Quantitative Study To Concrete Meso-damage Based On CT Image And Improved Differential Box Counting Method

Concrete CT images show the meso-damage state about concrete directly,but the recoginzed and reasonable method how to describe the damage state quantitatively has not been found yet.The structure,distribution and evolution behavior of meso-damage crack inside the concrete show the strong fractal characteristics.Fractal method can be used to describe the concrete damage process.Concrete internal damage evolution inside concrete on the damaged area can be described through fractal dimension D quantitatively,which has great significance to establish damage constitutive relationship.The main research content summarized as follows:（1）Concrete CT images of different stress stages under uniaxial compression and uniaxial tension were obtained by CT scanning.The process of crack initiation,cracking,coalescence and failure of concrete can be described by using binarization processing to concrete CT image.Through analyzing the concrete damage process,it is found that the distribution of micro crack in the concrete CT images has strong fractal characteristics.（2）Calculation methods of fractal dimension have been introduced in detail.These method’s applicable conditions have been discussed.It is concluded that the Differential Box Counting method is more appropriate to describe concrete damage process.（3）The fractal dimension of images can be accurately and efficiently estimated by traditional DBC method.But it also has disadvantage,for example,the height and label of boxes will affect the accuracy when the crack is covered by boxes.（4）The improved differential box counting method has been used to analyze the evolution process of the crack in concrete CT image.The fractal dimension trend of traditional DBC method and improved DBC method is the same.However,the improved DBC method avoids the traditional DBC method defects,for instance,boxes can not cover crack accurately.So it is concluded that the fractal dimension obtained by the improved DBC method can appropriately describe concrete damage evolution process.（5）The damage variable expression D_pi based on the fractal dimension results obtained from the improved differential box counting method has been established.The relation curvebetween D_pi and D of each stress stage in concrete damage process has been got,then its application has been verified.The result shows that the damage variable established based on the improved fractal dimension has the same change law as the fractal dimension.It can be used to quantitatively describe the process of damage development of each stress stage during concrete specimens evolution process and the concrete macro-mechanical properties can be reflected.The innovation is that the fractal dimension of each stress stage in the process of concrete damage by using the improved differential box counting method was calculated firstly.Then,damage variable expression based on fractal dimension has been established.