Analysis of fatigue resistance of PEI/SCF/PTFE antifriction composite based on structural and deformation damping factors

The loading of short carbon fibers (SCF) can significantly improve the fatigue properties of polymer matrix composites (PMC). PMCs are used in harsh conditions, where, in addition to mechanical properties, functional properties, for example, anti-friction, are of importance as well. For this purpose, special functional additives are additionally loaded into the composite, which, however, can significantly deteriorate the structure of the PMCs as well as their fatigue resistance. At the same time, assessment of damping is a common approach for studying the fatigue resistance of materials, including polymer matrix composites. However, the literature lacks relevant studies in which the relationship is established between the structure of PMCs with their fatigue resistance and damping properties. The paper studies polyetherimide (PEI)-based composites reinforced with SCF with a length of 200 μm and an aspect ratio of AR=20. Polytetrafluoroethylene (PTFE) with an average particle size of 3 μm was used as an antifriction additive. The mass fraction of components in the composition of the composites under study was as follows: in the two-component composite “PEI/SCF”: 90 % PEI + 10 % SCF; in three-component “PEI/SCF/PTFE”: 80 % PEI + 10 % SCF + 10 % PTFE. The purpose of the study was to establish the patterns of correlations “composition-structure-properties” under fatigue loading with the use of the parameters of mechanical hysteresis loops. Fatigue tests were carried out under load control mode at a maximum cycle stress of 35, 45 and 55 MPa for the three component PEI/SCF/PTFE composite, while it was 55, 65 and 75 MPa for the two-component PEI/SCF composite. The load pulse shape was sinusoidal with a cycle asymmetry coefficient of R=0, i.e. the minimum stress in all tests was taken to be zero. During the fatigue tests, two loading blocks were alternated: i) the main loading block with a frequency of 5 Hz and ii) measurement blocks (cycles) for recording the parameters of hysteresis loops with a loading frequency of 1 Hz. During the measuring cycle, load data was recorded and photographs of the sample surface were captured to determine strain and construct hysteresis loops. Based on the obtained mechanical hysteresis loops, the following parameters were determined: loop area, change of the dynamic modulus, and development of cyclic creep. The damping (loss) coefficient (factor) ψ was defined as the ratio of the hysteresis loop area to the area of elastic strain energy. Based on the results of the study, the fatigue properties of the PEI/SCF/PTFE antifriction composite were examined, while some recommendations on their improvement were given.