Direct tension and fracture resistance curves of ultra high performance marine composite materials

Fracture behavior is one of the most important,yet still little understood properties of ultra-high performance cementitious composites(UHPCC),a new marine structural engineering material. Research on the fracture and direct tension behavior of UHPCC was carried out.The constitution law of UHPCC was divided into three phases:pre-partial debonding,partial debonding,and pullout phases.A direct tension constitution law was constructed based on the proposed fiber reinforcing parameter as a function of fiber volume fraction,fiber diameter and length,and fiber bonding strength.With the definition of linear crack shape,the energy release rate of UHPCC was derived and the R-curve equation was calculated from this.Loading tests of UHPCC using a three-point bending beam with an initial notch were carried out.The predictions from the proposed R-curve were in good agreement with the test results, indicating that the proposed R-curve accurately describes the fracture resistance of UHPCC.Introduction of a fiber reinforcement parameter bridges the fracture property R-curve and micro-composites’ mechanics parameters together.This has laid the foundation for further research into fracture properties based on micro-mechanics.The proposed tension constitution law and R-curve can be references for future UHPCC fracture evaluation.

Tensile and tear-type fracture toughness of gypsum material:Direct and indirect testing methods

In this context,four specimens,i.e.(i)circumferentially notched cylindrical torsion(CNCT),(ii)circum-ferentially notched cylindrical direct tension(CNCDT),(iii)edge notch disc bend(ENDB)and(iv)three-point bend beam(3PBB),were utilized to measure the modesⅠandⅢfracture toughness values of gypsum.While the CNCT specimen provides pure modeⅢloading in a direct manner,this pure mode condition is indirectly produced by the ENDB specimen.The ENDB specimen provided lower KⅢc and a non-coplanar(i.e.twisted)fracture surface compared with the CNCT specimen,which showed a planar modeⅢfracture surface.The ENDB specimen is also employed for conducting pure modeⅠ(with different crack depths)and mixed modeⅠ/Ⅲtests.KIc value was independent of the notch depth,and it was consistent with the RILEM and ASTM standard methods.But the modeⅢfracture results were highly sensitive to the notch depth.While the fracture resistance against modeⅢwas significantly lower than that of modeⅠ,the greater work of fracture under modeⅢwas noticeable.

Phase field investigation on the initial planar instability with surface tension anisotropy during directional solidification of binary alloys

Phase field investigation reveals that the stability of the planar interface is related to the anisotropic intensity of surface tension and the misorientation of preferred crystallographic orientation with respect to the heat flow direction. The large anisotropic intensity may compete to determine the stability of the planar interface. The destabilizing effect or the stabilizing effect depends on the misorientation. Moreover, the interface morphology of initial instability is also affected by the surface tension anisotropy.

Unified analytical stressstrain curve for quasibrittle geomaterial in uniaxial tension, direct shear and uniaxial compression

Considering strain localization in the form of a narrow band initiated just at peak stress, three analytical expressions for stressstrain curves of quasibrittle geomaterial (such as rock and concrete) in uniaxial tension, direct shear and uniaxial compression were presented, respectively. The three derived stressstrain curves were generalized as a unified formula. Beyond the onset of strain localization, a linear strain-softening constitutive relation for localized band was assigned. The size of the band was controlled by internal or characteristic length according to gradient-dependent plasticity. Elastic strain within the entire specimen was assumed to be uniform and decreased with the increase of plastic strain in localized band. Total strain of the specimen was decomposed into elastic and plastic parts. Plastic strain of the specimen was the average value of plastic strains in localized band over the entire specimen. For different heights, the predicted softening branches of the relative stressstrain curves in uniaxial compression are consistent with the previously experimental results for normal concrete specimens. The present expressions for the post-peak stressdeformation curves in uniaxial tension and direct shear agree with the previously numerical results based on gradient-dependent plasticity.

The effect of anisotropic surface tension on the morphological stability of planar interface during directional solidification

This paper considers the effect of the anisotropic surface tension on the morphological stability of the planar interface during directional solidification. When the expression exhibiting the four-fold symmetry is included, the modified absolute stability criterion is obtained by employing the multi-variable expansion method. The linear stability analysis reveals that for the given temperature gradient, as the anisotropic surface tension parameter increases, the stability zone tends to decrease.

