【作者】 高炳军；

【导师】 陈旭；

【作者基本信息】 天津大学 ， 化工过程机械， 2005， 博士

【摘要】 利用自行设计的直管准三点弯曲实验装置、弯管面内弯曲加载装置,采用电阻应变法,在多轴疲劳实验机上对循环弯曲载荷作用下的20#钢内压直管、弯管进行了棘轮效应研究。利用自行设计的径向位移传感器监测了管子的径向变形。对于直管,发现棘轮应变首先沿环向产生,随着载荷的增加,轴向也将产生棘轮应变,但较环向小。随着棘轮应变的产生,直管圆截面变为椭圆截面。对于90~°弯管,无论是长半径还是短半径,最大棘轮应变发生在顶线位置处,为环向应变;对于45°弯管,距内缘线45°位置处的棘轮应变较顶线位置大。随着棘轮应变的产生,弯管圆截面变为椭圆截面。多载荷步加载时,以往棘轮应变历史会降低应有的棘轮应变速率,尤其在较大的载荷下先发生棘轮应变后,这种影响十分明显。利用直管准三点弯曲实验装置,确定了内压直管循环弯曲的棘轮边界。考察了现有循环塑性本构模型,发现能够适用于各种材料各种加载路径的本构模型还不存在,但对某类材料寻求较为适宜的本构模型完全可能。尤其Ohno-Wang模型及基于Ohno-Wang模型的改进模型的出现,很大程度上提高了材料及结构的多轴棘轮效应预测的准确性。利用ANSYS的用户编程特性UPFs,通过编写USERPL.F子程序,将Ohno-Wang模型及其修正模型嵌入ANSYS软件,实现了结构循环塑性分析。利用ANSYS程序对循环弯曲载荷作用下的内压直管与弯管进行了弹塑性分析,通过比较各模型对直管、弯管棘轮应变的预测,发现对于直管、弯管中的E90S及E45L,修正的Jiang-Sehitoglu模型的预测结果更好一些,而对于弯管中的E90L,Chen-Jiao-Kim模型预测值与实验值能够较好地吻合。对于循环弯曲载荷作用下的内压直管,讨论了确定棘轮边界的现有规范及方法,发现采用修正的Jiang-Sehitoglu模型按C-TDF方法确定的棘轮边界线能够较好地界定安定区。采用修正的Jiang-Sehitoglu模型按C-TDF方法确定了面内循环弯曲载荷作用下内压弯管E90L、E90S及E45L的棘轮边界。根据等强度原则推导了内压弯管的理论壁厚分布,并根据弯管的加工特点,提出一种假定的壁厚分布,可有效改善弯管抗棘轮应变能力。更多还原

【Abstract】 Ratcheting of pressurized straight pipes and elbows made of S20C wereexperimentally studied with multiaxial fatigue testing system and aquasi-three-point bending apparatus for straight pipe and in-plane bendingapparatus for elbow were designed. Ratcheting strains were acquired bymulti-channel strain processor with strain gauges and the radial deformation ofthe pipe was measured by self-designed radial displacement extensometers. Forstraight pipes, it is found that ratcheting initiates firstly in hoop direction and thatin axial direction follows with the increase of loading but less in magnitude. Thecircular cross section turns into ellipse as the ratcheting strain accumulates. It wasfound that, for 90o elbows, the maximum ratcheting strain occurs at the flank inhoop direction. For 45o elbows, ratcheting strain at 45o is larger than that at flanks.Ratcheting strain rate grows with the increase of reversed bending load or internalpressure for both different specimen with different loadings and same specimenwith multi-step loadings. In multi-step loading, ratcheting rate suffers from theratcheting history, especially for that with ratcheting history at higher levelloading. Ratcheting boundary for straight pipe is determined with the aid of thequasi-three-point-bending apparatus. Analysis of published rate-independent models for cyclic plasticity revealsthat model suitable for all materials and all loading paths is not available. Butsome models definitely satisfy a kind of material for most loading paths.Moreover, Ohno-Wang model and its modified models bring lights to theprediction of multiaxial ratcheting both for material and components.Taking advatanges of the User Programmable Features of ANSYS,elasto-plastic analysis for components was accomplished with user ANSYS inwhich Ohno-Wang model and its modified models were programmed.Ratcheting strains were predicted with the user ANSYS for pressurizedpiping under reversed bending with a number of models. It is found thatmodified Jiang-Setitoglu model predicts well for straights pipe and elbowsexcept for that with long radius which may be well predicted by Chen-Jiao-Kimmodel.Ratcheting boundary determination methods available in the current codeand literature were discussed for pressurized straight pipe under reversedbending. It is found that ratcheting boundary determined by EPFEA(Elasto-Plastic Finite Element Analysis) with Modified Jiang-Sehitoglu modelby ratcheting rate control method suggested by C-TDF divides the shakedownregion well. Ratcheting boundaries were determined for E90L, E90S and E45L.Based on the equal-strength of pressurized elbow, the theoretical equalstrength thickness distribution was deduced and an ideal thickness distributionwas suggested with the consideration of elbow processing characteristics.Ratcheting analysis of the elbows with this kind of suggested thicknessdistribution shows great resistance to ratcheting compared with equal thicknesselbows.更多还原