A Mental Challenge for Aero Engineers
A Mental Challenge for Aero Engineers
(OP)
Folks...
I'd like to try something different with this thread... and ask a loaded question as a test of YOUR practical engineering/manufacturing/maintenance knowledge.
I been thinking about, and asking others about, this question for almost 20-yrs now. This is a question that is designed to stimulate an awareness and understanding of FAILURE mechanisms, and other practical issues, for ALL engineers. I believe You will find this question looks simple... but don't think that it is: this question has MANY critical aspects.
What is YOUR response to the following question(s)?
It is standard aeronautical practice to: (a) attain 125-microinches** Ra machined finish [or better] on cut and machined edges/surfaces; (2) deburr holes and chamfer/radius edges; and (3) round-off [rasius] sharp [square-ish] exterior and interior corners.
WHY??? What engineering and practical benefits are derived from these standards practices???
[** Sorry...I am not sure what equivalent SI units for 125-Ra surface roughness are. 125-Ra is a typical U.S. Aerospace industry standard requirement.]
NOTE: I promise to provide MY answers to this question in about (2--3) weeks [29 Oct to 8 Nov 02], depending on rate of responses.
Regards, Wil Taylor
I'd like to try something different with this thread... and ask a loaded question as a test of YOUR practical engineering/manufacturing/maintenance knowledge.
I been thinking about, and asking others about, this question for almost 20-yrs now. This is a question that is designed to stimulate an awareness and understanding of FAILURE mechanisms, and other practical issues, for ALL engineers. I believe You will find this question looks simple... but don't think that it is: this question has MANY critical aspects.
What is YOUR response to the following question(s)?
It is standard aeronautical practice to: (a) attain 125-microinches** Ra machined finish [or better] on cut and machined edges/surfaces; (2) deburr holes and chamfer/radius edges; and (3) round-off [rasius] sharp [square-ish] exterior and interior corners.
WHY??? What engineering and practical benefits are derived from these standards practices???
[** Sorry...I am not sure what equivalent SI units for 125-Ra surface roughness are. 125-Ra is a typical U.S. Aerospace industry standard requirement.]
NOTE: I promise to provide MY answers to this question in about (2--3) weeks [29 Oct to 8 Nov 02], depending on rate of responses.
Regards, Wil Taylor





RE: A Mental Challenge for Aero Engineers
(2) Burrs increase stress concentration at hole edges, which already have three times the net section stress at the edge. Therefore, removing burrs decreases stress concentration, which increases fracture resistance and fatigue life. Lastly, burrs can interfere with proper seating of mechanical fasteners, so removing them reduces damage to fasteners and clamped components during assembly.
(3) Sharp corners increase stress concentration, so increasing radii decreases stress concentration, which increases fracture resistance and fatigue life.
Some practical benefits of (2) and (3) are reduced injury to humans contacting the objects.
RE: A Mental Challenge for Aero Engineers
Great start... very descriptive of one aspect!
Aero-Guys...
What are other good reasons exist????
Regards, Wil Taylor
RE: A Mental Challenge for Aero Engineers
Wow,I thought Corypad covered a lot of it. But I would also guess that since you have wires, water and oxygen lines, you would need to have the least amount of sharp edges as possible, to prevent something dangerous happening due to chafing.Also probably rounded off and polished surfaces look a lot better too, appearance wise.
Cheers,
Asanga
RE: A Mental Challenge for Aero Engineers
RE: A Mental Challenge for Aero Engineers
All good points... but there are still MANY more important aspects about this issue.
Hint: this question/issue/problem cuts across a wide span of engineering, fabrication and maintenance disciplines!
Regards, Wil Taylor
RE: A Mental Challenge for Aero Engineers
a) After fatigue life, one of the biggest issues I have with poor surface finish is that it introduces a billion new points for crevice corrosion on the surface. Also, a rough surface can make it difficult to get good results with non-destructive testing methods like die penetrants--especially when the roughness is in a pattern (like flycutting) that will most likely be causing the surface defects in the first place.
