# Study on the Properties and Fatigue Characteristics of Glass Fiber Composites Due to Porosity
## 3 Materials and Methods  
### Pores and Laminate  
In general, it is very difficult to define the size, shape, location, and distribution of pores occurring in RTM. Generally, pores generated in composite materials vary too much in shape and size to be defined as a single number, and it is known that the size and position of pores are affected by the speed of the resin tip, the flow rate of the fiber's capillary flow, and the volatile components or wettability of the polymer during the RTM process. In general, micro voids have a spherical shape, and as the size increases, they tend to have a spherical void shape.  
Pores are known to affect interlayer shear strength (ILSS), longitudinal compression direction, and transverse tensile strength, which are characteristics dominated by the matrix. In the case of glass fiber/epoxy specimens, unlike carbon/epoxy, the increase in stress due to a decrease in net area appears to play a decisive role rather than damage caused by large discontinuities due to voids [9; 17; 18], and in the case of woven composites, the effect of tensile stress due to pores is negligible in the fiber direction, but the tensile modulus and in-plane shear modulus in the inclined direction are shown to be reduced. And in the case of compression, it was found that the compression coefficient decreased or affected destruction as the pore content increased [7; 19].  
In this study, intra and inter-voids in glass fiber/epoxy composites were implemented and analyzed with RVE (Representative Volume Element), assuming a 40-200 \(\upmu\)m diameter sphere for the ease of analysis and due to the limitations of the analysis program [20; 21; 22; 23; 24; 25; 26].  
The shape determination and generation of the RVE were performed using the commercial software Digimat 2023. Within the Digimat program, the number and porosity of the voids can be defined, which in turn determines the diameter of the voids. The distribution of the voids was randomly dispersed within the program. The size of the RVE at this point is 75 \(\upmu\)m \(\times\) 75 \(\upmu\)m \(\times\) 75 \(\upmu\)m. The mesh modeling of the RVE was also conducted using the Digimat software. The boundary conditions were set to periodic to simulate continuous boundary conditions, and the load and solve conditions were applied as specified by Digimat.  
The specimen test for mechanical properties was performed using a static material testing system at the KSNU Advanced Technology Institute for Convergence.  
Figure 1: 1 MW class wind turbine blade using glass fiber.  
In this study, UD glass fiber and epoxy were selected as composite materials, and Table 1 shows their lamina properties. The table utilized the test results of specimens used in actual wind turbine blades, and similar values were applied as physical properties used for damage analysis in Digimat. Three composite lamination patterns of [UD 0\({}^{\circ}\), 90\({}^{\circ}\), 45\({}^{\circ}\)] were applied to the analysis of mechanical properties caused by pores. The number of stacked layers at each UD angle is shown in the table below, the fiber volume ratio is assumed to be 0.6, the matrix is assumed to be an isotropic material, and physical properties were obtained with a multiscale model by applying the Digimat program. The method of obtaining the property value simulates arbitrary defects in micro and meso modeling as the RVE, and the commercial program Digimat was used to determine and generate the shape of RVE.  
The ratio of pores was analyzed from 0.5% to 2.5%, and the distribution of pores was distributed randomly within the program.  
Based on the properties listed in Tables 1 and 2, RVE (Representative Volume Element) models of matrices and composite materials including porosities were created. Through the analysis of the RVE models, the mechanical properties of the materials containing porosities were obtained.