Novel damage constitutive models and new quantitative identification method for stress thresholds of rocks under uniaxial compression

Four key stress thresholds exist in the compression process of rocks,i.e.,crack closure stress(σ_(cc)),crack initiation stress(σ_(ci)),crack damage stress(σ_(cd))and compressive strength(σ_(c)).The quantitative identifications of the first three stress thresholds are of great significance for characterizing the microcrack growth and damage evolution of rocks under compression.In this paper,a new method based on damage constitutive model is proposed to quantitatively measure the stress thresholds of rocks.Firstly,two different damage constitutive models were constructed based on acoustic emission(AE)counts and Weibull distribution function considering the compaction stages of the rock and the bearing capacity of the damage element.Then,the accumulative AE counts method(ACLM),AE count rate method(CRM)and constitutive model method(CMM)were introduced to determine the stress thresholds of rocks.Finally,the stress thresholds of 9 different rocks were identified by ACLM,CRM,and CMM.The results show that the theoretical stress−strain curves obtained from the two damage constitutive models are in good agreement with that of the experimental data,and the differences between the two damage constitutive models mainly come from the evolutionary differences of the damage variables.The results of the stress thresholds identified by the CMM are in good agreement with those identified by the AE methods,i.e.,ACLM and CRM.Therefore,the proposed CMM can be used to determine the stress thresholds of rocks.

Effect of quenching cooling rate on residual stress and microstructure evolution of 6061 aluminum alloy

In this study,the cooling rate was manipulated by quenching with water of different temperatures(30,60 and 100℃).Surface and internal residual stresses in the quenched 6061 aluminum alloy samples were measured using hole-drilling and crack compliance methods,respectively.Then,the processability of the quenched samples was evaluated at cryogenic temperatures.The mechanical properties of the as-aged samples were assessed,and microstructure evolution was analyzed.The surface residual stresses of samples W30℃,W60℃and W100℃is−178.7,−161.7 and−117.2 MPa,respectively along x-direction,respectively;and−191.2,−172.1 and−126.2 MPa,respectively along y-direction.The sample quenched in boiling water displaying the lowest residual stress(~34%and~60%reduction in the surface and core).The generation and distribution of quenching residual stress could be attributed to the lattice distortion gradient.Desirable plasticity was also exhibited in the samples with relatively low quenching cooling rates at cryogenic temperatures.The strengthes of the as-aged samples are 291.2 to 270.1 MPa as the quenching water temperature increase from 30℃to 100℃.Fine and homogeneous β"phases were observed in the as-aged sample quenched with boiling water due to the clusters and Guinier-Preston zones(GP zones)premature precipitated during quenching process.

Effect of heat treatment on stress corrosion cracking, fracture toughness and strength of 7085 aluminum alloy

The influences of heat treatment on stress corrosion cracking （SCC）, fracture toughness and strength of 7085 aluminum alloy were investigated by slow strain rate testing, Kahn tear testing combined with scanning electron microscopy （SEM） and transmission electron microscopy （TEM）. The results show that the fracture toughness of T74 overaging is increased by 22.9% at the expense of 13.6% strength, and retrogression and reaging （RRA） enhances fracture toughness 14.2% without reducing the strength compared with T6 temper. The fracture toughness of dual-retrogression and reaging （DRRA） is equivalent to that of T74 with an increased strength of 14.6%. The SCC resistance increases in the order： T6〈RRA〈DRRA≈T74. The differences of fracture toughness and SCC were explained on the basis of the role of matrix precipitates and grain boundary orecioitates.

Effects of silicon content on microstructure and stress corrosion cracking resistance of 7050 aluminum alloy

Evolution of microstructure and stress corrosion cracking （SCC） susceptibility of 7050 aluminum alloy with 0.094%, 0.134% and 0.261% Si （mass fraction） in T7651 condition have been investigated. The results show that the area fraction of Mg2Si increases from 0.16% to 1,48% and the size becomes coarser, while the area fraction of the other coarse phases including Al2CuMg, Mg（Al,Cu,Zn）2 and A17Cu2Fe decreases from 2.42% to 0.78% with Si content increasing from 0.094% to 0.261%. The tensile strength and elongation of 7050-T7651 alloys is decreased with the increase of Si content by slow strain rate test （SSRT） in ambient air. However, electrical conductivity is improved and SCC susceptibility is reduced with the increase of Si content by SSRT in corrosion environment with 3.5% NaCl solution.

