材料动态力学性能实验研究技术进展

材料高应变率实验技术是应用于研究材料加载应变率在10^2-10^8S^-1。范围内力学响应的实验基础,在对材料高应变率实验技术综述的基础上,从基本理论、加载方法和测量技术等方面详细论述了SHPB实验技术的进展,分析了SHPB实验技术在实际应用中存在的问题,并提出了相变材料、贵金属及特殊材料等SHPB实验研究值得深入探索的方向。     

铝粉动态压缩动力学特性研究

为得到韧性粉末在动态压缩下的力学行为,采用分离式霍普金森压杆(split Hopkinson pressure bar, SHPB)对微米级铝粉进行了不同加载条件下的动态压缩实验。利用高速摄像机和红外测温系统(infrared temperature measurement system, ITMS)分别记录散斑场的发展和动态加载过程中铝粉试样的表面温度。从10^-4 s^-1～3 600 s^-1应变率范围内,铝粉应变率效应明显。数字图像相关法(digital image correlation, DIC)结果表明,试样整体变形不均匀,压缩前期试样压实由颗粒平动主导,后期转变为颗粒旋转和滑移主导。ITMS结果表明,与固体材料温升特性不同,加载后铝粉试样的辐射温度仍然上升。在铝粉试样中,由于有大量孔隙,颗粒在应力波驱动下加速运动,冲击能转化为颗粒动能,试样进一步压缩后,孔隙减少,颗粒运动受到限制,动能转化为颗粒内能,试样温度升高。     

铝粉动态压缩动力学特性研究EI北大核心CSCD

为得到韧性粉末在动态压缩下的力学行为,采用分离式霍普金森压杆(split Hopkinson pressure bar,SHPB)对微米级铝粉进行了不同加载条件下的动态压缩实验。利用高速摄像机和红外测温系统(infrared temperature measurement system,ITMS)分别记录散斑场的发展和动态加载过程中铝粉试样的表面温度。从10^(-4) s^(-1)~3600 s^(-1)应变率范围内,铝粉应变率效应明显。数字图像相关法(digital image correlation,DIC)结果表明,试样整体变形不均匀,压缩前期试样压实由颗粒平动主导,后期转变为颗粒旋转和滑移主导。ITMS结果表明,与固体材料温升特性不同,加载后铝粉试样的辐射温度仍然上升。在铝粉试样中,由于有大量孔隙,颗粒在应力波驱动下加速运动,冲击能转化为颗粒动能,试样进一步压缩后,孔隙减少,颗粒运动受到限制,动能转化为颗粒内能,试样温度升高。     

316L三点弯曲试样动静态断裂韧性对比实验研究

为研究316L型不锈钢的动静态断裂韧性,该文使用Instron-4505万能材料试验机对含有预制裂纹的316L型不锈钢试样进行准静态三点弯曲实验,测得其准静态断裂韧性应力强度因子K_I=53.34 MPam,同时基于霍普金森压杆(SHPB)技术对该材料三点弯曲试样进行动态断裂韧性的实验研究,测得其在3种不同加载率下的动态断裂韧性应力强度因子。结果表明:316L型不锈钢是率敏感材料,随着加载率的增加,该材料的动态断裂韧性呈下降趋势,且其断裂力学性能优良,可在工程实际中广泛应用。     

基于数字图像相关技术的混杂纤维混凝土动态抗拉性能试验研究

为研究不同纤维混杂比例下的钢-聚丙烯纤维混凝土(SPFRC)的动态抗拉性能,对六组不同纤维混杂比例的混凝土试件进行动态巴西劈裂试验,借助高速摄像仪和数字图像相关技术(DIC)对各组试样动态拉伸断裂过程进行记录和分析。研究表明：试件的应力时程曲线可以划分为缓速增长、快速增长、稳定、衰减和二次增长五个阶段,相较于素水泥砂浆试件,纤维混凝土试件在应力率二次增长后出现显著的稳定波动平台,纤维混凝土试件的动态拉伸破坏过程具有显著的延性特征;在总纤维掺量保持2%不变的条件下,混杂1%钢纤维与1%聚丙烯纤维的试件具有较优的动态抗拉强度和耗能能力;添加纤维可以减弱试件中心处的拉应变集中现象,混杂1.5%聚丙烯纤维和0.5%钢纤维的试件具有较优的开裂控制能力,平均开裂应变较素水泥砂浆试件提高了2.94倍。     