Any sort of surface treatment (plating, chemical conversion coat, etc.) will require more material on a rough surface to achieve an equivalent film thickness. Enough to make a difference over a production run.
A rough surface is harder to clean. Nitpicky, yes, but in the words of an old instructor, "Dirt is Weight"
2) A fastener hole with a good, sharp, burred corner will have obvious problems with seating when met with a fastener that has a radiused junction between head and shank.
Also, with riveted structure, friction (due to the clamping force of the fasteners) between faying surfaces in a joint serves a couple important functions. First, the friction provides a bit of 'shear preload'--the joint can take a certain amount of shear without loading the fasteners or sheet in bearing. The greater the friction, the more resistant the joint will be to working loose and smoking rivets. This ties in nicely to the second function: high frequency (engine) vibrations throughout the structure are damped or dissipated through joint friction. The greater the friction, the greater the high frequency fatigue resistance of a mechanically-fastened joint.
The upshot is: if a burr is sitting between the fastened sheets preventing good contact of the faying surfaces, much of this friction is lost.
3) Sharp edges and corners make any sort of forging or casting of a part difficult if not impossible.
Sharp outside corners on structure act as electrical charge concentrators, and can be a static discharge hazard. For the same reason, sharp corners can cause undesirable results in electroplating operations.
Sharp inside corners, in addition to being stress concentrators, are hard to clean (Dirt is Weight), and can collect pools of salty bilge water or alkaline cleaners and such.
I hope this came across well, I'm not having a very lucid day. Whatever the case, more material for discussion.
Regards
RE: A Mental Challenge for Aero Engineers
Your post was very good. I would even say lucid. I think Wil is getting some great stuff here.
RE: A Mental Challenge for Aero Engineers
Guys, Keep it up... there are still more facets to this problem just hiding in the wings to be uncovered!
Regards, Wil Taylor
RE: A Mental Challenge for Aero Engineers
However, also I think that in a high finished surface will be more easy to find cracks with visual inspections, than in a rough face...
Also, it will take less primer (i.e. alluminium parts) if the surface is not rough, and again we are talking about WEIGHT.
There is a special case, when the parts are subjected to relative movement. Obviously, the mating faces must be very fine machined to:
a) avoid friction
b) avoid heat due to friction. Excessive heat may change the properties of the material surface, with unpredictable consequences.
c) better lubrication. The active film in a fine machined surface will be more efficient because there will be more surface in contact with the lubricant. This will permit better heat transfer from the part to the lubricant
d) Excessive roughness may develop high material wear, leading to high play, and high replace frequencies of the parts.
Let me know what think you about the above...
Regards,
Sante
RE: A Mental Challenge for Aero Engineers
Guys, there are still many more reasons awaiting comment!
Hint: some of the other reasons have yet to be identified; while others are significant refinements/variations of existing comments.
Regards, Wil Taylor
RE: A Mental Challenge for Aero Engineers
Please excuse me if I am totally incorrect, as this is without any knowledge of physics, flow dynamics or any other of the staples of aero/astro engineering. If someone pleases, could they help me in my selection process of a good engineering school?
RE: A Mental Challenge for Aero Engineers
You've made a couple of worthy contributions! Great!
Everyone... there's still more to be brought to light! Keep it up!
Regards, Wil Taylor
RE: A Mental Challenge for Aero Engineers
If the fastener holes that are deburred are inspected with the use of HFEC (High Frequency Eddy Current) prior to installation into the aircraft, could those qualify as being zero timed?? I was looking through an older Boeing Structural Repair Document (D6-81987) and there was a statement about increasing the inspection threshold of these fastners (if the holes were zero timed) from 60,000 flight cycles to 100,000 flight cycles (only the Boeing 737-200 aircraft was mentioned). I bet that is a significant reduction of cost for a maintenance shop. Here of course they had specified an increase in '1/16 inch in diameter' which might not be practical in the case of smaller holes.
Anyway thanks to your question, I am starting to look at these maintenance books with a little more motivation
Cheers.
