Three‑dimensional numerical simulation of dynamic strength and failure mode of a rock mass with cross joints

To study the dynamic mechanical properties and failure characteristics of intersecting jointed rock masses with different joint distributions under confining pressure,considering the cross angleαand joint persistence ratioη,a numerical model of the biaxial Hopkinson bar test system was established using the finite element method–discrete-element model coupling method.The validity of the model was verified by comparing and analyzing it in conjunction with laboratory test results.Dynamics-static combined impact tests were conducted on specimens under various conditions to investigate the strength characteristics and patterns of crack initiation and expansion.The study revealed the predominant factors influencing intersecting joints with different angles and penetrations under impact loading.The results show that the peak stress of the specimens decreases first and then increases with the increase of the cross angle.Whenα<60°,regardless of the value ofη,the dynamic stress of the specimens is controlled by the main joint.Whenα≥60°,the peak stress borne by the specimens decreases with increasingη.Whenα<60°,the initiation and propagation of cracks in the cross-jointed specimens are mainly controlled by the main joint,and the final failure surface of the specimens is composed of the main joint and wing cracks.Whenα≥60°orη≥0.67,the secondary joint guides the expansion of the wing cracks,and multiple failure surfaces composed of main and secondary joints,wing cracks,and co-planar cracks are formed.Increasing lateral confinement significantly increases the dynamic peak stress able to be borne by the specimens.Under triaxial conditions,the degree of failure of the intersecting jointed specimens is much lower than that under uniaxial and biaxial conditions.

Prediction and Verifcation of Forming Limit Diagrams Based on a Modifed Shear Failure Criterion

The forming limit diagram plays an important role in predicting the forming limit of sheet metals.Previous studies have shown that,the method to construct the forming limit diagram based on instability theory of the original shear failure criterion is efective and simple.The original shear instability criterion can accurately predict the left area of the forming limit diagram but not the right area.In this study,in order to improve the accuracy of the original shear failure criterion,a modifed shear failure criterion was proposed based on in-depth analysis of the original shear failure criterion.The detailed improvement strategies of the shear failure criterion and the complete calculation process are given.Based on the modifed shear failure criterion and diferent constitutive equations,the theoretical forming limit of TRIP780 steel and 5754O aluminum alloy sheet metals are calculated.By comparing the theoretical and experimental results,it is shown that proposed modifed shear failure criterion can predict the right area of forming limit more reasonably than the original shear failure criterion.The efect of the pre-strain and constitutive equation on the forming limits are also analyzed in depth.The modifed shear failure criterion proposed in this study provides an alternative and reliable method to predict forming limit of sheet metals.

A Study of Pragmatic Failure of Address Forms in Cross-cultural Communication

As the process of globalization moves on,cross-cultural communication has been a part of people' s daily lives.It is quite important for people to know how to behave properly in many situations.The present study analyses the phenomena of pragmatic failure of address forms.The study may help enhance peoples' awareness on polite address in cross-cultural communi cation and improve their communicative competence.Foreign language teachers can get implications for their teaching methods,paying more attention on cultural teaching.In addition,the author hopes that equal cultural communication and multi-cultural concept can be achieved.

Failure mechanism and forming limit of tube axial compressive process

Based on minimum energy principle for plastic forming, tearing and buckling failures mechanisms are explored and criteria for them are developed by theoretical analysis and experiment. Combined with finite element software developed forming limit and effects of process parameters on failures are investigated and proper parameters for stable forming are determined. The results show that: 1) The failures and forming limit are mainly determined by geometry and materials parameters of tube blank, fillet radius or half conical angle of die. For the process under fillet die, there exists a maximum fillet radius within which no tearing failure happens, and a maximum radius and a minimum radius range within which no buckling failure happens. For the process under conical die, there exists a maximum and minimum half conical angle range within which no tearing and buckling failures occur. 2) For both forming processes, the higher the value of material strain hardening exponent or the lower the value of relative thickness, the more impossible for tearing and buckling failures to occur, and the larger the ranges of fillet radius and half conical angle. The experiment results verify the reliability and practicability of this research.

PREDICTION OF PATH-DEPENDENT FAILURE IN ALUMINUM SHEET METALS UNDER FORMING OPERATIONS

An approximate macroscopic yield criterion for anisotropic porous sheet metals is adopted in a failure prediction methodology that can be used to investigate the failure of sheet metals under forming operations. This failure prediction methodology is developed based on the Marciniak-Kuczynski approach by assuming a slightly higher void volume fraction inside randomly oriented imperfecte analysis. Here, a nonproportional deformation history including relative rotation of principal stretch directions is identified in a selected critical element of an aluminum sheet from a FEM fender forming simulation. Based on the failure prediction methodology, the failure of the critical sheet element is investigated under the non-proportional deformation history. The results show that thiven non-proportional deformation history.

