混凝土损伤演化声发射和超声波时-频-空联合响应

Time-frequency-space response of damage evolution for concrete based on AE-UT joint monitoring under step-loading

为了实现对混凝土结构受载损伤评估、全面认识其失稳前兆响应行为及演化机制,本文设计了混凝土材料分级加载条件下声发射和超声波主被动同步监测方案,结合试样内部裂隙发育过程分析了受载混凝土声发射和超声波时域-频域-空间多角度多参量联合时变响应特征,基于超声波速场分布特征推演了试样损伤场的时空响应规律,借助声发射微裂纹发育定位解释了试样损伤场的动态迁移路径及演化机制.结果表明:混凝土损伤发育经历了本身非均匀向均匀的同步性变化,再向非均匀的差异性变化过程,该损伤发育特征与声发射和超声波的时域-频域参量动态时变响应规律一致;而试样失稳主破裂空间位置与波速场狭长条带状低波速区及声发射高能量定位事件聚集区均相吻合;分级加载过程中应力及时间对试样损伤具有双重累积效应,表现出低值稳定—快速增加—平静—急速增加4阶段时变特征。声发射参量a_P和a_E能定量表征该效应,试样临破坏时a_P和a_E从0.6左右急速增大2.0,故可以把该参量视为混凝土失稳的前兆特征信号.

n order to evaluate the damage evolution process of loading concrete and fully understand their response behaviors and mechanisms for structures'instability,the experimental program of acoustic emission(AE)and ultrasonic(UT)joint monitoring for concrete samples under step-loading was designed and their time-frequency multi-parameter response characteristics corresponding to microcracks evolution were studied.Based on UT velocity of different testing points,the spatial distribution characteristics of velocity filed were obtained to deduce the spa-tial migration rule of damage field.By means of AE locating events of microcracks,the dynamic spatial migration path and its mechanism of damage field were reflected visually.Research results show that the loading concrete damage evolution process experiences the identity and diversity changes,that is,the spatial distribution of damage filed changes from intrinsic heterogeneity to homogeneity then to heterogeneity,which is consistent with the time-frequency dynamic time-varying response characteristics of AE as well as UT parameters.The narrow-long strip distribution area of low UT velocity and large energy AE events both are coincident with the future main-failure zone.There is a stress-time double cumulative effect to the damage in the step-loading process,which can be divided into stable stage,rapidly increasing stage,quiet stage and sharply increasing stage and can be characterized by AE parameters(aPand aE).When the loading concrete is near to failure,aPand aEincrease rapidly from about 0.6to 2.0,so they can be regarded as the precursor signal of concrete instability.