聚碳酸酯冲击压缩的实验研究

介绍了用SHPB装置获得聚碳酸酯冲击压缩应力应变曲线的实验研究,并采用二波法获得了PC材料的应力应变曲线。结果表明:聚碳酸酯材料在动态压缩时,对应变率较敏感,与静态压缩相比,在变形机制方面存在明显的差异;在研究不同温度对冲击压缩应力应变关系时,发现该材料对温度是比较敏感的。     

分层介质动态力学性能的实验研究

采用Φ75mm分离式霍普金森压杆实验装置,对由普通混凝土与泡沫混凝土组合的分层介质的动态力学性能进行了研究。结果表明,应力波通过普通混凝土时的衰减仅为其原来峰值的5%~10%,应力波通过泡沫混凝土时其峰值衰减为原来的1/5。由此说明在含泡沫混凝土与普通混凝土的分层介质中,泡沫混凝土提供削波与延缓应力波作用时间的作用,普通混凝土则为其提供较高强度。通过对分层介质应力衰减和动量变化的分析表明,分层介质在动态力学行为上表现为较大的塑性和应变率相关性,其应力衰减主要取决于泡沫混凝土材料。     

分层介质动态力学性能的实验研究

采用Φ75mm分离式霍普金森压杆实验装置,对由普通混凝土与泡沫混凝土组合的分层介质的动态力学性能进行了研究。结果表明,应力波通过普通混凝土时的衰减仅为其原来峰值的5%¹0%,应力波通过泡沫混凝土时其峰值衰减为原来的1/5。由此说明在含泡沫混凝土与普通混凝土的分层介质中,泡沫混凝土提供削波与延缓应力波作用时间的作用,普通混凝土则为其提供较高强度。通过对分层介质应力衰减和动量变化的分析表明,分层介质在动态力学行为上表现为较大的塑性和应变率相关性,其应力衰减主要取决于泡沫混凝土材料。     

冲击载荷下环氧树脂动态压缩力学响应实验研究

为了从细观角度研究碳/环氧复合材料的动态力学性能,利用分离式霍普金森压杆实验装置对其组分材料TDE86#环氧树脂体系进行动态冲击压缩实验,获得不同应变率加载条件下环氧树脂的应力一应变曲线,基于应力-应变曲线分析了应变率对环氧树脂动态压缩力学性能的影响.研究结果表明:环氧树脂具有明显的应变强化效应,随着应变率的增加,环氧树脂的强度基本没有变化,最大应力时的应变逐渐减小,动态模量和压缩刚度有很大程度的提高.     

陶瓷材料动态断裂韧性测试

为了测试陶瓷材料动态断裂韧性,利用Hopkinson压杆实验原理和改装的Hopkinson压杆装置,并将试件加工成单边切口梁进行了三点弯曲动态试验.利用改装的Hopkinson压杆装置可直接测得透射应力波,从而直接得到试件变形过程中作用在试件上的支反力.本文定义了无量纲挠度和挠度变化率,给出了几种陶瓷材料在不同挠度变化...     

A method for testing interlaminar dynamic fracture toughness of polymeric composites

Static and dynamic Mode I delamination fracture in two polymeric fiber composites was studied using a WIF test method. The dynamic test was conducted on a Split Hopkinson Pressure Bar apparatus. Crack speeds up to 1000 m/s were achieved. Dynamic fracture and crack propagation were modeled by the finite element method. Dynamic initiation fracture toughness of S2/8552 and IM7/977-3 composites were obtained. The dynamic fracture toughness of IM7/977-3 associated with the high speed propagating crack was extracted from the finite element simulation based on the measured data. It was found that the dynamic fracture toughness of the delamination crack propagating at a speed up to 1000 m/s approximately equals the static fracture toughness.     

