surface finishing operations are known to lower the SCC resistance of austenitic stainless steels and yet are unavoidable. AISI 316L SS samples were given for different surface finishing operations like turning, milling and grinding. The surface roughness in each case was determined using stylus profiler (contact mode). Lattice strain and phase changes were detected by X-ray diffractometry (XRD) studies. SCC susceptibility has been studied before and after surface buffing operation by exposing to magnesium chloride hexahydrate solution (MgCl2.6H2o) as per ASTM standard procedure G36 for 3h. Microstructural characterization before and after SCC tests were characterize by using field emission SEM (FESEM). Results highlights that AISI 316L SS in turned, milled, and ground condition were highly prone to Cl– induced SCC. But on the application of buffing, the SCC resistance of the samples was enhanced and no cracking was observed. Crack enhancement was due to a) reduced surface roughness b) minimum plastic strain and c) high compressive residual stresses generated during buffing operation.

An allowable cladding peak temperature for spent nuclear fuels in interim dry storage

Allowable cladding peak temperatures for spent fuel cladding integrity in interim dry storage were investigated, considering hydride reorientation and mechanical property degradation behaviors of unirradiated and neutron irradiated Zr-Nb cladding tubes. Cladding tube specimens were heated up to various temperatures and then cooled down under tensile hoop stresses. Cool-down specimens indicate that higher heat-up temperature and larger tensile hoop stress generated larger radial hydride precipitation and smaller tensile strength and plastic hoop strain. Unirradiated specimens generated relatively larger radial hydride precipitation and plastic strain than did neutron irradiated specimens. Assuming a minimum plastic strain requirement of 5% for cladding integrity maintenance in interim dry storage, it is proposed that a cladding peak temperature during the interim dry storage is to keep below 250 °C if cladding tubes are cooled down to room temperature.

Effect of Secondary Orientation on the Mechanical Behavior of a Unit Cell Model with a Film-cooling Hole in Single Crystal

A 3D unit cell model with a hole has been set up to analyze the stress and strain distribution near the hole of nickel-based single crystal. The emphasis is placed on the influence of secondary orientation on the mechanical behavior of cell model. The crystallographic slip theory was applied to calculate the elasto-plastic deformation. The results show that, the maximum Mises stress and the maximum resolved shear stress are strongly dependent on the secondary orientation. The minimum plastic strain energy density criterion predicts that the optimal orientation of the hole is <100> crystallographic orientation. In addition, the secondary orientation has limited influence on the stress distribution near the hole. But the influence of the secondary orientation on the plastic strain energy density distribution around the hole is obvious.

Subsurface characterisation of wear on mechanically polished and electro-polished biomedical grade CoCrMo

CoCrMo alloys have been widely used for metal-on-metal total hip replacements (THRs). However, the use of the metal-on-metal implants has recently been seriously called into question due to adverse local tissue reactions due to the response of the body to wear debris and corrosion products. It is important to understand the wear of metal-on-metal THRs, hence to reduce the wear rate and metal ion dissolution. A nanocrystalline layer has been reported on the topmost surface of both in vivo and in vitro CoCrMo THRs and is believed to play a key role in the wear resistance of the material. The current work provides a detailed study of surface damage of biomedical CoCrMo after reciprocating wear testing. Systematic differences in the starting surface structure were investigated through a comparison of a standard mechanical polished and an electropolished surface. Extensive use of cross-sectional transmission electron microscopy (TEM) was applied to evaluate the evolution of the nanocrystalline layer. It was found that the nanocrystalline layer was not observed in cross-section samples from the as-mechanically polished surface, however there was extensive formation of ε-martensite and mechanical twins. In contrast, the electro-polished surface exhibited minimal evidence of deformation. The nanocrystalline layer developed during sliding contact for both starting surfaces, but at different rates. For the mechanically polished surface, the nanocrsytalline layer was far more extensive than for the electro-polished surface. Thus, this suggests that the nanocrystalline layer forms through some high strain shear process from the prior ε-martensite structure, and that a minimum plastic strain is required in the surface before the nanocrystalline layer starts to form. The formation mechanisms are discussed in detail.

A method for estimating the metal residual life of the weld pipes of gas main pipelines

A method was proposed for estimating residual life T res with the use of the plastic strain ɛop accumulated by the metal of a pipe during its operation, the minimum critical plastic strain increment Δɛcr, upon whose accumulation during further after-inspection operation a mechanical property attains its maximally allowable regulatory value, and the known operating time T op of an inspected gas pipeline section. The method makes it possible to increase the reliability of the estimation of the metal residual life of inspected pipes and to exclude the factor of randomness due to the fact that the strain ɛop accumulated by the metal of a pipe and the minimum plastic strain increment Δɛcr are determined on specimens that are cut from an inspected pipe instead of those cut from an emergency stock pipe or from a thermally treated inspected pipe.

Improvement of the strength of a metastable austenitic stainless steel by cyclic deformation-induced martensitic transformation at 103 K

Specimens of a metastable austenitic stainless steel were cyclically deformed at 103 K under constant plastic strain control with constant plastic strain ranges Δε pl = 1% and 2%. The tests were performed both with a symmetrical plastic strain amplitude (mean plastic strain ε pl = 0 ) and with an asymetrical plastic strain amplitude ( ε pl > 0 ) with the minimum plastic strain equal to zero. The cyclic deformation behaviour was investigated, especially in the stage of cyclic hardening, and correlated with the deformation-induced formation of martensitic phases which could be detected by transmission electron microscopy. The effect of the transformed martensite on the mechanical properties of the steel was quantified by subsequent tensile tests at room temperature with the cyclically deformed specimens. In contrast to results of earlier studies at room temperature, no cumulative plastic “incubation” strain was necessary at 103 K to trigger the deformation-induced martensitic transformation. At Δε pl = 2% and ε pl = 0 the maximum cyclic range was attained after only 18 cycles. In the reference test with an asymmetrical plastic strain amplitude ( ε pl = 1% ) 23 cycles were required to reach the maximum cyclic stress range. Tension tests revealed an increase of up to 200% in the 1% offset yield stress and a 75% increase in the tensile strength at room temperature after cycling the specimens to the maximum stress range with either symmetrical or asymmetrical plastic strain amplitudes. The ductility and the toughness of the steel remained surprisingly high with an elongation after failure of 45%.

Elasto-Plastic behaviour in plane strain problems for granular media

In the paper a simple mathematical model for granular media is presented. It is assumed that the critical surface corresponds to the Coulomb-Mohr description of ultimate behaviour and the yield condition is a family of piecewise smooth surfaces with edges corresponding to the edges of the critical surface. The existence of a surface similar to the critical surface on which there are no plastic volumetric strain increments in assumed. The hardening parameter is assumed as a linear combination of the plastic volumetric strain and the integral of the difference of maximum and minimum plastic strain increments. An analytic solution for the case of compression and tension in plane strain case is given. The relations are generalized to the three-dimensional case with rotating principal axes of stresses. The solution for shear is given as an illustration. The behaviour when the stresses are on the edge are discussed and in detail the problem of compression in the axially-symmetric case is considered. From comparison of experimental results in compression in the triaxial test the parameters for sand are estimated.