Particle flow study on strength and meso-mechanism of Brazilian splitting test for jointed rock mass

A discrete element method （DEM） called particle flow code （PFC2D） was used to construct a model for Brazilian disc splitting test in the present study. Based on the experimental results of intact Brazilian disc of rock-like material, a set of micro-parameters in PFC2D that reflected the macro-mechanical behavior of rock-like materials were obtained. And then PFC2D was used to simulate Brazilian splitting test for jointed rock mass specimens and specimen containing a central straight notch. The effect of joint angle and notch angle on the tensile strength and failure mode of jointed rock specimens was detailed analyzed. In order to reveal the meso-mechanical mechanism of crack coalescence, displacement trend lines were applied to analyze the displacement evolution during the crack initiation and propagation. The investigated conclusions can be described as follows. （1） The tensile strength of jointed rock mass disc specimen is dependent to the joint angle. As the joint angle increases, the tensile strength of jointed rock specimen takes on a nonlinear variance. （2） The tensile strength of jointed rock mass disc specimen containing a central straight notch distributes as a function of both joint angle and notch angle. （3） Three major failure modes, i.e., pure tensile failure, shear failure and mixed tension and shear failure mode are observed in jointed rock mass disc specimens under Brazilian test. （4） The notch angle roles on crack initiation and and joint angle play important propagation characteristics of jointed rock mass disc specimen containing a central straight notch under Brazilian test.

Experimental research on the development of residual strain in seasonal frozen soil under freezing-thawing and impact type traffic loads

Vehicle load is among the main factors affecting the deformation of subgrade soil.In this research study,the concept of impact type traffic load is introduced to investigate the effects of vehicle load based on the dynamic stress and displacement time histories acquired from seasonal frozen subgrade soils.Using freezing-thawing and dynamic triaxial tests and considering the amplitude and loading sequence of impact type traffic load,the residual deformation characteristics of subgrade soil under impact type traffic loads and freezing-thawing cycles is studied.It was found that under impact type traffic load,the residual deformation of soils increased sharply as the amplitude of impact type traffic load increased.It was also found that the increase in the amplitude of impact type traffic load led to the increase of residual deformation in a scale of power and exponential function.The amplitudes of impact type traffic load affect the development stress-strain path of the residual strain.After the soil experienced the proper amount of pre-vibration of the light load,residual deformation decreased by 15%.After freezing-thawing,the residual strain of soil increased as the amplitude of the impact type traffic loads increased.Also,when the amplification effect of freezing-thawing on the residual strain was basically stable,the residual deformation increased by about 10%.The peak impact type traffic load had a large effect on soil deformation after the freezing-thawing process,leading to the observation that of the earlier the peaks,the stronger the effect of freezing-thawing.After the soil was subjected to preloading with a small load,the influence of the freezing-thawing cycles gradually stabilized.The results may be useful in preventing and controlling the risk of subgrade soil failure when construction takes place spring thaw periods.

Tensile strength of sea ice using splitting tests based on the digital image correlation method

The splitting test is a competitive alternative method to study the tensile strength of sea ice owing to its suitability for sampling.However,the approach was questioned to the neglect of local plastic deformation during the tests.In this study,splitting tests were performed on sea ice,with 32 samples subjected to the regular procedure and 8 samples subjected to the digital image correlation method.The salinity,density,and temperature were measured to determine the total porosity.With the advantage of the digital image correlation method,the full-field deformation of the ice samples could be determined.In the loading direction,the samples mainly deformed at the ice-platen contact area.In the direction vertical to the loading,deformation appears along the central line where the splitting crack occurs.Based on the distribution of the sample deformation,a modified solution was derived to calculate the tensile strength with the maximum load.Based on the modified solution,the tensile strength was further calculated together with the splitting test results.The results show that the tensile strength has a negative correlation with the total porosity,which agrees with previous studies based on uniaxial tension tests.

Approach to minish scattering of results for split Hopkinson pressure bar test

Split Hopkinson pressure bar(SHPB) apparatus, usually used for testing behavior of material in median and high strain-rate, is now widely used in the study of rock dynamic constitutive relation, damage evolvement mechanism and energy consumption. However, the possible reasons of sampling disturbance, machining error and so on often lead to the scattering of test results, and bring ultimate difficulty for forming general test conclusion. Based on the stochastic finite element method, the uncertain parameters of specimen density ρs, specimen radius Rs, specimen elastic modulus Es and specimen length Ls in the data processing of SHPB test were considered, and the correlation between the parameters and the test results was analyzed. The results show that the specimen radius Rs has direct correlation with the test result, improving the accuracy in preparing and measuring of specimen is an effective way to improve the accuracy of test and minish the scattering of results for SHPB test.

