大理岩动态拉伸强度及弹性模量的SHPB实验研究

提出了获取脆性材料动态拉伸强度及弹性模量的实验步骤及相关记录数据的分析方法。利用直径为100mm的分离式Hopkinson压杆径向冲击巴西圆盘和平台巴西圆盘试样,测试了大理岩在高应变率加载下的动态力学性能。应力波加载下动态劈裂拉伸圆盘在试样中心产生了约45/s的拉伸应变率。分析了实验的有效性并考虑了试样两个端面应力波波形差异的影响以提高实验结果的精度。结果表明准静态下的公式可适用于动态劈裂拉伸实验;大理岩的动态拉伸强度及弹性模量比静态时有明显的增加。     

大理岩的高应变率动态劈裂实验

首次把平台巴西圆盘试样引入动态劈裂试验。利用直径100mm的分离式Hopkinson杆对大理岩巴西圆盘和平台巴西圆盘试样进行了动态劈裂实验。结合有限元分析,得到了大理岩的动态劈裂破坏的拉伸强度。分析了巴西圆盘和平台巴西圆盘的典型破坏方式。结果表明,大理岩的动态拉伸强度随着应变率的提高而增加。利用圆盘中心粘贴的应变片来测大理岩等脆性材料的动态拉伸强度,是一种简便高效的试验方法。和巴西圆盘相比,平台巴西圆盘具有更大的优越性和更好的测量效果。     

大理岩动态劈裂拉伸的SHPB实验研究

利用直径100 mm的Hopkinson压杆和薄圆形铝片作为波形整形器,用不同弹速径向冲击平台巴西圆盘试样以研究大理岩的动态拉伸强度。分析了试样的应变率、破坏时间、破坏模式,以及破坏过程中的载荷-应变关系,得到了关于大理岩在高应变率下拉伸强度及弹性模量的一些结论。考虑了试样的尺寸大小及两个平台附近应力的时间不均匀性与空间不均匀性对实验结果的影响。同时,利用有限元法对平台巴西圆盘试样的动态应力分布进行了数值模拟,验证了动态劈裂拉伸实验的有效性。     

岩石巴西圆盘动态劈裂的流形元法模拟

利用二阶流形元法,通过引入裂纹产生与扩展判据,对冲击载荷作用下岩石平台巴西圆盘的动态拉伸劈裂过程进行了数值模拟,再现拉伸波作用下圆盘被劈裂的过程。模拟现象与实验结果相符,动态平衡时的应力分布与有限元结果基本一致。从而验证了流形元在模拟冲击载荷作用下材料动态破坏过程的有效性和可行性。     

不同加载边界下混凝土巴西劈裂过程及强度的DIC实验分析

平板、垫条、圆弧压模及平台试样等不同加载方式下, 脆性材料巴西劈裂实验结果的差异一直是人们所讨论的问题. 本文设计了混凝土平板直接加载与采用垫条加载实验, 采用理论、高速相机与数字图像相关法(digital image correlation, DIC)相结合的方法, 对试样表面应变场演化、起裂位置及裂纹扩展过程进行分析, 探讨了不同的加载边界对试样应变集中演化及劈裂拉伸强度结果的影响. 结果显示: (1)混凝土拉伸非线性特性导致的应变集中演化对不同加载条件非常敏感, 平板加载时应变集中较早在加载端起始并向中心演化, 即使满足巴西实验"中心起裂"条件, 其强度仍低于垫条加载情况, 两者差别达17.9%; (2)如果软垫条加载接触角合理设计, 与平板直接加载相比, 其应力场更稳定, 有利于保证圆盘试样的应变集中及断裂均从中心起始, 更好满足巴西劈裂实验条件; (3)仅校验巴西实验 "中心起裂"有效性条件是不充分的, 设计时必须谨慎考虑. 研究结果对脆性材料巴西劈裂实验设计、测试分析具有重要参考意义.      

