적층수에 따른 GFRP 피막 Al 평활재의 저주기 피로수명 평가

Low Cycle Fatigue Life Behavior of GFRP Coated Aluminum Plates According to Layup Number

Abstract

섬유 금속 적층판(Fiber metal hybrid laminate, FML)은 금속재료와 FRP의 접합으로 기존의 금속 소재가 가지지 못했던 뛰어난 물성과 가벼운 무게로 경제적인 구조용 재료로 사용된다. 그러나 섬유의 형태와 종류, 적층조건에 따라 물성의 차이가 크며, 파괴거동을 예측하기 어렵다는 단점이 있다. 본 논문에서는 Al6061-T6 합금에 직조형태의 유리섬유 플라스틱(GFRP, GEP118)을 적층피막한 복합재의 파손거동에 대해 연구한다. Al합금에 GFRP 1, 3, 5 겹을 피막한 3가지 조건으로 성형하고, 피막의 적층수를 변수로 정적시험과 저주기 피로시험을 병행하여 파손거동을 분석하였다. 저주기 피로시험에서는 변형률-수명 해석, 전변형률 에너지밀도법을 사용하여 분석하고, 피로수명을 예측하여 하이브리드 재료에 대한 수명예측성을 분석하였다. 인장해석 결과, GFRP 피막으로 인한 강화효과는 없었고, 피로시험시 나타나는 히스테리시스 형상은 GFRP피막 유무와 피막 수에 상관없이 모재인 Al합금의 거동을 따랐다. 저주기 피로시험 결과 GFRP의 피막으로 피로강도가 증가하였지만, GFRP의 두께에 따라 비례하여 증가하지는 않았다.

Fiber metal hybrid laminate (FML) can be used as an economic material with superior mechanical properties and light weight than conventional metal by bonding of metal and FRP. However, there are disadvantages that it is difficult to predict fracture behavior because of the large difference in properties depending on the type of fiber and lamination conditions. In this paper, we study the failure behavior of hybrid materials with laminated glass fiber reinforced plastics (GFRP, GEP118, woven type) in Al6061-T6 alloy. The Al alloys were coated with GFRP 1, 3, and 5 layers, and fracture behavior was analyzed by using a static test and a low cycle fatigue test. In the low cycle fatigue test, strain - life analysis and the total strain energy density method were used to analyze and predict the fatigue life. The Al alloy did not have tensile properties strengthening effect due to the GFRP coating. The fatigue hysteresis geometry followed the behavior of the Al alloy, the base material, regardless of the GFRP coating and number of coatings. As a result of the low cycle fatigue test, the fatigue strength was increased by the coating of GFRP, but it did not increase proportionally with the number of GFRP layers.

Fig. 1. Sketching order of GFRP and adhensive film

Fig. 2. Tensile test curves of aluminum alloy and Al/GFRP laminates

Fig. 3. Nominal stress-strain curves of aluminum alloy at the cycle of half lives

Fig. 4. Nominal stress-strain curves of aluminum alloy and Al/GFRP laminates at the cycle of half lives

Fig. 5. Nominal stress-strain curves of Al + 3 plies at 1,2 and 600 cycles

Fig. 6. Fatigue test results of aluminum alloy and Al/GFRP laminates

Fig. 7. Description of the plastic and elastic strain energy densities

Fig. 8. Total energy density method results of aluminum alloy and Al/GFRP laminates

Fig. 9. Comparison at each specimen type between experimental life and predicted life by total strain energy density method

Fig. 10. Fractography of GFRP-coated Al specimens at the tensile test

Fig. 11. Fractography of GFRP-coated Al specimen at the fatigue test

Table 1. Thickness of specimen and GFRP fraction

Table 2. Tensile test results of rolling direction of Al6061-T6 plate

Table 3. Average of the internal area of the hysteresis loop at half life

Table 4. Experimental constants in total strain energy density method

Table 5. Experimental constants in total strain energy density method