RELIABILITY ANALYSIS OF FRP LAMINATED PLATES WITH CONSIDERATION OF BOTH INITIAL IMPERFECTION AND FAILURE SEQUENCE

A probabilistic progressive failure analyzing method is applied to estimating the reliability of a simply supported laminated composite plate with an initial imperfection under bi-axial compression load. The initial imperfection and the strength parameters are considered as random variables. Ply-level failure probability is evaluated by the first order reliability method (FORM) together with the Tsai-Wu strength criterion and Tan criterion. Current stresses in the laminated structure are calculated by the classical lamination theory with the stiffness modified based on the last step ply failure. Probabilistically dominant ply-level failure sequences leading to overall system failure are identified, based on which the system failure probability is estimated. A numerical example is presented to demonstrate the methodology proposed. Through parameter studies it is shown that the deviation of the initial imperfection and some of the strength parameters largely influence the system reliability.

A two-dimensional limit equilibrium computer code for analysis of complex toppling slope failures

Evaluation of blocky or layered rock slopes against toppling failures has remained of great concern for engineers in various rock mechanics projects.Several step-by-step analytical solutions have been developed for analyzing these types of slope failures.However,manual application of these analytical solutions for real case studies can be time-consuming,complicated,and in certain cases even impossible.This study will first examine existing methods for toppling failure analyses that are reviewed,modified and generalized to consider the effects of a wide range of external and dead loads on slope stability.Next,based on the generalized presented formulae,a Windows form computer code is programmed using Visual C#for analysis of common types of toppling failures.Input parameters,including slope geometry,joint sets parameters,rock and soil properties,ground water level,dynamic loads,support anchor loads as well as magnitudes and forms of external forces,are first loaded into the code.The input data are then saved and used to graphically draw the slope model.This is followed by automatic identification of the toppling failure mode and a deterministic analysis of the slope stability against this failure mode.The results are presented using a graphical approach.The developed code allows probabilistic introduction of the input parameters via probability distribution functions(PDFs)and thus a probabilistic analysis of the toppling failure modes using Monte-Carlo simulation technique.This allows calculation of the probability of slope failure.Finally,several published case studies and typical examples are analyzed with the developed code.The outcomes are compared with those of the main references to assess the performance and robustness of the developed computer code.The comparisons demonstrate good agreement between the results.

围压和裂隙倾角对花岗岩力学行为的影响研究

含裂隙围岩的力学行为对高放废物地质处置库的长期稳定和安全至关重要。为研究围压和裂隙倾角对花岗岩围岩力学行为的影响,以完整花岗岩和含不同倾角贯通单裂隙的花岗岩为研究对象,开展了单轴压缩和三轴压缩试验研究。结果表明:0~10 MPa围压范围内,倾角30°和60°裂隙花岗岩分别发生了穿裂隙面破坏和沿裂隙面滑移破坏;随围压升高,倾角45°裂隙花岗岩由复合破坏转变为穿裂隙面破坏,后又在10 MPa围压条件下再次转变为倾斜“Z”字型裂纹的复合破坏。围压的存在制约了预制裂隙面的劣化作用。三轴压缩条件下低倾角裂隙花岗岩的抗压强度及变形发展曲线与完整花岗岩的结果相近,试样峰值破坏时的轴向应变在围压0,2,5,10MPa条件下分别约为0.38%,0.49%,0.59%和0.75%。沿裂隙面滑移破坏试样的抗压强度以及峰值破坏时的轴向应变均显著低于穿裂隙面破坏试样的相关结果,且其抗压强度与试验前后裂隙面的分形维数差值符合幂函数增长关系。

弯曲载荷下双金属机械复合管内衬层屈曲失效机理

双金属机械复合管是油气田集输管线防腐控制的主要措施之一,而内衬层的失效是制约复合管工程应用的关键难题。为研究双金属机械复合管内衬层屈曲失效机理,建立弯曲载荷下复合管力学模型,研究成型压力、工作内压及复合管结构参数对内衬层失效模式的影响规律。结果表明:增加成型后的残余接触压力有助于提高内衬层的抗屈曲能力;较高的工作内压能够延缓内衬层屈曲时间,减小其褶皱幅值;成型前复合管层间初始间隙减小有利于提高内衬层抗屈曲能力;随着外基管壁厚、内衬管壁厚、复合管内径的增加,内衬管的抗屈曲能力增强。

