Into composite pultrusion profiles
The so-called fiber reinforced composites (Fiber Reinforced-Polymer Composites, referred to as FRP or composite materials) are made of reinforced fiber materials (glass fiber, carbon fiber, aramid fiber, etc.), and matrix materials through winding, molding or pultrusion. formed composite material. FRP pultrusion profiles are long strips of FRP products that are continuously produced by the pultrusion process.
FRP is divided into many types according to different fibers and matrix materials. Although there are certain differences in performance, they generally have the characteristics of light weight and high strength, and have considerable advantages in the fields of construction, transportation, and manufacturing.
The application history of FRP in the field of civil engineering is relatively short. Existing studies have shown that the mechanical properties of FRP will show varying degrees of decline under long-term environmental effects. Therefore, research on its durability will directly affect the design and use of FRP. It is also of great significance to the promotion of FRP materials in the future.
Civil engineering structures often have a design service life of several decades, but the durability test of FRP in the laboratory environment can only be carried out for tens of months. Therefore, in order to test the durability of FRP, accelerated aging tests are often carried out in the laboratory with a limited time to simulate the performance of FRP after decades. Factors such as wind and rain in nature play a decisive role in the aging of FRP: water immersion/humidity, alkali solution, pickling solution, high/low temperature, ultraviolet radiation, freeze-thaw cycle, dry-wet cycle and their combination wait.
Since the 1970s, there have been many related experiments. To sum up, the main points are as follows:
>Solution condition: The aging of FRP under water immersion condition is serious. The intrusion of water will lead to the destruction of the interface between fiber and resin, thereby affecting the mechanical properties of FRP; the increase of water immersion time, alkaline environment and high temperature environment will aggravate the corrosion of composite materials.
>Dissolution of water: The dissolution of FRP by water is affected by many aspects, such as temperature and porosity.
>External stress effect: In the short term, the fibers are straightened under external stress, which is beneficial to the enhancement of FRP strength; but in the long run, the external stress will increase the water absorption of FRP, resulting in accelerated aging of the material.
>High temperature: High temperature has a significant impact on the compressive strength, mass loss and impact resistance of FRP.
>Low temperature: The influence of pure low temperature conditions is small, but in a humid environment, the combined effect of low temperature and water intrusion will cause cracks inside the FRP and separation of the interface between fibers and resins, which is not conducive to material durability.
>Natural environment: Under natural conditions, FRP often faces a variety of environmental conditions. Under the combined effects of temperature, water and ultraviolet rays, the post-curing effect of FRP will lead to an increase in the elastic modulus; at the same time, the erosion problem in seawater is often more serious than that on land.
>Test factors: FRP durability tests around the world have a large degree of dispersion, so it is difficult to draw a general conclusion directly from the test. The discrete degree of FRP material is mainly reflected in:
1. The aging mechanism is complex, and the corrosion of FRP may occur in the resin, the interface between the resin and the fiber, and the fiber;
2. There are many types of fibers and resins, and different FRPs are often composed of fibers and resins with different combinations and ratios, so the test results will be different;
3. Different FRP production processes will lead to differences in the fiber content and organizational structure of the material, which will affect the mechanical properties and durability of the material.
So, what is the durability of FRP pultruded profiles that everyone cares about?
Aging mechanism
Since FRP will age under the influence of the environment, how does aging happen?
There are both external and internal causes of aging. Under the action of different factors, the aging mechanism is also different.
1. Aging mechanism under water immersion/humidity conditions
>Fiber: Inorganic fibers such as carbon fiber and glass fiber will not absorb water, but the microcracks caused by resin water absorption will expand at the resin and fiber interface, eventually leading to fiber cracking. Water absorption by organic fibers will directly lead to expansion and cracking of fibers.
> Resin matrix: Water in the environment invades into FRP mainly through osmosis and capillary phenomena. Water intrusion will cause the resin to expand and cause microcracks. In addition, water intrusion causes plasticization and hydrolysis of the material, leading to softening of the material, etc. When dried, the plasticization of the material is partially reversible, but hydrolysis is irreversible, which can lead to permanent damage to the material.
>Fiber-resin matrix interface: The interface provides a convenient channel for water to enter. For FRP materials with poor interface quality, the channel effect is more obvious. Water absorbed along the fiber-matrix interface causes swelling of the interface and propagation of microcracks. In addition, fiber-matrix debonding will occur, and part of the matrix material may be dissolved in the water at the intrusion interface, which directly reduces the interlaminar shear strength of the FRP material.
> Effect of water intrusion: Water ingress into the resin matrix and fiber-matrix interface may further widen the initial cracks and create new cracks and voids, allowing additional water to be absorbed into the FRP material, exacerbating the degradation mechanism. Water intrusion affects the fiber-dominated properties, i.e. tensile properties, and significantly reduces the matrix- and interfacial-dominated properties, i.e. bending and shear properties.
2. Aging mechanism under the condition of acid-base solution
Alkaline and acidic solutions age FRP materials in a manner similar to water intrusion. Both solutions can penetrate into the material through osmosis, after which they induce swelling in the matrix and fiber-matrix interface, leading to the formation and propagation of microcracks, which reduce the strength and elastic modulus of the material. According to the existing research data, generally alkaline solution has a greater influence on the mechanical properties of FRP materials.
3. Aging mechanism under high temperature conditions
When the high temperature acts together with the solution, it can accelerate the aging effect caused by water, alkali, and acid solution; and the high temperature alone will also affect the mechanical properties of FRP.
High temperature will affect the viscoelasticity of the resin matrix of FRP. When the ambient temperature is close to or higher than the glass transition temperature of FRP, the resin matrix softens and cannot transfer stress between the fibers and the matrix, resulting in a decrease in the matrix elastic modulus and deterioration of the fiber-matrix interface. Therefore, at high temperature, the failure mode of FRP materials is controlled by fibers, which manifests as sudden brittle fracture of fiber bundles.
4. Aging mechanism under ultraviolet radiation
FRP is prone to chemical degradation under ultraviolet radiation, and aging starts from the "skin". UV radiation can cause surface oxidation of the resin matrix, destroy the chemical bonds between molecules and affect the surface gloss of the material. However, the degradation of mechanical properties due to UV radiation is limited to a depth of about 10 μm within the material, with almost negligible effects on the mechanical properties.
5. Aging mechanism under freeze-thaw cycle conditions
The coupling of water and freeze-thaw cycles can cause aging of soaked materials, but the aging of dry materials under freeze-thaw cycles is often negligible. Experiments have shown that the bending stiffness of GFRP materials (glass reinforced plastics) under freeze-thaw cycles does not decrease but increases, which is due to the hardening behavior of the resin matrix at low temperatures. Therefore, freeze-thaw cycles have no obvious adverse effects on the mechanical properties of FRP materials.
6. Aging mechanism under dry-wet cycle conditions
The aging mechanism of dry-wet cycle on FRP is similar to the aging mechanism caused by aqueous solution. Essentially, water or water-based solution initiates the aging process.
7. Aging mechanism under natural conditions
The aging of FRP under natural conditions is often the combined effect of the above mechanisms.
To sum up, the long-term environmental effects that cause FRP aging mainly include the above types, and these external conditions comprehensively act on the fibers, matrix, and interface of FRP. After qualitatively analyzing the aging mechanism, let's look at the specific test data.