A Study On Nondestructive Evaluation Of CFRP Porosity And The Development Of The System

The research is supported by the Project "Porosity Nondestructive Evaluation of airfoil Panels". The main objective is to find a methodology for carbon fibre reinforced plastics (CFRP) porosity testing and set up a novel portable ultrasonic measuring system.Chapter one is introduction. The ultrasonic testing and the ultrasonic signal processing are briefly introduced. Combined with the Project, the research necessities are discussed. The main contents of the dissertation are also given.Chapter two is the research of CFRP porosity evaluation by ultrasonic method. Firstly the background and the basic knowledge of the composites are introduced briefly. It is demonstrated that: 1) Voids defect is a kind of important defect in composites and there is no composites without voids; 2) The morphology and content of voids have an important influence on the mechanical property of CFRP; 3) It is necessary to have a porosity evaluation for CFRP when being used in high-tech fields. Secondly the traditional methods of porosity test and the theory and empirical equations of ultrasonic porosity test are discussed in this chapter. It shows that: 1) The destroyed ways of porosity measuring methods are superfluous and complicated, and also have a high cost; 2) The porosity of nondestructive evaluation (NDE) of ultrasonic is an effective and practical porosity measuring method, and it can also meet the real-time testing conditions; 3) It is necessary to consider the voids' morphology and its statistic features for further research of porosity evaluation theories. Thirdly a morphological study of voids and a statistical study of the morphological features in carbon fiber reinforced plastics are implemented using microscopy and image analyzing system. The result shows: 1) The voids morphological features are correlated with the CFRP porosity. They tend to be small and spherical when the porosity is low and often occur in the resin abundant regions. With the porosity increasing, the voids become elliptical or elongated along the carbon fibers, and even become much larger and flattened between layers; 2) The fraction of voids is nearly a logarithmic normal distribution with the voids' size measured in the layer plane. Lastly the research of porosity evaluation theory of ultrasonic and the experimental analysis are carried out. 1) Based on the pulse-echo mode, an ultrasonic attenuation model of CFRP porosity testing is established in...