Asanga
RE: A Mental Challenge for Aero Engineers
Your question does not track with the question I posed. Do You have any Comments to add? Based on the nature of Your question, I can sense You have a feeling for the potential seriousness of "burrs" and flaws!!! Add Your 2-Cents worth! There are still some serious issues of regarding affects of "burrs and sharp corners/edges" to be addressed.
Another Hint for all: EE/EL types should take this topic very seriously!
Asanga...
You have raised an interesting and VERY serious question: Please paste Your question [above] into a NEW POST for separate discussion!!!! Note: I used to watch Forrest Service S-2's, C-130s and PBYs getting "zero-timed" at a shop in Ca without removing permanent fasteners is critical locations... then I learned about DADTA... and Rouge Flaws. Yeoooouch.
Regards, Wil Taylor
RE: A Mental Challenge for Aero Engineers
This goes along the EE route mentioned above as well as other posts.
Roughness can equal friction as stated above. Friction can lead to electricity (Tribo-electric effect). Electricity can lead to corrosion.
That's all I have for now.
Tony
To pre-aero, University of Cincinnati (Good co-op program), Georgia Tech, St. Louis University, Purdue are all good schools for aero engineering. There are many many more, but I'm midwestern based so I'm midwestern biased.
Check the web, and look for aero design competitions, look for what schools participate and win, that should be a good indication of a decent school.
RE: A Mental Challenge for Aero Engineers
Come-on now, try harder!!!! You almost had a good point, except Your logic trail fell apart.
I'll expand the last EE-EL hint just-a-bit: "resistance".
Note for pre-aero: I went to Cal Poly [CPSU] San Luis Obispo [71-76]. Great Aero dept and getting better... but entry qualifications are stiff! Understand Cal Poly Pomona and Embry Riddle Aero departments are class acts also.
Regards, Wil Taylor
RE: A Mental Challenge for Aero Engineers
Burrs and surface roughness will both interfere with good, uniform surface contact between faying surfaces in a mechanical joint. This, of course, increases the electrical resistance of the joint and, if severe, can cause problems with electrical bonding of structure; interfering with effective grounding of electrical equipment and/or antennae, and become a miniature plasma cutter in the event of a lightning strike.
Regards
RE: A Mental Challenge for Aero Engineers
You found a very compact/precise way to describe this important facet of the "burrs and surface roughness" question!!!
NOTE: there is at least one other problem that burrs and roughness induce relative to the bonding/grounding comments within the previous thread. Astroclone alluded to it, but missed the mark, slightly. What is it??
Also there still are a few other issues that haven't been addressed (or haven't been fully addressed), yet!!!
The Famous Aero engineer, John Thorp, often stated that engineers must take calculated [not stupid] risks to "move forward and make thing happen". One of his favorite quotes was: "The tortise only makes progress when it's neck is stuck-out".
Who else willing to "risk" presenting their thoughts/ideas on this topic??
Regards, Wil Taylor
RE: A Mental Challenge for Aero Engineers
(1) Rougher surfaces or sharper exterior edges can scratch coated or painted surfaces during assembly and might allow hidden corrosion to spread underneath what might temporarily appear as good finishes.
(2) Or, the scratches of item 1 could create an accelerated galvanic-corrosion anode or cathode site, if all (most) other surfaces are coated (insulated).
(3) Higher values of surface roughness (and burrs) increase leakage rate under/around gaskets and seals.
(4) Nipping gaskets, seals, and o-rings on sharp edges during installation, or scouring them on rougher surfaces during operation of rotating equipment, could accelerate leakage of item 3.
(5) i278 already mentioned hard to clean, so I can't mention increased air drag or turbulence over rougher, dirtier surfaces, or something to that effect, without being redundant.
(6) Rougher surfaces could create a better adhesive surface to build up (and hold on to) more ice, overstressing the structure or adversely affecting operation.
