# Possibilities of Detecting Damage Due to Osmosis of GFRP Composites Used in Marine Applications
## 1 Introduction  
Interest in composite materials in many industries results from the possibility of designing their functional properties required in a specific application. This creates the possibility of new design solutions that have been previously unavailable with the use of traditional materials, e.g., metals. Fiber-reinforced composites are particularly attractive materials. The use of fibers in the composite makes the material stronger and stiffer [1]. It may have physical parameters comparable to metals but have a lower specific weight. Most often, composites employ textile materials joined together with plastics, such as a binder, to create multi-layered composite materials used, for example, in marine applications. Glass-fiber reinforced composites are most often used in the construction of vachts. More and more often there are constructions where there is a layer of rigid polyvinyl chloride foam between two layers of glass-fiber reinforced composite X. Damage to the composite structure may occur both as a result of technological errors in the production phase and during operation as material fatigue. The main type of damage to watercraft is damage to the composite structure caused by impacts, which is one of the most critical failures. The effects of impact damage can be considered as the internal fracture surface is characterized  
1. impact damage resistance which is related to the response and damage caused by impact [2],
2. impact damage tolerance, associated with the reduced stability and strength of the structure due to the damage [3, 4]. Another factor that contributes to the damage to the composite hulls of watercraft is the phenomenon of osmosis (Figure 1). Osmosis in hulls is the penetration of water through the gelcoat layer into the laminate. Even the best-made laminate is not a homogeneous structure. There are micro-air bubbles and microcracks, both in the resin itself and at the junction of resin and glass. In these voids of the laminate, water collects, which forms acetic and hydrochloric acids and glycol as a result of hydrolysis. Since glycol is a strongly hygroscopic substance, the amount of water absorbed into the hull increases and the degradation process accelerates. The molecules of water contaminated with compounds formed as a result of hydrolysis strive to equalize the degree of their contamination with the environment. As a result of diffusion, pure water molecules penetrate the laminate unhindered, accelerating the osmosis phenomenon. Hydrolysis and diffusion are more intense in seawater, which is alkaline (pH 8-8.5). As a result of these phenomena, the connections between the fibers of the glass reinforcement and resins were destroyed in the first place. Water spreads along the reinforcement fibers, separating them from each other. The effect of this phenomenon is the gradual degradation of the laminate manifested by the formation of blisters filled with a specifically fragrant, acidic liquid with a pH of 0 to 6.5. Osmotic blisters can vary in size--from the size of a pinhead to a diameter of 10 cm. As a result of osmosis, not only does the laminate structure gradually deteriorate but the water content in the laminate increases, and the weight of the hull increases. Changing the water content in the laminate has a direct impact on the depth of immersion, maneuverability, speed, and fuel consumption [5]. Damage resulting from impacts or technological errors in the production of composite hulls of floating units accelerate the formation of damage caused by the phenomenon of osmosis. Figure 2 shows osmosis foci on the surface of the yacht's hull after removing the gelcoat layer. In this picture you can see what damage can be caused by the phenomenon of osmosis. Figure 1: The process of the osmosis phenomenon [5]. So far, a commonly used method of detecting the effects of osmosis has been tapping, for example, with a hammer on the surface of the yacht's hull. The characteristic sound caused by tapping is information about the existence of damage due to osmosis.