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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(A) (C) Figure 3 - Dynamics of damage development in an open-hole specimen according to X-ray data (B)

In the works abovementioned, visual and X-ray inspection was used. X-ray monitoring makes it possible to determine the growth dynamics of interlayer delaminations and longitudinal cracks, and the visual method – the growth of the number and dimensions of transverse and longitudinal cracks on the specimen surface. It is obvious that even with successful establishment of a relationship between the rate of development of the above defects and the loading parameters, the use of a technique based on this dependence will have a very limited application, since the abovementioned nondestructive testing methods (NDT) is not usable for continuous monitoring of the state of the specimen. In addition, such kinds of monitoring the state of the specimen will increase the economic and time costs for conducting the experiment. This will call into question the possibility of carrying out numerous, multicyclic tests on fatigue life. In accordance with the data obtained by us, the starting point of the delamination process is the zone around the stress concentrator, in this case the hole. Due to the fact that in modern CFRP, the growth rates of the delaminations are high and their detection in the process of runtime by the methods of nondestructive testing is very difficult, according to the requirements of normative documentation [4], delaminations should not appear in aircraft structures and, if they arise, further use of the structure is prohibited. Thus, for the fatigue tests under tension, the criterion for the destruction of the specimen of the CFRP can be considered the moment of the beginning of the delamination in the zone of the concentrator. In the future it will be shown that this moment was recorded by monitoring the "opening" of the delamination. It should be noted that the inspiration for creating such a criterion was the methods based on measuring the degradation of the stiffness of the construction element. They are based on the experimentally discovered fact that a change in the stiffness of a construction element of the CFRP indicates the beginning of its destruction. Such methods are used, for example, to estimate the fatigue life of wind turbine blades [5]. The degradation of the stiffness of the construction element in this case is determined from the change in the slope in the "Load-displacement" relationship under regular loading. Thus, for carrying out fatigue tests and fix the whole picture of the stress-strain state change, a method of detecting the initiation of the delamination in the zone of hole is necessary, which must have the following key features:  Continuous measurement;  Indication of measured parameters in real time;  Providing precise information about the onset of stratification and further growth rate. 3. The second stage: debugging of the growth sensor of the delamination of the specimen in the zone of the hole, the development of the criterion In the second stage of the investigation, a method was used to detect the onset of delamination in the hole zone using a delamination sensor specially developed in TsAGI. The work of the sensor is based on continuous fixing the change of the specimen thickness around the hole during fatigue loading. For the sensor, different thresholds were set: from 0.05mm to 0.5mm of thickening of the specimen in the zone of the hole. When the established threshold on thickening in the program of the testing machine associated with the delamination sensor is achieved, the test is suspended. The ultrasonic inspection of the