混凝土类材料动态拉伸强度的微观力学模型

基于水泥砂浆试样动态劈裂拉伸实验,研究了不同加载速率下水泥砂浆材料动态劈裂时的裂纹发 生和扩展规律,提出一个微观力学模型。结果表明,微裂纹惯性是混凝土类材料动态拉伸实验中测量到的动 态拉伸强度随应变率的增加而提高的一种微观机制。     

不同加载边界下混凝土巴西劈裂过程及强度的DIC实验分析

平板、垫条、圆弧压模及平台试样等不同加载方式下,脆性材料巴西劈裂实验结果的差异一直是人们所讨论的问题.本文设计了混凝土平板直接加载与采用垫条加载实验,采用理论、高速相机与数字图像相关法(digital image correlation, DIC)相结合的方法,对试样表面应变场演化、起裂位置及裂纹扩展过程进行分析,探讨了不同的加载边界对试样应变集中演化及劈裂拉伸强度结果的影响.结果显示:(1)混凝土拉伸非线性特性导致的应变集中演化对不同加载条件非常敏感,平板加载时应变集中较早在加载端起始并向中心演化,即使满足巴西实验"中心起裂"条件,其强度仍低于垫条加载情况,两者差别达17.9%;(2)如果软垫条加载接触角合理设计,与平板直接加载相比,其应力场更稳定,有利于保证圆盘试样的应变集中及断裂均从中心起始,更好满足巴西劈裂实验条件;(3)仅校验巴西实验"中心起裂"有效性条件是不充分的,设计时必须谨慎考虑.研究结果对脆性材料巴西劈裂实验设计、测试分析具有重要参考意义.     

基于巴西盘试验的海冰拉伸强度研究

海冰拉伸强度是其基本力学性能之一,同时也是冰区船舶与海洋工程结构设计所需的重要参数.对于脆性材料的拉伸强度测试,巴西盘劈裂试验相比单轴拉伸试验在试样制备与加载上具有明显的优势.为研究海冰的拉伸强度特征,对渤海辽东湾沿岸的粒状冰开展了系统的巴西盘劈裂试验研究.在加载过程中与试样破坏后,分别对加载横梁的位移与加载力以及试样最终破坏模式进行了记录.同时,对试样的冰晶结构、盐度、温度以及密度进行了测量.通过改变加载速率、试样厚度与试样温度以研究不同参数对试验结果的影响.针对传统试验中试样的刚体假设,考虑了试样变形对应力状态的影响并将其引入了理论模型.试验过程中所有海冰试样均以劈裂模式破坏.试验结果表明,加载速率与试样厚度对拉伸强度的影响并不显著,但孔隙率的影响较为明显.当孔隙率由75‰降低至10‰时,拉伸强度由1.0 MPa升高至2.8 MPa.与单轴拉伸试验所测得数据对比,巴西盘劈裂试验所得到的拉伸强度随孔隙率的变化趋势相一致.但该方法所得到的粒状冰拉伸强度要高于预期结果.试验表明巴西盘劈裂试验中海冰试样的破坏模式与试验结果均较为合理,可成为海冰拉伸强度的有效测试方法.     

基于巴西盘试验的海冰拉伸强度研究

海冰拉伸强度是其基本力学性能之一, 同时也是冰区船舶与海洋工程结构设计所需的重要参数. 对于脆性材料的拉伸强度测试, 巴西盘劈裂试验相比单轴拉伸试验在试样制备与加载上具有明显的优势. 为研究海冰的拉伸强度特征, 对渤海辽东湾沿岸的粒状冰开展了系统的巴西盘劈裂试验研究. 在加载过程中与试样破坏后, 分别对加载横梁的位移与加载力以及试样最终破坏模式进行了记录. 同时, 对试样的冰晶结构、盐度、温度以及密度进行了测量. 通过改变加载速率、试样厚度与试样温度以研究不同参数对试验结果的影响. 针对传统试验中试样的刚体假设, 考虑了试样变形对应力状态的影响并将其引入了理论模型. 试验过程中所有海冰试样均以劈裂模式破坏. 试验结果表明, 加载速率与试样厚度对拉伸强度的影响并不显著, 但孔隙率的影响较为明显. 当孔隙率由75‰降低至10‰时,拉伸强度由1.0 MPa升高至2.8 MPa. 与单轴拉伸试验所测得数据对比, 巴西盘劈裂试验所得到的拉伸强度随孔隙率的变化趋势相一致. 但该方法所得到的粒状冰拉伸强度要高于预期结果. 试验表明巴西盘劈裂试验中海冰试样的破坏模式与试验结果均较为合理, 可成为海冰拉伸强度的有效测试方法.      