高掺量粉煤灰自密实混凝土四点弯曲试验离散元模拟分析

为了研究不同粉煤灰掺量下自密实混凝土四点弯曲试验的力学性能变化及破坏过程,在PFC2D中建立高掺量粉煤灰自密实混凝土离散元模型,并进行模拟加载,对比离散元模拟结果与试验结果,分析其破坏形态与接触力变化。研究表明:使用离散元可以很好地模拟高掺量粉煤灰自密实混凝土四点弯曲试验,所得结果与实际试验具有较好的一致性,模拟试件破坏裂缝为一条由底部产生向上扩展的竖直裂缝,与试验室所得具有相似性,相较于试验室试验仅能肉眼观测的不足,离散元可以清晰地反映裂纹产生过程及内部应力变化。

结构可靠度FORM方法的混沌动力学分析

引入混沌动力学理论讨论了FORM收敛失败的非线性动力学根源.给出了几个典型函数在参数区间上的可靠指标分岔图,展示了极限状态函数经过FORM迭代成为非线性映射后计算结果的周期振荡、分岔和混沌等复杂动力学现象,计算了非线性映射的Lyapunov指数.结果表明,极限状态函数设计点的曲率大小与FORM的收敛性没有简单的联系,判别FORM迭代计算收敛性的指标是非线性映射的Lyapunov指数.

基于GISSMO失效准则的1500 MPa热成形钢断裂失效模型分析

以CR1000/1500HS为研究对象,基于GISSMO断裂失效准则,设计表征不同失效形式的5种测试试样;基于万能拉伸试验机和DIC测试系统,获取不同试样拉伸过程的力-位移曲线和断裂时刻的极限塑性应变;基于LS-DYNA搭建各失效试样的仿真分析模型,提取各试样断裂区域的应力三轴度;拟合获得断裂失效曲线和塑性失稳曲线,迭代获取其他模型参数;采用车门防撞梁动态三点弯曲冲击试验和仿真模型对承载特性进行分析;对比GISSMO失效模型、传统应变失效模型与试验测试的力-位移曲线差异,以验证失效模型的准确性。结果表明:Swift和Hockett-Sherby混合硬化模型可准确描述材料的塑性变形行为,关键参数误差小于5%;GISSMO失效模型获得的车门防撞梁承载特性关键参数和承载力-位移曲线与试验测试结果保持一致,误差低于5%;而采用传统应变失效准则获得的结果误差较大,最大位移的误差达到30%;表明基于GISSMO失效准则的材料模型可以准确表征CR1000/1500HS的塑性变形行为及断裂失效行为。

心力衰竭患者出入量记录表的设计与应用

目的:设计心力衰竭患者出入量记录表并评价其临床应用效果,为提高心力衰竭患者的容量管理质量提供依据。方法:采用文献研究、小组讨论、头脑风暴等方法设计心力衰竭患者出入量记录表。便利选取四川省某三级甲等医院的住院心力衰竭患者为研究对象,比较应用出入量记录表前后患者出入量记录完整率、记录规范率、口服补钾及时率、居家记录依从率和出院3个月内的再入院率。结果:应用出入量记录表后,心力衰竭患者出入量记录完整率、记录规范率、口服补钾及时率及居家出入量记录依从率分别提升32.7%、10.1%、22.1%、19.9%,差异有统计学意义(P<0.05);应用出入量记录表前、后心力衰竭患者的再入院率比较,差异无统计学意义(P>0.05)。结论:心力衰竭患者出入量记录表的应用有助于规范出入量记录流程,提高记录完整率、规范率和居家记录依从率,为容量管理决策提供了科学参考。

夹杂物对42CrMo4钢塑性及韧性的影响分析

风电主轴材质42CrMo4钢内部夹杂物的存在导致大锻件热成形中易出现微裂纹等缺陷。针对不同种类的夹杂物进行研究,探究其在热成形中的损伤失效行为。对原铸态42CrMo4钢进行取样、抛磨、腐蚀后进行夹杂物形貌的SEM观察与EDS成分检测,并对基体损伤较大的夹杂物做纳米压痕实验以获得其真实的力学性能参数。结果表明:铸态42CrMo4钢室温下组织为铁素体和珠光体的共存组织,主要存在Al_(2)O_(3)、TiN、硅酸盐类夹杂物及少量的CaS类夹杂物。纳米压痕实验表征得到在相同载荷下TiN的残余压痕深度最小,说明其塑性变形最小,弹性回复能力最好,Al_(2)O_(3)次之;室温下基体弹性性能最差,其塑性变形最大。在夹杂物附近观察到了部分孔洞,表明夹杂物较基体硬度大,在成形过程中会破坏基体的连续性,形成裂纹等缺陷从而影响产品质量。

结构可靠度分析FORM迭代算法的混沌控制

利用混沌控制原理对FORM收敛失败进行控制.理清了全局性和局部性两类混沌反馈控制各种方法的内在联系,说明稳定转换法和自适应调节法属于全局混沌反馈控制方法,自适应调节法可视为稳定转换法的特例.参数调节混合法不过是松弛牛顿法的另一种表达形式,它们都属于局部混沌反馈控制方法.阐明了混沌反馈控制表达式与工程力学收敛控制迭代算法的对应关系.也揭示了这些迭代算法收敛控制措施的功效和局限性.提出了一个以稳定转换法为主联合松弛牛顿法的混沌反馈控制方法,对可靠度分析FORM迭代算法实现了周期振荡、分岔和混沌控制.

