Research On Impact Damage Characteristics Of Coal And Rock Based On Image Analysis

In coal mining projects,coal and gas outbursts and rockburst disasters are accompanied by the impact of coal or rock.The research on the damage characteristics of coal and rock mass under impact loading is more in line with the actual situation in coal mines.And tensile failure often occupies the major part in the process of failure as the tensile strength of coal and rock is far less than its compressive strength.Researchers have used the stress-strain curve to describe the failure process of coal and rock under impact tensile failure and studied its dynamic characteristics,and achieved fruitful research results.However,the stress-strain curve is used as an indirect method to describe the process of coal rock failure,and it cannot accurately reflect the process of coal rock failure.The unstable failure process of coal and rock under the impact loading conditions is often accompanied by the initiation and evolution of cracks.The cracks generated on the surface of coal and rock masses are not only an important expression for measuring the damage state of coal and rock under dynamic impact,but also The most direct manifestation of the current state of coal rock bodies.The crack propagation can better describe the destruction process of coal and rock.However,traditional image processing algorithms such as threshold segmentation cannot accurately identify and quantify the cracks in coal and rock,resulting not accurately describe the crack propagation in the process of coal rock failure.Therefore,this paper designs a method that can automatically identify and quantify the cracks of coal and rock,and then study the relationship between the crack propagation characteristics and the dynamic characteristics of coal and rock in the process of impact damage,and provide a new technology for further revealing the destruction process of coal and rock.In this paper,a split-type large-diameter Hopkinson pressure bar was used as the impact loading conditions,and disc-shaped sandstones and coal-like specimens in the vertical and parallel bedding directions were produced.The stress wave signals and video images of the coal rock were simultaneously acquired using a hyperdynamic strain gauge and a high frame rate camera.Through the CrackSHPB model based on deep convolutional neural network to identify and quantify cracks in the coal rock mass destruction process.In addition,the dynamic characteristics of the coal samples and the relationship between the dynamic characteristics and crack characteristics in sandstone and different bedding directions under different impact velocities in the process of dynamic split tensile failure are studied.Finally,the SHPB dynamic impact failure experiments of sandstone and coal were modeled,solved and analysed based on the theory of non-local near-field dynamics.The specific research content and results of this article are as follows:（1）Research on image processing method of coal and rock mass under SHPB impact loadingThree aspects of coal-rock destruction video data were studied based on digital image processing technology,super-resolution reconstruction algorithm and deep learning theory:（1）A coal mine image processing flow including“local filtering—mean curvature flow calculation—histogram equalization”is proposed.The results show that this method can effectively remove the noise interference on the surface of coal and rock,improve the contrast between coal and rock body cracks and its background and preserve the original morphological features of the crack.（2）Aiming at the problem of low resolution of coal and rock images captured during SHPB impact damage,three times super-resolution reconstruction of low-resolution coal-rock crack images was performed under the condition of ensuring image quality.（3）In order to realize the automatic identification of cracks in coal and rock under SHPB impact damage,a CrackSHPB model with 440,000 weight parameters was proposed and implemented in the TensorFlow deep learning framework.Using RatSnake open source labeling software,two high-quality datasets of coal-rock cracks under SHPB impact were created,including 310sandstone failure crack images and 500 coal-like failure crack images,respectively.Four indicators that can evaluate the advantages and disadvantages of the coal-rock fracture algorithm from different perspectives are presented:precision,recall,F1-score,and AUC ROC.Through the data set and the proposed evaluation indicators,the cracks in the coal-rock images were identified.The results show that the CrackSHPB model has achieved high accuracy in identifying various indicators of coal-rock cracks.In the crack recognition of BXY sandstone,CrackSHPB achieved accuracy rates of 89.5%,87.4%,88.1%and 84.0%on precision,recall,F1-score and AUC ROC,respectively.In the crack identification of BCM and BPM coal samples,CrackSHPB achieved 85.9%and 75.2%,72.1%and 96.8%,77.1%and 83.7%,74.8%and 93.8%accuracy rates on precision,recall,F1-score,and AUC ROC,respectively.（2）Dynamic characteristics of coal and rock mass under SHPB impact loadingThe stress wave data in the coal and rock SHBP impact failure test were collected by strain gage and super dynamic strain gauge,and the dynamic characteristics of coal rock mass under impact damage were studied.（1）EMD decomposition and HHT transformation were used to filter the experimental stress wave data.Then the“two-wave method”formula was used to calculate the stress-strain relationship,dynamic tensile strength,strain rate,failure strain,and strain of the coal rock mass.（2）The stress-strain curve of rock and coal in the process of impact failure is basically the same,and can be divided into initial compaction stage,elastic deformation stage,crack growth stage,rapid deformation stage and unloading stage.The difference is that the rock specimen usually reaches the stress peak before the coal sample,and the strain value of the rock into the plastic zone is usually only half the size of the coal sample,and the characteristics of the rock initial compaction stage are not obvious.The change trend of the strain-strain energy density curve between rock and coal is basically the same,which can be roughly divided into:slow growth phase,rapid growth phase,and steady maintenance phase.