# Study on the Properties and Fatigue Characteristics of Glass Fiber Composites Due to Porosity  
[PERSON]  
1 School of Mechanical Engineering, Kunsan National University, 558 Daehak-ro, Miryong-dong, Gunsan 54150, Joellabuk-do, Republic of Korea, [EMAIL_ADDRESS]  
[PERSON]  
2 Korea Construction Equipment Technology Institute, 52 Saemankum 2-ro, Osikdo-dong, Gunsan 54002, Joellabuk-do, Republic of Korea; [EMAIL_ADDRESS]  
[PERSON]  
1 School of Mechanical Engineering, Kunsan National University, 558 Daehak-ro, Miryong-dong, Gunsan 54150, Joellabuk-do, Republic of Korea, [EMAIL_ADDRESS]  
Footnote 1: [[https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci)]([https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci))  
Footnote 2: [[https://www.mdpi.com/journals](https://www.mdpi.com/journals)]([https://www.mdpi.com/journals](https://www.mdpi.com/journals))  
areas. The effective load-bearing area is decreased, leading to lower tensile, compressive, and shear strengths. The increased stress in the material surrounding the voids makes these regions more susceptible to yielding and failure. At the microscale, voids act as sites of stress concentration. When an external load is applied to the composite, the stress around the voids is significantly higher than in the surrounding material. This amplification occurs because the void disrupts the uniform distribution of stress, causing it to concentrate around the edges of the void. These high-stress regions are more likely to initiate micro-cracks. The sharp corners and edges of the voids can serve as points where cracks can start to form. Once initiated, micro-cracks can propagate through the matrix and along the fiber-matrix interface under further loading, leading to premature failure. The propagation of these micro-cracks reduces the overall strength and stiffness of the composite material. The strength and stiffness of fiber-reinforced composites rely on effective load transfer between the fibers and the matrix. Voids disrupt this interface, leading to poor bonding and a reduced load transfer efficiency. The presence of voids can cause debonding at the fiber-matrix interface, further reducing the effectiveness of load transfer. The composite becomes less stiff because the load is not efficiently transferred between the fibers and the matrix and ability to resist inter-laminar shear forces is compromised, making it more prone to delamination. Extensive research was conducted on the ILSS (interlaminar shear strength) of glass/epoxy composites affected by dispersed porosity, revealing that voids present between eachply impact the sensitivity of ILSS. It was also discovered that, aside from the content of voids, the size and location of the voids significantly influence ILSS [8]. Unlike carbon/epoxy specimens, glass/epoxy specimens with discontinuous large voids did not break easily, even if the voids were large compared to the surface area. Nevertheless, most of the damage was initiated above and below the voids. The stress concentration due to the reduction in net section area was predicted to be critical [9]. Furthermore, a significant reduction in ILSS was observed with more than 1% void content, with the voids being predicted to play a crucial role in crack propagation through the laminates. Inelastic deformation was also noted to be more pronounced in composite materials with a high void content [10; 11]. Delamination is challenging to detect and can lead to a substantial reduction in the stiffness and strength of the structure, particularly under compression. Research in this domain is vital for the damage tolerance and durability of composite laminated structures, focusing on understanding the initiation, propagation, and failure mechanisms of delamination under fatigue loads [12; 13; 14; 15]. Further complicating the behavior of composite materials under fatigue is their inherent anisotropy, leading to complex damage mechanisms and overall brittle behavior.