Article(id=1246023204863591310, tenantId=1146029695717560320, journalId=1241755870837649424, issueId=1246023204117005194, articleNumber=null, orderNo=null, doi=10.19636/j.cnki.cjsm42-1250/o3.2023.047, pmid=null, cstr=null, oa=null, hot=null, price=null, onlineType=0, articleFormat=0, articleType=null, articleTypeStr=null, receivedDate=1693238400000, receivedDateStr=2023-08-29, revisedDate=null, revisedDateStr=null, acceptedDate=null, acceptedDateStr=null, onlineDate=1775004682030, onlineDateStr=2026-04-01, pubDate=1713974400000, pubDateStr=2024-04-25, doiRegisterDate=null, doiRegisterDateStr=null, onlineIssueDate=1775004682030, onlineIssueDateStr=2026-04-01, onlineJustAcceptDate=null, onlineJustAcceptDateStr=null, onlineFirstDate=null, onlineFirstDateStr=null, sourceXml=null, magXml=null, createTime=1775004682030, creator=13701087609, updateTime=1775004682030, updator=13701087609, issue=Issue{id=1246023204117005194, tenantId=1146029695717560320, journalId=1241755870837649424, year='2024', volume='45', issue='2', pageStart='145', pageEnd='288', issueExtLink='null', onlineDate='null', pubDate='null', beforeIssueId=null, nextIssueId=null, price=null, status=1, issueComplete=1, articleOrder=1, issueType=-1, specialIssue=null, createTime=1775004681852, creator=13701087609, updateTime=1775004747143, updator=13701087609, preIssue=null, nextIssue=null, ext={EN=IssueExt(id=1246023478026027853, tenantId=1146029695717560320, journalId=1241755870837649424, issueId=1246023204117005194, language=EN, specialIssueTitle=, coverIllustrator=null, specialIssueEditor=, specialIssueAbout=), CN=IssueExt(id=1246023478026027854, tenantId=1146029695717560320, journalId=1241755870837649424, issueId=1246023204117005194, language=CN, specialIssueTitle=, coverIllustrator=null, specialIssueEditor=, specialIssueAbout=)}, issueFiles=null}, startPage=279, endPage=288, ext={EN=ArticleExt(id=1246023205136221073, articleId=1246023204863591310, tenantId=1146029695717560320, journalId=1241755870837649424, language=EN, title=Stress Triaxiality of a Notched Round Bar under Axial Loading, columnId=1244229834482757770, journalTitle=Chinese Journal of Solid Mechanics, columnName=Research Paper, runingTitle=null, highlight=null, articleAbstract=

Stress triaxiality is a parameter that expresses the stress state and can be used as a variable to characterize the plasticity and fracture damage model of materials. It plays an important role in structural strength and failure analysis. The round bar tensile test with a notch can be used to calibrate the parameters in the plastic and damage models. However, there are two different formulas in the literature to calculate the triaxiality of the minimum cross-sectional axis of a notched round bar under tensile loading, which were proposed by internationally renowned scholars Bridgman and Wierzbicki, respectively. Their differences often cause confusion in application. Through refined finite element numerical analysis, this article attempts to clarify the validity and applicability of the two formulas. The results show that the Bridgman formula is more accurate only in the elastic stage and in a specific a/R range. The Bao-Wierzbicki formula, on the other hand, is in good agreement with the experimental data and simulation results, which can be used to calculate the arithmetic mean value of triaxiality during the entire tensile process. Based on further analysis, a new revised stress triaxiality formula in the plastic stage under elastic-perfectly-plastic condition is proposed, and the notch geometry effect and strain-hardening effect are further discussed. It is pointed out that notch ratio can affect the neck stress field. The smaller is the notch ratio, the closer is the stress triaxiality value in the elastic stage to 1/3. When the notch ratio is too small, it can also affect the change of stress triaxiality throughout the entire tensile process. The strain-hardening effect can change the trend of stress triaxiality during the stretching process, and an increase in the strengthening modulus will lead to a decrease in the peak value of the plastic stage. The higher is the strengthening modulus, the faster is the decrease of stress triaxiality after entering the plastic stage.