An evaluation of force-based design vs.direct displacement-based design of jointed precast post-tensioned wall systems

The unique features of jointed post-tensioned wall systems, which include minimum structural damage and re-centering capability when subjected to earthquake lateral loads, are the result of using unbonded post-tensioning to attach the walls to the foundation, along with employing energy dissipating shear connectors between the walls. Using acceptance criteria defined in terms of inter-story drift, residual drift, and floor acceleration, this study presents a multiplelevel performance-based seismic evaluation of two five-story unbonded post-tensioned jointed precast wall systems. The design and analysis of these two wall systems, established as the direct displacement-based and force-based solutions for a prototype building used in the PREcast Seismic Structural Systems （PRESSS） program, were performed at 60% scale so that the analysis model could be validated using the PRESSS test data. Both buildings satisfied the performance criteria at four levels of earthquake motions although the design base shear of the direct displacement-based jointed wall system was 50% of that demanded by the force-based design method. The study also investigated the feasibility of controlling the maximum transient inter-story drift in a jointed wall system by increasing the number of energy dissipating shear connectors between the walls but without significantly affecting its re-centering capability.

基于自抗扰控制的圆纬机恒张力输纱控制系统设计

针对当前圆纬机积极式送纱方式中存在的纱线张力及输送速度不可任意调节、纱线张力波动大等问题,提出了一种可为圆纬机输送恒定张力纱线的输纱控制系统。构建了以无刷直流电动机速度环为内环、以纱线张力为外环的纱线恒张力双闭环反馈控制系统,通过自抗扰控制技术获取纱线实时动态指标,并对无刷直流电动机目标转速进行调整,最后通过磁场定向控制技术驱动无刷直流电动机。详细阐述了纱线张力的形成机制、纱线恒张力输送控制方案、系统硬件设计及控制算法。输纱测试表明,该控制系统可实现多种类型纱线的恒定张力输送。其中,棉纱张力波动标准差为1.5 cN(300 m/min测试条件),氨纶包芯纱的张力波动标准差为0.33 cN(240 m/min测试条件)。输纱速度的调节范围为0~600 m/min,纱线张力的控制范围为2.0~50.0 cN。

ANALYSIS ON THE PRINCIPLE OF SERICIN MODIFICATION WITH DIRECT STEAMING FOR COOKING COCOON

Results of circular dichromatic spectrographic analysis of sericin revealcd a modification in the character of sericinafter direct steaming for cooking dry cocoon.That is,the molecular structure of cocoon shell sericin transforms fromthe frowned random structure to β- fold structure.The sericin modification reduces undulation of the gummingforce of cocoon shell sericin. Therefore,it is one of the key ways to improve the neatness of raw silk.

肌肉跳动针感及其作用探析

针感是临床起效的关键,包含患者的主观感觉和医者手下的感觉,如酸麻胀重、沉紧涩等感觉。古籍多从“动”来描述得气感,针刺过程中产生的肌肉跳动现象就是一种常见的针下运动反应。肌肉跳动针感的产生是对经气的完全调动,患者能感知较强的酸胀感,医者手下明显有沉紧感,是一种可感知、可视化的客观得气状态。肌肉跳动针感具有明显的手法特点和作用,其手法似合谷刺,能增强针感,扩大刺激面,有效激发调动卫气,达到以动治静的目的。同时,其进针部位、局部反应与西医的软组织张力、激痛点理论相似,或可解释其作用机理。临床研究进一步证明了肌肉跳动针感的获得能更好地提高疗效。

提拉Trocar直接穿刺法在腹腔镜完全腹膜外无张力疝修补术中的应用

目的探讨腹腔镜完全腹膜外无张力疝修补术(TEP)中第一套管孔建立的方法及其临床应用价值。方法回顾性分析2020年8月—2022年3月本院收治的72例行腹腔镜完全腹膜外无张力疝补术患者的临床资料。按建立第一套管孔方法差异分为观察组(以提拉Trocar直接穿刺法建立第一套管孔)和对照组(Hasson法建立第一套管孔)两组,每组各36例。比较两组建立第一套管孔的时间、美观性、损伤程度、并发症的差异。结果观察组与对照组比较,建立第一套管孔的时间[(60.±6)s vs(156±10)s]差异具有统计学意义(P<0.05);美观性比较,术后半年温哥华瘢痕评价量表(VSS)评分[(3.62.±1.09)分vs(7.95±0.99)分],患者瘢痕自我评分(PSAS)[(11.52.±1.42)分vs(26.14±3.36)分],观测者瘢痕评分(OSAS)[(9.39.±1.43)分vs(18.70±1.71)分],其差异具有显著统计学意义(P<0.001)。损伤程度比较,观察组低于对照组,其差异具有统计学意义(P<0.05)。两组并发症发生情况比较,差异无统计学意义(P>0.05)。结论提拉Trocar直接穿刺法在TEP中建立第一套管孔,具有操作简单、安全快速、有效、美观、损伤小,值得推广。

