On the Strength of the Glass Fiber Reinforced Polyester : Damage of the Glass-Cloth by Pressing and Influence of Moisture and Dryness

Abstract

The tensile strength of the glass fiber reinforced polyester (FRP) increases by the glass content by volume in the linear relationship. However, it decreases when the glass content exceeds a certain critical point. According to the consideration that this is caused by the damaged glass fiber in the pressing procedure for forming, the tensile strength of the glass cloth was examined after being pressed by a known pressure, and it was revealed that the strength was reduced suddenly beyond a certain critical pressure. This pressing pressure coinsides with the forming pressure of FRP at its critical tensile strength, which leads us to the conclusion that the chief factor for this phenomenon should be the damage of glass fiber due to the forming pressure of FRP. In addition, by the experiments for dried and wet specimen, it was clarified that the critical pressure is higher for dried specimen than the wet, the strength is reduced by moisture not only for alkali glass fiber, but also non alkali ones and that the strength in wet condition is restored by heating or redrying. This was explained by means of design of experiment.