, correspAuthors=Feng Xi, authorNote=null, correspAuthorsNote=null, copyrightStatement=null, copyrightOwner=null, extLink=null, articleAbsUrl=null, sourceXml=null, magXml=null, pdfUrl=null, pdf=null, pdfFileSize=null, pdfExtLink=null, richHtmlUrl=null, mobilePdfUrl=null, reviewReport=null, pdfFirstPage=null, abstractGraph=null, abstractGraphContent=null, abstractVideo=null, citation=null, cebUrl=null, magXmlContent=null, mapNumber=null, authorCompany=null, fund=null, authors=null, authorsList=Qiangsheng Liu, Feng Xi, Zhemin Zhu), CN=ArticleExt(id=1246023209892560952, articleId=1246023204863591310, tenantId=1146029695717560320, journalId=1241755870837649424, language=CN, title=轴向拉伸载荷作用下缺口圆棒的应力三轴度, columnId=1241831201896469478, journalTitle=固体力学学报, columnName=研究论文, runingTitle=null, highlight=null, articleAbstract=

应力三轴度是表达应力状态的参量,可作为变量表征材料的塑性和断裂损伤模型,在结构强度和失效分析中发挥重要作用. 具有缺口的圆棒拉伸试验可用于标定塑性和损伤模型中的参数. 然而,文献中却存在两个不同的公式计算拉伸载荷作用时缺口圆棒最小截面轴心处的三轴度,二者分别由国际著名学者Bridgman和Wierzbicki先后提出,其不同性往往造成应用时的困惑. 本文通过精细化的有限元计算分析,意在澄清两个公式的有效性和适用性. 结果表明,Bridgman公式仅弹性阶段和特定a/R范围内较为准确,Bao-Wierzbicki公式与数值模拟结果及实验数据吻合较好,可用于计算整个拉伸过程中三轴度的算术平均值. 基于进一步的分析,本文提出了理想弹塑性条件下塑性阶段的应力三轴度新的修正公式,还讨论了缺口几何和应变强化效应:指出不同的缺口比例会影响到颈部应力场,缺口比例越小,弹性阶段的应力三轴度越接近1/3;而应变强化则会导致拉伸过程中应力三轴度的降低.

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Relevant data of the round notched bar specimens

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实验编号N1a(mm)R(mm)ηfη0εf
3.003.001.020.730.082
3.004.000.930.650.123
3.006.000.830.550.195
3.123.121.0290.730.739
3.126.250.8350.550.951
), ArticleFig(id=1246023215479374151, tenantId=1146029695717560320, journalId=1241755870837649424, articleId=1246023204863591310, language=CN, label=表1, caption=

缺口圆棒试件相关实验数据

, figureFileSmall=null, figureFileBig=null, tableContent=
实验编号N1a(mm)R(mm)ηfη0εf
3.003.001.020.730.082
3.004.000.930.650.123
3.006.000.830.550.195
3.123.121.0290.730.739
3.126.250.8350.550.951
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轴向拉伸载荷作用下缺口圆棒的应力三轴度
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刘强生 , 席丰 ** , 朱哲民
固体力学学报 | 研究论文 2024,45(2): 279-288
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固体力学学报 | 研究论文 2024, 45(2): 279-288
轴向拉伸载荷作用下缺口圆棒的应力三轴度
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刘强生, 席丰** , 朱哲民
作者信息
  • 山东建筑大学土木工程学院,建筑结构加固改造与地下空间工程教育部重点实验室,济南,250101

