混凝土材料的SHPB实验技术研究

为提高混凝土材料的SHPB实验结果的精度和可比性,结合混凝土的实验过程,对波导杆的弥散效应、试件内应力应变均匀性和试件的径向惯性效应进行了研究。研究结果表明：一维应力波假定和均匀性假定基本满足,混凝土试件是否有足够时间使应力应变达到均匀,取决于混凝土试件的最大应变值和应变率;实验中采用脉冲整形技术和恒应变率实验技术,并选用三波公式处理实验数据,可以增大混凝土的有效承载时间,减小试件惯性效应对实验结果的影响;当应变率接近或大于104时,需要考虑试件的径向惯性效应。     

单轴循环冲击下花岗岩力学特性与损伤演化机理

为研究循环冲击荷载下黑云母花岗岩的动态力学特性,利用改进的分离式霍普金森压杆,选取4种不同的入射波应力幅值对花岗岩试样进行等幅循环冲击,并对相关机理和试验现象进行探析.结果表明:入射波应力幅值为110.57和90.48 MPa时,随着冲击次数的增加,岩样的峰值应力逐渐降低,最大应变、平均应变率和损伤值均呈现增大趋势;入射波应力幅值为70.82 MPa时,花岗岩的峰值应力随着冲击次数的增加表现出先增强后降低的特性,而最大应变、平均应变率与损伤值则表现出相反规律;入射波应力幅值降为50.69 MPa时,岩样的力学性质基本不变,岩样未见明显的损伤.此外,研究还发现基于岩样静态压缩应力-应变曲线推求的静态裂纹起裂应力,经强度增长比例系数放大后可得到动态裂纹起裂应力,籍此能较好地解释上述循环冲击试验中所观测到的现象.     

改善Hopkinson压杆的波形分离技术

在阐述一维应力波的理论知识和Hopkinson压杆(split Hopkinson pressure bar, SHPB)系统的工作原理基础之上,采用波形整形器改善加载波形振荡,且在入射杆2个不同位置粘贴应变片来测量应变波形,利用大距离应变片粘贴方法的分离波形技术改善SHPB系统杆长的限制.结果表明,该方法能有效地分离叠加波形,得到完整的入射波和反射波,并且简化了试验的处理过程.同时,采用入射波、反射波和透射波三者之间的关系以及测试的原始波形和分离波形之间的比较来验证,说明了该波形分离技术的合理性及可行性.     

脆性材料SHPB试验中实现近似恒应变率加载必要性的研究

基于分离式霍普金森压杆（SHPB）试验的数值模拟,对脆性材料SHPB试验中是否需要实现近似恒应变率加载的必要性进行了研究。结果表明在脆性材料的SHPB试验中,即使整形入射应力波在反射应力波中产生了一个近似平台,脆性材料试样中仍旧存在显著的由惯性产生的径向围压,说明在SHPB试验中实现了由反射应力波中的近似平台所表征的近似恒应变率加载也不能表示SHPB试样中的应变率为近似恒定。因此,在脆性材料SHPB试验中应用波形整形技术并不能改变转折应变率表征SHPB试样中应力状态转变的特性。无论是否采用了波形整形技术,应用脆性材料SHPB试验结果时都应考虑惯性引起的径向围压的影响。     

Dynamic mechanical behaviour of ultra-high performance fiber reinforced concretes

Ultra-high performance fiber reinforced concretes （UHPFRC） were prepared by replacing 60% of cement with ultra-fine industrial waste powder. The dynamic mechanical behaviour of UHPFRC with different fiber volume fraction was researched on repeated compressive impact in four kinds of impact modes through split Hopkinson pressure bar （SHPB）. The experimental results show that the peak stress and elastic modulus decrease and the strain rate and peak strain increase gradually with the increasing of impact times. The initial material damage increases and the peak stress of the specimen decreases from the second impact with the increasing of the initial incident wave. Standard strength on repeated impact is defined to compare the ability of resistance against repeated impact among different materials. The rate of reduction of standard strength is decreased by fiber reinforcement under repeated impact. The material damage is reduced and the ability of repeated impact resistance of UHPFRC is improved with the increasing of fiber volume fraction.     