直接激光沉积的金属液滴凝固特性数值模拟研究

液滴凝固过程是直接激光沉积制造的重要部分,其在制造面上的最终形态将直接影响制造层的成型质量。为了明晰直接激光沉积制造过程中金属液滴的凝固现象及成型机理,建立了金属液滴在冷壁面上凝固的两相流动及传热数值模型,并在模型正确性验证的基础上对强弱辐射作用两种情况下液滴的凝固过程开展研究。结果表明:弱表面辐射作用下,液滴在制造壁面上呈振荡式铺展及收缩,液滴前缘最大铺展速度可达25 mm/s,其凝固呈由壁面区域向中心区域、边缘区域向中心区域凝固的态势;强表面辐射作用下,液滴凝固呈由四周外表面向内部凝固的态势,液滴无明显铺展及收缩运动,液滴凝固速度相较于弱辐射情况极大提升,但随着凝固向内部的深入,凝固率呈线性增大趋势。

直接拉伸、劈裂及单轴压缩试验下岩石的声发射特性

采用自行研制的岩石直接拉伸试验装置,对砂岩和石灰岩2种岩样进行直接拉伸、劈裂及单轴压缩试验。试验结果表明,2种岩石的声发射活动情况大体相同。在单轴压缩条件下,加载早期的声发射活动较为活跃,随着荷载的增加,许多试样的声发射率较加载初期有所下降,这通常被认为与试样中的裂隙压密有关。劈裂试验条件下岩样的声发射活动规律与单轴压缩条件下基本一致,所不同的是:在劈裂试验条件下,声发射活动在整个加载过程中持续不断,直至临近破坏时,声发射活动大量增加,即劈裂试验条件下未观察到与单轴压缩试验类似的“裂隙压密”阶段声发射率较高的现象,也未观察到压缩试验中试样发生微破裂时,声发射累计事件数出现阶跃、变形曲线上出现拐点的现象。在直接拉伸条件下,试样的声发射活动又有很大不同,在破坏发生前的整个加载过程中,观察到的声发射事件数和能率远少于单轴压缩和劈裂试验的结果。对于大多数试样,声发射事件仅在试样破坏时才能观察到。

岩石直接拉伸与压缩变形的循环加载实验与双模量本构模型

采用一种可对同一岩石试样进行单轴压缩和直接拉伸试验的岩石试样试验装置,对5种岩石进行了压缩和拉伸循环加载试验。结果表明,对于其中压缩平均变形模量ECA较拉伸平均模量ETA大得多的3种岩石(ECA/ETA分别等于2,2.5和6),拉伸下的卸载平均模量ETAUN远大于ETA,且ETAUN约等于ECA。因此,虽然这几种岩石压缩下的变形可近似视为弹性的,拉伸下的变形却是非弹性或塑性的。一些研究者对ET<<EC的岩石引用了压缩和拉伸下均为线弹性但ET≠EC的双线弹性本构模型。本文的结果则表明,已知一种岩石的ET≠EC,不等于双线弹性模型能适用于该种岩石。对于这类拉伸变形为塑性的岩石,必须发展更复杂的本构模型。

岩石直接拉伸与压缩变形的试验研究

采用自行研制的可对同一岩石试样进行单轴压缩和直接拉伸的试验装置,对3种岩石进行了压缩和直接拉伸试验。结果表明,这3种岩石的压缩弹性模量均大于拉伸弹性模量,压缩与拉伸弹性模量之比分别为1.1,1.4,2.5。压缩泊松比也大于拉伸泊松比。研究表明:直接拉伸下切线泊松比随荷载的增加而减小,与压缩下泊松比的变化相反。

砂岩直接拉伸蠕变特性及Burgers模型的改进与应用

应用自行研制的岩石直接拉伸装置对红砂岩进行了单轴直接拉伸蠕变试验,试验结果表明,在低应力状态下砂岩的直接拉伸蠕变由衰减蠕变和稳态蠕变组成,高应力状态下砂岩直接拉伸出现历时较短的加速蠕变阶段。在对蠕变特性分析的基础上对Burgers蠕变模型进行了改进,引入了一个非线性黏塑性体,并基于BFGS算法对拉伸作用下蠕变试验结果进行直接拟合。结果表明,基于BFGS算法的改进Burgers蠕变模型理论曲线与试验曲线吻合效果较好,引入模型的非线性黏塑性体其黏性系数ηN的初始值较大(1019以上),该非线性黏塑性体在砂岩的加速蠕变阶段起到主导作用;改进Burgers模型不仅可以描述砂岩直接拉伸状态下的衰减蠕变和稳态蠕变阶段,而且可很好地描述砂岩加速蠕变过程。