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** E-mail:.
Stress Triaxiality of a Notched Round Bar under Axial Loading
Qiangsheng Liu, Feng Xi** , Zhemin Zhu
Affiliations
  • School of Civil Engineering, Shandong Jianzhu University, Key Laboratory of Building Structural Retrofitting and Underground Space Engineering, Ministry of Education, Jinan, 250101
出版时间: 2024-04-25 doi: 10.19636/j.cnki.cjsm42-1250/o3.2023.047
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应力三轴度是表达应力状态的参量,可作为变量表征材料的塑性和断裂损伤模型,在结构强度和失效分析中发挥重要作用. 具有缺口的圆棒拉伸试验可用于标定塑性和损伤模型中的参数. 然而,文献中却存在两个不同的公式计算拉伸载荷作用时缺口圆棒最小截面轴心处的三轴度,二者分别由国际著名学者Bridgman和Wierzbicki先后提出,其不同性往往造成应用时的困惑. 本文通过精细化的有限元计算分析,意在澄清两个公式的有效性和适用性. 结果表明,Bridgman公式仅弹性阶段和特定a/R范围内较为准确,Bao-Wierzbicki公式与数值模拟结果及实验数据吻合较好,可用于计算整个拉伸过程中三轴度的算术平均值. 基于进一步的分析,本文提出了理想弹塑性条件下塑性阶段的应力三轴度新的修正公式,还讨论了缺口几何和应变强化效应:指出不同的缺口比例会影响到颈部应力场,缺口比例越小,弹性阶段的应力三轴度越接近1/3;而应变强化则会导致拉伸过程中应力三轴度的降低.

应力三轴度  /  缺口圆棒  /  数值模拟  /  缺口几何效应  /  应变强化效应  /  新的修正公式

Stress triaxiality is a parameter that expresses the stress state and can be used as a variable to characterize the plasticity and fracture damage model of materials. It plays an important role in structural strength and failure analysis. The round bar tensile test with a notch can be used to calibrate the parameters in the plastic and damage models. However, there are two different formulas in the literature to calculate the triaxiality of the minimum cross-sectional axis of a notched round bar under tensile loading, which were proposed by internationally renowned scholars Bridgman and Wierzbicki, respectively. Their differences often cause confusion in application. Through refined finite element numerical analysis, this article attempts to clarify the validity and applicability of the two formulas. The results show that the Bridgman formula is more accurate only in the elastic stage and in a specific a/R range. The Bao-Wierzbicki formula, on the other hand, is in good agreement with the experimental data and simulation results, which can be used to calculate the arithmetic mean value of triaxiality during the entire tensile process. Based on further analysis, a new revised stress triaxiality formula in the plastic stage under elastic-perfectly-plastic condition is proposed, and the notch geometry effect and strain-hardening effect are further discussed. It is pointed out that notch ratio can affect the neck stress field. The smaller is the notch ratio, the closer is the stress triaxiality value in the elastic stage to 1/3. When the notch ratio is too small, it can also affect the change of stress triaxiality throughout the entire tensile process. The strain-hardening effect can change the trend of stress triaxiality during the stretching process, and an increase in the strengthening modulus will lead to a decrease in the peak value of the plastic stage. The higher is the strengthening modulus, the faster is the decrease of stress triaxiality after entering the plastic stage.