(7) If water creeps under interfaces via higher surface roughness and fills up a cavity or interface, then freezes, it could create high stresses and/or accelerate material fracture, not to mention stress corrosion cracking at scores from the hidden, trapped water/chemicals (i278 already alluded to that one, too).
RE: A Mental Challenge for Aero Engineers
You've defined more facets of the question/problem... although vonlueke stretched a couple of aero points beyond where I would have imagined [more of a coatings quality issue... although I've seen atrocious/rough paint jobs that affected acft performance].
I'll throw-out a hint that should help expand the conversation: "stress analysis and gapping".
Regards, Wil Taylor
RE: A Mental Challenge for Aero Engineers
RE: A Mental Challenge for Aero Engineers
You nailed a problem I have seen repeatedly in the field: poor "workmanship habits" affecting parts fit-up and load transfer. This is the classic "shanking and sheet gapping” syndrome, caused by burrs and "liberated burrs" [chips].
Again, though, I have to ask... What else????
Regards, Wil Taylor
RE: A Mental Challenge for Aero Engineers
You send a piece out to be fabricated. You write a requisition specifying that parts be fabricated as per your drawing, and you list the drawing and its revision number. Your drawing has a note on it stating that it has been done to the ASME Y14.5-1994 standard.
All of this works out as a legal contract that defines what you will accept when the vendor leaves something on your loading dock.
If you don't specify a surface finish, then you must be prepared to accept any surface finish the vendor sends you, like, say, 500RMS? Actually, I am inclined to specify 63 as a standard finish. You should look at a surface comparator sometime. 125 is not that good.
Your drawings show sharp inside corners, and you know full well that machine tools cannot cut these. You must know what the maximum radius is going to me if you have closely fitted parts. The worst thing that can happen when you specify them is that the machinist will call you and tell you he can't do it. Now, you will have to fix your mating chamfers and radii. :(
Burrs and sharp edges are fairly obvious, and have been discussed, above.
JHG
RE: A Mental Challenge for Aero Engineers
You indirectly raised a couple of great points...
a) that mechanical-finishes cost big Buck$$$ and should be thoughtfully specified so that they MATCH the engineering requirements. Otherwise You probably will get what you asked for... but not what you wanted... or You can pay-too much for no value-added.
and
b) mechanical parts fit-up [nesting] can be grossly affected by these factors (including the allowable tolerances).
OH yeah... I am very concerned about verifying the quality of mechanical finishes! I've come close to a couple of fist-fights on the shop floor regarding "wild-assed" thumb-in-the-air estimates of finish quality!!! I NOW own (3) high quality surface roughness comparators from GAR Electroforming << http://www.garelectroforming.com/ >>
SH-6 [shot-blasted surface roughness]
S-22 [general-machined surface roughness]
G-63 [ID & OD (hole & lathe) turned finish surface roughness]
BUT, thats another war-story...
All points about burrs, sharp corners and surface roughness have NOT been raised Yet... Time for another hint:
"shot-peening"
Regards, Wil Taylor
RE: A Mental Challenge for Aero Engineers
As others have already mentioned, joint problems can arise from excessive surface roughness, and over-peening is yet another method for creating surface roughness.
RE: A Mental Challenge for Aero Engineers
Isn't it November yet Will???
Cheers
Asanga
RE: A Mental Challenge for Aero Engineers
Not exactly the answers I was fishing for.. but good added info!
At the end of this week I'll put my money where my mouth is and give my answers to this "question".
OK, this is going to be my last hint. It is a simple quote from and "old" M&P mentor...
"You can't paint a sharp edge"
Regards, Wil Taylor
RE: A Mental Challenge for Aero Engineers
RE: A Mental Challenge for Aero Engineers
I'm not sure what else you were looking for with regards to shot peening, but I will comment on the painting a sharp edge part. The physics, electrochemistry, etc. are well documented about applying a coating to a sharp edge. When using any type of electrically catalyzed process (anodizing, electrocoating, electrostatic spray painting, etc.) current density fluctuations prevent the build-up of a uniform coating thickness. Variations in coating thickness have many negative aspects, such as variable friction at joint surfaces, areas for localized corrosion/pitting/galvanic cells, etc. Corrosion fatigue and stress corrosion cracking are obvious concerns.
