St15钢板冲裂成因分析

在利用St15钢板生产表壳过程中有少部分原料被冲裂。采用日本扫描电子显微镜、英国牛津公司的能谱仪和EBSD、光学显微镜、拉力实验机检测结果表明,组织不均匀、{111}织构较漫散是冲裂的主要成因。

800 MPa级别DP钢冲裂失效机理分析

文章通过对800 MPa级别的DP钢冲裂零件进行网格分析,依据冲压时受力状态及应变情况,确定模具状态及定位尺寸对冲压过程的影响。分析结果表明:由于模具润滑条件及定位尺寸不精确,导致零件的变形部位产生应力集中,突破了成形极限导致零件破裂。另外,通过对裂纹扩展处进行扫描电镜分析,确定了导致失效产生裂纹的机理:800 MPa级别的DP钢裂纹的扩展一般都沿着马氏体和铁素体相的界面,正常级别的夹杂物并不是裂纹的扩展的主要原因。

地下室底板高水压冲裂加固修复施工技术

目前,高层建筑地下室底板开裂和渗漏现象时有发生,严重影响地下室的使用功能和安全性能。本文对地下室底板高水压冲裂修复加固技术进行研究,以佛山顺德某实际工程为例,阐述了地下室底板高水压冲裂修复加固的原理,提出了开孔泄压、注浆预加固、钢板/架加固等施工工艺的控制要点,总结了地下室底板高水压冲裂修复加固技术工艺流程、关键环节。结果表明:该技术实施效果良好,地下室底板施工质量和加固效果良好,未出现渗漏水现象。该技术可为其他类似地下室底板高水压冲裂的修复处理施工提供参考。

西江和北江三角洲区的水沙特点及河道演变特征

通过对珠江三角洲区有关水文泥沙资料的分析 ,初步揭示了西江和北江在流速与流量、水位、含沙量、断面宽深比之关系方面的差别。结合河道平面及断面形态和河道冲淤特性 ,探讨了其河型特点及成因。认为研究区西江为典型的网状河流 ,而北江仍处于网状河流的发育初期。该区网状河流多河道的形成遵循冲裂机制 ,人工堤围加速了河道沉积、抑制了河漫滩地的淤积 。

改善HP295钢板加工性能的研究

通过试验得出了HP295钢板加工开裂的主要原因是由于MnS夹杂的有害影响,提出了防止加工开裂的生产工艺措施。

Numerical simulation and optimization of process parameters in fineblanking process

Fineblanking process is a typical large localized plastic deformation process. Based on its forming characteristics, a numerical model is established and an elasto-plastic simulation is performed using the finite element method （FEM）. The re-meshing method is used when the severe element distortion occurs to facilitate further computation and avoid divergence. The McClintock fracture criterion is adopted to predict and determine the time and site of crack initiation and propagation. Based on this numerical model, the distribution and developing trend of the stress and strain in the shearing zone are predicted. Furthermore, the influence of several process parameters, such as punch-die clearance, edge radius of punch and die, V-ring force, counter force, etc., on the blanked quality is analyzed. The discipline is in accordance with the actual manufacture situation, which can be a guidance to optimization of process parameters.

农业部“十五”期间重点推广50项技术

5月10日,农来部在人民大会堂召开新闻发布会,隆重推出农业部在“十五”期间重点推广的50项农业技术。农业部副部长张宝文出席会议并作了重要讲话。

Calculating changes in fractal dimension of surface cracks to quantify how the dynamic loading rate affects rock failure in deep mining

The split-Hopkinson pressure bar(SHPB)and digital image correlation(DIC)techniques are combined to analyze the dynamic compressive failure process of coal samples,and the box fractal dimension is used to quantitatively analyze the dynamic changes in the coal sample cracks under impact load conditions with different loading rates.The experimental results show that the fractal dimension can quantitatively describe the evolution process of coal fractures under dynamic load.During the dynamic compression process,the evolution of the coal sample cracks presents distinct stages.In the crack propagation stage,the fractal dimension increases rapidly with the progress of loading,and in the crack widening stage,the fractal dimension increases slowly with the progress of loading.The initiation of the crack propagation phase of the coal samples gradually occurs more quickly with increasing loading rate;the initial cracks appear earlier.At the same loading time point,when the loading rate is greater,the fractal dimension of the cracks observed in the coal sample is greater.

