本 自生을 胡伟光의 工學碩士 學位 誕说 从認地师. In this research, glass fiber unsaturated polyester (GFRP) composites under different bending stress and immersed in hot water at a temperature of 80℃ were investigated. Then the water absorption ratio is analyzed and the bending stress data and flexural modulus of elasticity after the three-point bending test, failure time, life curve, normalized strength degradation model and are analyzed by experiment. fracture energy of composites.
Finally, the change of the material interface affected by the environment is observed with optical microscope and SEM, and the reasons for the change of material mechanical properties are explained.
Introduction
General background
Main research significance
Sun Bo, Li Yan's research shows that glass fiber epoxy resin in different temperatures of water aging, mechanical properties decreased significantly in the first 8 weeks. The decline in the first 4 weeks is rapid, and a year later it tends to be stable[7]. In this research, the unsaturated glass fiber polyester (GFRP) composite materials were immersed in hot water at a temperature of 80 ℃.
Experiment
- Experimental material
- VARTM method technology
- Composite materials cutting
- Bending stress
- Water environment at a temperature of 80℃
- Three-point bending test
In the VARTM process, the mold release agent is first covered with a one-sided mold, the adapted glass fiber is placed in the mold carefully, and the plastic tube is fixed at both ends of the fibers. Place the mold release membrane, insulating cloth and flow medium on the fiber, and a vacuum bag is placed on top to form a tight vacuum seal. Then sweep the system with a vacuum pump, and the mixture of unsaturated polyester and hardener is introduced into the structure with plastic pipe from one side to the other slowly.
The ultimate material strength of the bending stress can be obtained by three-point bending test, and at the same time the corresponding ultimate displacement is obtained. In this study, constant pressure is pressed on the sample by a pressure fixture, shown in Figure 1. The material's ultimate strength is pressed on the sample by a pressure fixture with different displacements.
Then remove the sample, after it has dried and weigh it, analyze the impact of the conditions on mechanical properties of materials through a three-point bending test by a universal testing machine (UTM).
Analysis and results
- Water absorption rate
- Bending modulus of elasticity
- Bending strength
- Extrapolation of time to failure (TF)
- Life curve (S-T)
12 shows the change of water absorption rate of the sample over time under different bending stresses. The distribution of the dots in the figure shows that the residual flexural strength of the material decreases as time increases, and decreases rapidly as pressure increases. When pressure is applied, the residual flexural strength of the material decreases rapidly within the first 100 H, decreasing slowly from 100 H to 800 H. It decreases more rapidly as pressure increases.
16 and the relationship between decreasing rate of the residual bending strength and the rate of pressure increase in 100 H shown in Fig. The rate of decrease of the residual bending strength is low from 100 H to 800 H, and it has no relationship with pressure, almost all of them are below 10%. Mechanical property degradation of composites has an exponential decay considering the physics of the problem.
17 The relationship between decreasing rate of the residual flexural strength and the rate of pressure increase in 100 H. Water contributes to the propagation of the crack, can cause the matrix to burst, and substrate hydrolysis leads to chain crack reconciliation cross-linking. Studies have shown that hygroscopicity almost does not affect the tensile properties of the unidirectional composite material.
And the rate of erosion has a great relationship with humidity, temperature, as well as the treatment of the fiber surface. The main temperature mechanism in the coefficient of thermal expansion of fiber and matrix do not match. It causes residual stress, which leads to growth of micro-cracks, extensive removal of the interface and reduction of the fracture toughness of the laminate.
For example, at 30% stress state, TF is the time when the magnitude of the residual strength of the specimen is equal to 30% of the ultimate strength of the material. The residual flexural strength of the sample is then measured using a three-point bending test. Using the cubic splicing method and data extrapolation, the time to failure at different stress states is calculated.
Physics of the problem in fatigue behavior of composites is very similar to stress corrosion cracking of composites. Using the present model, residual strength of the composite material under hot water in different stress conditions and arbitrary time can be predicted.
Optical microscope and SEM observation
The above phenomena explain one of the important reasons why the mechanical properties of materials are reduced under bending stress in the hot water environment is with the action of water molecules, fiber and resin separation becomes easier, and the transfer stress becomes weak. With the rising time, the fiber-resin bond becomes more and more weak, and it was clearly layered. There is a subtle difference between the content of fiber and resin can be observed on the surface.
And after dipping sample in high temperature water environment, fracture of samples has more fiber pull out, less adhesive resin, the failure mechanism of material in matrix cracks and interface decomposition [22]. 25 from top to bottom successively shows the sample fracture after the three-point bending experiment without being immersed in hot water (0 H) and immersed in hot water 100 hours (100 H) and 800 hours (800 H) at a temperature of 80 ℃ . And the pictures on the left are magnified 1000 times, and 2000 times on the right.
The resin shrinks around the fiber and the increases over time can be clearly observed using SEM. And the spaces between the fibers become larger, so the mechanical properties obviously decreased.
Conclusion
I would like to thank my parents for 26 years of breeding, love and support, as well as my older sister's help and support. I would also like to thank my advising professor, Yun-Hae Kim, for his guidance and help over the past two years. Finally, I would like to thank Doctor Jin-Cheol Ha and President Hee-Soo Yoon of the dongdo basalt company for helping me with my Korean life, and the members of the laboratory and school, I was very happy in their company.
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