（3）Both the maximum strain rate and the average strain rate of sandstone will affect the dynamic tensile strength of sandstone materials,and the maximum strain rate has more significant influence on the dynamic tensile strength than the average strain rate.The strain rate of coal has no obvious relationship with the dynamic tensile strength.For coal samples with vertical bedding loading,the average strain rate is mainly concentrated within 60-70s^-1.For coal samples loaded with parallel bedding,the maximum strain rate is mainly concentrated within 100-106s^-1.（4）The dynamic tensile strength of the coal sample in the direction of vertical bedding loading is generally higher than that of coal sample in the direction of parallel bedding,indicating that the dynamic tensile strength of coal is stronger in the vertical direction.With the increase of impact velocity,the dynamic tensile strength of the parallel stratified coal samples has to be generally larger than that of the vertical stratification direction.At the same time,the strain strain and strain rate of coal samples loaded in the direction of parallel bedding are larger than that of coal samples loaded in the direction of vertical bedding.（5）The impact velocity has a great influence on the dynamic characteristics of coal and rock mass.When the impact velocity is small,the maximum strain rate and dynamic tensile strength of sandstone remain unchanged.With the increase of impact velocity,the dynamic tensile strength of sandstone decreases rapidly and the maximum strain rate increases.The coal strain rate fluctuates within a small range of variation.（3）Crack propagation characteristics of coal and rock mass under SHPB impact loadingThrough the proposed CrackSHPB model,crack quantification and feature extraction methods,the cracks generated by the 33 groups of coal rock mass in the impact failure experiment were identified,quantified and characterized.（1）When the coal and rock mass appear cracks,the growth rate of the crack area and the propagation rate of the crack along the X-Y axis basically follow the uniform growth characteristics.（2）For the brazed disk fracture experiment of coal and rock,the cracks on the surface begin to crack from the center and gradually evolve to the two ends of the specimen,which is similar to the static failure mode and meets the Griffith strength criterion to some extent.For the coal sample with a certain angle of inclination between the surface bedding and the incident beam,the cracks are propagated in such a way that the specimen begins to crack from the center,and as the center crack expands,the bedding direction slowly generates secondary cracks.The cracks propagated together until the center crack broke through,resulting in the destruction of the coal and rock body.（3）As the existence of bedding,the main cracks produced during the destruction of coal and rock masses are distributed in the center,and secondary cracks are mostly on both sides of the loading axis,showing some dispersion.With regard to the damaged morphology,the coal samples loaded in the direction of parallel bedding are more diverse.（4）The cracks produced on the surface of the coal rock mass during SHPB impact failure have good fractal properties,and the fractal dimension gradually increases with the impact process.The fractal dimensions of sandstone and coal samples range from 0.8 to 1.2 and 0.6 to 1.4,respectively.（4）A comparative study of dynamic characteristics and crack propagation characteristics of coal and rock mass（1）The impact velocity has a great influence on the crack characteristics of coal and rock.With the increase of the impact velocity,the sandstone crack area gradually increases and then decreases slightly.The crack propagation rate along the Y direction increases as the impact velocity increases.The crack area of coal shows a trend of decreasing first and then increasing.The rate of crack propagation in Y direction shows a trend of decreasing first and then increasing.（2）After the coal and rock body was impacted,cracks did not appear immediately,but the stress gradually increased.At the same time,the crack appear’s time of coal after impact is much larger than that of sandstone,which indicates that it takes longer time for coal to enter the plastic zone.This is mainly because the coal includes both the elastic deformation stage and the compaction stage before entering the plastic zone.When cracks appear,the stress of coal rock gradually decreases with the increase of crack area,and the crack area and residual stress show a better quadratic function relationship.With the increase of the crack growth rate,the stress reduction rate of sandstone and coal increases gradually,which indicates that the crack area of coal and rock material increases faster after failure and the stress that it can withstand declines faster.In the relationship with the crack characteristics,the stress peaks of sandstone and coal all show a tendency of rising first and then decreasing with the increase of the crack area growth rate.（3）After the coal and rock body apperas crack by impact,the strain of sandstone and coal increases with the increase of the crack area.The difference is that the strain and crack area of the rock are linear,while the strain and crack area of coal meet the quadratic relationship.The average strain rate of the sandstone specimen shows a significant negative correlation with the growth rate of the crack area during the failure process.With the increase of the crack growth rate,the strain rate of coal fluctuates within a small range.（5）Application of non-local Peridynamics in simulation of impacting failure for coal and rock massThe simulation method proposed in this paper can clearly describe the crack initiation,propagation process and the shape of the crack using 7860 material point positions.The model established in this paper also simulate the secondary cracks.The simulated stress-strain relationship is basically consistent with the experimental result,which verifies the accuracy of the experiment and simulation.In the simulation process,the damage distribution,strain distribution,stress distribution,and displacement distribution of the entire coal and rock mass surface are calculated,and the results are analyzed in conjunction with the experimental results,explaining the reasons for the center cracking of coal and rock mass under impact loading.During the simulation,it was found that the stress-strain relationship calculated from the total strain energy of all undamaged material points（elastic parts）was also consistent with the experimental results even in the plastic zone.This also shows that after the coal and rock body enters the plastic zone,it can be considered that the residual stress is caused by the release of the elastic energy of the undamaged part of the material.