爆炸焊不锈复合钢制压力容器和封头RT波状“裂纹”的试验研究

爆炸焊不锈复合钢制压力容器和封头RT底片显示的波状“裂纹”问题已困扰压力容器行业多年并常引起争议。通过大量试验分析 ,证实所谓“裂纹”的问题主要是爆炸焊不锈复合钢板一种固有的“缺欠” ,不是加工变形制造、焊接等所至 ,希望与制造、监督检验、使用单位等有个共识 ,并供制修订“承压设备无损检测”

1Cr18Ni9Ti棒材“裂纹”缺陷的失效分析

本文通过对典型"裂纹"状缺陷进行外观检查、表面缺陷微观观察和能谱分析,以及对"裂纹"纵剖面进行光学金相观察、电子金相观察及能谱分析,确定了"裂纹"实为"面状损伤",表面可见剪切韧窝和刮擦痕迹,有颗粒镶嵌物和附着物;"面状损伤区"表面颗粒的组成主要为1Cr18Ni9Ti中Mn、Cr两种合金元素的氧化物;圆饼的正反表面缺陷之间存在两条连贯的轴向缺陷,即正反表面的缺陷实际是轴向缺陷的横截面显示。得出了1Cr18Ni9Ti棒材的次表面存在两条轴向分布的连续缺陷带,其在钢棒横向切割时被截成多节;缺陷为大量高O、Mn、Cr元素的颗粒氧化物组成,颗粒氧化物作为硬质颗粒,在机械加工中受刀具拖动刮伤金属表面,形成了"面状损伤区";缺陷颗粒物可能来源于合金冶炼过程或者由内部氧化孔洞在锻造时拉长而成的结论。

电渣冷拉轴承钢表面“裂纹”试验研究

电渣冷拉轴承钢综合质量优于普通电炉冷拉轴承钢,由于该钢生产工艺复杂,表面常出现"裂纹",严重影响成材率。本文对电渣冷拉轴承钢表面"裂纹"缺陷进行了多种试验研究,找出了原因,使得该钢的验收合格率达到98.65%。

Effect of sintering temperature and holding time on structure and properties of Li_(1.5)Ga_(0.5)Ti_(1.5)(PO_4)_(3)electrolyte with fast ionic conductivity

Li_(1.5)Ga_(0.5)Ti_(1.5)PO_(4))_(3)(LGTP)is recognized as a promising solid electrolyte material for lithium ions.In this work,LGTP solid electrolyte materials were prepared under different process conditions to explore the effects of sintering temperature and holding time on relative density,phase composition,microstructure,bulk conductivity,and total conductivity.In the impedance test under frequency of 1-10^(6) Hz,the bulk conductivity of the samples increased with increasing sintering temperature,and the total conductivity first increased and then decreased.SEM results showed that the average grain size in the ceramics was controlled by the sintering temperature,which increased from(0.54±0.01)μm to(1.21±0.01)μm when the temperature changed from 750 to 950°C.The relative density of the ceramics increased and then decreased with increasing temperature as the porosity increased.The holding time had little effect on the grain size growth or sample density,but an extended holding time resulted in crack generation that served to reduce the conductivity of the solid electrolyte.

A thermo-mechanical damage constitutive model for deep rock considering brittleness-ductility transition characteristics

This paper developed a statistical damage constitutive model for deep rock by considering the effects of external load and thermal treatment temperature based on the distortion energy.The model parameters were determined through the extremum features of stress−strain curve.Subsequently,the model predictions were compared with experimental results of marble samples.It is found that when the treatment temperature rises,the coupling damage evolution curve shows an S-shape and the slope of ascending branch gradually decreases during the coupling damage evolution process.At a constant temperature,confining pressure can suppress the expansion of micro-fractures.As the confining pressure increases the rock exhibits ductility characteristics,and the shape of coupling damage curve changes from an S-shape into a quasi-parabolic shape.This model can well characterize the influence of high temperature on the mechanical properties of deep rock and its brittleness-ductility transition characteristics under confining pressure.Also,it is suitable for sandstone and granite,especially in predicting the pre-peak stage and peak stress of stress−strain curve under the coupling action of confining pressure and high temperature.The relevant results can provide a reference for further research on the constitutive relationship of rock-like materials and their engineering applications.

