Creep Experiment And Non-constant Viscoelastic Model Of Rock

It is generally considered that the rheological model of the Earth's media is steady, that is,the parameters of the model do not increase with time changes.In nature, the mechanical parameters change clearly with time because of the Earth's tectonic movement and other factors.Therefore,the non-constant viscoelastic creep model is introduced and the paper will focus on this research and exploration.A series of creep experiments have been carried out on fault gouges,marbles, granites and sandstones with step-loading and cyclic multi-level loading methods.The duration of experiments ranges from several hours to dozens of days,and the temperature is between normal temperature to 500℃.In the process of experimental data,it is found that there is non-uniqueness in inversion of creep curve and parameters of rock.For the same set of experimental data,three models Standard Linear Model(SLM),Burgers and Logarithm can be used to inverse the dataset.The theoretical curves obtained by inversion are similar with the measured curves and they are highly correlated.This means that the creep curve has non-uniqueness in creep model.However,when the length of the experimental data was divided into different time segments,that is,the experiment ended at the assumption time and inverse the different segments.The mechanical model parameters of inversion obtained are not constant values and vary with time.So the creep curves have non-uniqueness in the model parameters too.In order to obtain reasonable creep model,two pieces of criterion are put forward to identify excellent model,they are "match" and "prediction".First of all,the data generated by the model should be a good match to the experimental data in the past. That is,there is a high correlation coefficient and small standard deviation.The other is the model should have the ability to predict better the future of creep.For this reason the experimental data will be divided into several different lengths of time.The first segment of experimental data is used to inverse and draws the theoretical curves;the second segment of experimental data is for comparison.The purpose of this division is not only comparison the degree of "match" between models and measured data in the first segment,but also checking up the "prediction" ability in the comparison of the second segment.It will be able to find a reasonable creep model by combination.For the first criteria,there are many studies,but there is little discussion on the second one. The non-constant viscoelastic creep model namely modified standard linear model can at the same time meet the two criterions:"match" and "prediction".It is better to use the standard linear model(SLM) simulating the earth medium crust.In a short period of time it shows elasticity,and it shows rheological properties in a long time.Inverse the different segments dataset and obtain the modulus ratio C or the modular E₂ and the viscosity coefficientηor the relaxation timeÏ„which change as time regularly. Then we can determine the specific form of correct function and introduce a viscoelastic model of variable parameters.The modified standard linear model can better meet the criterions "match" and "prediction".In fact,the essence of this modification is to change Newtonian(n = 1 ) assumption by compensating for the effect of time scale on rock mechanical parameters.For large-scale geophysical measured data such as the postglacial rebounding of ground,the non-constant viscoelastic creep model can well simulate the data and calculate the viscosity coefficient.