Microstructure and mechanical properties of TC4 titanium alloy K-TIG welded joints

The 12 mm-thick Ti−6Al−4V(TC4)titanium alloy plates were welded using keyhole tungsten inert gas(K-TIG)welding at various heat inputs.The microstructure,grain boundary(GB)characteristics and mechanical properties of the weld metal zone(WMZ)were analyzed.The test results show that the K-TIG welds are well formed,and no obvious defects are observed when the heat input is 2.30−2.62 kJ/mm.When the heat input gradually increases,αlaths increase in length,andα′phase and residualβphase are reduced.The electron backscatter diffraction(EBSD)test results indicate that the high-angle GB proportion in the WMZ increases with the increase of heat input.The tensile strength of the WMZ gradually decreases and the elongation of the WMZ increases when the heat input increases from 2.30 to 2.62 kJ/mm.The impact toughness of the WMZ increases as the heat input increases.

Fracture evolution in coalbed methane reservoirs subjected to liquid nitrogen thermal shocking

Thermal shocking effect occurs when the coalbed methane(CBM)reservoirs meet liquid nitrogen(LN2)of extremely low temperature.In this study,3D via X-ray microcomputer tomography(μCT)and scanning electron microscope(SEM)are employed to visualize and quantify morphological evolution characteristics of fractures in coal after LN2 thermal shocking treatments.LN2 thermal shocking leads to a denser fracture network than its original state with coal porosity growth rate increasing up to 183.3%.The surface porosity of theμCT scanned layers inside the coal specimen is influenced by LN2 thermal shocking which rises from 18.76%to 215.11%,illustrating the deformation heterogeneity of coal after LN2 thermal shocking.The cracking effect of LN2 thermal shocking on the surface of low porosity is generally more effective than that of high surface porosity,indicating the applicability of LN2 thermal shocking on low-permeability CBM reservoir stimulation.The characteristics of SEM scanned coal matrix in the coal powder and the coal block after the LN2 thermal shocking presented a large amount of deep and shallow progressive scratch layers,fracture variation diversity(i.e.extension,propagation,connectivity,irregularity)on the surface of the coal block and these were the main reasons leading to the decrease of the uniaxial compressive strength of the coal specimen.

Effect of metallic content on mechanical property of Ni/(10NiO-NiFe_2O_4) cermets

Ni/ （10NiO-NiFe2O4） cermets were fabricated by using cold pressing-sintering method. The phase composition and effect of metallic content on the mechanical properties such as bending strength, Vickers’ hardness, fracture toughness and thermal shock resistance were studied. The results show that the cermets consist of Ni, NiO and NiFe2O4. Within the range of metallic content from 0 to 17%（mass fraction）, the relative density decreases with the increase of metallic content and the decrease of sintering temperature, Vickers’ hardness decreases from 7097MPa to 4814MPa and the bending strength increases from 110MPa to 157MPa, and the fracture toughness reaches the optimal value of 5.11MPa·m 1/2 at the metallic content of about 10%. The residual strength after thermal shock testing falls sharply as the thermal shock temperature difference is above 200℃.The cermets samples, whose metallic content is 10% and 15%, respectively, exhibit promising property of thermal shock resistance at 960℃ with six cycles of heating and quenching testing.

