PSI - Issue 28

A.A. Lukyanchuk et al. / Procedia Structural Integrity 28 (2020) 2291–2296 A.A.Lukyanchuk / Structural Integrity Procedia 00 (2019) 000–000

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Although, one cannot exclude the effect on the fatigue of the proximity of the impact area to the free border of the specimen (impact damage area tends to half the specimen width), characteristic of some experimental specimens and rarely found in real design.

Table 1 - Summary table of the calculated and experimental parameters of the programs of non-regular loading

Design value of durability in flight blocks

One block loading program damageability

The average experiment. durability value in flight blocks

Loading program

А

S lgN

Specimens Type 1 First program Second program Specimens Type 2 First program Second program

14.28 11.11

0.07 0.09

8.89 3.96

0.62 0.36

0.21 0.19

5.55 4.16

0.18 0.24

5.87 3.65

1.01 0.88

0.15 0.19

In particular, to answer the question about the reason for the relatively low values of the sums of fatigue damage for programs with low loads, tests of the second type of specimens were conducted. According to the results of the investigation of second type specimens, the value of the sum of fatigue damage for the program with low amplitude loads was 0.88, which does not contradict the recommendations in literature sources that it is possible to take the value A = 0.5 in most cases [5,6,7]. Hence, based on the obtained experimental data for the second type specimens, it can be concluded that the use of the hypothesis of linear addition of damage, makes it possible to estimate the durability with accuracy acceptable for engineering calculations for a hub in the form of an impact, provided that the recommendations of the standards regarding the size of the stress concentrators on the specimens are observed. The value obtained for parameter A indicates a weak influence of nonlinear effects on the law of damage addition. As can be seen from the above comparison, the results for two loading programs of second type specimens have shown insignificant damaging effect of low amplitude loads on the material of the specimens with impact damage. In other words, there are no nonlinearities. 4. Conclusion. In the end of our discussion we can say that impact damage for modern CFRP’s should be investigated more thoroughly. At the same time, to estimate the fatigue life of specimens with impact damage complying with the above standards, a linear summation hypothesis can be applied. Cycles with low amplitudes do not introduce any non-linear effects. In the case of an evaluation of the fatigue life of specimens with impact damage, the dimensions of which exceed those recommended in the standards, a deviation from the linear summation hypothesis is possible, and in this case, correction factors need to be introduced, which requires additional experimental research under irregular loading. Acknowledgements The research was financed by Ministry of Education and Science of the Russian Federation, under agreement No.RFMEFI62518X0044 (075-11-2018-175). The authors wish to greatly acknowledge the project partner of agreement, Irkut Corporation. References [1]Konovalov V.V. and Pankov A.V. Verification of CFRP Components Fatigue Evaluation Procedure under Irregular Cycling Loading // 30th Congress of the International Council of the Aeronautical Science (ICAS-2016), DDC, Daejeon, Korea, September 25-30, 2016г. [2]Aviation regulations. Part 25. Transport aircraft airworthiness regulations. AR MAC, JSC «Aviaizdat», 2009 (Russian).