4 ms·
You are right. At the very basic levels we have PT. This is a test with dye sprayed on the weld and a developer sprayed on to show the defects. This is very com
by frankharv 7y ago
You are right.
At the very basic levels we have PT.
This is a test with dye sprayed on the weld and a developer sprayed on to show the defects. This is very commonly done.
Next we have UltraSonic(UT) testing. This requires expensive equipment and a good examiner to interpret.
Third we have X-Ray. This is used on welds that are very important as it is also hard to do and interpret.
It also requires the weld to be ground smooth to distinguish flaws from ordinary weld.
I am sure there are a variety of other non destructive testing (NDT )techniques. These are the common ones used in industry.
I do want to mention that pressure vessels are hard to make. Especially something so large.
This could actually be a design problem and not welds.
Pressure vessels are often tested at 5/3 their working pressure. So failures at testing are not uncommon.
- frankharv 7y agoI really don't understand the build method they use. Ideally you would use a turntable to rotate the cylinder while welding the sections together with a sub-arc machine. https://www.youtube.com/watch?v=j-hfExEmGsE https://www.youtube.com/watch?v=j-hfExEmGsE Elon blaming "settings" seems like a silly explanation. There should be weld engineers and destructive testing done before the first beads are welded on an actual tank.
- jeanvalmarc 7y agoI've set up automated welding cells, and the "settings" excuse sounds totally reasonable to me haha. There are indeed a million settings (weld weave pitch and amplitude, wire feed speed, linear feed speed, gas flow rate, amperage, etc.) that are hard to tune 100% correctly and don't have "analytic solutions" (i.e. you tune some values like a monkey until it gives the result you want). Seems like this tank failure was their "destructive testing," although most people would probably have used smaller samples to save money. As someone else said: pressure vessels are hard!
- gpm 7y ago> Ideally you would use a turntable to rotate the cylinder while welding the sections together with a sub-arc machine. https://www.youtube.com/watch?v=j-hfExEmGsE https://www.youtube.com/watch?v=j-hfExEmGsE Uh, at least in the video that steel is pretty thick. Starships skin is slightly less than 4mm thick... while in that video they have a 2cm deep groove, would the same technique actually work? For what it's worth, ULA stainless steel (<1mm thickness) upper stages are resistance welded instead https://www.youtube.com/watch?v=o0fG_lnVhHw&t=41m10s https://www.youtube.com/watch?v=o0fG_lnVhHw&t=41m10s That said, I think a lot of the decisions right now are based on "what can we quickly experiment with" not "what is a good long term plan". Building a 9m turntable (diameter of the rocket) would take a long time.
- frankharv 7y agoI agree a 9m turntable would take some time to make. But once made you could crank these vessels out very fast. 4mm skin sounds OK and I am sure there are some structural supports inside to keep it from collapsing. I am not a weld engineer so I cannot say if Sub-Arc would be the best technique. I know that ships skins are being built with it.
- erikpukinskis 7y agoYou’re talking about production engineering. You don’t really do that until your design starts to be nailed down. It’s not.
- nraynaud 7y agoTory Bruno in an interview says ULA does between 1.1 and 1.25 safety factor for pressure vessels.
- WalterBright 7y agoBoeing avoids welded seams on airplanes because they are very difficult to reliably inspect. Riveted seams, however, are straightforward to inspect.
- gpm 7y agoI assume riveted seams don't work for pressure vessels? Would the air plane cabin be welded?
- jacquesm 7y agoRivets were the fastener of choice for pressure vessels in the steam age.
- WalterBright 7y agoThe pressure bulkheads in an airliner are riveted.
- anonsivalley652 7y agoDye penetrant is another NDT that highlights cracks for visual inspection too, but it's proven to be less effective than ultrasonic and x-ray (hence the changes some years ago in the inspection and rebuild manuals by AD's for many (it should be all) turbofan engines). I knew a German engineer consultant who did this on piping, welds and pressure vessels in the field for nuclear plants. You can use all three of these NDT techniques, where are used in nuclear energy and turbofan engine applications, but still fail to detect weak welds or metal fatigue because of improper calculations or conditions in the real world. There's no substitute for DT to validate the engineering and manufacturing, such as by overpressing a pressure vessel (which are also covered by ASME BPVC). Oddly enough, I knew a civil engineer (PE) who was an expert witness and wrote code interpretations for BPVC to the tune of $40-60k per for a few hours of work (but having his experience on large industrial projects is an important prerequisite). IIRC, he had a massive house in Saratoga.