RE: A Mental Challenge for Aero Engineers
In structural analysis of parts, etc., the calculations do not take account about surface finishings... (at least, I think so... I have not so much experience in advanced structural analysis)... I do not know any equations or mathematical theory or tabulations about exact stress distribution in rough surfaces. Only what we can say is that there will be a high probability of all situations that every one has described in all previous contributions.
So, when we make the calculations, or when the computer programs make F.E.M. calculations (or other method calculation...) they do not take account of roughness, and it is supposed that the surface is perfectly finished. Only what we can do are estimations of roughness effects based in experience and fracture mechanics theory, for example.
So, if the surface is highly finished, the behavior of the surface, part, etc., will match better with the mathematical model, because the surface finishing is "quite perfectly smooth"...
Ok, I do not know if what I just wrote is all true, so I will wait replies to my contribution!
Regards,
Sante
RE: A Mental Challenge for Aero Engineers
I have not forgotten my promise to answer this question... by the end of this week. Please be patient... working OT has eating-up my spare [writing] time... I'll have my "comments" by Monday AM.
Regards, Wil Taylor
RE: A Mental Challenge for Aero Engineers
RE: A Mental Challenge for Aero Engineers
And the question is...
It is standard aeronautical practice to: (a) attain 125-microinches** Ra machined finish [or better] on cut and machined edges/surfaces; (2) deburr holes and chamfer/radius edges; and (3) round-off [rasius] sharp [square-ish] exterior and interior corners.
WHY??? What engineering and practical benefits are derived from these standards practices???
Ok Guys.. Just a little-bit longer... Hang-in-there!
Regards, Wil Taylor
RE: A Mental Challenge for Aero Engineers
So you don't rip your trousers, sleeves or skin when walking past or or working in confined spaces.
RE: A Mental Challenge for Aero Engineers
My penniesworth...
Jesus is The TRUTH
RE: A Mental Challenge for Aero Engineers
Come on already. Monday am has come and gone so maybe I should ramble and redundatize. Many good points have been reiterated but usually with a little twist adding some new flavor. In my previous reply, I take it for granted that most everyone understands that the hole punching force of high current density results in stress risers to enhance SCC and corrosion fatigue.
Rough surfaces provide less surface area of contact giving rise to higher and very localized contact stresses. If flavored with a little salt mixed in and throw in some corrosion this could be dynamite. Is this way redundant all over again?
Jesus is The TRUTH
RE: A Mental Challenge for Aero Engineers
here is my idea:
Sharp corners, burr holes etc. increase not only the stress but the strain as well.
Looking at the strain we can have three different situations:
1. The strain can be inside the linear behavior. (Under the yield limit)
2. The strain can be between the ultimate and the yield limit.
3. The strain can reach the ultimate limit
If the third situation is going to occur, the cracks can develop because the material failure. In this case, the crack can also reach the its “critical value”.
For this reasons, round the corners, deburr the holes, finish better the surfaces will help to pass from the third to the first situation.
Giuseppe
RE: A Mental Challenge for Aero Engineers
In the design of extreme Hi-pressure equipment we also use a single finish/machining specification on all high pressure (in excess of 10,000 psi)components. This simplifies and makes it easy and understandable for all parties concerned what is required, when and where. From the fabrication specialist thru quality control to the millwrights, welders, pipefitters, and other assembly specialists, including the "woodpeckers" putting the equipment together.
Have a nice day!
saxon
RE: A Mental Challenge for Aero Engineers
RE: A Mental Challenge for Aero Engineers
I am inclined to believe that “wk” stands for “wicked” and I will definitely draw such conclusion if you don’t satisfy our curiosity and give us the promised explanation, tomorrow.
Sir William this time I am teasing a little bit but you have been teasing us for more than a month and I’ve become really curious. My personal opinion is that the major reason for all those features is a reducion of stress concentration, namely fatigue resistance improvement.
