PSI - Issue 13

A.A. Lukyanchuk et al. / Procedia Structural Integrity 13 (2018) 1285–1290 A.A. Lukyanchuk and others / Structural Integrity Procedia 00 (2018) 000 – 000

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hole CFRP specimens with zero-to-tension stress cycle the delamination effect was obtained. Delamination was observed visually on the edges of the specimen at 90°-th layers. Thus, the application of the "classical" fracture criterion into two parts led to a large scatter of fatigue life. The root-mean-square deviation of the logarithm of fatigue life S lgN , characterizing the scatter, ranged from 0.7 to 0.9. Such data are unacceptable for obtaining reliable methods of estimating fatigue life of aviation constructions. Figure 1 shows the delamination of an open-hole specimen under zero-to-tension cycle. Since the S-N curves must be obtained for different stress rations, it was necessary to develop another failure criterion to carry out the fatigue tests of open-hole specimens in the case of a zero-to-tension loading, which makes it possible to reduce the scattering of fatigue. In particular, it was necessary to solve a number of tasks for registration: the moment of the appearance of the delamination; the place where the delamination begins; the propagation velocity of delamination; methods for fixing the onset and growth of the delamination; what moment is considered the destruction of the specimen. To solve them, a series of works was performed.

Figure 1 - Delamination on the edges of an open-hole specimen under tensile fatigue test

2. First stage: investigation of the moment of the beginning of delamination using a wire sensor

At the first stage of the research the wire sensor was chosen as one of the simplest solutions. Triggering (rupture) of the wire sensor (shown in Fig. 2(A)) occurred after the appearance of a delamination on the side surface of the specimen and its further increase to an actual opening of about 0.1 mm, followed by a stop of the testing machine.

(A) (B) Figure 2 – A)Broken wire sensor, specimen under tensile load; B) Delamination zones after fatigue runtime by zero-to-tension cycle Investigation of delaminated, but not destroyed by the "classical" criterion, specimens by ultrasonic testing showed: the delamination appears not uniformly in the area of the specimen, but in sections of different length and shape. Figure 2(B) shows the delamination zones. Obviously, usage of a wire sensor does not give a vision of the moment and place of the beginning of the delamination, and therefore more precise and detailed control methods are required. A picture similar to Figure 2(B) can be found in some sources [1-3]. For example, in Figure 3, the X-ray diffraction patterns of the specimen at the beginning of the test (A), during the test (B) and after the application of 10 6 load cycles (C) with a cycle maximum of 80% of the breaking load in static tests.