PSI - Issue 65

P.B. Severov / Procedia Structural Integrity 65 (2024) 215–224 P.B. Severov / Structural Integrity Procedia 00 (2024) 000–000

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material. This is because it was during this cycle that the second extremum was formed (see Fig. 20). An attempt to find a specific indicator of the beginning of the formation of the second extremum was unsuccessful. The inflection point of the non-linear component of the upper branch was considered as a possible candidate. However, the expectation was not met, as a more detailed analysis showed that each non-linear component of the upper branch in cycles 1 lc - 9 lc (Fig. 14) contains an inflection point near the right vertex of the loop. The intersection points of the dE xnl /dε x ↑ = dE x /dε x ↑ curves with the x-axis determine the locations of the inflection points of the non-linear components of the upper branches in cycles 4 lc - 9 lc (Fig. 8). 5. Discussion The degradation of the mechanical properties of carbon fiber-reinforced plastic (CFRP) under repeated loading is shown as a change in the linear and non-linear components of the stress-strain diagrams during loading and unloading. The change in the shape, size, and slope of the hysteresis loops is clearly due to the accumulation of various types of damages that occurs in CFRP as a result of external forces. The accumulation of damage should be viewed from the perspective of an evolutionary process that develops over time in areas where inelastic deformation and destruction are concentrated. The final stage of the evolution is catastrophic destruction. The topic of the elastic properties of CFRP has been discussed above. Even in loading cycles of 1 lc and 2 lc , CFRP is difficult to classify as elastic, as the upper and lower branches do not fully coincide (Fig. 12 and 18). As a result, a slight dissipation of mechanical energy is detected in the hysteresis loops (Fig. 5). The mismatch of the branches of the hysteresis loop in these cycles is associated with damages to CFRP in these cycles, which occurs at low loads mainly in the sections of increasing displacement. The presence of damages in the section of increasing displacement and their small number in the section of decreasing displacement is, strictly speaking, the root cause of a violation of the elastic properties of CFRP in loading cycles 1 lc and 2 lc . In this case, the sensitivity of the force sensor and the specimen elongation sensor should be high enough to respond to the damages that occur and account for their effect in the approximation equations of the branches of the hysteresis loop. The behavior of CFRP in loading cycles 1 lc and 2 lc can be described as quasi-elastic. Attention is drawn to the similarity of the non-linear components of the lower branches in loading cycles 1 lc to 9 lc (Fig. 14), which is due to the absence of or insignificant quantity of damages in the sections of decreasing displacement. The deformation of CFRP against the background of intense damages in the sections of increasing displacement definitely leads to a violation of non-linear elasticity and the dissipation of mechanical energy. On the contrary, the deformation of CFRP in the absence or insignificant quantity of damages in the sections of decreasing displacement occurs non-linearly elastically. A solid whose stress at each point is an unambiguous function of strain is called elastic. CFRP can be considered elastic when the stress-strain curves coincide in the sections of increasing and decreasing displacement in the absence of temperature influences. A hysteresis loop is not formed and mechanical energy is not dissipated as long as there are no damages or the level of accumulated damages does not affect the readings of the force sensor and the specimen elongation sensor. In each loading cycle, CFRP destruction occurs on the upper branch of the hysteresis loop, and this destruction increases from cycle to cycle. This is confirmed by the number of acoustic-emission pulses observed. On the lower branches of the hysteresis loops, acoustic-emission activity is significantly reduced due to a small quantity of damages. Thus, hysteresis loops are formed and develop with a violation of non-linear elasticity in sections of increasing displacement against the background of intense damages. The preservation of the similarity of the non-linear components of the lower branches of the hysteresis loops allows us to study the development of non-linear quasi-elastic deformation of CFRP during the slow accumulation of damages in multi cycle fatigue. 6. Conclusion The issues of the development of the non-linearity of the stress-strain dependence against the background of the accumulation of mechanical damages during repeated uni-axial quasi-static tension test until fracture of unidirectional CFRP in the direction of highest stiffness were considered.