Residual stress in the cylindrical drawing cup of SUS304 stainless steel evaluated by split-ring test

The residual stresses in the wall of a SUS304 stainless steel cylindrical drawing cup were evaluated by split-ring tests, and the influences of stamping die parame- ters on the residual stress were investigated. A new theoretical model of a split-ring test was developed to evaluate the resid- ual stress in a ring, which was verified to be reasonable and reliable by numerical simulations with ABAQUS code and by nanoindentation tests. Seven groups of split-ring tests were completed, and the residual stresses were calculated according to the theoretical model. The split-ring test results showed that the circumferential residual stress in the wall of the SUS304 stainless steel cylindrical drawing cup was very large and did not change with the different die comer radius. The circumferential residual stress first increased with the increase of drawing punch-die clearance, then was almost unchanged when the clearance increased greater than blank thickness 1 mm. Thus, a smaller clearance was suggested to be chosen to reduce the residual stress in the wall of the SUS304 stainless steel drawing cup.

钢支撑-钢筋混凝土掉层框架结构抗震性能试验研究

为改善坡地钢筋混凝土(RC)掉层框架结构抗侧刚度分布的不均匀性,设计了一栋在上接地层设置钢支撑的RC掉层框架结构,按1/4比例提取其顺坡向中榀框架下部5层子结构为试验对象,开展钢支撑-RC掉层框架拟静力试验,观测试件的破坏过程,并对比RC掉层框架试验所得试件的破坏形态,分析试件滞回性能、延性和刚度退化等抗震性能指标。结果表明:试件的最终破坏是以第4层柱底混凝土压溃,梁端混凝土剥落、底部钢筋断裂,顶层柱顶混凝土剥落为标志。与RC掉层框架试验结果相比,钢支撑-RC掉层框架试件上接地柱的破坏明显减轻,柱端破坏分布也更为均匀,避免了掉层框架结构“半层破坏模式”的出现,改善了结构的耗能和抗地震倒塌能力。但试件上接地层相邻上一层的变形和破坏程度较大,应对该楼层的柱端进行适当的抗震加强;设置钢支撑后,试件的滞回曲线较为饱满,并具有良好的延性(正、负向加载的位移延性系数分别可达5.13、5.69),正负向加载的刚度退化曲线也趋于对称。

湿陷性黄土地区碎石排水系统砂岩碎石遇水软化试验研究

为研究延安湿陷性黄土地区进行的平山造地工程碎石排水系统中砂岩的遇水软化特性及其对工程施工稳定性的影响,采用WDW-600微机控制电子万能机对现场所取砂岩岩样开展了岩石单轴压缩试验和圆盘劈裂试验,测定烘干状态、天然状态、浸水饱和状态三种不同含水量的砂岩的强度及软化特性,结果表明:该地区砂岩遇水后会发生较为明显的强度软化,其中岩石抗压强度软化程度稍大,烘干样、天然样和饱和样平均抗压强度分别为14.54MPa、10.58MPa和7.70MPa,软化系数为0.53;烘干样、天然样和饱和样平均抗拉强度分别为1.43MPa、1.30MPa和0.83MPa。试验增强了对延安湿陷性黄土地区砂岩遇水软化力学性能的认识,对岩石遇水软化后排水系统承载性能计算和稳定性分析具有参考价值。

微波和传统加热下混凝土劈裂抗拉性能分析

近年来,微波辅助混凝土粗骨料回收技术因其绿色性、高效性及低损伤性引起国内外学者广泛关注。该文采用工业微波多模谐振腔及传统马弗炉对圆柱体混凝土试样进行不同条件下的加热实验,探讨了混凝土在两种加热方式下的加热效率、宏观裂纹扩展与破坏模式、劈裂抗拉强度弱化规律以及加热参数对能耗和能效的影响规律。此外,借助扫描电镜分析了微波照射下混凝土的抗拉强度弱化机理。实验结果表明:微波加热技术具有更高的加热效率,能够有效地促使裂纹沿砂浆-骨料界面扩展,从而削弱混凝土强度;并且高微波功率下混凝土加热效率更高,强度损失更大,能量消耗更少。反之,传统加热下砂浆-骨料界面结构紧凑,未出现可见裂纹,加热过程能量利用率仅1%左右。