Effect of strain rate and water-to-cement ratio on compressive mechanical behavior of cement mortar

Effects of strain rate and water-to-cement ratio on the dynamic compressive mechanical behavior of cement mortar are investigated by split Hopkinson pressure bar(SHPB) tests. 124 specimens are subjected to dynamic uniaxial compressive loadings.Strain rate sensitivity of the materials is measured in terms of failure modes, stress-strain curves, compressive strength, dynamic increase factor(DIF) and critical strain at peak stress. A significant change in the stress-strain response of the materials with each order of magnitude increase in strain rate is clearly seen from test results. The slope of the stress-strain curve after peak value for low water-to-cement ratio is steeper than that of high water-to-cement ratio mortar. The compressive strength increases with increasing strain rate. With increase in strain rate, the dynamic increase factor(DIF) increases. However, this increase in DIF with increase in strain rate does not appear to be a function of the water-to-cement ratio. The critical compressive strain increases with the strain rate.     

Influences of Stress Wave Propagation upon Studying Dynamic Response of Materials at High Strain Rates

How the wave propagation analysis plays a key role in the studies of dynamic response of materials at high strain rates is analyzed. For the wave propagation technique, the followings are important: the loading and unloading constitutive relation presumed, the positions of the sensors embedded, the interactions between loading waves and unloading waves. For the split Hopkinson pressure bar (SI-IPB) technique, the assumption of one-dimensional stress wave propagation and the assumption of stress uniformity along the specimen should be satisfied. When the larger diameter bars are employed, the wave dispersion effects should be considered, including the high frequency oscillations, non-uniform stress distribution across the bar section, increase of rise time, and amplitude attenuation. The stress uniformity along the specimen is influenced by the reflection times in specimen, the wave impedance ratio of the specimen and the bar, and the waveform.     

Experimental and numerical study of failure behavior and mechanism of coal under dynamic compressive loads

A comprehensive understanding of the failure behavior and mechanism of coal is a prerequisite for dealing with dynamic problems in mining space. In this study, the failure behavior and mechanism of coal under uniaxial dynamic compressive loads were experimentally and numerically investigated. The experiments were conducted using a split Hopkinson pressure bar(SHPB) system. The results indicated that the typical failure of coal is lateral and axial at lower loading rates and totally smashed at higher loading rates. The further fractography analysis of lateral and axial fracture fragments indicated that the coal failure under dynamic compressive load is caused by tensile brittle fracture. In addition, the typical failure modes of coal under dynamic load were numerically reproduced. The numerical results indicated that the axial fracture is caused directly by the incident compressive stress wave and the lateral fracture is caused by the tensile stress wave reflected from the interface between coal specimen and transmitted bar.Potential application was further conducted to interpret dynamic problems in underground coal mine and it manifested that the lateral and axial fractures of coal constitute the parallel cracks in the coal mass under roof fall and blasting in mining space.     

易碎弹复合材料动态特性数值研究

利用Split Hopkinson Pressure Bar （SHPB）实验装置获取复合材料易碎弹试件的动态力学特性,并应用LS-DYNA软件对SHPB实验过程进行数值模拟.结合SHPB应力波理论,研究不同撞击速度对试件破坏程度、压杆轴向应变波形以及试件动态力学特性的影响.仿真结果表明：试件仿真结果与实验结果基本吻合,说明SHPB实验仿真是可行的;试件的破坏程度随着撞击速度的增大而显著,但压杆轴向应变波形以及试件动态力学特性随着撞击速度的增大变化不明显.     

