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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specimen was carried out and the changing of internal structure of the composite was fixed, after which the sensor was mounted back onto the specimen and the tests were continued. Data obtained by NDT method was used to develop a quantitative criterion value: tension  . As a result of the implementation of the set of measures, a criterion for the destruction of the specimen was developed for fatigue tests under tension on the basement of a delamination sensor: 0.05 tension mm   (1) The developed criterion provides a fatigue data with a dispersion S lgN not exceeding 0.3, which is acceptable for modern CFRPs and does not lead to an unreasonable increase in reliability factors for fatigue life.

Figure 4 - A specimen mounted on the test machine with a delamination sensor in the zone of the hole

To verify the fracture criterion, a delamination sensor was used in fatigue tests, which shown in Figure 4. An Instron extensometer with a 10 mm base was used with it. Figure 5 shows the typical dependence of the readings of the delamination sensor on the runtime (cycles). In the figure, the values min and max denote the values of the minimum and maximum values of the extensometer readings of each cycle.

Figure 5 - Graph of the dependence of the extensometer readings from the fatigue runtime

According to the experimental results, it was found that the failure criterion (1) (i.e. the displacement of the extensometer knives by 0.05 mm) corresponds to the largest number of specimens: 21 of 31. The remaining 10 specimens had different failures and errors during the development and debugging of the method. The results of ultrasonic testing, conducted after achievement of the developed criterion, showed the uniformity of the damages obtained: delamination in the open-hole zone of the same area in all the specimens studied (Figure 6). It should be noted that after reaching a thickening of 0.05 mm according to the readings of the delamination sensor, all the investigated specimens were subjected to further tests with a gradual increase of the thickening threshold to 0.075 and 0.1 mm. However, only nine specimens reached a thickening of 0.075 mm, while the area of emerging delaminations in the specimen became comparable with the area of the working space of the specimen. In remaining specimens, thickening stopped at a value of about 0.05 mm, while the delamination area continued to grow.