Controlling roof with potential rock burst risk through different pre-crack length:Mechanism and effect research

Roof pre-splitting is an effective method to control the roof with potential rock burst risk.In this study,three-point bending tests were carried out by using fine sandstone specimens with different pre-cracked lengths as test objects,and digital speckle correlation method(DSCM)and acoustic emission(AE)technology were used to track the entire process of crack propagation.The effect of pre-cracks on the fracture of rock beams was evaluated,and the mechanical mechanism of the rock beam fracture process was analyzed.The rock beam pre-splitting design method was developed,and the application effect of the method was proved by the microseismic monitoring data obtained from the 10303 working face of Jining No.2 coal mine in China.The results show that the loading time history curve of pre-cracked beams exhibits obvious residual characteristics.Compared with the intact rock beam,the tensile strength,and maximum tensile strain of 35 mm pre-cracked rock beam are decreased by 32.4% and 33.1%,respectively and the acoustic emission b value is increased by 30.2%.According to the pre-splitting design method of rock beam,the maximum and average microseismic energy of the 10303 working face after pre-splitting construction are reduced by 25.6% and 6.4%,respectively,with excellent prevention and control effect of thick roof.

Preparation and properties of a new cutting pick of coal shearers

Reliability and wear resistance of cutting picks play a significant role in coal mine exploitation with coal shearers.Tool bit separation,blade breaking,severe erosion of the cutting body and fatigue fractures are the main reasons for failure of cutting picks.We carried out carburization on a 30CrMnMo alloy to synthesize a new cutting pick material with improved mechanical properties and high wear resistance.The results indicated that carburization can effectively strengthen the surface of the 30CrMnMo alloy by forming a thick carburized layer and thus significantly improve the surface hardness and wear resistance.In addition,the excellent toughness of 30CrMnMo alloy as a substrate of cutting picks can prevent brittle ruptures and fatigue fractures under high impact stress conditions.The significant decrease in both frictional coefficient and rate of erosion of this carburized 30CrMnMo alloy suggests that this alloy is a potential material for cutting picks of coal shearers after rational carburization.

Influence of process parameters and defect analysis in hot deep drawing process of aluminum alloy AA6082

In this study, a novel punch toolset was developed to investigate the hot stamping of AA6082-T4 sheet. The effect of the process parameters, including forming temperature, punching velocity, friction coefficient, and blank holder force(BHF) on formability was quantified using Taguchi design, analysis of variance(ANOVA) and mathematical statistics. The finite element(FE) model has been established in software Pamstamp for simulation and analysis of their effects on the minimum thickness and thickness variation of the hot-stamped component. The major factors influencing the minimum thickness of the hot-stamped part has been found to be BHF and friction coefficient with influence significance of 35.3% and 34.88%, respectively. Additionally, punch velocity and BHF affect the thickness deviation significantly with influence significance of 40.43% and 35.42%, respectively. Furthermore, a serious thinning occurs on the punch corner region of the hot-stamped cup when the BHF is larger than 2.4 kN. The thickness deviation of the hotformed cup has been found to be firstly decreased and then increased with the increase of punch velocity. Low friction coefficient between punch and blank led to crack at bottom centre of the cup. Moreover, different type, phenomenon and mechanism of defects occurring during hot stamping process, such as crack and wrinkling, were discussed. The crack mode was dimple-dominated ductile fracture, which was induced by micro-void nucleation, growth and coalescence.

Radial explosion strain and its fracture effect from confined explosion with charge of cyclonite

Instrumented experiments were conducted in concrete models to study the explosion-induced radial strain and fracture effect of rock-like media under confined explosion with a charge of cyclonite. As a charge was exploded, two different radial strain waves were sequentially recorded by a strain gage at a distance of 80 mm from the center of charge. Through the attenuation formula of the maximum compressive strain(εrmax), the distribution of εrmax and its strain rate（ ） between the charge and gage were obtained. The effect of the two waves propagating outwards on the radial fracture of surrounding media was discussed. The results show that the two waves are pertinent to the loading of shock energy （Es） and bubble energy （Eb） against concrete surrounding charge, respectively. The former wave lasts for much shorter time than the latter. The peak values of εrmax and of the former are higher than those of the latter, respectively.