stress triaxiality  /  notched round bar  /  numerical simulation  /  notch geometry effect  /  strain-hardening effect  /  new revised formula
刘强生, 席丰, 朱哲民. 轴向拉伸载荷作用下缺口圆棒的应力三轴度. 固体力学学报, 2024 , 45 (2) : 279 -288 . DOI: 10.19636/j.cnki.cjsm42-1250/o3.2023.047
Qiangsheng Liu, Feng Xi, Zhemin Zhu. Stress Triaxiality of a Notched Round Bar under Axial Loading[J]. Chinese Journal of Solid Mechanics, 2024 , 45 (2) : 279 -288 . DOI: 10.19636/j.cnki.cjsm42-1250/o3.2023.047
所谓应力三轴度,定义为静水压力与Mises等效应力之比(一般用η表示),亦即无量纲静水压力. 作为描述应力状态的参量,在材料的塑性和断裂评估中起着关键作用. 如对于脆性或岩土类材料,其塑性依赖静水压力,因此其屈服准则和强化、软化规律都需要应力三轴度表达. 对于延性金属材料,虽然其初始屈服和早期的塑性与静水压力关系不大,但在大变形塑性阶段以及损伤起始和断裂发生时,应力状态的影响较为严重,因此,迄今为止有大量的文献建立依赖静水压力的塑性模型和损伤、断裂准则,如B-W塑性模型[1]、J-C失效模型[2]、X-W失效模型[3]等. 其中J-C失效模型考虑了应力三轴度温度及应变率对于材料的影响,将三轴度作为关键参量描述材料的断裂行为,由于其形式简单且参数易于标定等特点,在冲击侵彻问题中得到了广泛且成功的应用;B-W塑性模型考虑了静水压力对于金属材料塑性的影响,将应力三轴度作为屈服函数表达式中的重要变量,以更精确地描述材料的塑性行为;X-W失效模型同时考虑了三轴度和Lode角参数对于材料失效破坏的影响,通过不同应力状态下的简单实验标定参数. 所以与描述应力状态的另一个参数应力Lode角一样,应力三轴度在这些塑性和断裂模型中都是关键的参量. 因此,建立依赖静水压力的塑性和断裂模型需要应力三轴度参数的准确表征.
依赖应力三轴度的塑性和损伤模型需要借助不同简单应力状态下的实验结果标定参数. 如带缺口的圆棒拉伸试验,其颈部区域呈现复杂应力状态. 若试件拉伸过程应力状态保持不变,则相应的三轴度为常数,改变颈部缺口几何则相应三轴度也会改变. 通过改变缺口大小,可获得不同应力三轴度下的塑性和断裂性能实验数据. 然而在整个试件拉伸过程特别是进入颈缩阶段(见图1),应力三轴度难以保持恒定,所以初始与断裂时的应力三轴度是两个主要关注点. 已有文献表明[1],在一定范围内,断裂发生在构件应力三轴度最大处,对于单向拉伸实验,最小横截面轴心处为开裂点.
由于实验设备的局限性,横截面内部的应力-应变场不能直接进行测量[4],因此无法直接确定颈部轴心处的三轴度. 在理想弹塑性假定下,Bridgman[5]推导出了如下应力三轴度公式:
式中a为颈部最小截面半径,R为缺口半径.
Bao和Wierzbicki[6]通过有限元数值分析对公式(1)修正如下:
在之后的十几年中,不同学者对于两者的使用不尽相同. Bao[7]将公式(2)作为2024-T351铝合金圆棒的初始应力三轴度;Algarni[8]使用公式(1)作为初始应力三轴度,对B-W塑性模型进行了扩展并通过实验验证了修正后模型的正确性;Li[9]采用公式(1)作为初始应力三轴度,对断裂机理进行深入讨论;Bao和Wierzbicki[10]率先提出用平均应力三轴度ηav来表示在拉伸过程中变化的应力三轴度;Bai[11]采用公式(2)作为缺口圆棒初始的应力三轴度,比较了缺口圆棒与平槽板在相近的三轴度范围内的断裂轨迹;祁爽等[12]将公式(1)作为拉伸过程中的平均应力三轴度,比较了颈部最小截面上应力三轴度随截面中心处应变的变化规律;Alves[13]以公式(1)作为标准,进行了静态和动态拉伸实验,指出具有小缺口半径的试样失效部位发生在低三轴度区;余万千等[14]在确定初始应力状态下的应力三轴度采用了式(1),对于整个变形过程则采用了平均值进行计算,并确定了B-W模型的失效参数.