地质聚合物固化软土的力学特性及微观机理分析

为解决水泥固化软土强度低及水泥生产时能耗高、污染大等问题,拟采用矿渣、粉煤灰两种工业固体废弃物制备地质聚合物固化路基软土。通过无侧限抗压强度试验研究矿渣、粉煤灰占比对固化土强度的影响;采用X射线衍射(X-Ray Diffraction,XRD)及扫描电子显微镜(Scanning Electron Microscope,SEM)试验对固化土的物相组成及内部结构进行微观表征。结果表明:地质聚合物固化土具有早强特性,前驱体中矿渣与粉煤灰最优比例为9:1,对应试样28 d的抗压强度为8.2 MPa;矿渣水化产物是固化土强度提高的主要原因,粉煤灰适量替代矿渣后固化土微观结构更为致密,但过量替代后土体松散度增大。

基于Miner理论的汽车翼子板疲劳破坏研究

针对汽车翼子板路试开裂问题,分析了翼子板冲压成形和装配过程中产生的残余应力,研究了残余应力对疲劳破坏的影响。基于Miner理论对翼子板安装点附近的工艺缺口进行优化,降低了装配过程中产生的应力集中,有效提高了翼子板的疲劳寿命。该方法不需要基于路谱的疲劳仿真,在解决翼子板疲劳开裂问题的同时,为快速分析并解决工程问题提供了思路和理论依据。

铝合金/碳纤维超混杂薄壁吸能结构的制备及性能研究

纤维/金属超混杂复合管在提高整体结构碰撞吸能性能的同时又具有显著的轻量化效果,在吸能结构材料应用领域具有良好的应用前景和优势.通过热压成形与螺栓连接相结合的方法制备铝合金/碳纤维超混杂薄壁吸能结构,讨论不同尺寸的双耳端结构、螺栓连接位置与数目在轴向压溃下对薄壁吸能结构失效行为及吸能性能的影响.试验结果表明:双耳端尺寸与螺栓位置影响结构的层间结合性能,双耳长度为12、18 mm时容易造成应力集中,从而影响结构在轴向压溃下的失效形式与吸能特性;螺栓连接数目影响结构的强度与层间结合性能,5螺栓连接的薄壁吸能结构拥有最优吸能性能与失效形式.

航空发动机涡轮叶片热障涂层的现状与发展

热障涂层的研究对提高航空发动机涡轮叶片的工作效率和服役时间具有重要意义。本文重点介绍了热障涂层的结构、材料分类、制备及后处理技术、失效机制方面的研究现状,并对其未来发展进行了展望。高熵陶瓷材料等新型陶瓷材料因其优异的热稳定性及综合力学性能,具有良好的应用前景。激光表面改性技术具有效率高、形变小、局部选区处理等优势,可进一步提高热障涂层的安全可靠性。后续热障涂层的研究将聚焦高熵陶瓷材料、功能梯度材料等的研究及应用,充分采用数值分析及建模等深入研究涂层的失效与防护机制,为航空发动机涡轮叶片的发展奠定基础。

考虑裂隙影响的桩端岩溶顶板破坏形态与安全厚度

为了研究桩端岩溶顶板破坏形态和安全厚度受岩体质量的影响,借助离散元软件,建立含节理裂隙的岩溶顶板承载计算模型,揭示顶板损伤断裂破坏细观机制,明确含裂隙顶板破坏形态,以此为基础提出考虑裂隙影响的顶板安全厚度修正计算法。研究表明:完整岩体岩溶顶板表征出典型的脆性破坏,破坏临界位移仅为较破碎岩体的60%,对应的破坏荷载则大了67.5%,岩体裂隙会明显影响桩基安全冗余度的判断,实际工程应充分考虑;桩基荷载作用下含裂隙岩溶顶板发生渐进破坏,主要经历局部压缩、微裂隙闭合→新裂隙萌生、旧裂隙扩展→形成宏观裂纹并张开→生成贯通破坏面与断口,岩体割裂成块→破坏面发展,岩体割裂加剧→形成滑动带,顶板整体破坏;岩溶顶板因错综复杂分布的节理裂隙,其空间破坏形态并非遵循规则的椎台,也非以特定曲线为母线的旋转体,而是源于裂隙的空间不规则破坏体;提出了考虑岩体节理裂隙和桩基荷载作用的岩溶顶板安全厚度修正分析方法,与模型试验结果对比表明,修正计算具备合理性。