Influence of Milling Conditions and Negative Temperature on the Strength of Carbon Fiber Reinforced Polymer during Cyclic Bending Loading

The results of tests performed to estimate the change in the fatigue strength of carbon fiber reinforced polymer (CFRP) as functions of the milling conditions during its machining and of the influence of a negative temperature are presented. The tests are carried out on CFRP samples of three series, which differ in the cutting conditions (rate of milling cutter rotation at the same feed rate). Each series is prepared in three sets to estimate the strength change caused by holding at a positive temperature and at –50°C for 60 and 120 h. The tests are performed by cyclic bending three-point loading at a given bending flexure when stress changes are recorded every 5 × 10³ cycle. The total number of cycles is 10⁵ and the cyclic loading frequency is 5 Hz. The test results demonstrate that the CFRPs fabricated by milling at the lowest rate of milling cutter rotation have the best fatigue strength characteristics. A decrease in the rate of milling cutter rotation leads to an increase in the maximum bending flexure and to the best cutting surface quality. After holding at a negative temperature, the fracture of all series of samples occurs at a lower force and the maximum bending flexure increases. The influence of a negative temperature brings about an increase in the rate of decrease of strength with increasing loading cycles.