Data to speak
Based on the above several environmental conditions that have attracted the most attention and the data collected from the test, we can quantitatively analyze the influence of the environment on the tensile, compressive, bending and shear properties of FRP. Due to space limitations, only part of the data and analysis results are shown here.
1. Aging analysis under water immersion/humidity conditions
According to the current test data, the duration of durability tests under most solution conditions (including water immersion, alkali solution, pickling solution) is within one year, and the test temperatures are usually 20, 40, 60 and 80°C. For immersion/humidity conditions, the temperature ranged from 20 (room temperature) to 80°C, a temperature range sufficient to simulate the service temperature of FRP structures in real environments. However, shorter test times may affect the reliability of durability test results. Since the post-curing effect of the resin matrix and the slow degradation of the fiber-matrix interface and matrix in water require a long test time to fully reflect, this paper recommends the shortest The test period is 18 months.
The data from different tests are scattered, so only the aging trend of the material can be observed, that is, the moisture will cause the decrease of the tensile strength of FRP pultruded profiles, and this decrease behavior will be more obvious with the increase of exposure time and temperature.
2. Aging analysis under alkaline solution conditions
Test results for alkaline solutions were similar to those for submerged/humid conditions: most tests were within one year and exposure temperatures in these tests were typically 20, 40, 60 and 80°C. Except for a few trials that were conducted for 1.5 to 2.5 years, almost all trials were conducted for a shorter period of time.
It can be seen from the figure below that the dispersion of the test data is large, and it is impossible to draw conclusions that can guide the design; moreover, in the existing test data, there are also serious contradictory results, which is the observed aging effect in the test. caused by incomplete completion. Therefore, it is recommended to conduct longer tests in the future to fully observe the aging behavior of FRP in alkaline solution.
In this paper, the pH value of the alkaline solution is divided into two categories:
1) Weak alkaline solution with a pH value of about 8, simulating seawater or salt water environment;
2) A strong alkaline solution with a pH value of about 13, simulating the concrete pore environment or other harsh alkaline environments. The general trend is that the mechanical properties of FRP pultruded profiles in alkaline solution will decrease with the increase of test time and temperature. In addition, the strong alkaline environment may lead to more severe aging of FRP materials than the weak alkaline environment.
3. Aging analysis under high/low temperature conditions
Typical test conditions include exposing FRP materials to high or low temperature air. The test temperature varies from -100 to 700°C, and most tests are carried out at temperatures below 200°C.
Lower temperatures may harden the FRP material, resulting in improved mechanical properties. Conversely, high temperatures may soften the resin matrix, destroying its ability to transmit stress between the fibers and the matrix. In general, the residual tensile strength and temperature show a linear negative correlation.
4. Aging analysis under natural conditions
Natural conditions are a combination of various conditions that synergistically affect the mechanical properties of FRP pultruded profiles. In existing trials, the shortest exposure time was 100 days, while the longest exposure time was 8 years. Natural conditions included simulated natural environments under laboratory conditions, seasonal conditions in Switzerland, urban environments in Portugal, and arid environments in Saudi Arabia. The environment around the world will have different effects on the mechanical properties of FRP pultruded profiles. Therefore, it is recommended to select and conduct accelerated aging tests according to the actual use environment of FRP materials.
Existing standards
In the past decade, the application standards of FRP pultrusion profiles are being established in different countries and/or regions around the world. For example, in the national standard "Fiber Reinforced Composite performance.
However, the existing engineering design standards and specifications in various countries have not yet covered all environmental factors that may affect the performance of FRP materials. Therefore, this paper proposes to improve the existing design methods to further consider the influence of all types of environmental effects on FRP pultruded profiles.
Suggestions and Prospects
The durability of FRP pultruded profiles is superior, and extensive and in-depth research is urgently needed. The current theoretical prediction model is limited by limited test data and cannot be widely used in the prediction of FRP durability; moreover, although many durability tests have been carried out in the past few decades, the results are often not comparable with each other, thus come to a unified conclusion. Based on the summary and analysis of relevant research data in the past 20 years, this paper recommends:
1. For all types of accelerated aging tests, longer test times are recommended.
2. It is recommended to strengthen the aging analysis of the transverse properties and compressive properties of pultruded profiles.
3. When evaluating and predicting aging behavior, it is recommended to consider the effects of fiber and matrix type, fiber content and specimen thickness.
4. It is recommended to consider each environmental effect separately in the aging test (for a period of time in the future, it is not recommended to carry out the aging test of the combined environmental effect), so as to truly understand the aging mechanism of the material and guide the design and use of the FRP structure.
5. Suggest improvements, unification of standard test methods, and development of more accurate durability prediction models in the future.
The research on the durability of FRP is like a jigsaw puzzle, and the experiments of every scholar at home and abroad add an important piece to it. We believe that with the joint efforts of the academic community and the full support of all sectors of society, the research on FRP durability will eventually synthesize a beautiful picture and vigorously promote the wide application of FRP materials in the future.
Looking back on history: my country's scientific and technological workers conducted systematic research on the durability of FRP as early as 44 years ago, and this article pays tribute to the older generation of scientific and technological workers.