• <tr id="yyy80"></tr>
  • <sup id="yyy80"></sup>
  • <tfoot id="yyy80"><noscript id="yyy80"></noscript></tfoot>
  • 99热精品在线国产_美女午夜性视频免费_国产精品国产高清国产av_av欧美777_自拍偷自拍亚洲精品老妇_亚洲熟女精品中文字幕_www日本黄色视频网_国产精品野战在线观看 ?

    Effect of mandrel on the forming quality of thin walled tube

    2015-12-19 08:48:11ChaoWUYongYANZhiHUHubeiKeyLaboratoryofAdvancedTechnologyforAutomotiveComponentsWuhanUniversityofTechnologyWuhan430070ChinaHubeiCollaborativeInnovationCenterforAutomotiveComponentsTechnologyWuhan430070China
    機床與液壓 2015年12期
    關(guān)鍵詞:頭球芯棒管件

    Chao WU,Yong YAN,Zhi-i HU,(Hubei Key Laboratory of Advanced Technology for Automotive Components,Wuhan University of Technology,Wuhan 430070,China)(Hubei Collaborative Innovation Center for Automotive Components Technology,Wuhan 430070,China)

    Effect of mandrel on the forming quality of thin walled tube

    Chao WU1*
    ,Yong YAN2,Zhi-1i HU1,2
    (1Hubei Key Laboratory of Advanced Technology for Automotive Components,Wuhan University of Technology,Wuhan 430070,China)
    (2Hubei Collaborative Innovation Center for Automotive Components Technology,Wuhan 430070,China)

    Mandrel shows a great influence on the quality of the tube during tube bending process.Especially,the selection of mandrel type directly affects the forming quality of the tube.In the paper,a 3D simulation model of stainless steel tube was established by finite element platform ABAQUS/Explicit.The influence of five kind mandrels on the quality of bended tube was investigated by simulation and experiment.It is found that the cylinder,round and arc mandrels show a great advantage in controlling the outer wall thinning compared with ball and round ball mandrels.Arc mandrel is benefit to reduce wall thinning of tube bending,while the ball mandrel exhibits the best performance among all the five kind mandrels during tube bending.The finite element simulation results show good agreement with the experimental results,and provide a theoretical guidance for the selection of the mandrel type in the actual production.

    Thin-walled tube,NC bending,Mandrel,F(xiàn)inite element simulation

    Hydromechatronics Engineering

    http://jdy.qks.cqut.edu.cn

    E-mail:jdygcyw@126.com

    1 Introduction

    As the thin-walled bending tube could meet the lightweight and low cost requirements of the products,it has a wide range of applications in aviation,aerospace,automotive and other fields[1].However,the thin-walled tube is hollow in structure and the radiusthickness ratio(the ratio of diameter and thickness)is large.It is easy to appear forming defects such as excessive outer wall thinning,wall instability wrinkling and cross section distortion in the bending forming process,which largely affects the forming quality and performance characteristics of the bending tube.To meet the needs of engineering,these defects should be avoided or controlled within a narrow range.It has been proved that a reasonable selection of mandrel could control these defects effectively[2].

    The mandrel is placed inside the tube as a role of support and constraints during the tube bending process.There are various forms of the mandrel,such as the cylinder mandrel,round mandrel,arc mandrel,ball mandrel and the combination of several forms. The influence of the mandrel forms on the quality of thin-walled bending tube is great.To choose the proper mandrel,nowadays the domestic and foreign researchers mostly concentrate on the traditional bending process.Moreover,the small diameter tubes are used as the main study object,which lack the relevant reports for larger diameter thin-walled tube NC bending. In the actual production and processing,it still be dominated by empiricism[3-5].A theoretical analytic model of the mandrel in the bending process was established[6].The influence of mandrel parameters on the tube NC bending process was analyzed,and the range of theoretical parameters of the mandrel was obtained.But the article was only based on the one form of mandrel,which lacks the comparison with different kinds of mandrels.The effect of three kinds of man-drels(cylinder mandrel,round mandrel and arc mandrel)on the quality of bended tube was discussed[7].The influence rule of tube bending under the support of these forms of mandrel was achieved.But the discussion to the forms of mandrel was not comprehensive enough,and the common mandrel in the current wasn’t covered.

    The tube NC bending technology is a new process which is developed from the traditional tube bending craft combined with CNC technology.It can make the tube bending process intelligent,flexible and automated,and meet the requirements for high precision and high efficiency of the product[8].Finite element numerical simulation method is the use of finite element analysis and related critical forming metrics in the actual metal forming process,obtaining the forming limit of metal[9].The finite element simulation can be used to characterize the thin-walled tube and bending die by selecting the appropriate elements.Thus the contact relationship between the tube and die can be described reasonably and the movement of each bending die can be simulated effectively.As a result,the wall thickness distribution and the stress and strain distribution can be obtained.Therefore,the forming defects can be predicted.

    The paper was based on the finite element analysis theory.A 3D simulation model of tube NC bending was established by finite element platform ABAQUS. Based on the explicit integration algorithm,the finite element numerical simulation was used to simulate the tube bending supported by different kinds of mandrels.Finally,an experiment was made as a contrast,in order to provide a theoretical guidance for the choice of the mandrel type in the actual production.

    2 Stress and strain analysis

    In the action of bending moment M,the stressstrain condition of the tube is shown in Fig.1.In the process of tube bending,the outer wall of the tube suffers a two-way tensile stress,and the inner wall is subjected to the action of three-way compressive stress.The outer wall of tube is thinning under the action of the tangential tensile stress σθ;the instability wrinkling occurs on the inner wall of tube in the role of the tangential compressive stress σo;the cross section tends to distort under the action of tensile and compressive stress at the same time.

