Functionalization of Adhesives and Application Requirements Analysis III

4 Aircraft Structural Bonding - Durability of Use

The structural bonding of aircraft is a kind of very complicated bonding in the form of joints, stress conditions, adhered materials and the use of atmosphere, etc. It has high requirements on the performance of adhesives and has many contents. With the development of aviation industry, the requirements are still Changes, content continues to increase, and demand is more demanding. The development of aerospace structural adhesives includes almost all aspects of high performance and functionality. Due to the characteristics of its application, it also puts forward more important requirements for the use of durability, making its functionalization move toward higher goals. The development of aerospace structural adhesives can be roughly divided into four stages [11,12].

Stage 1 (from the beginning of the 20th century to the end of the 1930s) - Performance Improvement Stage This stage mainly uses natural rubber. The Wright Brothers first aircraft wood and textiles were bonded using animal protein based glue. The adhesive has poor moisture resistance and was later replaced with a milk-based casein glue. Although moisture resistance is improved, in a humid atmosphere, it will fail for a long time. The fall of the Fokker aircraft in 1931 marked the end of this phase. Want to use natural rubber to solve the problem of moisture resistance and other issues, went into a dead end.

Phase 2 (2nd World War - early 1970s) - High-performance, functional stage At this stage, advances in synthetic polymer science and technology have created favorable conditions for improved adhesive properties. The high performance and functionalization of adhesives have met various high performance requirements in use, completing the transformation of plastics from natural rubber to synthetic rubber; the transformation of sticky materials from wood and steel to aluminum alloys and composites; The joint structure consists of simple sheet metal to sheet metal and honeycomb sandwich panel composite structure, from the hole to the non-porous cellular transformation. The types and series of structural adhesives have gradually increased to basically meet the performance requirements. At this time, the durability of the solution will be put on the agenda.

Stage 3 (from the early 1970s to the end of the 1980s) - Resolving the use of durability - High functionalization stages Early aircraft structural bonding has not drawn attention to durability. In the first stage of the Wright Brothers aircraft, although the animal glue had poor water resistance, the flight time was short at that time and it was only when the weather was good. The problem was not discovered. The crash of the Fokker aircraft is generally said to be the destruction of the wood structure. Is it the destruction of the wood or the damage of the glue? No one knows. In the second phase of the first major jet passenger crash, the De Havilland Vitus aircraft crashed. The main bearing structure was glued with a synthetic elastomer-modified phenolic. The cause of the accident was the durability of the rubber. not good. However, it was later proved that the accident was caused by metal fatigue, and the durability of this type of rubber was quite good. Because the oxide layer formed by the surface pretreatment method was weak, the early epoxy-aluminum bonded structure had poor durability, and the allowable stress was significantly reduced under the humidity and cyclic load. People get revelations from it: To solve the problem of glue joints, we must not consider the performance of adhesives in isolation. We must use glue joints as a system and integrate various performance requirements to make them accept the durability test. The biggest achievement of the aerospace structure bonding at this stage was the introduction of Boeing's aluminum surface phosphoric acid anodizing pretreatment method and the implementation of the US Air Force Primary Adhesively Bonded Structure Technology (PABST) program. The PABST plan is based on the above-mentioned viewpoints. Based on the work of phosphoric acid anodizing, it first improved the adhesive preparation technology, confirmed it in a number of laboratories, and finally turned into a full-scale test of the fuselage, including a hot and humid environment. The large number of repeated low-frequency experiments under the conditions, repeated and comprehensive research, and finally succeeded. Its significance lies in: it indicates the direction for the highest durability of aluminum materials; it promotes the improvement of the overall performance of the fuselage structure in terms of cost, weight, integrity, and durability; and it successfully develops a series of durable and aluminum plates and Honeycomb sandwich structure with matching adhesives; broadens ideas for the development of other adhesives such as titanium alloy, resin matrix composites.

Phase 4 (from the 1990s to the present) - Resolving compatibility with the environment - The highly functional and intelligent phase of the PABST program is of great significance in the history of the development of aerospace adhesives, laying a solid foundation for the application of adhesives in this field. However, in recent years, the development of adhesive technology has not stopped. If the previous stage was considered to improve the performance of the adhesive with a joint system, this stage mainly considers the entire system as the system to consider the high performance and functionalization of the adhesive. Taking the fighter as an example, the jet fighter has developed to the 4th generation. Its design philosophy has undergone great changes. The style of air combat has evolved from tail-attack and close combat to full-scale attack and mid-range combat. Fourth-generation fighter aircraft such as F-22 require high mobility, stealth, supersonic cruise, high reliability and long life. This puts forward newer requirements and content for the “higher, faster and farther” goals of the year, and the high performance and functionalization of adhesives must be adapted to it; in addition, the military’s cost savings and sustainability Starting from the development strategy, the United States Air Force put forward the requirements of “older, cheaper, and more green” (13) based on the original high-performance requirements. Therefore, in addition to the continuous emergence of adhesives with superior adhesive properties and performance in recent years, new developments have been made in the development of environmentally compatible adhesives. For example, high-temperature adhesives containing no carcinogenic and suspicious substances such as asbestos and methylenebis (4-aniline) have been successfully developed, while solvent-based primers are developing toward low-VOC primers or VOC-free inorganic primers and water-based adhesives. [14]. The 5th-generation military aircraft being brewed is an intelligent aircraft. With sophisticated design and materials, aviation glue has to develop.

5 Intelligent - Functional Development

(1) Proposed intelligent

Adhesive applications from simple to complex, performance requirements from low to high, adhesives from high performance to functional development. In the process of functionalization, there is currently a new feature that has attracted attention. This is what special biological materials have. General materials will only deteriorate and deteriorate under the long-term effects of load and external factors, and they will not have the ability to repair or regenerate. However, biological materials have this function. The material assigned to this function in the industry is an intelligent material whose "intelligence" is achieved through three interconnected parts: the sensor part that senses the external influence, which is equivalent to the biological sensory nerve, has judgment Function; The Processor part that makes correct judgments and proper processing is equivalent to the brain and has judgment function; The Actuator part that plays a role of recovery and regeneration is equivalent to biomuscular, and has response function. . The three interact with each other to form their intelligence. Adhesives are one of the materials, so they are interested in this function possessed by the organism and require intelligence because they have encountered problems that are difficult to solve during the bonding process.

Bonding as a connection method has many advantages over mechanical connection methods such as screwing, riveting, and the like. Connections do not require attachments, and have a compact structure, a beautiful surface, and sealing performance, and are widely used in the assembly of lightweight products. However, it has two major drawbacks: First, the bond is broken, usually resulting from the stress concentration of the adhesive layer, and once it is destroyed, it cannot be stopped and it will continue to extend; and after that, the adhesive cannot be peeled off and disintegrated. No bond reversibility. If these two problems are not resolved, the application and promotion of bonding will be limited, and its solution will be based on intelligence.
(to be continued)