Simulation of temperature and stress in 6061 aluminum alloy during online quenching process

The cooling curves of 6061 aluminum alloy were acquired through water quenching experiment. The heat transfer coefficient was accurately calculated based on the cooling curves and the law of cooling. The online quenching process of complex cross-section profile was dynamically simulated by the ABAQUS software. The results suggest that the heat transfer coefficient changes during online quenching process. Different parts of the profile have different cooling velocity, and it was verified by water quenching experiment. The maximum residual stress of the profile was predicted using FEM simulation based on ABAQUS software The relations between the temperature and stress were presented by analyzing the data of key points.

Effect of flow stress—strain relation on forming limit of 5754O aluminum alloy

A modified Swift type flow stress—strain relation was presented in order to describe the uniaxial tension test curve reasonably. The FLD-strain （forming limit diagram made up of limit strain） of 5754O aluminum alloy sheet was calculated based on the two flow stress—strain relations using Yld2000-2d yield function. By comparing the theoretical and experimental results, it is found that the calculated FLD-strain based on the modified Swift flow stress—strain relation can reasonably describe the experimental results. However, though the common Voce flow stress—strain relation can describe the deformation behavior during homogenous deformation phase accurately, the FLD-strain calculated based on it is obviously lower than the experimental result. It is concluded that the higher the hardening rate of sheet metal is, the higher the forming limit is. A method for determining the reasonable flow stress—strain relation is recommended for describing the material behavior during inhomogenous phase and the forming limit of sheet metal.

Preliminary Study on the Adaptability of Cucumis hystrix Chakr. under Aluminum Salt Stress

[Objective] The study was carried out to research the adaptability of Cucumis hystrix Chakr.under aluminum salt stress.[Method] Using Cucumis hystrix Chakr.and three cultivated cucumbers as tested materials,the change of physiological indexes including POD activity,SOD activity,electrolyte leakage and the content of malondialdehyde（MDA）,proline（Pro）content and soluble sugar were studied.[Result] The POD activity,SOD activity,proline content and soluble sugar content of Cucumis hystrix were higher than these of cultivated cucumbers,while electrolyte leakage and MDA content were lower,and it showed that Cucumis hystrix had better adaptability under aluminum salt stress.[Conclusion] Our study could lay a foundation for the improvement of cultivated cucumber by using the excellent characteristics of Cucumis hystrix.

Prediction of vulnerable zones based on residual stress and microstructure in CMT welded aluminum alloy joint

Numerical simulation and experimental results were employed for the identification of the most vulnerable zones in three-pass cold-metal-transferring （CMT） welded joint. The residual stress distribution in the joint was predicted by finite element （FE） method, while the structural morphology of distinctive zones was obtained through metallographic experiments. The highest principal stress made the symmetric face of the joint most sensitive to tensile cracks under service conditions. Whereas, the boundaries between the weld seam and the base plates were sensitive to cracks because the equivalent von Mises stress was the highest when the first interpass cooling was finished. The third weld pass and the inter-pass remelted zones exhibited the modest mechanical performances as a result of the coarse grain and coarse grain boundary, respectively. The most vulnerable zones were regarded to be the crossed parts between the zones identified by numerical and experimental methods.

Effect of pre-deformation on the stress corrosion cracking susceptibility of aluminum alloy 2519

The effects of pre-deformation and strain rate on the stress corrosion cracking （SCC） behavior of aluminum alloy 2519 in air and in 3.5% NaCI water solution were investigated by means of slow strain rate tension （SSRT）, scanning electron microscopy （SEM）, and transmission electron microscopy （TEM）. The results indicate that the alloy is susceptible to SCC in 3.5% NaCI water solution and not in air. At the same pre-deformation, the alloy is more susceptible to SCC at 1.33 × 10^-5 s^-1 than at 6.66 × 10^-5 s^-1. Moreover, it is more susceptible to SCC at free pre-deformation than at 10% pre-deformation at the same strain rate. The number of 0 precipitated along the grain boundaries is reduced and distributed discontinuously, at the same time, the precipitate-free zones （PFZ） become narrow and the susceptibility to stress corrosion cracking is reduced after 10% pre-deformation.

Effect of quenching rate on microstructure and stress corrosion cracking of 7085 aluminum alloy

The influence of quenching rate on microstructure and stress corrosion cracking （SCC） of 7085 aluminum alloy was investigated by tensile test, slow strain rate test （SSRT）, combined with scanning electron microscopy （SEM）, transmission electron microscopy （TEM） and electrochemical test. The results show that with decreasing the quenching rate, the size and inter-particle distance of the grain boundary precipitates as well as precipitation free zone width increase, but the copper content of grain boundary precipitates decreases. The SCC resistance of the samples increases first and then decreases, which is attributed to the copper content, size and distribution of grain boundary precipitates.

