Fracture criterion for tested CFRP specimens under tension

Abstract

The development of the theory-experimental methodology to evaluate the fatigue life is one of the essential stages to substantiate the strength and to verify the certification of aircraft from the POV of fatigue and service lives. To develop such a methodology, it is necessary to obtain the S-N curves for different stress ratios. However, the fatigue test by zero-to-tension stress cycle of an open-hole specimen manufactured of carbon fiber reinforced polymer (CFRP) until final fracture (broken into 2 parts) shows a big scatter of fatigue lives and sometimes it is impossible due to fibers long lives despite of fractured matrix. For example, in our research, the root-mean-square deviation of the logarithm of fatigue life SlgN was in the range 0.7 ÷ 0.9. In order to obtain reliable resource characteristics of the structure, at this kind of dispersion values, a lot of time and material costs are required. In order to solve this problem, the task was formulated to create a new fracture criterion, which will allow reducing dispersion of fatigue life. Our analysis showed that the fracture process starts from interlayer delamination at hole edge and propagates towards the outer side of specimen under test. The delamination initiation at the hole edge is proposed to be taken as failure initiation moment; then the data obtained are to be used to estimate the S-N curve. To fix the appearance of delamination, a special sensor was developed. A method was developed to detect the delamination initiation while monitoring the transverse displacement near the free edge of hole. This method provided a drastic decrease of fatigue life scatter up to SlgN = 0.2÷0.3 that is acceptable to estimate the fatigue life; at this, two modes of fracture process in CFRP are taken into consideration: initiation and propagation of delamination, which are characterized by different fracture mechanisms.