Ultra-low cycle three-point bending fatigue of glass fabric reinforced plastic

In the present work, the static strength and ultra-low cycle fatigue (less than 100 cycles) of plain weave fiberglass reinforced plastic STEF (TM) under three-point bending along the warp and the diagonal are studied. For this glass fiber reinforced plastic (GFRP), force-displacement loading diagrams are studied in combination with acoustic emission data. Digital image correlation (DIC) showed that the maximum bending strain is close to the failure strain of the tensile specimens. Bending fatigue tests showed progressive displacement (cyclic creep) and a nonmonotonic change of specimen stiffness. This is associated with two processes: straightening in warp di-rection and micro-damage - local delaminations and failures of the matrix and fibers. Acoustic events (piezo-sensor signals) were divided into low-and high-frequency groups. The loading in the diagonal direction is accompanied only by low-intensity, low-frequency (up to 300 kHz) events. When bending was applied in the warp direction, the acoustic image had intense low-and high-frequency events (up to 500 kHz). The presence of a high-frequency component in the frequency spectrum can be a diagnostic sign of a weakening of the material and approaching fracture, which is valuable for structural health monitoring. Under cycling with constant force amplitude, low-frequency events were repeated under loading and unloading stages, which related to friction at the fiber-matrix level. Thus, the Kaiser effect is not present for this GFRP. During unloading, acoustic events are also observed. This indicates the creep of the material under cyclic loading. The bending fatigue limit with the base of 100 cycles life was about 70% of the ultimate strength.