Carbon fiber reinforced plastic (CFRP) laminates are being used in stiffness-critical structural components. In addition to the application, because of the demand for reduced structural mass, CFRP laminates are tried to be used for some strength critical applications. To promote the further application of this material, it is important to evaluate the characteristics of delamination CFRP laminates since one of the strength problems of the laminates is less impact strength for the delamination. For the evaluation of the delamination, we are given two kinds of information of the delamination, such as its size and form. The former has been used by many researchers, and it has been revealed that the area of delamination is proportional to the impact energy subjected to the specimen. Geometrical form of delamination, however, has been hardly discussed in quantitatively. On the other hand, fractal geometry is considered to be a self-similar structure with infinite repeating procedures. It is well known that a number of forms in nature has fractal characteristics. The forms of the delamination could be evaluated by means of fractal dimensions as a parameter of the evaluation of impact delamination. In this study, fractal dimensions of delamination in CFRP laminates were investigated. To examine the influence of damage area on fractal dimensions, CFRP specimens were subjected to impact load with variation in test speed. After the impact test, the ultrasonic inspection test were conducted and obtained C-scan image data of delamination. Fractal dimensions of delamination were estimated by using the box counting method. It was demonstrated that the relationship between the size and number of boxes that cover the contour of delamination profile is linear in logarithmical scales. This implies that the contours of delamination has fractal characteristics. It was also demonstrated that fractal dimensions of delamination decreased with increasing area of delamination. This relationship shows linearity in the case that few cracks were observed in the specimen. The relationship becomes out of linear as a lot of cracks were generated and the rate of the decrease becomes large. As a consequence, it was concluded that fractal dimension is effective to quantitative evaluation for the status of delamination induced by impact load.
Publication Source: Trends in NDE Science & Technology; Proceedings of the 14th World Conference on Non-Destructive Testing, New Delhi, 8-13 December 1996.Vol. 3, pages 1795 - 1798 Publisher:Ashgate Publishing Company
NDTnet