Although I have designated myself as “aeronautics” the more correct choice would be “propulsion” (unfortunately not offered as an option) in other words I am rather familiar with turbojet engines. The mentioned practices are a must in the manufacturing of turbojet parts. In particular concerning dynamic parts which suffer from L(ow) C(yclic) F(atigue). Even major casings (eg: Combustion Chamber Outer Casing) could be “victims” of LCF failure. I do remember a failure of a CCOC because of a poor surface finish. Since it is a kind of “pressure vessel” (modern engine compressor pressure ratio is above 20:1) a few machining notches initiated cracks and finally a LCF failure of the casing occurred.
Mr Taylor it is your turn now.
RE: A Mental Challenge for Aero Engineers
RE: A Mental Challenge for Aero Engineers
I don’t know if I am too late or I am repetitious, but here is my cent:
I think that one other reason to decrease roughness, have rounded edges in holes, deburr, etc, is to Increase Friction. As far as I remember, a bolt or a rivet should fasten through means of an increased normal force, and therefore increased friction between surfaces to be joined.
If you have had any experience with very polished surfaces you will have noted the adhesive effect present. In fact, when the surfaces are not rough, there are less “peaks” to be cracked and valleys to be filled, everything resembles more like low dunes, and there are more contact points between the surfaces.
While the absence of roughness points to microscopically flatness, the absence of burrs, points to a macroscopically flatness, much in the same direction. These burrs will flatten in a non controlled way over time loosen the connection, and increasing all the factors above mentioned.
Sancat
RE: A Mental Challenge for Aero Engineers
Bruv
RE: A Mental Challenge for Aero Engineers
One thing is for sure, it has been a long interesting discussion! I feel sorry to get in so late.
Mr. Taylor you are asked to take the "micro" and address to the forum the conclusions in a clear way, some of which you knew since the very beginning, of course enriched with some find answers, you got here.
Let me give an humble contribuition, following BRUV's last reply, just a few closing general thougths:
i)Speaking about aerodynamic characteristics, sometimes you need some roughness level, not necessary and not always quasi-smooth surfaces, all over the aircraft.
ii)The aerodynamics characteristics distribuition, on the aircraft surfaces, must be in balance with coherent and optimized surface treatment(s), that applies for all components.
iii) When you design, build an aircraft, you buid an "entity" with initial physical characteristics, with initial forces, effects inside it, with an history, which must deal correctly along its life-time, with all random involving working conditions concerning.
Best Regards
zzzo
RE: A Mental Challenge for Aero Engineers
Before somebody starts looking for a tree to hang me from, let me add that I've worked with a couple aircraft design folks but not had the pleasure of actually working on a project. I have done designs where a few 'A' size pencil drawings could get a $50,000 dollar order out the door, to the complex CAD drawings required to build a motorcycle. The aircraft industry is unique in that if a part fails, there are many lives at risk. Further, the history of this set of finish criteria provides a solid legal footing when one of those parasites (lawyers) decides to pad his pockets at our expense.
This may be the Best place for "If it works, don't fix it!"
Keep the wheels on the ground
Bob
showshine@aol.com
RE: A Mental Challenge for Aero Engineers
If you have a sharp corner, and you blast it, you 'roll' the corner over, creating a sort of burr which is bad for most of the reasons already mentioned up above.
Other reasons would be wear on sharp edges, or burrs breaking off through usage and contaminating whatever they are part of. (Not good for pipework)
But of course, there are times when a sharp edge is needed. Labyrinth seals in gas turbines spring to mind, as do squealer tips on compressor blades....
Excessive accuaracy is a sign of poor breeding. -Socrates.
RE: A Mental Challenge for Aero Engineers
OK, my 2 cents worth. Those three aeronautical practices are to minimize stress concentration and intensity and instead spread them evenly so as to not create any easy point for failure.
How's that for a summary? Are you gonna take us up on the answer, Taylor?
