PSI - Issue 52

Haseung Lee et al. / Procedia Structural Integrity 52 (2024) 252–258 Haseung Lee, Hyunbum Park / Structural Integrity Procedia 00 (2019) 000 – 000

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voids and meso-voids are done by the micro flow at the TOW level related to heterogeneous media of the preform. Macro-voids are formed by macroscopic flow related to homogeneous media of the preform by Chen D et al.(2015) and Tahir MW et al.(2014). It was shown that even a small amount of the voids of FRC have an effect on mechanical behavior, particularly the characteristic governed by matrix such as inter-layer shear strength (ILSS), longitudinal compression strength and transverse tensile strength. In particular, it was shown to be vulnerable to fatigue behavior by Uhl et al.(2015). Olivier et al.(1995) confirmed that the voids existing between each ply affect the high sensitivity of ILSS. In addition, they confirmed that the size and location of void in addition to the voids content had a significant effect on the ILSS. Koushyar et al.(2012) confirmed a significant decrease in ILSS by voids for a content of more than 1%, and Voids were predicted to play an important role in crack propagation through the laminate. And inelastic deformation was found to be more prominent in composite materials with higher void content. Wisnom et al.(1996) conducted extensive studies on the ILSS of glass/epoxy composites under the influence of dispersed voids. Unlike Carbon/Epoxy specimens, the Void of glass/Epoxy specimens hardly started to break even when there was a discontinuous Void with a large Void compared to the cross section. Even so, most of the damage started above and below the void. It is predicted that the stress concentration due to the decrease in net cross-sectional area was deterministic. In the case of continuous FRC (glass/epoxy), Srinivasulu et al.(2014) conducted an analytical approach through FE simulation to the effect of voids of glass/epoxy composites, and confirmed a decrease in effective tensile modulus and Poisson ’ s ratio as porosity increased. If there are defects such as these voids when manufacturing composite products, they are repaired, or disposed of if impossible to repair. This leads to have a feature that shows higher defect rate compared to other industries. In addition, higher process cost is generated. The failure condition formula is used to predict the laminate strength, and the failure condition is expressed using the stress or strain components of the lamina expressed in the material coordinate system due to the orthotropic properties of the composite. Traditionally, maximum strain condition, maximum stress condition, Tsai-Wu failure condition, Hashin failure condition, Puck failure condition, etc. have been used, and recently LaRC failure condition and Daniel failure conditions have been proposed and used. This study assumed that there were the void defects in the glass fiber composites mostly used for the wind turbine blades. The material property change by defects was predicted through DIGIMAT, which is a commercial program, and it was to predict the behavior in full-scale structural analysis on the composite structures. 2. Analysis 2.1. Voids and Laminates In general, it is very difficult to define the size, shape, position and distribution of the voids generated in RTM(Resin Transfer Molding). The voids have so a wide variety of shapes and sizes to determine with one single number. During the RTM process, it is possible to predict the size and position of typical voids depending on the velocity of the resin tip. However, the velocity of the resin tip changes due to the position of the resin injection port, venting port and runner etc., and is influenced by the volatile constituents or wettability etc. of polymer. Micro-voids usually show a spherical form, and the greater the size increase, they tend to show spherical void shape. It has been known that voids have an effect on the ILSS, longitudinal compression strength and transverse tensile strength etc. which are the characteristics governed by the matrix. In case of the glass fiber/epoxy specimen, it is shown that the stress increase caused by the cross-section decrease plays a decisive role rather than the damage due to a great discontinuity resulted from voids unlike the carbon/epoxy by Wisnom et al.(1996). In case of the woven composite, it is shown that the influence of tension caused by voids is small enough to ignore in the fiber direction but the tensile modulus in the slope direction and the in-plane shear modulus decrease. For compression, it is shown that, as the void content increases, the compressive modulus decreases or it has an effect on failure by Mstsuzaki R. et al.(2014).