Study On Creep Characteristics Of Mudstone Based On Loading Rate Effect

The creep is one of the main reasons that cause the failure of engineering rock mass.In fact,the loading rate has the obvious effect on the creep characteristics of rock.In this paper,the loading rate effect of creep is studied by combining laboratory test,theoretical analysis and numerical simulation.In order to study the stress-strain characteristics of mudstone under variable loading rate and the influence of loading rate on the strength and creep characteristics of mudstone,a set of uniaxial compression tests at constant and variable loading rates and the step loading creep tests on mudstone were performed at 4 different loading rates.It was shown that the stress-strain curve of mudstone can be divided into two stages: elastic stage and failure stage.In elastic stage,the elastic modulus increases with the increase of loading rate.There is an obvious step-down phenomenon of stress-strain curve in failure stage.Under the condition of variable rate loading,the stress-strain relationship of mudstone shows obvious iso-velocity characteristics-in the variable rate uniaxial compression test,The stress-strain curve changes abruptly,when the loading rate changes suddenly,the stress-strain curve after the change keeps the trend until the next change,There is a linear relationship between stress variation and stress level.Under the condition of constant rate loading,mudstone exhibits obvious loading rate effect,the different loading rates are corresponding to the different stress-strain curve.Peak strength of mudstone increases with the increase of loading rate.The maximum strain decreases with the increase of loading rate.The creep curve of mudstone can be well fitted by piecewise cubic polynomial,and the creep rate curve of mudstone can be obtained by deriving the polynomial.The loading rate have a notable influence on the creep deformation value and the creep rate.When the loading rate is higher,the creep deformation is greater and the creep rate is higher,however with the increase of stress level,the effect of loading rate on creep deformation value and creep rate decreases and vice versa.Under the same loading rate,the maximum creep rate decreases with the increase of stress level.However,with the increase of loading rate,the trend of decreasing maximum creep rate becomes slower.The creep rate attenuates as the time increases,and the attenuation process can be divided into the linear attenuation,the logarithmic attenuation and the stabilizing stages at a higher loading rate,The creep deformation value in the linear and logarithmic creep stages accounts for more than 70% of the total creep in a relatively short time.however the difference between the first two stages is not obvious at lower loading rate.A viscoelastic-plastic constitutive model based on loading rate is established with strain rate as internal variable in the theoretical framework of component model.The model is used to identify the creep test curve of mudstone,and the parameters of the model are obtained.Comparing with the experimental results,it is found that the viscoelastic-plastic constitutive model based on loading rate can simulate the loading rate effect and creep characteristics well.