    Fig.1 Stress-strain condition of tube bending

    The wall is easily to crack if the outer wall thinning of tube is too large during the bending process,as shown in Fig.2(a).When the inner wall thickening of tube is serious,the wall instability wrinkling may appear due to the material cannot maintain a stable deformation,as shown in Fig.2(b).And the combining action of large tensile and compressive stress leads to the lateral diameter of the tube increasing,the normal diameter reducing,and makes it elliptical,as shown in Fig.2(c).

    Fig.2 The faiIure modes of the tube bending process

    3 Finite element model

    The process of tube forming is complex,mainly involves three nonlinear problems of mechanics:geometric nonlinearity,namely large displacement,and large rotation or large deformation occurred during the tube bending process;physical nonlinearity,that is material nonlinearity;boundary nonlinearity,that is non-linear relationship caused by the contact friction between the die and workpiece[10-11].Therefore,based on the finite element platform ABAQUS/Explicit,a finite element modeling was built to describe the process in this paper,which can solve the nonlinear problems of tube bending simulation effectively.

    3.1 Geometric model

    According to the theory of NC tube bending,combined with the actual bending process,a three-dimensional model was used to describe the tube,clamp die,die insert,pressure die,wiper die,bend die,mandrel(flexible balls)and other components.The established geometric model is shown in Fig.3. Among them,the tube which will be out of shape is deformable body,and the rest are rigid bodies.

    Fig.3 Finite eIement modeI for NC tube bending

    3.2 Material parameters

    The 304 stainless steel was used to simulate the tube NC bending.The outside diameter of the tube(D)is 60 mm;the wall thickness of the tube(t)is 0.73 mm;the bending radius(R)is 120 mm.The density of tube(ρ)is 7 800 kg/m3;the elastic modulus(E)is 207 GPa;the Poisson's ratio(λ)is 0.28. The uniaxial tensile test was conducted according to GB/T228-2002“tensile test methods for metallic materials at room temperature”.The basic parameters in the mechanical properties of the tube were obtained. The material model is shown in formula(1)and the tensile specimen is shown in Fig.4.

    The material hardening occurs in the process of tube NC bending.Thus the exponent function is used to characterize the hardening behavior of material[12]

    where K is intensity factor,1 426 MPa;ε0is prestrain,0.028;n is hardening exponent,0.5.

    Fig.4 TensiIe specimen materiaI

    The 304 stainless steel stress-strain curve was obtained through data processing,as shown in Fig.5.

    3.3 Element type

    Since the four-node double curvature shell elements S4R(4-node doubly curved thin shell,reduced integration,hourglasscontrol,andfinitemembrane strains)has a feature of hourglass control and reduced integration,the element type is used to describe the tube.Five integration points was chosen in the thickness direction.The discrete rigid shell elements R3D4(4-node 3D bilinear rigid quadrilateral)was used to describe the dies,such as bend die,pressure die,wiper die,clamp die,die insert,and mandrel(flexible balls)[13].

    Fig.5 The stress-strain reIation of tube

    3.4 Contact and friction conditions

    The appropriate contact algorithm can make the motion of dies in the simulation closer to the actual bending process.The contact interfaces contain tube/ clamp die,tube/die insert,tube/pressure die,tube/ wiper die,tube/bend die and tube/mandrel(balls). Surface to surface contact mode was selected to describe the contact algorithm between dies and tube. According to the actual process,the“small sliding mode”was chosen to characterize the movement type between tube and clamp die and the“finite sliding mode”was used to describe the rest of the contact interface[14].The Coulomb friction was selected to describe the contact between the tube and dies.The coefficients of friction between the contact interfaces are shown in Table 1.

    TabIe1 The friction condition of contact interface

    3.5 Boundary conditions

    Based on the ABAQUS platform,the two kinds of constraints,those are Rotation and Velocity,were used to apply load among dies.The dies,such as bend die,clamp die and die insert,could only rotate around the bending center,and all the remaining five degrees of freedom were constrained.The pressure die could only move along the bending direction,the moving speed of pressure die was equal to the linear velocity of bend die in the bending process.All the mobile and rotational degrees of freedom of wiper die were constrained in the bending process.The mandrel was fixed in the bending process.If it contains balls,the corresponding connecting unit is used to describe the relationship between the mandrel and balls.

    4 Mandrel type

    Since the mandrel plays an important role in the bending process,the mandrel is usually used in the tube NC bending.There are many types of mandrel shapes.For the tube with different relative bending radius R/D or radius-thickness ratio D/t,the proper mandrel should be chosen[15].According to the structure characteristic of the mandrel,the mandrel can be roughly classified into two categories:one is the integral mandrel,such as cylinder mandrel(a),round mandrel(b)and arc mandrel(c);the other is articulated mandrel(there are several parts in front of mandrel),such as ball mandrel(d)and round ball mandrel(e).

    The structure of cylinder mandrel is simple,the cost of that is low,and it is easy to manufacture.The end of the round mandrel is hemispherical and the end of arc mandrel is near parabolic.The front of ball mandrel is formed by a plurality of balls.The relationship between the mandrel and the ball or the each ball is similar to the hinge connection.Thus the ball can be adaptive rotated with the rotation of dies in a certain angle.Round ball mandrel is a deformed structure of ball mandrel,but the hemispherical ball is used at the end.

    These common forms of the mandrel all can play a supporting role in tube bending.But to the different types of mandrel,there are certainly differences in the supporting effect.It lacks of persuasiveness and credibility only by theoretical analysis.In order to achieve the optimal supporting effect,it also needs to obtain the effects of different forms of mandrel to the quality of the tube by the finite element simulation results.

    Fig.6 The common types of mandreI

    5 Numerical analysis of mandrels

    The five kinds of mandrels were used in the finite element simulation models respectively.The bending angles of the tube were 90°.In order to make the tube inside better supported by mandrel,a reasonable overhang of the mandrel should be selected.For the cylinder mandrel,round mandrel and arc mandrel,the overhang of the mandrel was 10 mm.And for the ball mandrel,the Mandrel parameters show great influence on the forming of the tube,as shown in Fig.7. Therefore,the reasonable parameters were chosen. The outer wall thinning and inner wall thickening were used to measure the change of the wall thickness,and the computation formulas were as follows.

    where t is original wall thickness of tube(mm);tminis the minimum wall thickness(mm);tmaxis the maximum wall thickness(mm).

