That night, Philip and Messerschmidt worked on the design until half past five in the morning, and finally got the preliminary plan ready. In the course of his discussion with Philip, Messerschmidt was once again impressed by Philip's profound knowledge and unique and original perspective, and he even doubted whether he was not suitable to be an aircraft designer.

When the two discussed the overall design of the commercial transport aircraft, Philip's proposal of "aircraft structural fatigue and damage tolerance design" really shocked Messerschmidt, a concept he had never heard of.

Messerschmidt patted his somewhat confused head, looked at Philip with surprised eyes, and said: "Philip, why do you always have so many strange things in your head, and every time you come up with a concept, it almost makes me, a doctor of aeronautical engineering, a little confused, alas, it seems that you are a real master of aircraft design, and we are all old and can't keep up with the pace." What is the concept of fatigue and damage tolerance design of aircraft structures, you can talk about it carefully, and I can also learn new knowledge. ”

Why did Philip introduce the fatigue and damage tolerance design of aircraft structure into the overall design plan of the commercial transport aircraft, this is because from the later changes in aircraft structure design ideas, many technological advances and design concept evolution in the history of aircraft design come from the lessons and inspiration of flight accidents, although flight accidents are the last thing aircraft designers want to see, but I have to admit that even in the 21st century, when the aviation industry is highly developed, Humans are still in the stage of exploration of flight and aircraft design. As an expert in military engineering, Philip is very clear about the evolution of aircraft structural design, and every change is a lesson in blood, so Philip does not want the design problems of the commercial transport aircraft he designed, especially the structural problems.

Philip clearly remembers that the British Comet jet airliner, which is more famous in aviation history, was caused by a series of crashes due to structural design problems, and finally the Comet airliner was grounded. In one year, there were three Comet airliners disintegrated and crashed in the air, and later investigation and research showed that because the comet used a pressurized cockpit, due to the lack of experience in the design of the pressurized structure of the cockpit, the long-term flight and frequent take-off and landing made the body repeatedly bear the alternating stress of pressurization and decompression, which caused metal fatigue, which was the cause of the comet airliner disintegration and crash. This time the commercial transport aircraft bidding will use a pressurized cockpit design, so this problem cannot be avoided. You must know that the accident of the comet airliner directly ruined the future of this airliner and its designer, de Havilland, and Philip did not want the newly established Dragon Soul Ordnance Complex to suffer the same fate as de Havilland.

The development of aircraft structural design ideas has undergone five major changes, which Philip studied in college. Before the fifties of the twentieth century, the structural design of aircraft was basically designed according to static strength, but after the fifties and before the sixties, the idea of safe life was slowly adopted, and after the seventies, the idea of damage-safety design was proposed, that is, the design of damage tolerance.

Seeing Messerschmidt's expectant face, Philip sorted out his thoughts and said: "The load of the aircraft structure not only has the characteristics of static load, but also the nature of periodic load, such as the airtight pressurized cockpit we want to use, its pressurized load is also a periodic load, under the action of repeated loads, even if the stress level is very low, the aircraft structure may be fatigue damaged, resulting in catastrophic accidents." Therefore, only the static strength of the aircraft design can not ensure the safety of the aircraft in flight, especially the commercial transport aircraft, which is loaded with a large number of passengers, once an accident occurs, its impact is very large, and may even ruin the future of an aircraft manufacturing company. So we're going to introduce the fatigue design concept and the damage tolerance design concept. ”

Messerschmitt thought for a moment and said, "But why don't we have many accidents with the planes we design according to the current structural design method?" ”

Philip nodded and said: "Dr. Wiley, the fatigue problem of the aircraft designed before this is not prominent, because only the strength and stiffness requirements were considered in the design, but due to the low level of structural design, a large strength reserve is set aside, and the safety factor is high, so that the stress level of the aircraft in use is relatively low." In this way, the fatigue problem of the aircraft structure is masked. For example, Junkers' JU52/3m transport aircraft, its structural weight reaches about 5 tons, the maximum take-off weight is about 10 tons, and the load capacity is only more than one ton, such a structural design level is very low, of course, this is in my opinion. In order to overwhelm Junkers in this bidding, it is very important to be significantly ahead of the competition in terms of structural design, and we have adopted a pressurized cockpit, and in order to achieve the same number of passengers and load capacity while the structural weight is only about half of the opponent's, it is necessary to introduce the structural fatigue and damage tolerance design of the aircraft. ”

Messerschmidt suddenly realized: "Philip, I finally understand when I hear you say this!" This is an opportunity to lead the way in aircraft design, and with this design concept and methodology, we are far ahead of other aircraft manufacturing companies in terms of design! Philip, you are so impressive! So what exactly should we do? ”

Philip yawned and said slowly: "Fatigue design is based on no cracks, only considering the life of no cracks, in order to improve the fatigue strength of the structure, the materials can be appropriately selected, the stress level can be controlled, the fatigue quality can be improved through the detailed design, the stress concentration caused by the discontinuity of shape, material and load, and the quality can be controlled in the production process." For example, when designing some openings on the aircraft, it is necessary to take into account the problem of stress concentration, and now the aircraft is usually designed with a sharp shape such as a square corner, so that the stress will be concentrated, and we must use the method of rounding or elliptical opening, so that the stress is not so concentrated, so that it is not easy to crack, and ensure the service life and safety. ”

After taking a sip of water to moisten his throat, Philip continued: "The damage tolerance design only considers the crack life. The crack propagation life from the initial crack to the critical crack is the total life of the structure, and the reliability can be improved through quality control, non-destructive testing and reasonable material selection by introducing an inspection cycle related to the crack propagation rate to control the crack propagation life and residual strength. ”