Riveting technology – "threading the needle and threading the thread" to assemble the fuselage
Ship hull welding can withstand decades of seawater erosion, while aircraft skins are tightly held together by tens of thousands of rivets. Once the riveted structure of an old aircraft is damaged, can people directly use a welding gun to repair it? Many people often take it for granted that it can, but the answer to the actual situation is far from that simple.
Aluminum alloy is as brittle as glass when welded
After the aluminum alloy is welded, the grains inside the heat-affected zone will coarsen, causing the hardness value to drop from about 150 to about 80, and the brittleness will become particularly obvious. Since the strength of the weld area is far lower than the strength of the base material, this is equivalent to forming a weak link called a "soft rib" on the fuselage. When impacted by external forces, this area is the first to cause problems.
Defects such as pores, slag inclusions and lack of fusion often occur during the welding of aluminum alloys, resulting in a much higher repair rate than steel. Even if there are only a few pores of 0.5 mm in size, it is enough to directly reduce the local load-bearing capacity by 70%. This is equivalent to the fact that during flight, can old riveted structures be repaired with welding instead? Can old riveted structures be repaired with welding? It leaves behind hidden dangers that cannot be seen.
Flight vibrations can tear welds apart
During cruise operations, the aircraft skin must withstand repeated stretching movements, and high-frequency vibrations will occur inside the fuselage several times per second. The high and low temperatures of the external ambient temperature must constantly switch alternately. In this state, the welding seam is prone to stress concentration problems, so small micro-cracks will slowly expand and evolve along the position of the welding feet. After experiencing hundreds of thousands or more cyclic load tests, the overall structure of the aircraft can no longer support the load.
In the accident of Japan Airlines Flight 123 in 1978, the riveted area at the rear of the fuselage broke, causing the tail to fall off. In the end, 520 people were killed. Once the weld cracks at high altitude, the consequences of mid-air disintegration are also very fatal and irreversible.
Rivet precision welding is a few streets away
Rivets are stamped in batches on the production line, and the tolerances are controlled at the micron level, so the size of each rivet is almost exactly the same. After they are installed, they become standard parts. The quality is stable and can be traced back to the source. Even if a million of these products are mass-produced , the old riveted structure can be replaced by welding during repairs , and there will be no distortion.
Thermal deformation caused by welders during the operation is uncontrollable. As a result, a long weld may have a deviation of several millimeters, which is manifested as a bend of several millimeters. For an aircraft skin with a thickness of only two millimeters, a deviation of several millimeters means that it is a deformation accident. During assembly, the hole positions simply do not match up.
The rivets can withstand the pressure of 1100 MPa.
In riveted connections, there are no stress concentration points like welds. Therefore, its fatigue life is much higher than that of welding. Under long-term vibration environment, riveted structures are not prone to cracks. Its anti-vibration performance and anti-fatigue performance are more outstanding.
The design pressure of modern passenger aircraft skin riveting joints can reach 1100 MPa. At this level, it is difficult for welded joints to achieve the same reliability and service life. This gap is structural.
If it breaks, just replace one without throwing away the whole rack.
The connection method of welding is a permanent connection. Once a certain place is burned out, the entire skin will be scrapped, and the only way to replace the entire skin is to replace it. But if it is a rivet, you only need to remove the rivet and only replace the problematic single piece. The price of a skin made of composite materials can easily reach hundreds of thousands of yuan, so the cost saved by doing this is not a small amount.
Welding repair work requires preheating, and heat treatment is required after welding. After that, ultrasonic flaw detection is required. The entire cycle time is calculated in days, so the cost of grounding is very high. The riveting repair method only requires a few new nails and the work can be completed on the same day. The efficiency of the ground crew is several times higher than the previous method.
The old riveted structure really cannot be welded casually.
Those aircraft have been in service for more than twenty or thirty years, and the fatigue on the skin has accumulated little by little, the coating has aged, and the rivet holes have been worn. If you directly use a welding gun to weld, it is equivalent to introducing new thermal stress, which is like opening another knife on the old scar. The risk is directly superimposed on the old risk.
According to the clear provisions in the Aviation Maintenance Manual (SRM), if the riveted structure is damaged, the priority should be to repair the riveting or replace the skin section. In addition, the welding process is basically only allowed to be used for decorative parts that are not load-bearing. In load-bearing areas, welding is absolutely not allowed to replace the original riveting process.
When something goes wrong with the structures you have on hand that are made up of old-fashioned riveting methods – the riveting technology – "threading needles and threads" to assemble the fuselage, your first reaction will be to seek help from a riveter, or will you be planning to pick up a welding gun and try to solve the problem through welding?
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