Measurement of transient deformations using digital image correlation method and high-speed photography: application to dynamic fracture

The digital image correlation method is extended to the study of transient deformations such as the one associated with a rapid growth of cracks in materials. A newly introduced rotating mirror type, multichannel digital high-speed camera is used in the investigation. Details of calibrating the imaging system are first described, and the methodology to estimate and correct inherent misalignments in the optical channels are outlined. A series of benchmark experiments are used to determined the accuracy of the measured displacements. A 2%-6% pixel accuracy in displacement measurements is achieved. Subsequently, the method is used to study crack growth in edge cracked beams subjected to impact loading. Decorated speckle patterns in the crack tip vicinity at rates of 225,000 frames per second are registered. Two sets of images are recorded, one before the impact and another after the impact. Using the image correlation algorithms developed for this work, the entire crack tip deformation history, from the time of impact to complete fracture, is mapped. The crack opening displacements are then analyzed to obtain the history of failure characterization parameter, namely, the dynamic stress intensity factor. The measurements are independently verified successfully by a complementary numerical analysis of the problem.     

层板复合材料动态断裂韧性测试的SHPB技术研究

为测试层板复合材料的断裂韧性，对传统的Hopkinson压杆测试系统进行了改进，建立了应力波载荷作用下材料动态断裂韧性的测试方法。该方法采用三点弯曲试样进行动态断裂试验，应用动态断裂韧性测试系统ANSYSED5．4确定动态应力强度因子的响应曲线，进而测试材料动态断裂韧性。对层板复合材料试验结果的分析表明，设计的测试装置有效，建立的测试方法是对层板复合材料断裂韧性测试的有益尝试，有较大的参考价值。     

A method of extracting dynamic fracture toughness from CT tests

Recent research on methods of determining the dynamic fracture resistance for rapidly propagating cracks has progressed to the point where standardized testing procedures are being proposed. Because direct measurements of dynamic fracture resistances cannot be made, the resistances must be inferred from other measurement. In the present paper, an analysis which is fundamental to a method whereby the dynamic fracture toughness as a function of crack speed can be inferred from measurements of the specimen's end displacement at initiation of crack growth and of the total crack extension is described. All of the reference curves required for making the inference are developed for a standardized CT specimen. The procedure for using these curves to establish the dynamic fracture toughness-crack speed relationship and the minimum fracture toughness is given.

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Résumé Les recherches récentes sur les méthodes de détermination de la résistance à la rupture dynamique en présence de fissuration en cours de propagation rapide ont progressé à ce point que des procédures d'essai standards sont à présent proposées. Comme des mesures directes des résistances à la rupture dynamique ne peuvent être effectuées, les résistances doivent être déduites d'autres mesures. Dans le mémoire, on décrit une analyse qui constitue la base d'une méthode par laquelle la ténacité à la rupture dynamique en fonction de la vitesse de fissuration peut être déduite de mesures de déplacement de l'extrémité de l'échantillon lors de l'amorçage de la propagation de la fissure, et de mesures de l'extension totale de la fissure. Les courbes de référence requises pour établir cette déduction sont développées dans le cas d'éprouvettes standards CT. La procédure d'utilisation de ces courbes pour établir les relations qui lient la ténacité à la rupture dynamique et la vitesse de propagation de la fissure ainsi que la ténacité minimum à la rupture sont fournies.

     

A method for dynamic fracture toughness determination using short beams

This paper deals with dynamic fracture toughness testing of small beam specimens. The need for testing such specimens is often dictated by the characteristic dimensions of the end product. We present a new methodology which combines experimentally determined loads and fracture time, together with a numerical model of the specimen. Calculations are kept to a minimum by virtue of the linearity of the problem. The evolution of the stress intensity factor (SIF) is obtained by convolving the applied load with the calculated specimen response to unit impulse force. The fracture toughness is defined as the value of the SIF at fracture time. The numerical model is first tested by comparing numerical and analytical solutions (Kishimoto et al., 1990) of the impact loaded beam. One point impact experiments were carried out on of commercial tungsten base heavy alloy specimens. The robustness of the method is demonstrated by comparing directly measured stress intensity factors with the results of the hybrid experimental-numerical calculation. The method is simple to implement, computationally inexpensive, and allows testing of large sample sizes, without restriction on the specimen geometry and type of loading.     