Dynamic mechanical property of hybrid fiber reinforced concrete(HFRC)

The uniaxial compressive response of steel-polypropylene hybrid fiber reinforced concrete(HFRC) and steel fiber-reinforced concrete(SFRC) was analyzed under high strain rate loading with a 74 mm diameter split Hopkinson pressure bar(SHPB).The experimental investigation focused on recorded data and resulted in distinguishing the strain rate that mobilized different ductility of steel-polypropylene hybrid fiber reinforced concrete(HFRC) and steel fiber-reinforced concrete(SFRC).64 specimens of HFRC and SFRC with higher static compressive strength were tested at the strain rates changing from 20 to 120 s-1.The static compressive strength and dynamic stress-strain curves of the two materials were obtained at 4 different strain rates and the failure stress,and peak strain and peak toughness were also analyzed.The results show that HFRC has quite good dynamic mechanical property and clear strain-rate effect,and the failure mechanism of HFRC and SFRC was also compared based on the specimens’ failure modes in static and dynamic compressive tests.     

玄武岩纤维地聚物混凝土的高温动态力学特性

采用自主研制的高温100 mm SHPB试验装置,研究了玄武岩纤维增强地聚物混凝土(BFRGC)的瞬时高温动态力学特性.采用厚度为1.0 mm,直径为30,35,40,45,50 mm的铝片作为整形器对入射波进行整形,保证了试验过程的有效性.结果表明:不同温度下,玄武岩纤维地聚物混凝土(BFRGC)的动态抗压强度、峰值应变和能量吸收特性随应变率的提高近似线性增长;200℃时BFRGC的动态抗压强度相对于常温时大幅度提高;随着温度的升高,BFRGC的峰值应变显著提高,且均大幅度高于常温时的峰值应变值;200～600℃时BFRGC的吸能特性明显优于常温状态,而800℃时能量吸收特性明显降低.     

Q235、Q345钢结构材料的低周疲劳性能

对Q235、Q345钢的低周疲劳性能进行了研究。采用轴向应变控制方法,在岛津电液伺服疲劳试验机上测定了2种钢低周疲劳过程中的循环应力响应特征、循环应力与应变的关系,通过拟合Basquin公式和Coffin-Manson公式得到了2种钢的疲劳寿命公式,据此计算了Nf=100周时的循环能量吸收率σa.Δεt值,并与别的钢筋进行了比较。通过断口扫描发现,Q235钢裂纹起源于试样表面,由于第二相质点和夹杂物的存在,形成微孔洞和微裂纹,互相连接形成疲劳断裂;Q345钢裂纹起源于表面,然后通过不断扩展形成微裂纹,再连接成宏观裂纹,最终导致材料断裂。     

混凝土SHPB试验的波形整形材料研究

针对SHPB试验中试件的应力均匀性问题,计算分析了试件达到应力均匀时波在试件中来回透射-反射所需的最低次数,以及相对应的入射脉冲上升沿最短时间。通过Ф50mmSHPB试验装置对CAO混凝土进行冲击压缩试验,得到铜片、铝片、黄油、橡胶4种波形整形材料冲击后的入射波形,并进行分析比较。结果表明：橡胶是良好的波形整形材料,且其波形呈半正弦状。     

混凝土压缩试验的改善及动态损伤

为了探求混凝土动态压缩试验入射波形的改善方法,从而进一步研究冲击荷载作用下混凝土动态损伤及力学性能,采用常规的大直径分离式霍普金森压杆(split Hopkinson pressure bar, SHPB)进行混凝土的动态压缩试验。然而入射脉冲的弥散效应和严重的高频震荡现象会对试验结果会产生较大的误差,所以试件在破坏前应力均匀分布要求且保持恒应变率加载是保证试验有效性及试验结果可靠性的关键。通过黄铜整形器改善入射波形以减小试验误差,然后利用理论公式与试验相结合的方式,通过控制变量,进一步研究气压、冲击次数等对混凝土动态损伤的影响。研究结果表明:较薄的小直径实心铜片整形器可以提高试验准确度;随着重复冲击次数的增加,混凝土损伤度逐渐提高,动态弹性模量减小,抵抗冲击的能力减弱。     