    Fig.7 The forming tube under different parameters of the baII mandreI

    5.1 Effect of mandrels on wall thinning

    After the simulation finished,the tube was cut along the centerline of the tube.Then the thickness values of the outer wall from the pressure die to clamp die were read by the Postprocessor of ABAQUS.In according to formula(2),the wall thinning of the tube was calculated,and got the data processed.Then the data was imported to the Graphics software(Origin Pro),thus the wall thinning curves were obtained. The contrast curves of outer wall thinning under the support of five kinds of mandrels were obtained,as shown in Fig.8.

    As shown in Fig.8,under the support of articulated mandrel,such as ball mandrel and round ball mandrel,the outer wall thinning of tube is significantly higher than which of the integral mandrel,such as cylinder mandrel,round mandrel and arc mandrel. The effect on wall thinning is little difference between ball mandrel and round ball mandrel,but the rest three have some differences.Compared with other mandrels,arc mandrel is benefit to reduce wall thinning of tube in the bending process.

    Fig.8 Effects of different mandreIs on waII thinning of the tube

    Fig.9 is the diagram of tube supported by round mandrel(articulated mandrel)and ball mandrel(integral mandrel)respectively.As shown in the figure,the support angle is bigger when the tube is supported by ball mandrel(support angle is the angle between the line which is from bending center O to the tangent point of the front of mandrel and original bending plane that go across bending center O),which makes the contact area between tube and the internal wall of tube increase.Thus the frictional resistance increases when the tube rotates around the bend die in the bending process,so the tensile stress of the outer tube will increase.That results in the material fiber of tube outside increased,so the wall thinning is larger.

    5.2 Effect of mandrels on wall thickening

    Due to its structural limitations of the mandrel,the severe wrinkling defects will appear in the inner wall of bending tube when it is supported by integral mandrel,such as cylinder mandrel,round mandrel and arc mandrel,as shown in Fig.10(Avg 75%is the default threshold criteria.If the relative nodal variation for each node is less than averaging threshold,the contributing values from elements are averaged at that node).

    Fig.9 The supporting diagram of the tube by different mandreIs

    Fig.10 The forming diagram under different mandreIs

    By the analysis,it can be seen that when the tube is supported by integral mandrel,the plastic deformation zone of tube get small rigid bound area by mandrel.Thus the tangential pressure stress of the tube inside is large and the gap between the mandrel and inner wall of tube is wide,which provides sufficient space for the accumulation of material in the bending process.Thus leads to the serious wrinkling in the inner wall after tube bending.

    Due to the severe wrinkling of the forming tube,it cannot play a normal function in engineering applications.Thus it has no practical significance to discuss the effects of these mandrels on wall thickening of the tube.To obtain the impact of mandrel structure on the inner wall thickening,just two mandrels(ball mandrel and round ball mandrel)are discussed here.

    The simulation results were calculated in according to formula(3),and got the data processed.The inner wall thickening contrast curve under two kinds of mandrels supported are obtained,as shown in Fig.11.

    As shown in Fig.11,the effect on wall thickening is approximate between ball mandrel and round ball mandrel,all of which are small.The inner wall does not appear obvious wrinkling,and it shows that the inner wall of the tube can be better constrained by the articulated mandrel.However,there are some differences in wall thickening at different positions.When the measurement position is greater than 45°,ball mandrel shows advantage in control wall thickening.

    Fig.11 Effects of different mandreIs on waII thickening of the tube

    5.3 Comparison with different mandrels

    The effect of five kinds of mandrels on outer wall thinning,inner wall thickening and instability wrinkling of the tube has been discussed.It is found that integral mandrel,such as cylinder mandrel,round mandrel and arc mandrel,is benefit to reduce wall thinning of tube bending.But the severe wrinkling will occur at the inner wall of tube.While the instability wrinkling could get better controlled under the support of articulated mandrel(ball mandrel and round ball mandrel).Though the thinning of the outer wall is slightly larger than that supported by the integral mandrel,the wall thinning can be controlled within the range allowed by the industry standard as long as the mandrel parameters are reasonable selected. Thus the performance of tube can be satisfied.

    In addition,the bending section of tube can be fully supported and restricted under the support of articulated mandrel.Thus the cross section distortion can be significantly improved.The supporting diagram is shown in Fig.12.

    Fig.12 The supporting diagram of the tube by baIIs

    In the process of tube bending,the lateral of the tube suffers tangential tensile stress,and the inner is subjected to the tangential compressive stress.Under the role of tensile and compressive stress at the same time,the cross section has a tendency to distortion. However,the ball of ball mandrel can reach the sensitive area where the section of tube is easy to be out of shape.The support forces N′1and N′2,which are the forces that the balls give to the inner wall,are in the opposite direction of tangential tensile stress N1and tangential pressure stress N2respectively.Therefore,the appropriate choice of the ball can offset part of the normal compressive stress,and the cross section distortion gets better controlled.

    Integrating the indicators,it is indicated that the tube exhibits the better performance under the support of articulated mandrel(ball mandrel and round ball mandrel),as shown in Fig.13.Especially the wrinkling and cross section distortion get a great degree of improved.These two forms of defects will seriously restrict the actual use of performance of tube.To ensure the well comprehensive quality of tube,now the articulated mandrel,such as ball mandrel,is widely used as an internal support structure in CNC bending machine.

    Fig.13 The forming diagram under different mandreIs

    6 Experimental verification

    It needs to be tested by experiment to determine whether the numerical results are of practical significance.Compared to several other mandrels,the ball mandrel exhibits the best performance.Therefore,the ball mandrel is chosen as the research object of the experiment in the paper.

