Fractal Box Dimension Characterization And Analysis Of Ultrasonic Response Characteristics Of Composite Pore Morphology

Composite materials are widely used in aerospace,automobile and medical equipment due to their high specific strength,high specific modulus and high fatigue resistance.Pore is a common defect in composite materials.Pore content and pore morphology have a significant impact on the mechanical properties of composite materials.Therefore,pore detection is of great significance.In the ultrasonic testing of composite pores,due to the complexity of pore morphology,the pore content is not uniquely corresponding to the ultrasonic characteristic parameters.Therefore,it is necessary to study the influence of pore morphology on the ultrasonic characteristics,and the pore morphology needs to be characterized.Therefore,this paper uses the fractal box dimension to quantitatively characterize the pore morphology,and then uses the fractal box dimension to analyze the influence of pore morphology on the ultrasonic characteristic parameters.The specific contents of this study are as follows:Firstly,random pore distribution model is established by introducing random medium theory.The effectiveness of the random pore model is verified by comparing the pore morphology characteristics of the random pore model and the real composite material.On this basis,the fractal box dimension was used to characterize the pore morphology of the established random pore model,and it is found that the fractal box dimension can better characterize the pore morphology of the composite material,and it is more conducive to studying the influence of pore morphology on ultrasonic testing.Secondly,the ultrasonic response characteristics of composites with different porosity and pore morphology were simulated.The results show that when the porosity exceeds 2.4%,the influence of pore morphology on ultrasonic characteristic parameters increases,indicating that the greater the porosity,the greater the influence of pore morphology on ultrasonic characteristic parameters.When the porosity remains unchanged,the increase of pore length and width in linear ultrasonic testing can cause the increase of ultrasonic attenuation coefficient a.The increase of length size in nonlinear ultrasonic testing will cause the increase of relative nonlinear coefficient β’,and the increase of pore width size will weaken the relative nonlinear coefficient β’.Then,the fractal box dimension is used to analyze the change of ultrasonic characteristic parameters caused by different pore morphology.The results show that the coincidence rate of pore length and width of two different porosity is more than 74%,which has poor separability.When fractal box dimension is used to characterize,the coincidence rate of pore morphology with two different porosity is less than 5%.At the same time,the experimental results also verify the simulation results,that is,the fractal box dimension can better characterize the pore morphology.Finally,in order to reduce the accuracy of porosity detection caused by the complexity of pore morphology,the fractal box dimension and ultrasonic attenuation coefficient α are combined to characterize the porosity of composite materials.The results show that when the fractal box dimension is combined to characterize the porosity of composite materials,the error range is reduced by 35%～64%,and the detection accuracy of porosity is greatly improved.The research results have certain guiding significance for the detection of porosity of composite materials.