Fatigue
Fatigue can be defined as stress collected in atomic structure depending on movement. This stress can cause fatigue failure depending on time. Sixty-five percent of all failures relate to fatigue failure. There are two major parameters such as fatigue strength and fatigue limit. These are parameters related to the atomic structure of the material. This type of failure is very important for the mechanism of the material and the lifetime of the material. Especially, aviation systems have a big history related to fatigue failure such as De-Havilland accident and Air-India Express Flight 812 crash.
Fatigue Limit
Fatigue limit is defined as which material has an endless lifetime below a certain stress amplitude. These materials are ferrous and some titanium alloy. This situation depends on Body Center Cubic atomic structure because it can emit stress that is collected on the atomic structure during movement for certain amount.
Fatigue Strength
Fatigue strength is defined as which material has broken for a certain stress cycle. There is no endless life-time for the material. Therefore, the material can not emit stress on its atomic structure regarding structure. These material is non-ferrous alloys.
Aircraft Wing Body Fatigue Fracture
There are many fracture types, excess deformation, ductile fracture, brittle fracture, impact or dynamic load, creep, relaxation, fatigue, and others. Fatigue failure is the most important fracture mechanism regarding aviation systems. Therefore, air behaves as a dynamic load during flight, and it can be continuous. Thus, stress can accumulate on the body and wings under high pressure during flight. After that, some specific processes will be started regarding the atomic structure of the material such as crack initiation, crack propagation, and fracture. Crack initiation is the most dangerous part of total fracture. Therefore, electrons of elements are separated on the atomic scale during stress differences occur. Crack can occur due to fatigue, as a consequence of manufacturing, and metallurgical discontinuities. Materials used for aviation system passes many test and control to eliminate the type of crack problem. However, fatigue of material is affected badly during dynamic air load and high pressure. When crack initiation on the body or wing, the crack propagation process will start. This process occurs with smaller steps, these steps are examined with the Paris-Erdogan Law. When the crack reaches a critical point for fracture, plastic deformation will happen suddenly. Thus, many people’s life and airplane were lost due to fatigue failure. There will be many design criteria to prevent this situation. I will explain these designs later.
De-Havilland Accident
The de Havilland DH.106 Comet was the world’s first commercial jet airliner. Developed and manufactured by de Havilland in the United Kingdom, the Comet 1 prototype first flew in 1949. It featured an aerodynamically clean design with four de Havilland Ghost turbojet engines buried in the wing roots, a pressurised cabin, and large windows. For the era, it offered a relatively quiet, comfortable passenger cabin and was commercially promising at its debut in 1952.
Within a year of entering airline service, problems started to emerge, three Comets being lost within twelve months in highly publicised accidents, after suffering catastrophic in-flight break-ups. Two of these were found to be caused by structural failure resulting from metal fatigue in the airframe, a phenomenon not fully understood at the time; the other was due to overstressing of the airframe during flight through severe weather. The Comet was withdrawn from service and extensively tested. Design and construction flaws, including improper riveting and dangerous concentrations of stress around square cut-outs for the ADF (automatic direction finder) antennas were ultimately identified. As a result, the Comet was extensively redesigned, with structural reinforcements and other changes. Rival manufacturers heeded the lessons learned from the Comet when developing their own aircraft.
Although sales never fully recovered, the improved Comet 2 and the prototype Comet 3 culminated in the redesigned Comet 4 series which debuted in 1958 and remained in commercial service until 1981. The Comet was also adapted for a variety of military roles such as VIP, medical and passenger transport, as well as surveillance; the last Comet 4, used as a research platform, made its final flight in 1997. The most extensive modification resulted in a specialised maritime patrol derivative, the Hawker Siddeley Nimrod, which remained in service with the Royal Air Force until 2011, over 60 years after the Comet’s first flight.
wikipedia
Simulations
I prepared some simulations regarding to fatigue behavior. I used same design and different parameters.