用圆孔平台巴西圆盘确定岩石拉伸强度的非局部应力方法

对用圆孔平台巴西圆盘确定岩石拉伸强度的方法进行了研究.用有限元法分析了试件加载直径上的双向应力分布.由于加载直径靠近孔边处存在较大的拉伸应力梯度,如果以加载直径与中心圆孔相交点的最大拉伸应力来确定岩石的拉伸强度σt,拉伸强度的试验值会随着中心圆孔与圆盘半径之比r/R的增加而减小.采用非局部应力方法,考虑双向应力状态作用下发生破坏的Griffith强度准则的等效应力σG,在加载直径上选取材料特征尺寸,让σG"在特征尺寸上积分后取均值后得到岩石的拉伸强度σt.利用该方法得到的圆孔平台巴西圆盘测得的σt值不随r/R的变化而变化,其平均值大约为平台巴西圆盘测试值的1.23倍左右.     

Dynamic Tensile Response of a Microwave Damaged Granitic Rock

Background

Understanding the dynamic tensile response of microwave damaged rock is of great significance to promote the development of microwave-assisted hard rock breakage technology. However, most of the current research on this issue is limited to static loading conditions, which is inconsistent with the dynamic stress circumstances encountered in real rock-breaking operations.

Objective

The objective of this work is to investigate the effects of microwave irradiation on the dynamic tensile strength, full-field displacement distribution and average fracture energy of a granitic rock.

Methods

The split Hopkinson pressure bar (SHPB) system combined with digital image correlation (DIC) technique is adopted to conduct the experiments. The overload phenomenon, which refers to the strength over-estimation phenomenon in the Brazilian test, is validated using the conventional strain gauge method. Based on the DIC analysis, a new approach for calculating the average fracture energy is proposed.

Results

Experimental results show that both the apparent and true tensile strengths increase with the loading rate while decreasing with the increase of the irradiation duration; and the true tensile strength after overload correction is lower than the apparent strength. Besides, the overload ratio and fracture energy also show the loading rate and irradiation duration dependency.

Conclusions

Our findings prove clearly that microwave irradiation significantly weakens the dynamic tensile properties of granitic rock.

     

加载角度对层理页岩裂纹扩展影响的实验研究

采用分离式霍普金森压杆（SHPB）系统对页岩进行冲击实验,研究层理角度对页岩动态断裂过程的影响,在裂尖设置裂纹扩展计,借助高速摄影和数字图像相关（DIC）技术对页岩中心切槽半圆盘弯曲（NSCB）试件断裂的全过程进行研究,得到了不同加载角度下页岩的动态起裂韧度、裂纹扩展速度、断裂过程中应变场和水平位移场的变化规律。实验发现:不同加载角度下,页岩的动态起裂韧度具有显著的各向异性,加载角度与动态起裂韧度呈正相关;加载角度对试样的裂纹扩展速度具有显著影响,与裂纹扩展速度呈负相关;当冲击速度较低时,切槽方向是裂纹扩展的优势方向,而当冲击速度较高时,试样会产生沿层理弱面的次生裂纹,次生裂纹对试样的断裂具有显著影响。     

Dynamic Tensile Testing of Soft Materials

Determination of dynamic tensile response of soft materials has been a challenge because of experimental difficulties. Split Hopkinson tension bar (SHTB) is a commonly used device for the characterization of high-rate tensile behavior of engineering materials. However, when the specimen is soft, it is challenging to design the necessary grips, to measure the weak transmitted signals, and for the specimen to achieve dynamic stress equilibrium. In this work, we modified the SHTB on the loading pulse, the equilibrium-monitoring system, and the specimen geometry. The results obtained using this modified device to characterize a soft rubber indicate that the specimen deforms under dynamic stress equilibrium at a nearly constant strain rate. Axial and radial inertia effects commonly encountered in dynamic characterization of soft materials are also minimized.     