Simulation on dynamic characteristics of TC4 cutting with crack defects

Titanium alloys play an important role in aerospace and other fields.However,after precision forging and cold rolling process,some defects will appear on the subsurface of titanium alloy bars,thus reducing the surface quality and precision of turning process.This study aimed at exploring the effect of crack defects on TC4 cutting.Firstly,the finite element cutting simulation model of TC4 material with crack defects was established in ABAQUS.Then,the cutting parameters such as cutting force,stress concentration,chip morphology,residual stress were obtained by changing the variables such as the size and height of crack defects.Finally,the turning experiment was carried out on centerless lathe.The results show that the cutting force changes abruptly when the defect position is located on the cutting path,the maximal stress occurs at the tip of the defect,and the mutation of stress value is more serious with the increase of defect size;the buckling deformation of chip morphology occurs and becomes less serious with the increase of the distance between the defect position and the workpiece surface;the surface residual stress near the defect is related to the stress when the tool is close to the defect,the larger defect size and the closer to the machined surface,the greater the residual stress.Therefore,under certain processing conditions,the TC4 material should avoid large size defects or increase the distance between defects and the machined surface,so as to obtain better and stable surface quality.

Failure mechanisms and destruction characteristics of cemented coal gangue backfill under compression effect of non-uniform load

Backfill mining is one of the most important technical means for controlling strata movement and reducing surface subsidence and environmental damage during exploitation of underground coal resources. Ensuring the stability of the backfill bodies is the primary prerequisite for maintaining the safety of the backfilling working face, and the loading characteristics of backfill are closely related to the deformation and subsidence of the roof. Elastic thin plate model was used to explore the non-uniform subsidence law of the roof, and then the non-uniform distribution characteristics of backfill bodies’ load were revealed. Through a self-developed non-uniform loading device combined with acoustic emission (AE) and digital image correlation (DIC) monitoring technology, the synergistic dynamic evolution law of the bearing capacity, apparent crack, and internal fracture of cemented coal gangue backfills (CCGBs) under loads with different degrees of non-uniformity was deeply explored. The results showed that: 1) The uniaxial compressive strength (UCS) of CCGB increased and then decreased with an increase in the degree of non-uniformity of load (DNL). About 40% of DNL was the inflection point of DNL-UCS curve and when DNL exceeded 40%, the strength decreased in a cliff-like manner;2) A positive correlation was observed between the AE ringing count and UCS during the loading process of the specimen, which was manifested by a higher AE ringing count of the high-strength specimen. 3) Shear cracks gradually increased and failure mode of specimens gradually changed from “X” type dominated by tension cracks to inverted “Y” type dominated by shear cracks with an increase in DNL, and the crack opening displacement at the peak stress decreased and then increased. The crack opening displacement at 40% of the DNL was the smallest. This was consistent with the judgment of crack size based on the AE b-value, i. e., it showed the typical characteristics of “small b-value-large crack and large b-value-small crack”. The research results are of significance for preventing the instability and failure of backfill.

Dynamic Phase Field Description on Ductile Fracture Process

In comparison to discrete descriptions of fracture process,the recently proposed phase field methodology averts the numerical tracking strategy of discontinuities in solids,which enables the numerical implement simplification.An implicit finite element formulation based on the diffuse phase field is extended for stable and efficient analysis of complex dynamic fracture process in ductile solids.This exhibited formulation is shown to capture entire range of the characteristics of ductile material presenting J2-plasticity,embracing plasticization,cracks initiation,propagation,branching and merging while fulfilling the basic principle of thermodynamics.Herein,we implement a staggered time integration scheme of the dynamic elasto-plastic phase field method into the commercial finite element code.The numerical performance of the present advanced phase field model has been examined through several classic dynamic fracture benchmarks,and in all cases simulation results are in good agreement with the associated experimental data and other numerical results in previous literature.

Experimental studies on pillar failure characteristics based on acoustic emission location technique

Acoustic emission （AE） technique is a useful tool for investigating rock damage mechanism, and is used to study the temporal-spatial evolution process of microcracks during the similar pillar material experiment. A combined AE location algorithm was developed based on the Least square algorithm and Geiger location algorithm. The pencil break test results show that the location precision can meet the demand of microcrack monitoring. The 3D location of AE events can directly reflect the process of initiation, propagation and evolutionary of microcracks. During the loading process, stress is much likely concentrated on the area between pillar and roof of the specimen, where belongs to danger zone of macroscopic failure. When rock reaches its plastic deformation stage, AE events begin to decrease, which indicates that AE quiet period can be seen as precursor characteristic of rock failure.