    Experimental conditions:with a stainless steel tube as the research object,the outer diameter of the tube is 60 mm,the wall thickness is 0.73 mm,the bending radius is 120 mm,and the diameter of the mandrel is 57.6 mm.With three balls at the front of mandrel,the diameter of the ball is 57.5 mm,the thickness of the ball is 19 mm,and the interval between balls is 3 mm.The special lubricant is used to lubricate mandrel,balls,wiper die and wall of tube in the experiment.Besides,it must ensure that there is no relative sliding between the clamping die and tube.

    The experiment of thin-walled tube bending was carried out on CNC bending machine.After the completion of the experiment,the special cutting device was used to cut the forming tube along the centerline of the tube.The cutting sample is shown in Fig.14. The wrinkling on the inner wall is a typical defect in the tube bending.Under this experimental condition,the wrinkling also occurred in the simulation.To some extent,the inner wall has been validated by comparing the wrinkling.On the other hand,the out wall thinning was chosen to verify the reliability of simulation by numerical calculations.Thus the accuracy and effectiveness of the simulation results would be better verified.The calipers was used to measure the thickness of outer wall at five positions(0°,30°,45°,60° and 90°).The results were calculated in according to formula(2).The wall thinning obtained by numerical simulation are 7.7%,14.9%,15.3%,14.8%and 10.8%,while the wall thinning achieved by the experiment are 7.8%,15.1%,15.8%,13.7%and 10.2%.The contrast curve between simulation results and experimental measurements is shown in Fig.15.

    As shown in Fig.15,under the same conditions,that is,the original parameters stay the same in the numericalsimulationandexperiment,thewall thinning is approximate at the same position.The maximum error of the wall thinning is 7.4%.The finite element simulation results shows good agreement with the experimental results,which indicates the thin-walled tube NC bending can be simulated by the finite element simulation reliably.Besides,the finite element numerical simulation method can reduce the number of tests and cost,which has certain guiding significance for the actual production.

    Fig.14 The experiment sampIe

    Fig.15 Comparison between the simuIation data and experimentaI resuIts

    7 Conclusions

    In order to study the effect of different mandrels on the forming quality of thin-walled tube,the finite element numerical simulation methods,combined with experimental validation,are used in the paper.The following conclusions are drawn from the present study.

    1)The outer wall thinning of tube is significantly higher than that of the integral mandrel under the support of articulated mandrel,such as ball mandrel and round ball mandrel.

    2)Among the five kinds of mandrels,arc mandrel is benefit to reduce wall thinning of tube bending,while the instability wrinkling and cross section distortion can be better controlled under the support of ball mandrel.

    3)The distortion of the cross section significantly improved as the bending section of tube is supported by balls during tube bending.It is found that the type of mandrel is the main factor that influences the cross section distortion of the tube.

    4)The ball can be adaptive rotated with the rotation of the tube under the support of the ball mandrel. Thus,the forming quality of the bending tube is better.

    5)The thin-walled tube NC bending can be simulated by the finite element simulation reliably for the finite element simulation results showing good agreement with the experimental results.

    Acknowledgements

    This paper is supported by the Fundamental Research Funds for the Central Universities(2014-IV-042).

    [1]Ye H M,Zhang W,F(xiàn)eng L,etal.Numerical simulation and experimental studies of the mandrel parameters in the thinwalled pipe rotary bending[J].Modern Manufacturing Engineering,2013(10):73-78.

    [2]Lee H,Van Tyne C J,F(xiàn)ield D.Finite element bending analysis of oval tubes using rotary draw bender for hydroforming applications[J].Journal of Materials Processing Technology,2005,168(2):327-335.

    [3]Tang N C.Plastic-deformation analysis in tube bending[J].International Journal of Pressure Vessels and Piping,2000,77(12):751-759.

    [4]Hu F T.Theory and experimental research on shaped tube and profile bending process without die[J].Qiqihar:The Northeast Heavy Machinery Institute,1995.

    [5]Hu Z,Li J Q.Computer simulation of pipe-bending processes with small bending radius using local induction heating[J]. Journal of Materials Processing Technology,1999,91(1):75-79.

    [6]Shen S J,Yang H,Li H,etal.Experimental study on role of mandrel in thin-walled tube NC bending process with small bending radius[J].Journal of Plasticity Engineering,2008,14(4):29-34.

    [7]Tang D,Li D Y,Peng Y H.Finite element analysis to mandrels in rotary draw bending of copper tube[J].China Mechanical Engineering,2008(S1).

    [8]Hu J,Wang Y H,Wu Z Y,etal.Current state and developing trends of CNC[J].Mechanical Engineer,2000(3):5-7.

    [9]Yan J,Yang H,Zhan M,etal.Forming limits under multi-index constraints in NC bending of aluminum alloy thinwalled tubes with large diameters[J].Sci China:Tech Sci,2010(6):601-618.

    [10]Zhong Z H,Li G Y.Computer simulation and application of sheet metal forming process[M].Beijing:Beijing Institute of Technology Press,1998.

    [11]Yang J F,Zhang Z G.Study of metal plastic forming methods[J].Metal forming technology,1990(2:):55-59.

    [12]Zhang J J,Yang H,Zhan M,etal.Effects of pressing die's boosting function on wall thinning and springback in NC bending process of thin-walled tube with large diameter[J]. Journal of Plasticity Engineering,2008,15(1):60-65.

    [13]Wang Z K,Yang H,Li H,etal.Springback laws of large diameter 316L stainless steel tube in NC bending[J]. Materials Science&Technology,2012,4:010.

    [14]Li H,Yang H,Zhan M,etal.Effect mechanism of friction between tube and various dies in thin-walled tube NC bending process[J].2005.

    [15]Jin G M.Selection and use of mandrel for tube bending[J].Forging&Stamping Technology,1999,24(2):33-35.