煤的直接拉伸力学特性及声发射特征试验研究

为探究直接拉伸荷载作用下煤岩力学特性和损伤演化特征,使用MTS815岩石力学测试系统及PCI-Ⅱ声发射测试系统,对原煤试件进行单轴直接拉伸和实时声发射测试,分析了煤的直接拉伸力学特性及声发射特征,建立了基于声发射的损伤模型。试验结果表明:因煤体孔隙裂纹分布差异,抗拉强度离散性明显,试件平均抗拉强度为0.66 MPa;随拉伸荷载持续作用,煤体损伤累积,试件峰后承载能力逐步下降,但仍保持一定的抗拉承载能力。煤体声发射事件在峰值应力区附近沿破坏面急剧增加和汇合,预示了宏观裂纹面的形成和煤体破坏的发生;峰后试件振铃计数率和声发射能率阶段性反复升降,试件破坏进程逐步推进;声发射测试可良好展示煤体直接拉伸荷载条件下损伤出现、发展和导致破坏的完整演化过程。

岩石非线性拉、压蠕变模型及其参数识别

基于岩石单轴拉、压作用下加速蠕变曲线的幂函数拟合分析,推导了二元件黏塑性蠕变模型中黏性元件的非线性表达式,并将建立的非线性黏塑性模型与Burgers蠕变模型串联建立一个新的非线性黏弹塑性拉、压蠕变模型。分析显示该模型能够描述岩石在直接拉伸、压缩作用下的衰减蠕变、稳态蠕变和加速蠕变3个阶段。结合对重庆红砂岩单轴直接拉伸与单轴压缩蠕变试验结果,通过Matlab编写Quasi-Newton优化算法(BFGS)实现了岩石非线性黏弹塑性拉、压蠕变模型的参数辨识,辨识参数相关性系数平方R2均在96%以上。将模型与修正西原模型比较发现非线性黏弹塑性拉、压蠕变模型线型吻合程度均高于修正西原模型,表现出拉、压蠕变特性的适用性,从而进一步验证了模型描述岩石拉伸以及压缩蠕变特性的正确性。

土工直接拉伸试验装置的研制及应用

针对现有土工直接拉伸试验装置的不足,研制了一套卧式直接拉伸试验装置;装置由拉伸加载系统、数字液压伺服控制系统、数据采集分析系统3部分组成。试验装置可开展多种易成型材料的直接拉伸试验,能精确测试材料的抗拉强度并能给出拉应力–应变曲线;巧妙设计了加载夹具解决了试验材料与拉伸装置的连接难题;能精确连续控制试验拉力,并能获得材料抗拉强度峰后段的拉应力–应变曲线。使用新研制的试验装置开展了黏土、铁晶砂相似材料和水泥砂浆的直接拉伸试验,试验结果表明:在黏土直接拉伸试验所涉及的含水率范围内,当干密度保持不变时,黏土的抗拉强度随含水率的增加逐渐降低,而峰值拉应变则随含水率的增加逐渐增加;黏土达到峰值强度后,并不会突然断裂而是存在一个软化段,此时土体仍然具有一定的承载能力;铁晶砂相似材料的抗拉强度随着松香酒精溶液浓度的增加而提高;随灰砂比的提高,水泥砂浆的抗拉强度和拉伸弹性模量均相应提高。

岩石直接拉伸破坏过程及其分形特征的三维数值模拟研究

通过建立带有残余强度的弹脆性拉伸损伤细观力学模型,运用并行有限元计算手段,模拟不同均匀程度的岩石试样在直接拉伸应力作用下的破坏演化过程,得到拉伸断裂位移–应力全过程曲线,并进行理论和试验对比分析验证。通过分析非均匀性对拉伸过程中非线性力学行为及断裂破坏形态,最后讨论了拉伸断裂破坏的分形特征。数值试验结果表明,非均匀程度对岩石直接拉伸破坏过程中的非线性行为有很重要的影响。岩石细观非均匀程度越高,宏观表现出的直接拉伸强度和初始弹性模量越高,破坏的脆性程度越强,但是残余变形能力和残余强度越低,断口表现出更为光滑的形态。三维空间的破坏分形分析表明,分形维数随着拉伸的进行而逐渐增加,当岩石完全拉伸断裂之后分形维数增加到一个恒定的最大值,而分形维数的变化情况同样反映了岩石的非均匀程度和脆性拉伸破坏程度。

类岩石材料表面裂纹复合型断裂准则探讨

开展含表面裂纹的类岩石材料试件的直接拉伸试验,分析表面裂纹倾角对试件峰值强度及破坏过程的影响。由于倾斜表面裂纹处于Ⅰ,Ⅱ,Ⅲ复合变形状态,为了进一步研究表面裂纹的断裂判据,利用断裂力学软件计算半圆形表面裂纹前缘的应力强度因子,分析表面裂纹前缘3种应力强度因子的变化规律。根据最小应变能密度准则、最大应力准则以及改进的最大应力准则分别预测半圆形表面裂纹的起裂角,并与前述试验结果进行对比,结果表明,考虑Ⅲ型应力强度因子影响的改进的最大应力准则预测的起裂角与试验结果更接近。