可以看出对于这两个公式在文献中都被广泛应用,但目前并未形成一致的看法. 应用不同的公式,将对塑性和断裂模型产生不同的影响,最终得出不同的计算结果和结论. 因此,有必要进行全面而深入的分析比较. 本文利用ABAQUS非线性有限元程序建立精细化模型,对缺口圆棒的单向拉伸行为进行数值模拟,重点考察颈部轴心处应力三轴度的变化规律,并将Bridgman公式(1)和Bao-Wierzbicki公式(2)的结果与有限元数值结果进行对比,检验二者的适用性;在此基础上,提出理想弹塑性条件下塑性阶段的应力三轴度公式,并深入讨论试件尺寸效应和应变强化效应.
利用ABAQUS/CAE对缺口圆棒建立不同的有限元模型,确保分析结果的可靠性. 建立的两种有限元模型分别为:
模型一:由于拉伸缺口圆棒为轴对称问题,因此采用CAX4R轴对称单元,同时由于与平行颈部最小截面处的x-z面也为对称面,因此可取试件1/4. 边界约束条件为:在网格的左侧采用轴对称约束(图2中标为A-C),在网格底部施加对称边界条件(图2中标为S-C),模型沿y轴正向施加轴对称载荷,如图2(a)所示.
模型二:采用C3D10实体单元,为提高计算效率,取试件的1/8进行建模,边界约束条件为:在网格左侧相对于y-z平面,网格后方相对于x-y平面及网格底部相对于x-z平面施加对称边界条件(图二中标为S-C),模型沿y轴正向施加轴对称载荷,如图2(b)所示.
根据塑性理论,小变形条件下保持单调加载是实现简单加载即应力分量比值不变的必要条件,亦即应力三轴度保持恒定,所以应该使用ABAQUS/Standard标准通用静态分析步. 为了输出详尽的计算数据,将增量步长设置为固定增量步长,步长为0.01.
由于Bridgman的计算公式是在材料理想弹塑性假定下得到的,所以校验加载过程也采用理想弹塑性模型. 为了考察应变强化效应,相关的计算则采用各向同性线性强化弹塑性模型,σs表示初始屈服强度. 相关材料参数为:杨氏模量E为210 GPa,线性强化切线模量Et取不同值以考察强化效应;σs为880 MPa,泊松比υ取0.3.
参照文献[15]中的1045钢圆棒拉伸试验数据,采用CAX4R单元和C3D10单元建模,进行从加载至破坏全过程的数值仿真,其最终破坏结果对比见图3. 可以看出:数值解能够较好地呈现出试验最终的破坏结果;光滑和带缺口两种情况下的载荷位移曲线见图4,两种不同单元有限元模型的载荷位移曲线均能够与试验曲线相吻合,因此文中所建立的精细化模型能够准确描述圆棒试件在拉伸载荷作用下的复杂应力状态.
对于不同的a/R值,通过两种有限元模型分别进行了多组计算,将数值解与Bridgman公式(1)和Bao-Wierzbicki公式(2)进行比较,可校验缺口圆棒在理想弹塑性条件下的应力三轴度,见图5. 结果表明,两种单元数值解在弹性阶段对应的应力三轴度值均与Bao-Wierzbicki公式偏离较大,但与Bridgman公式相近,结果充分说明了Bridgman公式解并不依赖于单元类型的改变,精确度得到验证,将其作为确定缺口圆棒弹性阶段的应力三轴度η0是合理的;而当进入塑性后,如时,所对应的应力三轴度值有显著的上升,此阶段公式(1)、(2)已不再适用,后续分析将引入平均应力三轴度进一步考察公式的适用性. 由于两种数值解在弹性阶段与Bridgman公式基本吻合,因此后续的模拟均采用计算效率较高的CAX4R单元.
接下来考虑材料应变强化效应. 采用材料线性应变强化模型,所得到的数值解与Bao-Wierzbicki公式的比较结果见图6. 其中对于平均应力三轴度采用两种不同的方法,即分别由文献[7]和[2]建议的如下公式(3)、(4)进行计算:
其中εf分别表示断裂应变和等效塑性应变,η0ηf分别表示弹性阶段和断裂时所对应的应力三轴度,ηav1为积分平均值,ηav2为算术平均值. 相关数据见表1[9,16].
考虑强化后的数值结果见图6,可以看出整个拉伸过程中ηav2ηbw相近,误差均小于5%. 但与ηav1误差较大,最大误差达到了30%;所以,对于缺口圆棒的整个拉伸过程,使用Bao-Wierzbicki公式可计算应力三轴度的算术平均值.