Stress corrosion cracking behaviour of 7xxx aluminum alloys: A literature review

Stress corrosion cracking （SCC） is degradation of mechanical properties under the combined action of stress and corrosive environment of the susceptible material. Out of eight series of aluminium alloys, 2xxx, 5xxx and 7xxx aluminium alloys are susceptible to SCC. Among them, 7xxx series aluminium alloys have specific application in aerospace, military and structural industries due to superior mechanical properties. In these high strength 7xxx aluminium alloys, SCC plays a vital factor of consideration, as these failures are catastrophic during the service. The understanding of SCC behaviour possesses critical challenge for this alloy. The main aim of this review paper is to understand the effect of constituent alloying elements on the response of microstructural variation in various heat-treated conditions on SCC behavior. Further, review was made for improving the SCC resistance using thermomechanical treatments and by surface modifications of 7xxx alloys. Apart from a brief review on SCC of 7xxx alloys, this paper presents the effect of stress and pre-strain, effect of constituent alloying elements in the alloy, and the effect of environments on SCC behaviour. In addition, the SCC behaviours of weldments, 7xxx metal matrix composites and also laser surface modifications were also reviewed.

Butt-welding Residual Stress of Heat Treatable Aluminum Alloys

This study, taking three types of aluminum alloys 2024-T351, 6061-T6 and 7075-T6 as experimental materials, conducted single V-groove GTAW （gas tungsten arc welding） butt-welding to analyze and compare the magnitude and differences of residual stress in the three aluminum alloys at different single V-groove angles and in restrained or unrestrained conditions. The results show that the larger the grooving angle of butt joint, the higher the residual tensile stress. Too small grooving angle will lead to dramatic differences due to the amount of welding bead filler metal and pre-set joint geometry. Therefore, only an appropriate grooving angle can reduce residual stress. While welding, weldment in restrained condition will lead to a larger residual stress. Also, a residual stress will arise from the restraint position. The ultimate residual stress of weldment is determined by material yield strength at equilibrium temperature. The higher the yield strength at equilibrium temperature, the higher the material residual stress. Because of its larger thermal conductivity, aluminum alloy test specimens have small temperature differential. Therefore, the residual tensile stress of all materials is lower than their yield strength.

Evolution of residual stress field in 6N01 aluminum alloy friction stir welding joint

Based on the characteristics of friction stir welding( FSW) and Coulomb friction work theory,the residual stresses field of FSW joints of 6 N01 aluminum alloy( T5),which was used in high speed train,were calculated by using the ANSYS finite element software. During the FEM calculation,the dual heat source models namely the body heat source and surface heat source were used to explore the evolution law of the welding process to the residual stress field. The method of ultrasonic residual stress detecting was used to investigate the residual stresses field of the 6 N01 aluminum alloy FSW joints. The results show that the steady-state temperature of 6 N01 aluminum alloy during FSW is about 550 ℃,and the temperature mutates at the beginning and at end of welding. The longitudinal residual stress σ_x is the main stress,which fluctuates in the range of-25 to 242 MPa. Moreover,the stress in the range of shaft shoulder is tensile stress that the maximum tensile stress is 242 MPa,and the stress in the outside of shaft shoulder is compressive stress that the maximum compressive stress is 25 MPa. The distribution of the tensile stress in the welding nugget zone( WNZ) is obviously bimodal,and the residual stress on the advancing side is higher than that on the retreating side. With the increasing of the welding speed,the maximum temperature decreased and the maximum residual stress decreased when the pin-wheel speed kept constant. With the increasing of the pin-wheel speed,the maximum temperature of the joint increased and the maximum residual stress increased when the welding speed was constant. The experimental results were in good agreement with the finite element results.

EFFECT OF STRUCTURAL PARAMETERS ON THE THERMAL STRESS OF A NiFe_(2)O_(4)-BASED CERMET INERT ANODE IN ALUMINUM ELECTROLYSIS

Inert anode has been a hot issue in the aluminum industry for many decades. With the help of FEA （finite element analysis） software ANSYS, a model was developed to simulate the thermal stress distribution working condition of an inert anode. To reduce its thermal stress, the effect of some parameters on the thermal stress distribution was investigated, including the anode height, the anode radius, the hole depth, the hole radius, and the radius of inner chamfer and outer chamfer. The results showed that in the actual working condition of an inert anode, there existed a large axial tensile stress near the tangent interface between the anode and bath, which was the major cause of anode breaking. Increasing the anode height and reducing the hole depth properly seemed to be beneficial for the stress distribution. With the increase of anode radius, the stress distribution became better first and then deteriorated, the reasonable value was between 0.045 to 0.06m. The hole radius had a significant effect on the stress and a smaller radius would reduce the thermal stress. The effect of the radius of the inner chamfer and the outer chamfer was less than other parameters.

