Powerandpassion
New Member
My interest is 1930's steel framed aircraft using tubular steel construction with steel plate gussets/fish plates. These structural concepts carry through to many contemporary designs. CAD drafting, 3D laser scanning and laser cutting are wonderful productivity tools that allow the recreation of small runs of antique steel fuselage members. This would have been cost prohibitive or impossible to imagine ten years ago. These tools allow the recreation of wonderful aircraft. Anecdotally , I have acquired an impression that laser cut parts are not accepted in aviation circles. At core, I surmise that heat affected zones at laser cut boundaries are considered to render such parts unacceptable.
In the course of other engineering work I conduct business with laser cutters producing work for racing teams using 4130 tubes. I would accept that the forces that racing car bodies are subjected to are equal or in excess to those that aircraft are subjected to. Laser cutters report no issue with the supply of laser cut 4130 components into the car racing industry.
I can understand that new technologies can be too complex for some to assimilate, and that a defensive response is to ride on the wings of hard earned credibility to dismiss an approach that requires you to launch a boat on unknown seas. This is OK, as long as it does not disadvantage or discourage progress. I can understand that the laser cutting technology of 2000 may have been cumbersome, and may have adversely affected materials, but I see the laser cutting technology of 2013 as entirely different.
Putting on my Spock ears, the only way I can pecieve that laser cutting may affect a material is via heat. In the case of a high tensile steel, this may soften the material in the heat affected zone. So what ? ( I am not a metallurgist, so this is where a metallurgist can put me right ).
In order to understand where a thought has come from, I try to go to the source. I figure most aeronautical engineers are afraid of things cracking up. Steve Austin, astronaut, a man barely alive. Challenger. Sally Ride. De Havilland Comet, lots of things falling after fluttering wings. I figure that cracks are what keep the engineer awake at night.
I figure stress cracking, or more realistically corrosion/stress cracking is the issue. The segue to laser cutting is the creation of weakened boundary zones on laser cut parts that allow stress cracking to originate. My gut feel is that a soft boundary zone may actually mitigate against the birthing of cracks in fishplates.
I have before me "Ferrous Metallurgy in Aeronautics - A complete survey of the qualities and characteristics of modern steels for Aero Work" by WH Hatfield, Aircraft Engineering May 1935. Relax, I also have a beer. Hatfield was the dude from England's Vicker's steel that I guess most influenced ferrous material selection for things like the Hawker Hurricane, which had high tensile stainless fish plates rivetted to a high tensile steel tube construction. I figure that this stuff was absolutely cutting edge in the 1930's, requiring imaginative and open minded engineers exploring all the possibilities of fascinating new materials and processes. But their virgin, firework thoughts have now become the ossified dogma of the staus quo. I think Hatfield would have been driven to poetry by laser cutting and its possibilities.
I figure the fear of cracking really started with the move from timber based aircraft to metal based aircraft. In the opera citare Hatfield writes about the stresses created by punching and shearing metal parts. The staus quo accepts punched and sheared parts as being OK. The staus quo is comfortable with the creation of expensive punching and shearing tooling by vanishing artists, which, by its cost, holds back aircraft restoration. But the author of the staus quo warns : "there are a number of processes employed in fabrication, particularly with sheet materials, which are liable to leave the parts concerned in a state of internal stress. Any cold pressing or cold forming operation is liable to do so....it should be remembered that stresses of this kind are additional to the service stress, and they are apt to be ignored in calculations when designing."
Put with colour, hidden microscopic stress zones created on material boundaries by punching or shearing operations affected by corrosion develop into microscopic cracks that open up on a sunny day and cause your wings to fall off above a preschool. So perhaps laser cutting may prevent this? Hatfield writes about how they tested for this in 1935 : " Two pieces of 2.5 inch square were cut from high tensile stainless steel strip of two different qualities. Erichsen impressions (The depth of impression in millimeters required to fracture a cupped sheet metal supported on a ring and deformed at the center by a spherically shaped tool) were put on these four samples and, recognising that this left the samples in a state of internal stress, they were placed in a solution designed to cause accelerated corrosion. The solution was 50% HCl cold, and the pieces were left overnight. Numerous small cracks developed in two of the samples."