现考察Bridgman公式所适用的几何尺寸范围,数值解与Bridgman公式解的比较见图7. 可以看出,当缺口圆棒试件的几何尺寸改变时,所得到的模拟结果并非都能满足Bridgman公式(1). 譬如:当颈部a的半径保持恒定且缺口半径R逐渐减小时,应力三轴度在偏离Bridgman解. 模拟结果表现为下降趋势,公式(1)此时不再适用,即公式(1)在有限的a/R值范围内具有准确性. 具体表现为,当a/R小于1.0时,数值解总能够与公式吻合. 当a/R值在1.0和1.2之间时,数值解逐渐偏离公式,但误差保持在5%以内. 当a/R超过2.0时,误差继续增加. 当a/R达到2.0时,误差达到26%,曲线完全偏离. 对于误差原因,进行6组小缺口情形分析.
由于拉伸过程中颈部中心处的应力状态几乎保持不变,三个主应力呈线性增长,所占比例均保持稳定,因此将三个主应力大小量化为主应力占比,定义为主应力分量占三个主应力之和的比重.
拉伸过程中的η0及颈部中心处的主应力占比见图8. 从中可以看出,随着R值减小,a/R比值增大,初始应力三轴度η0在下降,主应力σ1所占的比例在上升. 当a/R达到20时,初始应力三轴度为0.3478,接近于单向拉伸时的应力三轴度0.33,而此时所对应的主应力σ1占比99%,而σ2σ3仅为0.5%,与σ1相比非常小,此时颈部中心近似可看成单向应力状态,σ2σ3对应力场影响可忽略不计. 结果表明当颈部半径R非常小时(Ra),Bridgman公式(1)不再适用,初始应力三轴度需要借助有限元分析精确给出.
对于理想弹塑性材料,考虑六个不同缺口情形(a为1.0 mm,R分别为1.00,1.19,1.40,1.59,2.04,2.38 mm),记作N2,编号分别为A~F. 通过模拟所得到的η随位移变化的曲线及情形A的公式解见图9. 可以看出,对于整个拉伸过程应力三轴度的变化,可根据是否产生塑性应变(PEEQ)将其分为弹性区和塑性区. 在圆棒颈部中心即将进入塑性时,应力三轴度会发生急剧上升变化,比较于弹性阶段大幅增长. 在进入塑性阶段后,应力三轴度变化幅度较小,可近似为定值,因此将PEEQ为0.05时的应力三轴度与η0进行比较,见图10,结果表明,随着R的增大,情形A在塑性阶段的应力三轴度比弹性阶段的值增长了67%,情形F则达到了82%.
对于情形A而言,弹性阶段满足小变形条件下单调加载,能够保持应力三轴度维持不变;随着载荷增加,在未进入塑性阶段之前,即等效塑性应变(PEEQ)为0时,应力三轴度就已发生改变. 由于此时的加载条件已经无法满足理想状态下的单调加载,应力分量不再保持按比例增长,因此三轴度必然会产生变化. 在进入塑性阶段之后,两公式均与有限元数值解有一定偏差.
由于进入塑性阶段后应力三轴度会显著上升然后维持平稳状态,相应的稳态值明显高于公式(1)(2). 因此,在塑性阶段,这两个公式并不准确. 为此通过最小二乘法进行非线性拟合,得到如下公式:
公式(5)与公式(1)、(2)所画出的ηa/R的变化及数值解见图11. 三种公式所构成的曲面见图12,可以看出拟合公式所对应的曲面要高于已知公式的曲面,并且与数值解吻合,能够反映出理想弹塑性模型中应力三轴度的真实值.
综上所述,对于不同的塑性应变强化情形,在拉伸过程中η所使用的公式见图13. PEEQ为0时,依然采用Bridgman公式确定弹性阶段的应力三轴度;PEEQ不为0时,对于理想弹塑性模型,则使用上文给出的ηpl公式(5);对于线性强化模型,使用平均值计算应力三轴度的大小则更为合理.
对于相同aR,缺口所占比例是否会影响到中心点处的应力场,进而造成应力三轴度的差异,下面将进行讨论. 考虑如图14中几种情形,用角度θ表示缺口部分所占比例,改变θ,比较在相同a/R时,三轴度的变化. 选取a=1.5 mm,1~4组不同的R,分别为6.13,4.26,3.15,2.38 mm.
η0θ的变化见图15. 结果表明,当R为6.13时,角度θη0没有影响,随着R的减小,θ的影响范围逐渐扩大. 对于2组和3组,当θ小于45°时,η0发生变化;对于4组,当θ小于55°时,η0开始下降. 对于相同的a/R组,三轴度随着θ的减小而增加. 此处最小只给出θ为15°时的η0,假设θ趋于无穷小,此时缺口的比例几乎为0,因此相应的η0应无限接近1/3. 当θ为15°时,受影响组对应的η0均降至0.4以下,最终值接近0.33,即R越小,θ引起的影响越大,变化越剧烈. 由此可以看出,η0会受到缺口比例的影响从而造成数值上的差异.
对于拉伸全过程η随位移的变化,见图16. 结果表明,当θ分别为45°、60°、90°时,三种工况的η0存在差异,但随着位移增长,加载后期η值趋于一致,能够与公式(5)相符合;当θ为30°时,η0较另外三组数值较小,后期的η也大幅下降. 由此表明,缺口比例不仅会影响η0,在比例过小时还会影响到全过程中应力三轴度的变化,在进行实验设计时需考虑缺口比例的影响.