    芯棒形式對薄壁管彎曲成形質(zhì)量的影響

    吳 超1*,嚴 勇2,胡志力1,2
    1.武漢理工大學(xué)現(xiàn)代汽車零部件技術(shù)湖北省重點實驗室,武漢 430070
    2.汽車零部件技術(shù)湖北省協(xié)同創(chuàng)新中心,武漢 430070

    薄壁管彎曲成形過程中芯棒對彎管的成形質(zhì)量影響較大,芯棒形式的選取是否合理直接關(guān)系著管件成形質(zhì)量的優(yōu)劣。采用有限元平臺ABAQUS/Explicit建立不銹鋼管三維仿真模型,討論了圓柱式、圓頭式、弧式、球窩式和圓頭球窩式5種形式的芯棒對彎管成形質(zhì)量的影響,并對球窩式芯棒進行了實驗驗證。結(jié)果表明:在球窩式和圓頭球窩式等關(guān)節(jié)式芯棒的支撐下彎曲管件的外壁減薄率比圓柱式、圓頭式以及弧式芯棒等整體式芯棒高;弧式芯棒較有利于減輕管件彎曲后外壁減薄情況;球窩式芯棒有利于獲得綜合性能更好的成形管件;有限元仿真模型能夠較可靠地模擬實際數(shù)控彎曲成形,為生產(chǎn)加工芯棒的選取提供了理論指導(dǎo)。

    薄壁管;數(shù)控彎曲;芯棒;有限元仿真

    10.3969/j.issn.1001-3881.2015.12.009Document code:A

    TG386.43

    21 November 2014;revised 12 February 2015;accepted 8 March 2015

    *Corresponding author:Chao WU,Master degree candidate.