Investigation of viscoelastic fracture fields in asphalt mixtures using digital image correlation

In this work we have studied the fracture behavior of asphalt mixtures, a heterogeneous mix of hard aggregates (usually in the form of crushed quarried rock) with a petroleum based asphalt binder, used in paving applications. Specifically, we studied the dependence of asphalt mixes’ fracture response on loading rate, temperature, and recycled content—the latter used primarily to replace virgin materials like aggregates and binder. Fracture tests were conducted on semi-circular bend edge cracked specimens obtained from mixes with different compositions, and the fracture event was recorded with a camera to allow for digital image correlation (DIC) measurements. DIC, with a spatial resolution of about 40 \(\upmu \)m/pixel, measured the far-field strain and displacement fields developing around a preexisting notch tip. Our focus here is on characterizing the material behavior by quantifying its viscoelastic response and fracture properties. The elastic–viscoelastic correspondence principle was used to extract viscous and elastic components from the full-field DIC-measured strain and displacement fields. Various energy dissipation mechanisms other than the fracture itself were evaluated. Stress–strain response and energy dissipated in the far-field regions were quantified. The pseudo-elastic stress intensity factor was then used to study the fracture properties, and quantify the effects on fracture properties of loading rate, temperature, and recycled content in the binder. It was seen that the viscoelastic characteristics of the material were a dominant factor in the material behavior obtained at room temperature. In general, the elastic component of the displacement was only up to about 30% of the total displacement, indicating a strong influence of viscoelasticity in this state. Loading rate, temperature and recycled asphalt shingles (RAS) content all affected the viscous response by introducing more elastic response when loading rate or recycled content increased or when temperature decreased. It became clear from these macroscopic measurements that the increase of RAS content considerably embrittles the material producing less viscous effects and less energy dissipated in the far-field, almost comparable to reductions associated with the loading rate increase (from 6.25 to 50 mm/min) or the temperature change (\(-12\) to \(25\,{^{\circ }}\)C).     

Characterization of dynamic microgyroscopes by use of temporal digital image correlation

The advanced mechanical testing of microelectromechanical systems (MEMS) is necessary to provide feedback of measurements that can help the designer optimize MEMS structures and improve the reliability and stability of MEMS. We describe a digital image correlation (DIC) method for dynamic characterization of MEMS using an optical microscope with a high-speed complementary metaloxide semiconductor-based camera. The mechanical performance of a series of microgyroscopes is tested. The DIC method is employed to measure the microgyroscope in-plane displacement with subpixel accuracy. Use of the DIC method is less restrictive on the surface quality of the specimen and simplifies the measurement system. On the basis of a series of temporal digital images grabbed by a high-speed camera, the stability characteristic of the microgyroscopes is analyzed. In addition, the quality factors of the microgyroscopes are determined and agree well with other experimental methods.     

On image gradients in digital image correlation

In digital image correlation (DIC), the unknown displacement field is typically identified by minimizing the linearized form of the brightness conservation equation, while the minimization scheme also involves a linearization, yielding a two‐step linearization with four implicit assumptions. These assumptions become apparent by minimizing the non‐linear brightness conservation equation in a consistent mathematical setting, yielding a one‐step linearization allowing a thorough study of the DIC tangent operator. Through this analysis, eight different image gradient operators are defined, and the impact of these alternative image gradients on the accuracy, efficiency, and initial guess robustness is discussed on the basis of a number of academic examples and representative test cases. The main conclusion is that for most cases, the image gradient most common in literature is recommended, except for cases with: (1) large rotations; (2) initial guess instabilities; and (3) costly iterations due to other reasons (e.g., integrated DIC), where a large deformation corrected mixed gradient is recommended instead. Copyright © 2015 John Wiley & Sons, Ltd.     

Application of digital image correlation method for analysing crack variation of reinforced concrete beams

The Digital Image Correlation (DIC) method is a fast-growing emerging technology that provides a low-cost method for measuring the strain of an object. In this study, the feasibility of using this method to observe cracks developed in reinforced concrete beams will be explored so that a practical application can be proposed. The DIC method has been applied for analysing the field of surface displacement and strain; it is not applicable for measuring non-continuous field of displacement. However, if a singular point (i.e., crack points) can be considered as the area of concentrated strain by imitating the treatment of micro-cracks using the finite element method, the region of concentrated strain field based on analyses of digital images can be applied for determining the locations of cracks. Laboratory results show that cracks developed in reinforced cement beams can be observed with a good precision using the von Mises strain field, and that smaller grids lead to clearer crack images. In addition to identifying visible cracks, the DIC image analysis will enable researchers to identify minute cracks that are not visible to naked eyes. Additionally, the DIC method has more accuracy and precision than visual observation for analysing crack loadings so that earlier warnings can be realized before cracks develop in the specimen.