由于Bridgman是在理想弹塑性假定下得到的公式(1),对于应变强化效应并不在公式的考虑范围之内. 通过改变切线模量Et与弹性模量E的比值,研究不同强化程度对三轴度变化的影响,其中Et/E=0对应理想弹塑性,Et/E=100%则对应完全线弹性. η随位移的变化见图17. 结果表明,对于弹性-线性强化塑性材料,强化模量的增大会大幅改变应力三轴度的变化曲线,具体表现为强化模量越大,应力三轴度在进入塑性阶段之后下降的越快;进入塑性后应力三轴度曲线会出现峰值,且强化模量的升高,会使得塑性区应力三轴度的峰值下降;当Et/E达到20%时,曲线的峰值并不明显,与Et/E=1%、10%相比大幅降低,Et/E=0和100%时,曲线均近似保持水平;当Et/E≠0、100%时,随着位移的增长,η曲线接近完全弹性时对应的η0.
有限元数值结果充分说明了应变强化效应对于应力三轴度的影响很大. 对于整个拉伸过程的应力三轴度而言,即使是相同尺寸的试件,应变强化程度不同所得出的值差异是较大的,因此对于实验中所得到的应力三轴度进行平均化是有必要的.
本文通过有限元计算,考察了文献中的两个计算公式即Bridgman公式及Bao-Wierzbicki公式的有效性和适用性,在此基础上提出了新的修正公式,并讨论了缺口尺寸效应和应变强化效应对拉伸过程中应力三轴度的影响. 所得到的主要结果和结论为:
(1)对于有明显强化效应的材料可使用Bao-Wierzbicki公式计算拉伸过程中应力三轴度的算术平均值;Bridgman公式适用于计算特定a/R范围内弹性阶段的应力三轴度,对于拉伸全程并不适用.
(2)缺口比例不仅会影响弹性阶段的应力三轴度,在比例过小时还会影响到全过程中应力三轴度的变化,缺口比例缩小,会使得整个拉伸过程中的η降低,直接影响断裂参数的准确性.
(3)应变强化效应会改变拉伸过程中应力三轴度的变化趋势,与理想弹塑性相比,使得进入塑性后的应力三轴度曲线出现峰值,且强化模量的升高会导致塑性区的峰值下降,强化模量越高,进入塑性后的三轴度下降越快.
(4)对于理想弹塑性材料,Bridgman公式和Bao-Wierzbicki的预测结果均偏小,本文给出了新的修正公式,能够较好的预测理想弹塑性条件下塑性区的应力三轴度.
  • 国家自然科学基金项目(12172198)
  • 山东省研究生教育优质课程建设项目(SDYKC20158)
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doi: 10.19636/j.cnki.cjsm42-1250/o3.2023.047
  • 接收时间:2023-08-29
  • 首发时间:2026-04-01
  • 出版时间:2024-04-25
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  • 收稿日期:2023-08-29
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国家自然科学基金项目(12172198)
山东省研究生教育优质课程建设项目(SDYKC20158)
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    山东建筑大学土木工程学院,建筑结构加固改造与地下空间工程教育部重点实验室,济南,250101

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2种不同金属材料的力学参数

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鹅膏菌科Amanitaceae 2 11 5.26 鹅膏菌属 Amanita 10 4.78
小菇科 Mycenaceae 2 12 5.74 丝盖伞属 Inocybe 5 2.39
多孔菌科 Polyporaceae 8 14 6.70 蜡蘑属 Laccaria 5 2.39
红菇科 Russulaceae 3 23 11.00 小皮伞属 Marasmius 6 2.87
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