Three dimensional analytical solution for finite circular cylinders subjected to indirect tensile test

This paper derives a new three-dimensional (3-D) analytical solution for the indirect tensile tests standardized by ISRM (International Society for Rock Mechanics) for testing rocks, and by ASTM (American Society for Testing and Materials) for testing concretes. The present solution for solid circular cylinders of finite length can be considered as a 3-D counterpart of the classical two dimensional (2-D) solutions by Hertz in 1883 and by Hondros in 1959. The contacts between the two steel diametral loading platens and the curved surfaces of a cylindrical specimen of length H and diameter D are modeled as circular-to-circular Hertz contact and straight-to-circular Hertz contact for ISRM and ASTM standards respectively. The equilibrium equations of the linear elastic circular cylinder of finite length are first uncoupled by using displacement functions, which are then expressed in infinite series of some combinations of Bessel functions, hyperbolic functions, and trigonometric functions. The applied tractions are expanded in Fourier–Bessel series and boundary conditions are used to yield a system of simultaneous equations. For typical rock cylinders of 54 mm diameter subjected to ISRM indirect tensile tests, the contact width is in the order of 2 mm (or a contact angle of 4°) whereas for typical asphalt cylinders of 101.6 mm diameter subjected to ASTM indirect tensile tests the contact width is about 10 mm (or a contact angle of 12°). For such contact conditions, 50 terms in both Fourier and Fourier–Bessel series expansions are found sufficient in yielding converged solutions. The maximum hoop stress is always observed within the central portion on a circular section close to the flat end surfaces. The difference in the maximum hoop stress between the 2-D Hondros solution and the present 3-D solution increases with the aspect ratio H/D as well as Poisson’s ratio ν. When contact friction is neglected, the effect of loading platen stiffness on tensile stress in cylinders is found negligible. For the aspect ratio of H/D = 0.5 recommended by ISRM and ASTM, the error in tensile strength may be up to 15% for both typical rocks and asphalts, whereas for longer cylinders with H/D up to 2 the error ranges from 15% for highly compressible materials, and to 60% for nearly incompressible materials. The difference in compressive radial stress between the 2-D Hertz solution or 2-D Hondros solution and the present 3-D solution also increases with Poisson’s ratio and aspect ratio H/D. In summary, the 2-D solution, in general, underestimates the maximum tensile stress and cannot predict the location of the maximum hoop stress which typically locates close to the end surfaces of the cylinder.     

材料超高温动态拉伸SHTB实验方法的有效性分析

针对高温拉伸分离式Hopkinson杆实验技术,通过数值模拟、实验验证以及几种典型材料的高温动态拉伸性能测试相结合的方法,对此实验技术中存在的几个关键问题进行了深入研究。结果表明:对于平板状钩挂式拉伸试样,通过标距段尺寸优化后,应力分布均匀,流动应力曲线与螺纹拉伸试样一致,且应力上升段后没有剧烈跳动;通过精确气动控制,在加载脉冲到来同时,可实现有效的试样快速同步组装和加载;当试样温度为1 200 ℃时,在冷加载杆与高温试样接触以及应力波加载试样的整个过程中,试样平均温度下降约1.3%,而加载杆端温升低于180 ℃。为了验证此实验技术,对3D打印TC4、镍基单晶高温合金DD6进行了最高温度约1 200 ℃时的高温动态拉伸力学性能实验测试。     

基于Hopkinson压杆的M型试样动态拉伸实验方法研究

采用传统分离式Hopkinson压杆进行M型试样的动态拉伸实验，可避免试样与杆的连接问题，但该方法并未得到发展和验证。本文中，采用有限元数值分析和实验方法，对M型试样动态拉伸实验进行分析和改进。结果表明:（1）改进的封闭M型试样，可以增强试样整体刚度，有效减少试样畸变引起的附加弯矩对拉伸标段的影响，方便通过Hopkinson压杆加载实现一维拉伸变形；（2）采用试样刚度系数修正法，可消除M型试样整体结构的弹性变形对测试的影响，精确获得试样拉伸标段的塑性应变；（3）高加载率下，建议采用波形整器加载，可显著减少试样结构引起的载荷震荡现象、改善两端的应力平衡，获得准确的动态拉伸应力应变曲线，实现5 900 s?1甚至更高应变率下的动态拉伸实验。研究方法可为M型试样拉伸实验设计和应用提供参考。     

