PSI - Issue 59

Serhii Drobyshynets et al. / Procedia Structural Integrity 59 (2024) 601–608 Serhii Drobyshynets et al./ Structural Integrity Procedia 00 (2019) 000 – 000

606 6

  , , cr b cyc to fb R is

fb R ,  =

 fb R , 

and

. The value of the total strain in the stress interval from

  fb = 0.00004921. The share of elastic deformations in the specified interval is el R ,  = 0.00003396. Such deformations correspond to a limiting elasticity coefficient of R  = 0.67. Then q = 0.29. Finally, for the steel fibre reinforced concrete under study, the formula for determining low-cycle fatigue takes the form

0.29

1

n



cyc

1 0.25

.

(7)

 



, fb cyc

0.29

n

cyc

The main number of prisms was tested by repeated loading with measurements at the degrees of longitudinal deformation. For the P-8 prism, the relative level of repeated loading was assumed to be  = 0.95. This prism collapsed on the second cycle. Prism P-9 at  = 0.91 withstood two loading cycles and collapsed on the third cycle (Fig. 3).

 fb , М P а

20 25 30

15

5 10

5

 fb *10

0

0

50

100

150

200

Fig. 3. Deformation diagram of prism P-9.

On the first cycle, at the maximum stress value of  fb = 28.4 MPa the deformations of prism P-9 reached  fb = 0.00001537, almost the limit value of the average of three prisms that were loaded once to failure, determined theoretically. It should be noted that at the first stage of unloading, there was not a decrease in deformations, but a slight increase. Obviously, at this level of loading, there is a progressive increase in plastic deformation. After complete unloading, the residual deformations were 0.0000291. In the second cycle, the total deformations reached  fb = 0.00001746 and a slight increase in deformations was also observed at the first unloading stage. It should be noted that in the second cycle, the loading and unloading curves are already convex to the deformation axis, indicating the development of pseudoplastic deformations. In subsequent tests, the level of cyclic stresses was reduced, and the number of cycles withstood was determined. Fig. 4 shows the deformation diagram of the P-18 prism, for which the level of cyclic stresses was  = 0.77 and which collapsed on the 51st cycle. In the P-18 prism, on the first loading cycle at  fb = 23.8 MPa the deformations were  fb = 0.00001095, and at full unloading, the residual deformations were 0.0000128, which is significantly less than in the P-8 prism. On the third cycle, when loaded, the deformations were  fb = 0.00001107 (without considering residual deformations). The same