基于离散元法与分形维数的沥青混合料间接拉伸试验

Experimental on indirect tensile test for asphalt mixture based on discrete element simulation and fractal dimension

为掌握沥青混凝土内部损伤的演化过程及其裂缝扩展规律,首先在MTS万能试验机上进行沥青混合料间接拉伸试验,获取试验过程中的力、位移和时间参数;同时采用工业CT设备扫描试验前、后的试件,并通过灰度化、图像增强和阈值分割等数字图像处理技术,提取沥青胶砂、粗集料和空隙等组成部分信息;然后,基于所提取的信息建立离散元模型,进行间接拉伸仿真试验,观察试件裂缝的扩展过程,并采集仿真试验过程中的荷载、位移及单元颗粒间连接断裂[WTBX]数量Ncb等参数;最后,对比分析了室内试验和仿真试验的荷载-位移曲线特点;并基于Ncb提出了内部损伤度Dm,基于试件整体的裂缝图像提出了计盒维数指标Dc,分析了损伤度与裂缝扩展的关系。研究结果表明:室内试验与仿真试验的间接拉伸强度接近、荷载-位移曲线变化趋势相同且裂缝分布规律相似,验证了仿真试验的可行性。裂缝的扩展可分为萌生、扩展、贯通和贯通后4个阶段。在萌生和扩展阶段,细观损伤度Dm较低,而在贯通阶段较高。计盒维数Dc与裂缝扩展也密切相关,Dc在裂缝扩展的各阶段内单调变化且规律明显。因此,可利用计盒维数Dc对[WT]CT扫描得到试件的裂缝特征进行定量评价,确定裂缝的扩展程度,并基于裂缝与损伤的联系间接评价沥青混凝土内部损伤。该研究为沥青路面损伤评价提供了新的思路,即可根据路面裂缝状况评价其损伤程度。

In order to investigate the evolution process of internal damage and the law of crack propagation in asphalt concrete,the indirect tensile test of asphalt mixture was carried out in the laboratory by using MTS universal test machine.The force,displacement and time data during the test were obtained.At the same time,the specimens were scanned by industrial CT equipment,and the scanning images before and after the indirect tensile test were obtained.In addition,the constituents such as asphalt mortar,coarse aggregate and air void were extracted by digital image processing techniques such as grayscale,image enhancement and threshold segmentation.Then,the discrete element model was built based on the constituents information,and indirect tensile simulation test was carried out.the development of cracks of the model was observed.The load,displacement and number of fractures of the joints Ncb were obtained.Finally,the development of the cracks was analyzed.The characteristics of load-displacement curves in the simulation test and that in the laboratory test were compared.Based on Ncb,the internal microscopic damage variable Dm was present,the box-counting dimension variable Dc was proposed based on the crack image of the whole specimen,then the relationship between meso damage degree and crack development was established.The results show that the strength of the simulation test and that of the indoor test is very close.The trends of the load-displacement curve and the distribution law of these two tests are similar,and the feasibility of simulation test is verified.The development of cracks can be divided into four stages,in clude initiation,development,interconnection and after interconnection.Dm is lower during the initiation and expansion stages but higher during the interconnection stage.In addition,Dc is also closely related to the fractal characteristics of the crack.Dc changed monotonously in each stage of the crack,and the trend is obvious.In short,according to Dc,the development of the crack can be determined.Therefore,the mesoscopic damage inside the asphalt concrete can be evaluated indirectly.It also provides a new idea for pavement damage evaluation,which can be used to evaluate the degree of pavement damage according to the pavement crack condition.3 tabs,14 figs,28 refs.