    E-mail:wuchaoktzdk@163.com

    猜你喜歡
    頭球芯棒管件
    足球?qū)⑻蕴^球?
    海外文摘(2023年1期)2023-01-11 01:04:20
    芯棒環(huán)形傷產(chǎn)生原因及整改措施
    鋼管(2021年5期)2021-03-09 01:32:58
    限動芯棒在線速度模型的研究與應(yīng)用
    鋼管(2020年4期)2020-10-29 06:13:12
    Mn-Ni-Mo鋼埋弧焊增材制造管件的性能和組織研究
    架空導(dǎo)線用纖維增強樹脂基復(fù)合材料芯棒的彎曲力學(xué)性能仿真
    電線電纜(2018年3期)2018-06-29 07:41:00
    芯棒對鋼管質(zhì)量的影響
    細長薄壁管件車削變形誤差控制研究
    頭球真會導(dǎo)致記憶下降
    百科知識(2016年23期)2016-12-24 16:09:37
    45°彎頭管件注塑模設(shè)計
    中國塑料(2015年7期)2015-10-14 01:02:52
    核電站核級管件的制造與使用
    一区二区三区免费毛片| 久久欧美精品欧美久久欧美| 又黄又爽又免费观看的视频| 日韩中文字幕欧美一区二区| 两性午夜刺激爽爽歪歪视频在线观看| 俺也久久电影网| 久久久成人免费电影| 亚洲国产精品sss在线观看| 国产老妇女一区| 在线天堂最新版资源| 亚洲中文字幕一区二区三区有码在线看| 成人国产综合亚洲| 日韩中文字幕欧美一区二区| 国产极品精品免费视频能看的| 色5月婷婷丁香| 一级毛片久久久久久久久女| 亚洲av美国av| АⅤ资源中文在线天堂| 精品乱码久久久久久99久播| 国产精品一及| 国产精品三级大全| 丰满乱子伦码专区| 99久久中文字幕三级久久日本| 男人舔女人下体高潮全视频| 美女大奶头视频| 亚洲成人中文字幕在线播放| 亚洲欧美日韩无卡精品| 此物有八面人人有两片| 成人欧美大片| 国产真实伦视频高清在线观看 | 国产亚洲精品av在线| 国产高清三级在线| 可以在线观看毛片的网站| 欧美高清成人免费视频www| 日韩欧美在线乱码| 内射极品少妇av片p| 特级一级黄色大片| 日韩国内少妇激情av| 成人国产综合亚洲| 国内精品美女久久久久久| 成人国产一区最新在线观看| 亚洲最大成人av| 在线免费十八禁| 久久久久久久亚洲中文字幕| 免费观看在线日韩| 国产av不卡久久| 99久久精品国产国产毛片| 九九爱精品视频在线观看| 少妇熟女aⅴ在线视频| 人人妻人人澡欧美一区二区| 欧美日韩乱码在线| 国产精品久久电影中文字幕| 亚洲av二区三区四区| 3wmmmm亚洲av在线观看| 性插视频无遮挡在线免费观看| 国产精品久久久久久久久免| 99久久九九国产精品国产免费| 精华霜和精华液先用哪个| 99九九线精品视频在线观看视频| 美女 人体艺术 gogo| 国产激情偷乱视频一区二区| 亚洲乱码一区二区免费版| 久久天躁狠狠躁夜夜2o2o| 免费看av在线观看网站| 国产精品乱码一区二三区的特点| 丰满的人妻完整版| 99在线视频只有这里精品首页| 别揉我奶头 嗯啊视频| 国产精品98久久久久久宅男小说| 国产高清激情床上av| 少妇的逼水好多| 色哟哟哟哟哟哟| 丰满的人妻完整版| 变态另类丝袜制服| 嫁个100分男人电影在线观看| 亚洲成a人片在线一区二区| 中文资源天堂在线| 午夜福利高清视频| 男女做爰动态图高潮gif福利片| 精品久久久久久久久av| 夜夜夜夜夜久久久久| 亚洲在线自拍视频| 国产乱人伦免费视频| 一级a爱片免费观看的视频| 亚洲av成人av| 国产高清有码在线观看视频| 国产综合懂色| 亚洲,欧美,日韩| 亚洲国产精品sss在线观看| 亚洲天堂国产精品一区在线| 国产毛片a区久久久久| 中文字幕av成人在线电影| 精品一区二区免费观看| 久99久视频精品免费| 天堂影院成人在线观看| 日韩欧美在线乱码| 国内揄拍国产精品人妻在线| 91在线精品国自产拍蜜月| 精品日产1卡2卡| 国产精品人妻久久久久久| 99视频精品全部免费 在线| 免费电影在线观看免费观看| 成人午夜高清在线视频| 91精品国产九色| 97人妻精品一区二区三区麻豆| 精品乱码久久久久久99久播| 久久久久性生活片| 麻豆av噜噜一区二区三区| 一个人看视频在线观看www免费| 悠悠久久av| 男女边吃奶边做爰视频| 国产精品嫩草影院av在线观看 | 中文字幕高清在线视频| 久久精品人妻少妇| 欧美一区二区亚洲| xxxwww97欧美| 少妇熟女aⅴ在线视频| 亚洲精品亚洲一区二区| 亚洲一级一片aⅴ在线观看| 真人一进一出gif抽搐免费| 中文字幕免费在线视频6| 国产色爽女视频免费观看| 日本成人三级电影网站| 一a级毛片在线观看| 免费搜索国产男女视频| 精品午夜福利视频在线观看一区| 在线天堂最新版资源| 午夜福利欧美成人| 嫩草影院入口| 女的被弄到高潮叫床怎么办 | 国产精品人妻久久久影院| 国产黄色小视频在线观看| 一进一出抽搐动态| 久久人妻av系列| 亚洲中文字幕日韩| 18禁黄网站禁片免费观看直播| 亚洲精华国产精华精| 婷婷丁香在线五月| 国产探花极品一区二区| 免费不卡的大黄色大毛片视频在线观看 | 给我免费播放毛片高清在线观看| 亚洲精品乱码久久久v下载方式| 又紧又爽又黄一区二区| 久久99热这里只有精品18| 久久国内精品自在自线图片| 国产色爽女视频免费观看| 国产毛片a区久久久久| 精品乱码久久久久久99久播| 中亚洲国语对白在线视频| 91麻豆av在线| 麻豆久久精品国产亚洲av| 99久久中文字幕三级久久日本| 能在线免费观看的黄片| 日韩一区二区视频免费看| 亚洲专区国产一区二区| 天堂动漫精品| 欧美色视频一区免费| 欧美一级a爱片免费观看看| 亚洲 国产 在线| 亚洲av第一区精品v没综合| 欧美国产日韩亚洲一区| 一个人看的www免费观看视频| 日韩欧美三级三区| 久久久久九九精品影院| 尾随美女入室| 黄片wwwwww| 桃红色精品国产亚洲av| 亚洲三级黄色毛片| 亚洲国产精品sss在线观看| 欧美日韩乱码在线| 一卡2卡三卡四卡精品乱码亚洲| 欧美日韩综合久久久久久 | 国产精品女同一区二区软件 | av在线蜜桃| 久久热精品热| 无遮挡黄片免费观看| 亚洲美女视频黄频| 18禁在线播放成人免费| 内地一区二区视频在线| 亚洲人成网站在线播| 久久精品国产99精品国产亚洲性色| 亚洲欧美激情综合另类| 老司机午夜福利在线观看视频| 亚洲第一电影网av| 日本三级黄在线观看| 观看免费一级毛片| 成人一区二区视频在线观看| 成人精品一区二区免费| 成人毛片a级毛片在线播放| av天堂在线播放| 亚洲一区二区三区色噜噜| 亚洲国产精品sss在线观看| 18禁在线播放成人免费| 人人妻人人看人人澡| 神马国产精品三级电影在线观看| 欧美性猛交黑人性爽| 人妻久久中文字幕网| 国产探花极品一区二区| 成人三级黄色视频| 一本一本综合久久| 色5月婷婷丁香| 亚洲一级一片aⅴ在线观看| 一本久久中文字幕| 日韩在线高清观看一区二区三区 | 男人舔女人下体高潮全视频| 亚洲欧美清纯卡通| 尤物成人国产欧美一区二区三区| 久久热精品热| 丰满乱子伦码专区| 精品久久久久久久久久久久久| 小说图片视频综合网站| 一本久久中文字幕| 成人美女网站在线观看视频| 男人舔女人下体高潮全视频| 禁无遮挡网站| 免费看光身美女| 美女大奶头视频| 日本一二三区视频观看| 亚洲欧美日韩高清在线视频| 久久精品国产自在天天线| 女人十人毛片免费观看3o分钟| 非洲黑人性xxxx精品又粗又长| 日本成人三级电影网站| 亚洲国产欧洲综合997久久,| 免费看日本二区| 国产探花极品一区二区| 天堂√8在线中文| 人妻少妇偷人精品九色| 一个人看的www免费观看视频| 高清日韩中文字幕在线| 久久久久性生活片| 少妇熟女aⅴ在线视频| 最新中文字幕久久久久| 禁无遮挡网站| 日韩一区二区视频免费看| 久久精品国产亚洲av香蕉五月| 国产伦人伦偷精品视频| 成人一区二区视频在线观看| 色在线成人网| 人妻少妇偷人精品九色| 不卡视频在线观看欧美| 亚洲国产欧美人成| 欧美丝袜亚洲另类 | 黄色视频,在线免费观看| 亚洲欧美激情综合另类| 日本免费一区二区三区高清不卡| 美女xxoo啪啪120秒动态图| 观看免费一级毛片| 亚洲专区中文字幕在线| 最近视频中文字幕2019在线8| 婷婷丁香在线五月| 久久热精品热| 男女视频在线观看网站免费| 少妇熟女aⅴ在线视频| 黄色女人牲交| 久久精品国产99精品国产亚洲性色| 精品福利观看| 免费观看人在逋| 成年女人毛片免费观看观看9| 精品日产1卡2卡| 亚洲中文字幕日韩| 变态另类丝袜制服| 国产日本99.免费观看| 中文字幕av在线有码专区| 国产蜜桃级精品一区二区三区| 国产成人影院久久av| 中文字幕久久专区| 免费观看在线日韩| 综合色av麻豆| 亚洲美女搞黄在线观看 | 1000部很黄的大片| 丰满乱子伦码专区| 亚洲在线观看片| 精品人妻偷拍中文字幕| 久久午夜福利片| 中文字幕久久专区| 日本黄大片高清| 午夜福利在线观看吧| 直男gayav资源| 午夜影院日韩av| 免费搜索国产男女视频| 日韩一区二区视频免费看| 亚洲国产精品sss在线观看| 日本爱情动作片www.在线观看 | 色综合亚洲欧美另类图片| 亚洲七黄色美女视频| 欧美国产日韩亚洲一区| 1000部很黄的大片| 午夜免费激情av| a级一级毛片免费在线观看| 中文亚洲av片在线观看爽| 我要搜黄色片| 亚洲久久久久久中文字幕| 午夜福利18| 中亚洲国语对白在线视频| 不卡视频在线观看欧美| 日韩欧美三级三区| 级片在线观看| 亚洲精品亚洲一区二区| 久99久视频精品免费| 国产探花极品一区二区| 99九九线精品视频在线观看视频| 国产aⅴ精品一区二区三区波| av国产免费在线观看| 国产精品久久视频播放| 亚洲,欧美,日韩| 日韩欧美三级三区| 最近在线观看免费完整版| av福利片在线观看| 欧美日韩中文字幕国产精品一区二区三区| 日韩大尺度精品在线看网址| 精品人妻视频免费看| 精品无人区乱码1区二区| 一个人看视频在线观看www免费| 亚洲性夜色夜夜综合| 成人亚洲精品av一区二区| 村上凉子中文字幕在线| 春色校园在线视频观看| 亚洲av日韩精品久久久久久密| 国产69精品久久久久777片| 白带黄色成豆腐渣| 免费电影在线观看免费观看| 夜夜夜夜夜久久久久| 成人高潮视频无遮挡免费网站| 日韩强制内射视频| 国产精品永久免费网站| 亚洲美女视频黄频| 亚洲国产色片| 永久网站在线| 亚洲精华国产精华液的使用体验 | 中文字幕免费在线视频6| 亚洲精品一区av在线观看| 午夜福利在线观看吧| 看十八女毛片水多多多| 久久久久久大精品| 天堂av国产一区二区熟女人妻| 久久国产乱子免费精品| 成人二区视频| 12—13女人毛片做爰片一| 亚洲最大成人手机在线| 成人高潮视频无遮挡免费网站| 欧美日韩乱码在线| 十八禁国产超污无遮挡网站| 国产高清视频在线观看网站| 亚洲精品456在线播放app | 亚洲无线在线观看| 国产欧美日韩精品一区二区| 少妇人妻一区二区三区视频| 欧美丝袜亚洲另类 | 国产亚洲精品久久久com| 中文字幕高清在线视频| 丰满乱子伦码专区| 亚洲av成人精品一区久久| 亚洲综合色惰| 夜夜夜夜夜久久久久| 亚洲真实伦在线观看| 日日干狠狠操夜夜爽| netflix在线观看网站| 国产高清激情床上av| 亚洲男人的天堂狠狠| 两个人的视频大全免费| 午夜精品在线福利| 亚洲色图av天堂| 亚洲av熟女| av专区在线播放| 免费高清视频大片| 日本在线视频免费播放| 久久婷婷人人爽人人干人人爱| 日本撒尿小便嘘嘘汇集6| 两性午夜刺激爽爽歪歪视频在线观看| 亚洲av熟女| 精品久久久噜噜| 国产精品国产三级国产av玫瑰| 亚洲一区高清亚洲精品| 免费看a级黄色片| 在线观看午夜福利视频| 日本熟妇午夜| 极品教师在线免费播放| 色5月婷婷丁香| 免费高清视频大片| 日日摸夜夜添夜夜添小说| 亚洲美女视频黄频| 97热精品久久久久久| 国产老妇女一区| 欧美性感艳星| 亚洲欧美日韩东京热| 欧美精品国产亚洲| 亚洲中文日韩欧美视频| 色吧在线观看| 精品一区二区三区av网在线观看| av在线观看视频网站免费| 国产69精品久久久久777片| 亚洲色图av天堂| 舔av片在线| 男插女下体视频免费在线播放| 国产久久久一区二区三区| 精品人妻1区二区| 性色avwww在线观看| 