冲击荷载下煤的塑性硬化-软化过程动态损伤本构模型

Dynamic damage constitutive model of plastic hardening-softening process of coal under impacting load

CO_(2)气相压裂已经成为我国高瓦斯突出煤矿一种有效的瓦斯综合治理技术,是低频、低速、长波非炸药爆破技术,破坏煤体的机理和过程与炸药爆破高应变率冲击作用不同,可能更接近于中应变率冲击条件下的破坏机制。为研究CO_(2)气相压裂冲击作用下煤体破坏特征,利用分离式霍普金森压杆(SHPB)试验系统对阳泉矿区新元矿的煤样进行10~100 s^(-1)中应变率条件下的动态冲击试验,分析煤在冲击荷载下脆性破坏向延性破坏转化的特性和应力-应变曲线峰值前硬化、峰值后软化机制。引入受应变率控制的动态弹性模量E_(D),建立煤的塑性硬化-软化过程动态损伤本构模型,并给出模型参数的物理意义及拟合方法,验证模型的正确性。结果表明:(1)冲击荷载下煤的动态应力-应变曲线近似为理想弹塑性变形过程,表现为应力峰值前的应变硬化与峰值后的应变软化特性。其内部裂隙损伤演化破坏过程呈现Ⅰ-无损伤、Ⅱ-微裂隙演化、Ⅲ-宏观裂纹成核、Ⅳ-宏观裂纹扩展及崩塌破坏4个阶段。(2)构建的方程描述了煤塑性硬化-软化全过程,发展了应力-应变曲线BC段应变软化过程的精确描述方法。(3)新模型仅含有5个动力参数,具有力学意义明确、参数少和拟合计算方便的优点。其中动态弹性模量E_(D)、极限应变ε_(s)、峰后应变ε^(B)_(I)随应变率ε的增大而增大,脆性指数n随应变率ε的增大而整体减小;同时ε_(s),ε^(B)_(I)控制损伤累积和宏观裂纹成核过程。(4)与前人研究成果对比分析表明,新模型精度高、普适性更强,同时适用于表征中、高应变率冲击条件下煤的塑性软化损伤破坏特征。

The CO_(2)gas fracturing has become an effective comprehensive treatment technology for high gassy and outburst coal mines in China,which is a low-frequency,low-velocity,long-wave,non-explosive technology.The mechanism and process of destroying coal are different from the high strain rate impact of explosive,and may be closer to the damage mechanism under medium strain rate impact.In order to study the coal failure characteristics under the impact of CO_(2)fracturing,the separated Hopkinson pressure bar(SHPB)test system was used to conduct the impacting tests on coal samples taken from Xinyuan coal Mine,in Yangquan Mining Area,Shanxi province under the conditions of medium strain rate(10-100 s^(-1)).Through the analysis on the characteristics of coal brittle failure to ductile failure and the mechanism of pre-peak hardening and post-peak softening of the stress-strain curve,the dynamic damage constitutive model describing plastic hardening-softening mechanism was established,including the dynamic elastic modulus E_(D)controlled by the strain rate.Moreover,the physical meaning and fitting method of the model parameters were given to verify the correctness of the model.The research results show that(1)the dynamic stress-strain curve of coal is approximately the ideal elastoplastic deformation process under the impacting load,which shows the characteristics of pre-peak strain hardening and post-peak softening.The process of internal fracture damage evolution is defined in four stages:Ⅰ-no damage,Ⅱ-micro-crack evolution,Ⅲ-macro crack nucleation,andⅣ-macro crack propagation,collapse failure stage.(2)The constructed equation of model can describe the whole process of plastic hardening and softening of coal,especially developing an accurate description method of the strain softening process in the BC section of the stress-strain curve.(3)The new model contains only five dynamic parameters,which has the advantages of clear mechanical meaning,fewer parameters,and convenient fitting calculation.Among the five parameters,the dynamic elastic modulus E_(D),ultimate strainε_(s)and post-peak strainε^(B)_(I)increase with the increase of strain rateε,while the brittleness index n decreases as the strain rate increases.Furthermore,the processes of damage accumulation and macro crack nucleation are controlled by ultimate strainε_(s)and post-peak strainε^(B)_(I).(4)Compared with previous research,the new model has higher accuracy and stronger universality,and is also suitable for describing the plastic softening damage and failure characteristics of coal under medium and high strain rate impacting conditions.