用于动态断裂试验的高温炉设计及温度场模拟

根据在分离式霍布金森压杆上进行岩石的高温动态断裂试验的要求设计了一个高温炉,并通过ANSYS程序对高温炉温度场进行模拟,得到了高温炉的温度场分布图。此外,还对包括试件和压杆（入射杆与透射杆）在内的试验系统温度场进行了模拟,比较了试件与压杆接触加热和分离加热两种计算结果。结果表明,采用压杆与试件分离加热方式可使试件获得均匀温度场。     

应变速率对天津海积软土力学性能的影响

采用WF应力路径三轴仪,对天津滨海海积软土进行了不同围压、不同应变速率的三轴固结不排水剪切试验（CU）,分析了不同应变速率对海积软土试样的应力一应变关系、有效应力及试样破坏形态的影响．结果表明：CU试验应力一应变关系呈硬化特征,剪应力峰值随着应变速率的增大而增大,而孔压变化值则随着应变速率的增大而逐渐减小;不同应变速率的有效应力路径在相同固结压力下基本一致,因而正常固结黏土的有效应力路径具有唯一性;同时,应变速率对总应力强度有所影响,总应力的内摩擦角随着应变速率的增大而增大．     

Relationship between diameter of split Hopkinson pressure bar and minimum loading rate under rock failure

In order to investigate the relationship between bar diameter and loading rate of the split Hopkinson pressure bar（SHPB） setup under the failure of rock specimen and realize the medium strain rate loading of specimen, new SHPB setups with different elastic bar＇s diameters of 22, 36, 50 and 75 mm were constructed. The tests were carried out on these setups at different loading rates, and the specimens had the same diameter of elastic bars and same ratio of length to diameter. The test results show that the larger the elastic bar＇s diameter is, the less the loading rate is needed to cause specimen failure, they show good power relationship, and that under the same strain rate loading, specimens are broken more seriously with larger diameter SHPB setup than with smaller one.     

镍基高温合金GH4169试件有限元分析与寿命预测

对光滑薄壁管进行了多轴恒幅比例加载疲劳试验,并运用弹塑性有限元法对该加载条件下的光滑薄壁管进行了分析和计算。结果表明,用有限元法计算的试件应力应变值与试验结果吻合,并用此方法对缺口试件危险点应力应变状态进行了分析。最后,用局部应力应变法预测该试件的疲劳寿命,得到较为理想的预测结果。     

近海环境下锈蚀箍筋约束混凝土本构模型

采用人工气候环境模拟技术模拟近海环境,对36组约束混凝土棱柱体试件进行加速腐蚀,开展轴压试验,研究箍筋锈蚀对约束混凝土峰值应力、峰值应变和应力-应变曲线形状的影响. 通过对试验结果的回归分析,得到考虑箍筋锈蚀率影响的混凝土峰值应力和应变修正系数拟合公式,以形状系数作为应力-应变曲线形状参数,基于Mander模型确定考虑箍筋锈蚀率影响的形状系数的计算公式,建立近海环境下的锈蚀箍筋约束混凝土本构模型. 与试验结果对比发现：采用该模型计算得到的各试件峰值应力、峰值应变及应力-应变曲线形状均与试验结果符合较好,表明建立的本构模型能够较准确地反映锈蚀箍筋约束混凝土力学性能,可以用于该环境下的锈蚀RC结构剩余承载力及抗震性能评估.     

Failure of rock under dynamic compressive loading

Split Hopkinson Pressure Bar（SHPB） test was simulated to investigate the distribution of the first principal stress and damage zone of specimen subjected to dynamic compressive load. Numerical models of plate-type specimen containing cracks with inclined angles of 0°,45° and 90° were also established to investigate the crack propagation and damage evolution under dynamic loading. The results show that the simulation results are in accordance with the failure patterns of specimens in experimental test. The interactions between stress wave and crack with different inclined angles are different; damage usually appears around the crack tips firstly; and then more damage zones develop away from the foregoing damage zone after a period of energy accumulation; eventually,the damage zones run through the specimen in the direction of applied loading and split the specimen into pieces.