Fatigue crack initiation and early propagation behavior of 2A97 Al-Li alloy

The fatigue crack initiation and early propagation behavior of 2A97 Al-Li alloy was studied. The smooth specimens were fatigued at room temperature under constant maximum stress control when stress ratio （R） is 0.1 and frequency （f） is 40 Hz. Microstructure observations were examined by optical microscopy, transmission electron microscopy, scanning electron microscopy and electron back scattered diffusion, in order to investigate the relationship between microstructure and fatigue crack initiation and early propagation behavior of 2A97 alloy. The results show that the fatigue cracks are predominantly initiated at inclusions and coarsen secondary phases on the surface of 2A97 alloy. The fatigue crack early propagation behavior of 2A97 alloy is predominantly influenced by the interactions between grain structure and dislocations or persistent slip bands （PSBs）. When the misorientation of two neighbouring grains is close to the orientations of the favorable slip plane within these two grains, high-angle grain boundary severely hinders the PSBs passing through, and thus leads to crack bifurcation and deflection.

Fatigue behavior and life prediction of A7N01 aluminium alloy welded joint

Fatigue characteristics of A7N01 aluminium alloy welded joint were investigated and a fatigue crack initiation life-based model was proposed. The difference of fatigue crack initiation life among base metal, weld metal and heat affected zone (HAZ) is slight. Furthermore, the ratio of fatigue crack initiation life (Ni) to fatigue life to failure(Nf) is a material dependent parameter, 26.32%, 40.21% and 60.67% for base metal, HAZ and weld metal, respectively. Total fatigue life predicted using the presented model is in good agreement with the experimental data and that using Basquin’s model. The observation results of fatigue fracture surfaces, using scanning electron microscope (SEM), demonstrate that fatigue crack initiates from smooth surface due to welding process for weld metal, blowhole in HAZ causes fatigue crack initiation, and the crushed second phase particles play an important part in fatigue crack initiation in base metal.

Failure mechanism of as-cast Mg-6Zn-2Er alloy during tensile test at room temperature

Tensile test of the as-cast Mg-6Zn-2Er alloy was conducted at room temperature. The results indicate that the alloy is inclined to failure when the strain reaches 5.6%. The coarse secondary phases are responsible for the failure, especially for the Mg3Zn3Er2 phase （W-phase）. It is indicated that the existence of the W-phase activates the stress concentrations due to the incapacity of W-phase for the load transfer, which results in the void at the inner of the W-phase. In comparison, the interface between the matrix and the secondary phase is stable. In conclusion, the characters of the secondary phases with respect to size, distribution, morphology and type, play an important role in the plastic deformation behavior of the alloy.

Fatigue fracture mechanism of AZ31B magnesium alloy and its welded joint

Fatigue test was carried out on AZ31B magnesium alloy. Under 2×10 6 cycle times, the fatigue strengths of base metal （BM）, butt joint （BJ）, transverse cross joint （TJ）, lateral connection joint （LJ） are 66.72, 39.00, 24.38 and 24.40 MPa, respectively. The crack propagation behavior of the alloy was analyzed by optical microscopy. The AZ31B magnesium alloy base metal has a smooth crack propagation macroscopic path. However, the microscopic path is twisted and some cracks have two forks, and the crack propagation is transgranular. The crack initiates in the weld toe and the crack propagates along the HAZ for the BJ and TJ; for the LJ crack initiates in the fillet weld leg. The fatigue fracture mechanisms were analyzed by SEM. The fatigue fracture surface consists of quasi-cleavage patterns or cleavage step and a brittle fracture occurs. Numerous secondary cracks are observed; some fatigue striations exist in butt joint and its size is about 5 μm.

Microstructure and fracture toughness of a TiAl-Nb composite consolidated by spark plasma sintering

A TiAl-Nb composite was prepared by spark plasma sintering （SPS） at 1250 °C and 50 MPa for 5 min from prealloyed TiAl powder and elemental Nb powder in a molar ratio of 9：1 for improving the fracture toughness of TiAl alloy at room temperature. The microstructure, phase constitute, fracture surface and fracture toughness were determined by X-ray diffractometry, electron probe micro-analysis, scanning and transmission electron microscopy and mechanical testing. The results show that the sintered samples mainly consist of γ phase, O phase, niobium solid solution （Nbss） phase and B2 phase. The fracture toughness is as high as 28.7 MPa？m1/2 at room temperature. The ductile Nbss phase plays an important role in absorbing the fracture energy in front of the cracks. Moreover, B2 phase can branch the propagation of the cracks. The microhardness of each phase of the composite was also tested.

