Low and high cycle fatigue behavior and corresponding failure mechanisms of CFRP and FML composites fabricated under optimized curing conditions

Carbon fiber reinforced polymers (CFRPs) have received growing attention in recent years with their advanced mechanical properties. Fiber metal laminates (FMLs) are hybrid materials that combine the advantages of CFRP and metallic materials. Similar to the vast majority of engineering materials, the most important mechanism limiting the life of these two advanced materials is fatigue. However, due to the nature of fatigue, which requires intensive labor and time, studies on it are quite limited. The literature predominantly reports low-cycle fatigue results for such materials, often without sufficient correlation to damage mechanisms. Moreover, most studies rely on conventional production techniques. In contrast, this thesis employed hot press molding, enabling precise control of curing parameters. Accordingly, the study makes novel contributions by investigating the independent and synergistic effects of these parameters on both low-cycle and high-cycle fatigue behavior, along with the use of stainless steel wire mesh to produce lightweight FMLs. In static tests, the effects of curing temperature (30 °C, 60 °C, 90 °C), curing time (3 and 5 hours) and reinforcement orientation ([0°/90°] and [±45°]) on strength were examined. Results showed strength improvement with increased curing time and temperature up to a critical point; surpassing the glass transition temperature led to strength deterioration. The strength decreased to about one-fourth in the reinforcement at [±45°] orientation configuration. This finding showed that off-axis reinforcements significantly reduced the load carrying capacity. Fatigue tests were performed at 85%, 80%, 75% and 70% levels of material strength. While FMLs showed linear degradation remaining in the low-cycle region, CFRPs reached high-cycle behavior, completing 10 million cycles at 70% stress without failure. Microscopic analyses showed that fiber fractures and matrix cracks were prominent in low cycle failures, but progressive damage mechanisms such as delamination and fiber pullout dominated the high cycle fatigue failures. The curve observed in the graph of CFRP-60-5-70% samples exhibited a sudden increase in deformation at the beginning, then continued to increase with a decreasing slope. Over time, this increase approached a horizontal trend, and the curve showed low amplitude but regularly repeating trough-like fluctuations. This behavior provided important clue about the mechanisms behind the long fatigue life of CFRP. Keywords: CFRP, FML, fatigue, curing conditions, failure mechanisms