Study On Damage Evolution Law Of Roadside Backfilling Concrete Under Cyclic Loading And Unloading

The gob-side entry retaining technology is one of the important development directions of coal pillarless mining.Due to the influence of repeated mining disturbance and creep deformation,the filling body near the gob-side entry retaining is easy to reduce the bearing capacity and even failure,which brings great problems to the stability control of surrounding rock of gob-side entry retaining.In order to study the influence of periodic load on roadside filling body,the damage deformation law of roadside filling concrete under cyclic loading and unloading was analyzed by theoretical analysis,laboratory rock mechanics experiment and ultrasonic detection.The damage constitutive model of roadside filling concrete under cyclic loading and unloading was established.The main research results are as follows:(1)The cylindrical concrete samples were prepared by selecting the material ratio of roadside filling concrete,and the mechanical properties of different ages were obtained by uniaxial compression test and scanning electron microscope test.The results show that with the increase of age,the internal structure of the roadside filling concrete specimen is gradually dense,the uniaxial compressive strength and elastic modulus increase nonlinearly,the Poisson‘s ratio decreases nonlinearly,and the brittleness increases.After instability and failure,the main failure mode is shear failure,and the roadside filling concrete specimen still has certain bearing capacity after peak,and the ductility is strong.(2)The cyclic loading and unloading experiments of the roadside filling concrete specimens were carried out by using the electro-hydraulic triaxial servo instrument MTS815.The results show that with the increase of cyclic loading and unloading times,the plastic hysteresis loop appears in the concrete,and the area of the hysteresis loop presents a decrease-stability-sudden increase,and the plastic strain increment presents a stage characteristic of sudden decrease-stability-sudden increase.Based on the plastic strain,the damage parameters are defined.According to the inflection point of cumulative damage parameters,the damage evolution process of roadside filling concrete specimens is divided into three stages:0～15%cycle is the deceleration increase stage.Because the first cycle of loading and unloading breaks through the yield stress of concrete specimens,large deformation occurs,but the increment of damage parameters decreases gradually,and the cumulative damage parameters decrease.15～85%cycle is the stage of uniform increase.In this stage,the increment of damage parameters is basically stable in a small range.The cumulative damage parameters increase uniformly and the cracks develop stably.85%cycled to the accelerated increase stage of failure.Due to a large number of damage accumulation,the internal cracks of the filled concrete specimen developed completely,and the cracks gradually penetrated.The large cracks led to the increase of deformation,and the cumulative damage parameters increased rapidly until the failure and instability of the specimen.(3)By analyzing the energy dissipation law of roadside filling concrete under cyclic loading and unloading,the energy evolution process of concrete specimens under cyclic loading and unloading is divided into initial dissipation stage,accumulation stage and rapid dissipation stage according to the Wk inflection point of dissipation energy rate.0～10%cycle is the initial dissipation stage.The plastic deformation of the first cycle is large and the energy dissipation is serious.However,with the increase of cyclic loading and unloading times,the increment of plastic deformation decreases sharply and the dissipation energy rate decreases sharply.10～80%cycle is the stable dissipation stage,the crack density and expansion phenomenon coexist in the specimen,the plastic deformation increment remains stable,the energy dissipation is small,the dissipation energy rate is maintained at 0.02～0.08,and there is a small fluctuation.80%of the cycle to failure is the stage of rapid dissipation.The micro-cracks gradually expand into large cracks,and the macro-cracks are gradually formed between large cracks and large cracks.At this stage,the energy is rapidly dissipated and the dissipation energy rate increases sharply.(4)The ultrasonic time domain and frequency domain parameters of roadside filling concrete under cyclic loading and unloading are extracted.The wave velocity and amplitude show stage characteristics with the increase of cyclic loading and unloading times.The variation law of characteristic parameters in the process of cyclic loading and unloading is highly correlated with the damage accumulation process of roadside filling concrete,and the development stage of filling concrete cracks is divided according to the time-domain characteristic parameters.The periodic characteristics show that at the initial stage of cyclic loading and unloading,the upper limit of initial loading stress exceeds the yield stress,which leads to the early fracture development and faster speed.With the increase of cyclic loading and unloading times,the fracture development speed increases slowly and tends to be stable.At the later stage of cyclic loading and unloading,the accumulation and number of cracks increase sharply before the instability and failure of the specimen,and the cracks gradually penetrate.At this time,the parameters of ultrasonic time domain and frequency domain appear obvious mutations.Compared with the damage evolution stage division determined by the macroscopic plastic strain,the damage evolution characteristic points represented by the ultrasonic time domain and frequency domain parameters are more advanced.Therefore,the mutation of ultrasonic characteristic parameters can be used as a precursor information for the deterioration of the bearing performance of roadside filling concrete.(5)Based on Weibull statistical distribution function,the definition of ultrasonic wave velocity damage variable was introduced,and the damage constitutive model of roadside filling concrete under cyclic loading and unloading was established according to Mises yield criterion.The loading and unloading curves at different stages were optimized and fitted by Auto2 Fit curve.By comparing the experimental data,the fitting degree of cyclic loading and unloading curve represented by the damage constitutive model is greater than 0.99,which can accurately characterize the stress-strain relationship of filling concrete under cyclic loading and unloading.The periodic characteristics of distribution scale m and morphological parameter F0 changing with cycles are similar to the damage evolution law of filling concrete,which further confirms the strong adaptability of damage constitutive model.The research results can provide basic basis for the prediction of deformation and instability of concrete filling body near gob-side entry retaining.