Formability of aluminum-silicon coated boron steel in hot stamping process

Flow behavior of the Al-Si coated boron steel was investigated with Gleeble-3500,in comparison with the uncoated one.Effect of deformation conditions on the coating integrity was characterized by optical microscopy.Facture surfaces of the coated steels were inspected under SEM.Experimental results indicate that the ultimate tensile strength and ductility of the Al-Si coated boron steel are lower than those of the uncoated steel under test conditions.Extensive cracks occur in the coating after tensile tests;the width and density of cracks are sensitive to the deformation temperatures and strain rates.The bare substrate exposed between the separate coating segments is oxidized.Appearance of the oxide degrades the Al-Si coating adhesion.Remarkable difference between formability of the coating layer and the substrate is confirmed.The formability of the Al-Si coating could be optimized by controlling the phase transformation of the ductile Fe-rich intermetallic compounds within it during the austenization.

Causes and control of welding cracks in electron-beam-welded superalloy GH4169 joints

Welding joint of GH4169 alloy with a good formation was obtained. No macroscopic defects occurred in the joint. The weld had mainly a dendritic structure; the base metal was a solid solution of Ni, Cr, and Fe, and the strengthening-phase particles such as Ni3Nb were dispersively distributed along the grain boundary. The average tensile strength of the joint reached 743.7 MPa, and the Vickers hardness of the weld exceeded HV 300. Because of the segregation of the low-melting compound Ni3Nb at the grain boundary of the fusion zone, liquid cracks tended to occur as a result of welding stress. The formation of liquid cracks was inhibited by adding an alloying element, Mn, to the welding bath, because Mn diffused to the fusion zone and the high-melting phase Mn2Nb formed, and thus the overall properties of the joint were improved.

Anti-Plane Elasticity Problem and Mode Ⅲ Crack Problem of Cubic Quasicrystal

The fracture theory of cubic quasicrystal was developed. The exact analytic solution of a Mode Ⅲ Griffith crack in the material was obtained by using the Fourier transform and dual integral equations theory, and so the displacement and stress fields, the stress intensity factor and strain energy release rate were determined. The results show that the stress intensity factor is independent of material constants, and the strain energy release rate is dependent on all material constants. These provide important information for studying the deformation and fracture of the new solid material.

Flexural destructive process of unidirectional carbon/carbon composites reinforced with in situ grown carbon nanofibers

Unidirectional carbon/carbon（C/C） composites modified with in situ grown carbon nanofibers（CNFs） were prepared by catalysis chemical vapor deposition. The effect of in situ grown CNFs on the flexural properties of the C/C composites was investigated by detailed analyses of destructive process. The results show that there is a sharp increase in the flexural load-displacement curve in the axial direction of the CNF-C/C composites, followed by a serrated yielding phenomenon similar to the plastic materials. The failure mode of the C/C composites modified with in situ grown CNFs is changed from the pull-out of single fiber to the breaking of fiber bundles. The existence of interfacial layer composed by middle-textured pyrocarbon, CNFs and high-textured pyrocarbon can block the crack propagation and change the propagation direction of the main crack, which leads to the higher flexural strength and modulus of C/C composites.

Effect of T7951 secondary aging treatment on crack propagation behavior of 7055 aluminum alloy

The crack propagation rates of T6 peak aging and T7951 secondary aging 7055 aluminium alloys were tested under stress ratios (R) of 0.6, 0.05 and ?1, respectively. The microstructures and fracture surfaces were analyzed by TEM and SEM. The results reveal that the crack propagation rate is affected by the stress ratio and microstructure such as the distribution, dimension and volume fraction of matrix precipitates, grain boundary precipitates and precipitate free zone. For both heat-treated specimens, crack propagation rate increases with the improvement of R when it is a positive value while crack propagation rate at R=?1 is much similar to that at R=0.06. The crack growth rates exhibit no obvious difference in lower stress intensity factor range (ΔK), while the difference starts to be obvious when ΔK exceeds certain value. The fracture analysis testifies a better fracture toughness for 7055-T7951 with a smaller striation space in Paris region.

Production of super-high strength aluminum alloy billets by low frequency electromagnetic casting

The effects of low frequency electromagnetic field on the macro-physical fields in the semi-continuous casting process of aluminum alloys and the microstructure and crack in the billets were studied and analyzed by the numerical and experimental methods.Comparison of the results for the macro-physical fields in the low frequency electromagnetic casting（LFEC） process with the conventional DC casting process indicates the following characters due to the application of electromagnetic field：an entirely changed direction and remarkably increased velocity of melt flow;a uniform distribution and a decreased gradient of temperature;elevated isothermal lines;a reduced sump depth;decreased stress and plastic deformation.Further,the microstructure of the billets is refined remarkably and the crack in the billets is eliminated in LFEC process because of modification of the macro-physical fields induced by the application of low frequency electromagnetic field.