预制裂隙砂岩巴西劈裂特性试验研究

为研究裂隙对岩石宏观破裂形态的影响,开展了单裂隙、共线双裂隙、平行双裂隙3种裂隙类型下砂岩的巴西劈裂试验,分析3种裂隙类型不同裂隙倾角下的破裂形态并对其分类,同时分析不同裂隙倾角下3种裂隙类型抗拉强度的变化。试验结果表明:裂隙会削弱岩石的抗拉强度,裂隙数量、裂隙倾角及裂隙间几何排布对试样的破裂形态及抗拉强度造成影响;单裂隙、共线双裂隙与平行双裂隙砂岩试样分别存在5种、4种与3种类型的破裂形式,岩石非均质性导致同一裂隙倾角下试样的破裂形态有所不同;单裂隙与共线双裂隙砂岩的抗拉强度总体在0°裂隙倾角时最大,随着裂隙倾角的增大,单裂隙砂岩抗拉强度呈先下降后上升再下降的变化规律,共线双裂隙砂岩抗拉强度呈现下降趋势,而平行双裂隙砂岩的抗拉强度在45°裂隙倾角时最大。

实时监测铣削力的分体式智能刀柄设计与切削测试

实时监测切削过程可提升表面质量和切削效率,为解决台式测力仪价格昂贵、布置装置过程费时费力等问题,提出了一种可同时测量三轴切削力和相应扭矩的分体式智能刀柄设计方法,搭建了实时监测铣削力的无线传输智能刀柄系统,并基于Kistler 9257B测力平台开展动态测试,验证了智能刀柄铣削力数据的准确性。

超高性能混凝土-石材界面黏结性能试验研究

超高性能混凝土(UHPC)加固修复既有石结构中,UHPC与石材之间优异的黏结性能是保证加固效果的关键。因此,为探究UHPC-石材界面的黏结性能,通过劈裂抗拉试验和双L型剪切试验,考察不同界面处理方式对UHPC-石材界面黏结性能的影响。结果表明,UHPC-石材界面破坏模式主要分为三种,其中发生模式C破坏(石材UHPC破坏)时界面黏结强度最大。界面键槽处理方式能显著提高UHPC-石材界面黏结强度、界面剪切刚度,且在改变破坏形态的同时有效提升试件的延性,当键槽深度为20 mm时,试件表现出较好的界面黏结性能。另外,试件剪切荷载-位移曲线包含滑移前阶段和滑移后阶段,凿毛试件在滑移后阶段没有经历塑性变形。相同界面处理方式下,UHPC-石材的界面抗剪强度与界面劈裂抗拉强度呈正相关关系。

超高性能混凝土动态压缩试验与损伤分析

为探究超高性能混凝土(ultra-high performance concrete,UHPC)在动态压缩下的损伤特性,该文利用霍普金森压杆装置研究了不同应变率(52~368 s-1)下的动态压缩力学性能,并基于数字图像相关技术对试件的破坏过程进行分析。结果表明:在冲击荷载下,UHPC试件破坏后,仍能保持完整;UHPC动态抗压强度和韧性表现出明显的应变率敏感性;随着应变率的增加,钢纤维能够更好地吸收冲击能量并抑制临界应变的增大;裂缝发展受应变率影响较大,裂缝最终宽度随应变率的增加不断增加,低应变率下裂缝最终宽度并非其最大宽度。该研究可为UHPC服役性能和损伤评估提供理论支撑。

Al/Hf ratio-dependent mechanisms of microstructure and mechanical property of nearly fully dense Al—Hf reactive material

This study proposed three types of Al—Hf reactive materials with particle size ratios(a),which were almost completely dense(porosity of<5.40%)owing to their preparation using hot-pressing technology.Microstructure characteristics and phase composition were analyzed,and the influence of particle size ratios on dynamic mechanical behavior and damage mechanism were investigated.The prepared sample with a=0.1 exhibited continuous wrapping of the Hf phase by the Al phase.Hf—Hf contact(continuous Hf phase)within the sample gradually increased with increasing a,and a small amount of fine Hf appeared for the sample with a=1.The reactive materials exhibited clear strain-rate sensitivity,with flow stressσ0.05and failure strainεfincreasing approximately linearly with increasing strain rate.ε.It is found that the plastic deformation of the material increased with increasing strain rate.As a increased from 0.1 to 1,the flow stress gradually increased.Impact failure of the material was dominated by ductile fracture with a large Al phase plastic deformation band for lower a,while brittle fracture with crushed Hf particles occurred at higher a.Finally,a constitutive model based on BP neural network was proposed to describe the stress-strain relationships of the materials,with an average relative error of 2.22%.