国产在视频线在精品| 他把我摸到了高潮在线观看| 婷婷丁香在线五月| 久久这里只有精品中国| 日本撒尿小便嘘嘘汇集6| 国产精品无大码| 男人的好看免费观看在线视频| 国产熟女欧美一区二区| а√天堂www在线а√下载| 日本熟妇午夜| 亚洲精品国产成人久久av| 日韩欧美在线乱码| 国产欧美日韩精品亚洲av| 看片在线看免费视频| 久久精品国产亚洲av香蕉五月| 国产大屁股一区二区在线视频| 久久人人精品亚洲av| 欧美成人免费av一区二区三区| 老熟妇仑乱视频hdxx| 18禁黄网站禁片午夜丰满| av天堂中文字幕网| 久久国内精品自在自线图片| 国语自产精品视频在线第100页| 成人无遮挡网站| 国产精品人妻久久久久久| 欧美日韩乱码在线| 久久久久久久久久久丰满 | 长腿黑丝高跟| 久久久久久久久中文| 久久精品国产清高在天天线| 午夜精品一区二区三区免费看| 久久精品91蜜桃| 色播亚洲综合网| 在线a可以看的网站| 国产黄色小视频在线观看| 国产白丝娇喘喷水9色精品| 免费看美女性在线毛片视频| 欧美不卡视频在线免费观看| 制服丝袜大香蕉在线| 欧美日韩瑟瑟在线播放| 亚洲综合色惰| 国产av麻豆久久久久久久| 久久精品综合一区二区三区| 日韩大尺度精品在线看网址| 国产精品爽爽va在线观看网站| 国产色婷婷99| 狂野欧美激情性xxxx在线观看| 国产高清三级在线| 亚洲精品国产成人久久av| 久久人妻av系列| 97超视频在线观看视频| 天堂av国产一区二区熟女人妻| 日韩欧美免费精品| 啦啦啦韩国在线观看视频| 欧美性猛交黑人性爽| 亚洲av电影不卡..在线观看| av中文乱码字幕在线| 亚洲欧美精品综合久久99| 日本五十路高清| 亚洲精品456在线播放app | 久久久久性生活片| 亚洲国产精品久久男人天堂| 亚洲精品一区av在线观看| 成人亚洲精品av一区二区| 午夜影院日韩av| 亚洲欧美激情综合另类| 色综合色国产| 日本免费a在线| 中文字幕久久专区| 长腿黑丝高跟| 熟妇人妻久久中文字幕3abv| 长腿黑丝高跟| 欧美绝顶高潮抽搐喷水| 嫩草影院入口| 国产黄片美女视频| 成人性生交大片免费视频hd| 亚洲性久久影院| 亚洲黑人精品在线| 欧美日韩综合久久久久久 | 久久草成人影院| 黄色女人牲交| 中文字幕免费在线视频6| 亚洲精品色激情综合| 精品人妻一区二区三区麻豆 | 精品久久久久久,| 无人区码免费观看不卡| 精品久久久久久久久亚洲 | av国产免费在线观看| 韩国av在线不卡| 黄片wwwwww| 免费看日本二区| 久久中文看片网| 亚洲美女搞黄在线观看 | 成人特级av手机在线观看| 天堂网av新在线| 免费看美女性在线毛片视频| av专区在线播放| 有码 亚洲区| 午夜福利成人在线免费观看| 亚洲国产精品sss在线观看| 久久精品国产自在天天线| 99久久久亚洲精品蜜臀av| 毛片一级片免费看久久久久 | 久久久久久久久久久丰满 | 国产一区二区三区视频了| 一区福利在线观看| 精品久久久久久久末码| 久久国产精品人妻蜜桃| 美女免费视频网站| 免费搜索国产男女视频| 少妇熟女aⅴ在线视频| 国产一区二区激情短视频| 日本熟妇午夜| а√天堂www在线а√下载| 亚洲第一区二区三区不卡| 免费人成视频x8x8入口观看| 久久久久久九九精品二区国产| 亚洲欧美精品综合久久99| 久久久久久九九精品二区国产| 可以在线观看的亚洲视频| 91久久精品电影网| 男女视频在线观看网站免费| 欧美不卡视频在线免费观看| 亚洲熟妇中文字幕五十中出| 淫妇啪啪啪对白视频| 欧美黑人欧美精品刺激| 人妻丰满熟妇av一区二区三区| av在线天堂中文字幕| 波多野结衣高清无吗| 国产高潮美女av| 亚洲成a人片在线一区二区| 国产免费av片在线观看野外av| 可以在线观看毛片的网站| 久久6这里有精品| 国内精品美女久久久久久| 国产精品伦人一区二区| 欧美精品国产亚洲| 久久久久国产精品人妻aⅴ院| 给我免费播放毛片高清在线观看| 成人三级黄色视频| 中文字幕久久专区| 久久久久久久精品吃奶| 久久热精品热| 日本在线视频免费播放| 久久久精品欧美日韩精品| 波野结衣二区三区在线| 一进一出抽搐动态| 国内揄拍国产精品人妻在线| 精品久久久久久久人妻蜜臀av| 男女视频在线观看网站免费| 嫩草影视91久久| 亚洲精品乱码久久久v下载方式| 床上黄色一级片| 欧美日韩瑟瑟在线播放| 国产精品美女特级片免费视频播放器| eeuss影院久久| 日韩一区二区视频免费看| 男女视频在线观看网站免费| 亚洲av中文av极速乱 | 人人妻,人人澡人人爽秒播| 亚洲男人的天堂狠狠| 亚洲av日韩精品久久久久久密| 看十八女毛片水多多多| 亚洲人成伊人成综合网2020| 色吧在线观看| 免费看光身美女| 99精品久久久久人妻精品| 3wmmmm亚洲av在线观看| 日日夜夜操网爽| 一夜夜www| 成人毛片a级毛片在线播放| 蜜桃亚洲精品一区二区三区| 丰满的人妻完整版| 国产在线精品亚洲第一网站| 亚洲性夜色夜夜综合| 国产精品亚洲一级av第二区| 免费看av在线观看网站| 国产精品,欧美在线| 91狼人影院| 波野结衣二区三区在线| 久99久视频精品免费| 精品一区二区免费观看| 日韩在线高清观看一区二区三区 | 久9热在线精品视频| 91av网一区二区| 免费在线观看日本一区| 最近最新免费中文字幕在线| 久久人人精品亚洲av| 免费观看精品视频网站| 国产精品爽爽va在线观看网站| 日韩中文字幕欧美一区二区| 婷婷丁香在线五月| 男女做爰动态图高潮gif福利片| 亚洲国产日韩欧美精品在线观看| 久久午夜亚洲精品久久| 18禁黄网站禁片免费观看直播| 国产精品永久免费网站| 日韩欧美免费精品| ponron亚洲| xxxwww97欧美| 久久精品国产清高在天天线| 免费看光身美女| 美女cb高潮喷水在线观看| 久久人人精品亚洲av| 国产精品电影一区二区三区| 美女xxoo啪啪120秒动态图| 午夜福利在线观看免费完整高清在 | 女同久久另类99精品国产91| 午夜激情欧美在线| 午夜福利18| 免费一级毛片在线播放高清视频| 午夜福利在线观看吧| 又粗又爽又猛毛片免费看| 国产高清三级在线| 成人午夜高清在线视频| 国产精品综合久久久久久久免费| 亚洲天堂国产精品一区在线|