Feature of flow stress of aluminum sheet used for can during hot compression

The flow stress feature of aluminum sheet used for pressure can during plastic deformation at elevated temperature was studied by isothermal compression test using Gleeble 1 500 dynamic materials testing machine. The experimental results show that the steady state deformation is remarkable when the material is deformed in the temperature range of 350～500 ℃ at strain rates within the range of 10 -2 ～10.0 s -1 . The material is sensitive to positive strain rate. A hyperbolic sine relationship is found to correlate well the flow stress with the strain rate, and an Arrhenius relationship with the temperature. Semi empirical constitutive equations of the flow stress are derived from all experimental data for tested material during plastic deformation at elevated temperature by polyelement linear regression analysis. [

Correspondence between Susceptibility to SCC of 7050 Aluminum Alloy and Passive Film-induced Stress at Various pH Values

The passive film-induced stress and the susceptibility to SCC of 7050 aluminum alloy in 3.5%sodium chloride solution at various pH values were investigated by slow strain rate testing（SSRT） and flowing stress differential method.The results showed that the passive film-induced stress and the susceptibility to SCC decreased with increasing pH values when pH≤7,while they increased with increasing pH values when pH7.However,the corrosion type was interpreted as exfoliation corrosion when pH=l and 14,and there was no film formed on the surface of the specimens.The whole variation plots of film-induced stress and the SCC susceptibility with pH values were both presented as a valley shape.The symbol and amount of the film-induced stress were related to the compositions of the passive film,which were analyzed using X-ray photoelectron spectroscopy（XPS）.

An investigation on the performance of stress corrosion cracking in aluminum-copper alloy welded joint

With the resistance to stress corrosion of the base metal as a reference, the contrast result of stress corrosion cracking （ SCC） susceptibility of aluminum-copper alloy 2219 and 2014 welded joints under different welding processes （ VP-TIG welding, HF-TIG hybrid welding, laser-TIG hybrid welding and laser HF-TIG hybrid welding） is obtained via the slow strain rate testing （ SSRT） , scanning electron microscope （ SEM） and microstructure observation auxiliary technologies. Test results show that the joints of aluminum alloy 2219, welded by hybrid welding processes, have superior resistance to stress corrosion compared to those welded by the VP-TIG welding process in varying degrees, especially, the joint welded by the laser HF-TIG hybrid welding process, where the resistance to stress corrosion is almost the same as that of the base material. However, the HF or laser hybrid welding effect is not significant under the same welding conditions for welded joints of aluminum alloy 2014.

MODIFIED LAYER REMOVAL METHOD FOR MEASUREMENT OF RESIDUAL STRESS DISTRIBUTION IN THICK PRE-STRETCHED ALUMINUM PLATE

The integrated structure parts are widely used in aircraft. The distortion caused by residual stresses in thick pre-stretched aluminum plates during machining integrated parts is a common and serious problem. To predict and control the machining distortion, the residual stress distribution in the thick plate must be measured firstly. The modified removal method for measuring residual stress in thick pre-stretched aluminum plates is proposed and the stress-strain relation matrix is deduced by elasticity theory. The residual stress distribution in specimen of 7050T7451 plate is measured by using the method, and measurement results are analyzed and compared with data obtained by other methods. The method is effective to measure the residual stress.

Flow stress equation for multipass hot-rolling of aluminum alloys

A series of simple axisymmetric compression tests were carried out on the computer servo controlled Gleeble 1 500 machine when strain rates ranged between 0.05 25 s -1 and deformation temperature 300 500 ℃. The results show that flow stress is related to the Zener Hollonom parameter Z and strain, as well as the static recrystallization fraction between passes during multipass hot deformation of 5182 aluminum alloy. Hence, a modified exponential flow stress equation was presented by considering the values of ln A and β as functions of strain, and by using the uniform softening method and incorporating the static recrystallization fraction between passes to consider the effects of residual strain during multipass hot rolling of 5182 aluminum alloy. The validity of the equation was examined by a typical non isothermal multipass deformation test.

Investigation of Stress Corrosion Cracking Initiation of 7A52 Aluminum Alloy

The stress corrosion cracking（SCC） behaviour of 7A52 aluminum alloy in air and in 3.5% NaCl solution was researched by slow strain rate test（SSRT） and SEM-EDS. The SCC susceptibility was estimated with the loss of the reduction in area. The experimental results indicate that the SCC susceptibility of 7A52 aluminum alloy in 3.5% chloride solution is the highest at strain rate of 1×10-6 s-1. The lowest one is under the condition of 1×10-5 s-1. Stress concentration and anode dissolving around Al-Fe-Mn intermetallics initiate micropores which will result in microcracks. The existence of intermetallics in the microstructure may play an important role in understanding the SCC initiation mechanisms of 7A52 aluminum alloy.