Predicting the Dynamic Compressive Strength of Carbonate Rocks from Quasi-Static Properties

In this study, the split Hopkinson pressure bar testing method was used to quantify the dynamic strength characteristics of rocks with short cylindrical specimens. Seventy dynamic compression tests were conducted on 10 different carbonate rocks with the split Hopkinson pressure bar apparatus. Experimental procedure for testing dynamic compressive strength and elastic behaviour of rock material at high strain rate loading is presented in the paper. Pulse-shaper technique was adopted to obtain dynamic stress equilibrium at the ends of the sample and to provide nearly a constant strain rate during the dynamic loading. In addition to dynamic tests, the physical properties covering bulk density, effective porosity, P-wave velocity and Schmidt hardness of rocks, and mechanical properties such as quasi-static compressive strength and tensile strength were determined through standard testing methods. Multiple linear regression analyses were carried out to investigate the variation of dynamic compressive strength depending on physical and mechanical properties of rocks and loading rate. A three parameter model was found to be simple and provided the best surface fit to data. It was found that dynamic compressive strength of rocks increases with increase in loading rate and/or increase in rock property values except porosity. Statistical tests of regression results showed that quasi-static compressive strength and Schmidt hardness are most significant rock properties to adequately predict the dynamic compressive strength value among the other properties. However, P-wave velocity, quasi-static tensile strength of rocks could also be used to estimate the dynamic compressive strength value of rocks, as well, except the bulk density and effective porosity.     

Flattened Brazilian Disc Method for Determining the Dynamic Tensile Stress-Strain Curve of Low Strength Brittle Solids

An indirect tensile testing method is proposed to measure the full dynamic tensile stress-strain curve of low strength brittle solids. In this method, the flattened-Brazilian disc (FBD) sample is loaded by modified split Hopkinson pressure bars (SHPB) system. Low amplitude dynamic forces were measured with a pair of piezoelectric force transducers embedded in the incident bar and the transmitted bar. The evolution of tensile stress at the center of the disc sample was determined through finite element analyses using the measured stress in SHPB as inputs. In a traditional Brazilian test, a strain gauge is mounted at the center of the specimen to measure the tensile strain, which is difficult to apply for low strength brittle materials. Thus, two types of non-contact methods, the Digital Image Correlation (DIC) technique and the Laser Gap Gauge (LGG), were used to measure the strain. The DIC method was used to monitor the displacement and the strain map of the specimen during the test, from which the strain at the center of the specimen can be obtained. The accuracy of the DIC results was assessed, and the displacement and strain uncertainties of our system were 0.003 mm and 0.003, respectively. LGG was used to monitor the expansion of the disc perpendicular to the loading axis, from which the average tensile strain is deduced. The numerical simulation revealed that the tensile strain at the center of the specimen is proportional to the average tensile strain and that the ratio is not sensitive to the material elastic parameters. The strain measured through LGG was compared with that measured by the DIC method using photos captured with a synchronized high-speed camera. The result of the LGG method was 20 % smaller than that of the DIC process. However, the latter was limited by the number of frames of the high-speed camera. The feasibility of this methodology was demonstrated using a polymer-bonded explosive (PBX).     

冲击拉伸实验装置的研制及其试验研究

材料动力学试验技术远比准静态力学中的复杂,为了模拟各种速率的冲击加载过程,试验装置设计就成为关键问题之一.特别是针对材料动态拉伸性能的测试,目前的冲击拉伸装置还没有统一标准,因此本文基于一维弹性应力波原理设计了一套双气室间接杆-杆型冲击拉伸试验装置.该装置采用了双气室对称布置的方式,通过气体转换器实现气路的转换,克服了现有气动式冲击拉伸设备结构复杂、密封要求严格的缺点.本文利用该装置对2A12T4铝合金试件的冲击拉伸性能进行了测试,并数值分析了应力波在杆系和试件中的传播效应.通过试验测试和数值分析论证了该冲击拉伸装置实验的可靠性和设计的合理性.