American Welding Society Forum
After the disaster that happened to the Columbia, which shocked all the civilized world, television has shown here in Brazil some moments of a survey that was made on the Columbia prior to its launching.
During the survey, quite a few cracks measuring up to 1 inch in length were discovered on the outside surface of the shuttle. The television went on showing that the cracks were repaired by TIG welding. Before and after welding the region was carefully cleaned by means of a wire brush. For us who are familiar with welding operations there was no mistery; the general public I suppose didn't understand anything of what was going on.
My question, Gentlemen who are welding specialists, is this:
is it possible that the welding of the cracks changed the metallurgical structure of the HAZ (grain size, martensite presence etc.) to such an extent that fatigue after a flight of 16 days produced the break of the affected region, which in turn produced the disaster?
Giovanni S. Crisi
Sao Paulo - Brazil
When someone mentions fatigue I tend to think of it like bending a wire coat hanger back and forth until it breaks. Basically, the metal is cold worked to the point it hardens, becomes very brittle and breaks. If you were to later weld on such material, my guess is that the welding would tend to anneal the material and restore some of the original properties. Weld overlays have been used successfully to restore pressure boundary integrity of piping affected by stress corrosion cracking and fatigue cracking in power plants for many years and their performance has been good.
Having said this however, there is much unknown about the details of why they chose to weld the cracks, what their cause was, how critical the location was to structural integrity, and what base metal and filler metal were involved. Is it possible the effects of welding contributed to the failure? It is feasible that some type of undesireable microstructure could be produced. It doesn't seem very likely given the amount of attention that NASA devotes to understanding materials and the rigorous welding procedure qualification requirements of that industry.
Good question Giovanni.
Just want to streighten out your understanding of fatigue. (Hope you don't mind.) Bending a wire coat hanger repeatedly till it breaks is NOT fatigue. It is excessive cold work leading to work hardening to the point that the ductility is lost, resulting in brittle fracture.
By definition, fatigue occurrs when a material is repeatedly loaded to a stress BELOW the material's yield point. In most cases this is a stress significantly below the yield point. In the case of the coat hanger, the stress is obviously above the yield strength.
In the case of fatigue, cracks are initiated and grow. Any thermal treatment of the material does NOT repair the cracks. Once the defects are there, they are there.
You are correct of course. I guess I was thinking about the alternating stress from bending in opposite directions and failure after a number of bending cycles as being similar to fatigue failure. But fatigue does occur at stresses below the yield point and this example does not fit the definition of fatigue. I stand corrected. Thanks for pointing out the difference.
What ever happend to the Shuttle will likely be lost in the fact the parts that are left are scattered over hundreds of miles. Speculation will likely have to do when trying to figure out what may have happend. With all that expense$$ and pressures to meet schedules and deadlines, I hope something wasn't overlooked or covered up to keep this thing on schedule for lift off. Keeping tiles on the bottom of this thing seem to be a problem from the very first shuttle. Glue don't seem to like temps of over 3000F. NASA has strict welding procedures and quality controls in place, I hope welding wasn't the cause of the thing coming apart minutes before landing. I wonder what types of materials(metals) are used for the structure, and skins of the shuttle?
There also were cracks found in the propellant pipe liners that fed the main engines. I don't think these cracks would have caused a re-entry mishap, but it does point out that the shuttles aren't getting any younger.
There were some articles in Aviation Week and Space Technology that covered the discovery of the cracks, and the subsequent weld repairs.
Hi Prof Crisi
In entering this speculative reasoning, we must of course be sensitive to the feeling of some of the members of this board regarding this matter. (Especially the Americans) Before going any further, I want to express my regret at what happened.
In addition, we must understand that we are only speculating here, and as such it is a purely theoretical exercise.
To answer the question, yes it could have had a major influence, especially if the outer surface has been made from an Al alloy. Does anyone know what these outside "skins" of the shuttles are made from?
In addition, if the material cracked in this area, it must surely have a high stress acting in this area. Merely welding it up would have a negative effect on the strength of this area. If these were fatigue cracks (again speculating) then it is a sign that the fatigue life of this component had been exceeded. My experience on structures operating in fatigue has shown that once this point is reached, repairs are not effective in obtaining long term positive results. It will just crack again somewhere else.
It will certainly be interesting to know why the disaster occurred.
Thanks, guys for being sympethetic towards our(american's) feelings. Being in America and knowing and seeing first hand how "$$big projects$$" put alot of pressure on people to push to stay on schedule due to the enormous expense, I only hope patching, to the skin fractures, wasn't done to keep on schedule, rather than halting things until better solutions could have been made. After reading the papers and articles about how the shuttle had lots of resistance on the left side of the craft, I wonder if tiles were missing or fractures to the skin was causing this craft to try and turn left before breaking up. The speed at which this craft enters the atmosphere must stress the skin enormously. The skin temp must go through a wide heat range as well(severe cold to over 3000F). I wonder if it bends in and ripples as the air is forced to go around it. If this is the case, then fatigue could certainly be the cause of cracking, and as others have stated, welding up these fractures may only cause it to fail elsewhere. Speculation can make your mind run wild sometimes, so I'll stop typing for now, because I too am only speculating. Others on this board are more familiar with aviation and the types of stresses that an aircraft encounters, could possibly give better explanation for the skin fractures. These shuttles are given a life expectancy of 100 trips in and out of orbit, I wonder if NASA will refigure the data or improve on the materials used so these 100 trips could become a reality.
I believe such procedure were looked into carefully and thoroughly. My guess is a ramdom factor is the cause of the disaster, ie a bird strike or a impact of something in space.
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