So perhaps the thing to do is to crop some samples of high tensile steel in a sharp guillotine, a blunt guillotine and via laser cutter, and test them for corrosion cracking on their boundaries as described above. I will bet a six pack of Coopers stout that there will be more stress cracking in the guillotined samples. I would be interested in what you think.
In the course of other engineering work I conduct business with laser cutters producing work for racing teams using 4130 tubes. I would accept that the forces that racing car bodies are subjected to are equal or in excess to those that aircraft are subjected to. Laser cutters report no issue with the supply of laser cut 4130 components into the car racing industry.
I can understand that new technologies can be too complex for some to assimilate, and that a defensive response is to ride on the wings of hard earned credibility to dismiss an approach that requires you to launch a boat on unknown seas. This is OK, as long as it does not disadvantage or discourage progress. I can understand that the laser cutting technology of 2000 may have been cumbersome, and may have adversely affected materials, but I see the laser cutting technology of 2013 as entirely different.
Putting on my Spock ears, the only way I can pecieve that laser cutting may affect a material is via heat. In the case of a high tensile steel, this may soften the material in the heat affected zone. So what ? ( I am not a metallurgist, so this is where a metallurgist can put me right ).
In order to understand where a thought has come from, I try to go to the source. I figure most aeronautical engineers are afraid of things cracking up. Steve Austin, astronaut, a man barely alive. Challenger. Sally Ride. De Havilland Comet, lots of things falling after fluttering wings. I figure that cracks are what keep the engineer awake at night.
I figure stress cracking, or more realistically corrosion/stress cracking is the issue. The segue to laser cutting is the creation of weakened boundary zones on laser cut parts that allow stress cracking to originate. My gut feel is that a soft boundary zone may actually mitigate against the birthing of cracks in fishplates.
I have before me "Ferrous Metallurgy in Aeronautics - A complete survey of the qualities and characteristics of modern steels for Aero Work" by WH Hatfield, Aircraft Engineering May 1935. Relax, I also have a beer. Hatfield was the dude from England's Vicker's steel that I guess most influenced ferrous material selection for things like the Hawker Hurricane, which had high tensile stainless fish plates rivetted to a high tensile steel tube construction. I figure that this stuff was absolutely cutting edge in the 1930's, requiring imaginative and open minded engineers exploring all the possibilities of fascinating new materials and processes. But their virgin, firework thoughts have now become the ossified dogma of the staus quo. I think Hatfield would have been driven to poetry by laser cutting and its possibilities.
I figure the fear of cracking really started with the move from timber based aircraft to metal based aircraft. In the opera citare Hatfield writes about the stresses created by punching and shearing metal parts. The staus quo accepts punched and sheared parts as being OK. The staus quo is comfortable with the creation of expensive punching and shearing tooling by vanishing artists, which, by its cost, holds back aircraft restoration. But the author of the staus quo warns : "there are a number of processes employed in fabrication, particularly with sheet materials, which are liable to leave the parts concerned in a state of internal stress. Any cold pressing or cold forming operation is liable to do so....it should be remembered that stresses of this kind are additional to the service stress, and they are apt to be ignored in calculations when designing."
Put with colour, hidden microscopic stress zones created on material boundaries by punching or shearing operations affected by corrosion develop into microscopic cracks that open up on a sunny day and cause your wings to fall off above a preschool. So perhaps laser cutting may prevent this? Hatfield writes about how they tested for this in 1935 : " Two pieces of 2.5 inch square were cut from high tensile stainless steel strip of two different qualities. Erichsen impressions (The depth of impression in millimeters required to fracture a cupped sheet metal supported on a ring and deformed at the center by a spherically shaped tool) were put on these four samples and, recognising that this left the samples in a state of internal stress, they were placed in a solution designed to cause accelerated corrosion. The solution was 50% HCl cold, and the pieces were left overnight. Numerous small cracks developed in two of the samples."
So perhaps the thing to do is to crop some samples of high tensile steel in a sharp guillotine, a blunt guillotine and via laser cutter, and test them for corrosion cracking on their boundaries as described above. I will bet a six pack of Coopers stout that there will be more stress cracking in the guillotined samples. I would be interested in what you think.