Damage constitutive model of lunar soil simulant geopolymer under impact loading

Lunar base construction is a crucial component of the lunar exploration program,and considering the dynamic characteristics of lunar soil is important for moon construction.Therefore,investigating the dynamic properties of lunar soil by establishing a constitutive relationship is critical for providing a theoretical basis for its damage evolution.In this paper,a split Hopkinson pressure bar(SHPB)device was used to perform three sets of impact tests under different pressures on a lunar soil simulant geopolymer(LSSG)with sodium silicate(Na_(2)SiO_(3))contents of 1%,3%,5%and 7%.The dynamic stressestrain curves,failure modes,and energy variation rules of LSSG under different pressures were obtained.The equation was modified based on the ZWT viscoelastic constitutive model and was combined with the damage variable.The damage element obeys the Weibull distribution and the constitutive equation that can describe the mechanical properties of LSSG under dynamic loading was obtained.The results demonstrate that the dynamic compressive strength of LSSG has a marked strain-rate strengthening effect.Na_(2)SiO_(3) has both strengthening and deterioration effects on the dynamic compressive strength of LSSG.As Na_(2)SiO_(3) grows,the dynamic compressive strength of LSSG first increases and then decreases.At a fixed air pressure,5%Na_(2)SiO_(3) had the largest dynamic compressive strength,the largest incident energy,the smallest absorbed energy,and the lightest damage.The ZWT equation was modified according to the stress response properties of LSSG and the range of the SHPB strain rate to obtain the constitutive equation of the LSSG,and the model’s correctness was confirmed.

高应变率荷载作用下钢筋与ECC的黏结滑移关系研究

采用水泥基复合材料(engineering cementitious composite,ECC)替代混凝土可以提高结构在偶然荷载作用下的抗连续倒塌性能,但在悬链线大变形阶段钢筋与ECC间可能会出现黏结滑移破坏。基于分离式霍普金森压杆(SHPB)试验装置,进行了钢筋与ECC的动态黏结滑移性能试验,分析了ECC强度等级、应变率、钢筋直径对极限黏结强度、刚度和滑移量的影响规律,获得了高应变率下钢筋与ECC的平均黏结滑移曲线,得到其黏结滑移破坏模式。并与静态黏结滑移试验进行对比,得到了动态黏结强度增强因子。进一步,通过钢筋开槽内贴应变片法,获得不同锚固位置的黏结应力及相对滑移的分布规律,提出黏结位置函数。最后,根据试验结果得到钢筋与ECC平均黏结滑移本构,进而乘以黏结位置函数得到考虑锚固位置影响的动态黏结滑移本构,为ECC结构构件设计及有限元分析提供试验依据和理论参考。

不同倾角裂隙混凝土试件动态力学破坏特性试验研究

为了探讨含不同倾角裂隙的混凝土试件在冲击荷载作用下的力学特性及破坏规律,采用分离式霍普金森压杆(split Hopkinson pressure bar, SHPB)对含有5种不同倾角裂隙的混凝土试件进行了冲击试验,借助高速摄像系统实时捕捉了裂纹扩展和动态破坏过程。结果表明:倾角为30°的裂隙试件的反射波波幅相对最大,倾角为0°的裂隙试件的反射波波幅相对较小。当预制裂隙倾角为0°时,冲击波耗散较少,透射波波幅较大;当预制裂隙倾角为60°时,冲击波耗散较多,即当应力波传播方向与预制裂隙夹角为60°时试件最容易发生破坏。冲击荷载下新形成的裂纹整体上均沿着预制裂隙尖端起裂、扩展和贯通,破坏模式主要为压应力作用下的张拉破坏;在相同预制裂隙宽度下,长度为20 mm预制裂隙的破坏形式比10 mm长度预制裂隙试件更加简单,形成新裂纹的数量相对较少,且更容易沿预制裂纹尖端方向向外起裂;巴西圆盘试件在冲击荷载的破坏形式与预制裂隙长度密切相关,与裂隙宽度关联性较弱。

黏胶纤维/锦纶/氨纶三组分织物的定量测试方法

为了探寻黏胶纤维/锦纶/氨纶三组分织物最优的定量测试方法,设计5种试验方案,并对这5种方案的试验结果进行分析比较。结果表明:方案1手工拆分法,方案2、3手工拆分与化学溶解相结合方法、方案5化学溶解法(先用二甲基乙酰胺溶解氨纶,后用质量分数为80%的甲酸溶解锦纶,剩余黏胶纤维)均为可行方案;但前3种方法适用受限,而方案5化学溶解法适用于所有组织结构的织物,为日常检测中最优选的测试方法。

非均匀沥青混合料不同影响因素下劈裂性能研究

为研究不同影响因素下非均匀沥青混合料的力学特性,在不同加载速率和试验温度条件下,开展不同离析程度沥青混合料的劈裂试验。结果表明:同一级配类型沥青混合料发生不同程度离析后,混合料内部沥青胶浆含量越多,劈裂强度受温度和加载速率变化影响越显著;开级配沥青混合料发生集料离析后,内部细集料越多,结构沥青越多,则劈裂强度越高。AC-13沥青混合料在各离析程度下比OGFC-16沥青混合料表现出更优的劈裂性能。