Ji-Woong Kang

The Experimental Evaluation of the Mixed Mode Delamination in Woven CFRP/GFRP Laminates under MMB Test

Blades of horizontal axis are nowadays made of composite materials. Generally, composite materials satisfy design provides lower weight and good stiffness, while laminate composites have often damages as like the delamination and cracks at the interface of laminates. The box spar and tail parts of a blade are composed of the CFRP/GFRP hybrid laminate composites. However, delamination and the interfacial crack often occur in the interface of CFRP/GFRP hybrid laminate composites under the mixed mode fracture condition, especially mode I and mode II. Therefore, there is a need for the evaluation of the mixed mode fracture behavior during the delamination of CFRP/GFRP hybrid laminates. This study shows the experimental results for the delamination fracture toughness in CFRP/GFRP hybrid laminate composites. Fracture toughness experiments and estimation are performed by using DMMB(Dissimilar mixed mode bending) specimen. The materials used in the test are a commercial woven type CFRP(Carbon fiber reinforced plastic) prepreg(CF3327) and UD type GFRP(Glass fiber reinforced plastic) prepreg(HD224A). A CFRP/GFRP hybrid laminate composite is composed by the 10 plies CFRP and GFRP prepreg for DMMB. A thickness of CFRP and GFRP layer is 2.5mm and 3.0mm, respectively. Also the fulcrum location which is a loading parameter is changed from 80 to 100mm on the specimen of length 120mm because it defines the ratio of mode I to mode II. In this study, the effects of the fulcrum location are evaluated in the viewpoint of energy release rate in mode I and mode II contribution. The results show that the delamination crack initiates at higher displacement and lower load according to the increase of the fulcrum location ratio. And the variation of the energy release rate for mode I and II contributions for the mode mixity are shown.

THE STRESS ANALYSIS AND THE CRACK BEHAVIOR ACCORDING TO THE CHARACTERISTIC OF THE INTERFACIAL REGION IN FIBER REINFORCED MMC

High performance composite reinforced with unidirectional continuous fibers are used in applications requiring high stiffness, high strength and light weight. Because of the high stiffness of the reinforced continuous fiber, the longitudinal performance of such unidirectional composites is greatly enhanced, but their transverse performance is so weak. The nature of the fiber/matrix interface is one of the important factors which determine the unique properties of the fiber reinforced metal matrix composites (MMCs). So, the current study is focused on the fracture behavior of the interface. Both stress state of the interface and crack parameters of the perpendicular crack to the interface for unidirectional fiber reinforced metal matrix composites under the transverse loading are investigated by using elastic-plastic finite element analysis. Different fiber volume fractions (5~60%) and arrangement (square and hexagon) of fibers were studied numerically. The fiber/matrix interface was treated as multi thin layer with different material properties. The fiber is assumed as linear elastic SiC and the matrix is assumed as elastic-plastic Ti-15-3 Titanium alloy. The results show that the stress distributions of the multi thin layer model have much less changes compared with a single interface case. And the properties of the interfacial zone affect the stress distribution, crack behavior and mechanical behavior of the fiber reinforced metal matrix composite.

The Effects of the Initial Crack Length and Fiber Orientation on the Interlaminar Delamination of the CFRP/GFRP Hybrid Laminate

Abstract : Considering the wind power system and the rotor blades which are composed of much technology, the wind power blade would be the most dangerous part because it revolves at high speed and weighs about dozens of tons, if the accident happens. Therefore, the light weight composite materials have been replacing as substitutional materials. The object of this study is to examine the delamination and damage for CFRP/GFRP hybrid composite that is used for strength improvement of a wind power blade. The influence of the initial crack length and fiber orientation for the interlaminar delamination was exposed for the blade safety. Plain woven CFRP instead of GFRP was inserted into the layer of the box spar for improving the strength and blade life. DCB(Double Cantilever Beam) specimen was used for evaluating fracture toughness and damage evaluation of interlaminar delamination. The material used in the experiment is a commercial material known as CF 3327 EPC in plain woven carbon prepreg(Hankuk Carbon Co.) and UD glass fiber prepreg(Hyundai Fiber Co.). From the results, crack growth rate is not so different according to the variation of the initial crack length. ModeⅠ interlamainar fracture toughness of fiber direction 0is higher than that of 45. Interlaminar fracture has an effect on fiber direction and K decreased with lower value according to increasing initial crack length. Also energy release rate fracture toughness was evaluated because CFRP/GFRP hybrid composite with a different thickness is under the mixed mode loading condition. The interlaminar fracture was almost governed by mode I fracture even though the mixed mode.

THE INVESTIGATION OF THE ELASTIC-PLASTIC BEHAVIOR USING THE PROPERTY GRADIENT OF THE INTERFACIAL REGION IN FIBER REINFORCED MMC

In this study, the interfacial perpendicular crack behavior and stress state of the unidirectional fiber reinforced Metal Matrix Composites (MMCs) are investigated by using FEA under the transverse loading. The fiber assumed as isotropic linear elastic SiC and the matrix is assumed as isotropic elastic-plastic Ti. The fiber/matrix interface is modeled as multi thin layer with different linear material properties. The behavior of perpendicular crack to the interface according to the change of the interface characteristics and thickness are evaluated.

AN ELASTIC-PLASTIC FINITE ELEMENT ANALSYS OF THE INTERFACE WITH PERPENDICULAR CRACK USING THE PROPERTY GRADIENT IN FIBER REINFORCED MMC

The effective utilization of the strength and stiffness of the fiber reinforced MMCs depends on efficient load transfers from the matrix to fibers through the interfacial region. However, during the fabrication and afterward utilization of composites, so many numbers of micro crack may extend, especially at the interface, even before any load has been applied. Thus, in this study, the interfacial perpendicular crack behavior and stress state of the unidirectional fiber reinforced MMCs are investigated by using FEA under the transverse loading. The fiber/matrix interface is modeled as multi thin layer with different linear material properties. The behavior of perpendicular crack to the interface according to the change of the interface characteristics and thickness are evaluated.