PSI - Issue 65

P.B. Severov / Procedia Structural Integrity 65 (2024) 215–224

222

P.B. Severov / Structural Integrity Procedia 00 (2024) 000–000

8

distances between the rates of change in the slopes of the non-linear components of the branches are determined by equations, the graphs of which are shown in Fig. 9. If you look closely at the curves in Fig. 14, you will notice the following. For the first two loading cycles 1 lc and 2 lc , an almost complete coincidence of the non-linear components of the upper and lower branches can be observed in Fig. 18. This, combined with the almost complete coincidence of their linear components in each of these cycles, as shown in Fig. 12 and 13, indicates the manifestation of the elastic non-linear properties of CFRP in these cycles. In the 3 lc cycle, the difference between the curves is more noticeable, as in the previous cycles and in the 3 lc loading cycle, there was a certain acoustic emission activity observed. As the number of loading cycles increases, the difference between the non-linear components of the upper and lower branches becomes more significant (Fig. 19). In this case, the non-linear components of the lower branches maintain their non-linear properties, while the non linear components of the upper branches tend to straighten out. The inelastic non-linear properties of CFRP are manifested, as confirmed by the significant dissipation of mechanical energy in the hysteresis loops, as shown in Fig. 5. In loading cycles 7 lc - 9 lc (Fig. 20), the non-linear properties of the lower branches continue to be preserved, while the straightening of the upper branches develops in a slightly different form - a second extremum is formed in the area of positive stress values. The appearance of the second extremum indicates the transition of the material to a qualitatively new state with a high level of accumulated damages. The non-linear elasticity of the material is disrupted in the sections of increasing displacement (upper branches of the hysteresis loops), since it is in these sections that the highest number of acoustic emission pulses are recorded.

Fig. 18. Cycles 1 lc - 3 lc . Non-linear components of the upper σ xnl ↑ and lower σ xnl ↓ branches

Fig. 19. Cycles 4 lc - 6 lc . Non-linear components of the upper σ xnl ↑ and lower σ xnl ↓ branches

Fig. 20. Cycles 7 lc - 9 lc . Non-linear components of the upper σ xnl ↑ and lower σ xnl ↓ branches

The formation of a second extremum in the area of positive stress values in the non-linear component of the upper branch indicates the transition of the CFRP to a hazardous state with a significant level of accumulated damages. The 7 lc loading cycle can be seen as the boundary between the acceptable and dangerous states of the