Simultaneously measuring initiation toughness of pure mode Ⅰ and mode Ⅱ cracks subjected to impact loads

In order to simultaneously measure the initiation toughness of pure mode Ⅰ and mode Ⅱ cracks in one specimen,a large-size double-cracked concave-convex plate(DCCP)specimen configuration was proposed.Impacting tests were implemented in the drop plate impact device.Strain gauges were employed to measure impact loads and crack initiation time.The corresponding numerical model was established by using the dynamic finite difference program AUTODYN,and the experimental-numerical method and ABAQUS code were utilized to obtain the initial fracture toughness of the crack.Using experiments and numerical research,we concluded that the DCCP specimen is suitable for measuring the initial fracture toughness of pure mode Ⅰ and mode Ⅱ cracks at the same time;the dynamic initiation toughness increases with the increase of loading rate and the crack initiation time decreases with increasing loading rate;the initiation toughness of mode Ⅱ crack is 0.5 times that of mode Ⅰ crack when subjected to the same loading rate.For the pre-crack in the vicinity of the bottom of a sample,when its length increases from 20 to 100 mm,the dynamic initiation toughness of the pure mode Ⅰ crack gradually decreases,and the longer the lower crack length is,the easier the crack would initiate,but the dynamic initiation toughness of pure mode Ⅱ crack varies little.

Degradation mechanism of rock under impact loadings by integrated investigation on crack and damage development

Failure of rock under impact loadings involves complex micro-fracturing and progressive damage. Strength increase and splitting failure have been observed during dynamic tests of rock materials. However, the failure mechanism still remains unclear. In this work, based on laboratory tests, numerical simulations with the particle flow code(PFC) were carried out to reproduce the micro-fracturing process of granite specimens. Shear and tensile cracks were both recorded to investigate the failure mode of rocks under different loading conditions. At the same time, a dynamic damage model based on the Weibull distribution was established to predict the deformation and degradation behavior of specimens. It is found that micro-cracks play important roles in controlling the dynamic deformation and failure process of rock under impact loadings. The sharp increase in the number of cracks may be the reason for the strength increase of rock under high strain rates. Tensile cracks tend to be the key reason for splitting failure of specimens. Numerical simulation of crack propagation by PFC can give vivid description of the failure process. However, it is not enough for evaluation of material degradation. The dynamic damage model is able to predict the stress-strain relationship of specimens reasonably well, and can be used to explain the degradation of specimens under impact loadings at macro-scale. Crack and damage can describe material degradation at different scales and can be used together to reveal the failure mechanism of rocks.

Prediction of crack location and propagation in stretch flanging process of aluminum alloy AA-5052 sheet using FEM simulation

The stretch flanging process is significantly affected by various geometrical,material and process parameters.The punch-die clearance and initial flange length are main parameters which have major effects on the edge crack location and strain distribution along die profile radius in the flange.Non-axisymmetric stretch flanging process of AA-5052 sheet metal blanks was carried out by numerical simulation to predict the deformation behavior of flange,location and propagation of crack in flange and to investigate the effect of punch die clearance,flange length,die and punch profile radius and friction in the stretch flanging process.The experimental investigations were made to validate the simulations results.The results reveal that the crack length increases with the increase in the flange length.It is found that the flange length has a significant effect in circumferential direction as compared with the radial direction.The punch die clearance has the most significant effect in crack propagation in comparison with flange length.The circumferential strain is found to be larger in the case of punch having the profile radius less than the die profile radius,which leads to faster edge crack propagation.A close agreement is found between simulation and experimental results in terms of location of edge crack and forming load.

Energy transfer theory and application research of accumulation energy district on coal and rock mass

Using research results on physical properties of primary coal and rock mass, in this paper, response characteristic function y（t）, amplitude frequency characteristic H（ω） and phase-frequency characteristic φ（ω） were used to describe dynamic response of stress impulse signal. At the same time, with the help of computer simulation analogue technique, applying these characteristic parameters to research the rock-burst forecasting, this paper deduced response characteristic function model. This function model is valuable for rockburst tendency test. According to these researches, develops monitoring recording system to acquire rockburst precursor signals. This system can monitor stress impulse signal dynamically and continuously. By applying the forecast information systems can forecast rockburst successfully.