RE: A Mental Challenge for Aero Engineers
RE: A Mental Challenge for Aero Engineers
My regrets for not communicating sooner. Most of November and December have been terribly unnerving and distracting for me.
My Dad was crying when he called to inform me that my Mother was on the edge of dying in early November. She was helo evacuated from their AZ desert home to Phoenix for a number of exotic tests and treatments for her heart disease. She was finally allowed to go home in mid-late November with an “ICD” implant... then, I had a terrible cold so I was unable to visit her for another (2) weeks ( early Dec). I am afraid that her “full” recovery at 78-yrs is not going to happen… and my 84-yr+ Dad is not “up” to the challenge of caring for her. My sister and I are now faced with the many challenges of helping with our folks deal with their new/stark realities, at long distance.
And on top of all this I still had to work some OT.
I am trying to reconstruct my original answer, which was corrupted in early Nov when I saved the wrong file and “blew-away the good file”. I am sure this has never happened to anyone else. I HOPE have my “long-awaited answer“ by the end of this week… before the Christmas holidays.
Regards, Wil
PS: I still haven't reviewed all the latest imputs... plz be patient.
Regards, Wil Taylor
RE: A Mental Challenge for Aero Engineers
RE: A Mental Challenge for Aero Engineers
RE: A Mental Challenge for Aero Engineers
I hope that you have had some good fortune with your folks wktaylor
Now my contribution to the "puzzle" If an overly rough surface causes corrosion could this joint develop a static charge? The two conductive metal surfaces separated by a dielectric (oxide). Add some movement/vibration and hey presto....static charge. I am not in the aircraft biz but I do get to enjoy the outcome of big static discharges on occasion
Ben Roberts
Broadcast Engineer
Sentient being.
RE: A Mental Challenge for Aero Engineers
yates
RE: A Mental Challenge for Aero Engineers
RE: A Mental Challenge for Aero Engineers
This was one nice exercise he started.
Regards, David
RE: A Mental Challenge for Aero Engineers
It is WISE to understand that one is dealing with a set of well founded assumptions with STANDARD PRACTICES. Whilst they are often a good starting point for design, it is important to recognise that PARTICULAR circumstances may dictate the selection of different practices for particular applications.
Just by-the by, with damage toleranct design now the accepted standard for new aircraft (the new standard practice as it were):
1. don't forget that safe-life and fail-safe approaches still have perfectly valid applications, just not everywhere, and,
2. that each manufacturer has their own suite of standard practices (usually detailed in the SRM ) for hole preparation and fastener installation that result in a presumed joint condition for the purposes of test conduct and the setting of inspection thresholds and intervals. eg. HLT type fasteners which burnish the hole during installation which should not be replaced by HL types without specific clearance, since the HL's do not have the same effect upon the hole.
It has been noted by many people over many years that paint will not provide good cover on a sharp corner, and will fail first on a sharp corner. I have no doubt that some of the esoteric explanations above for why this occurs are accurate, but try explaining the reasons to a painter or a sheet-metal worker and watch their eyes glaze over. The use of a standard practice is easily understood, and readily complied with.
Mr Tayor appears to have some significant personal issues on his plate at present. I wish him well and hope that his parents are not still distressed. I am in Australia, but if there is anything that I can do to assist I would be pleased to do so.
RE: A Mental Challenge for Aero Engineers
Come to meet us again, just come up and say something!
All the best to you, and this very peculiar and really interesting forum,
zzzo
RE: A Mental Challenge for Aero Engineers
Been away a while myself, and was sorry to read your post from last December about your mother's illness. Speaking for all of us who've enjoyed your helpful and professional comments, we send our best hopes for you and your family. I will include you all in my prayers.
RE: A Mental Challenge for Aero Engineers
RE: A Mental Challenge for Aero Engineers
I hope all is well on the home front and your doing well.
After seeing your challenge, I thought what the heck why not??
Almost without exception fatigue cracks start @ the surface of a part rather than internally. One possible reason may be that the highest stresses are usually found at the surface (e.g. bending and torsion) and the surface is vulnerable to stress raisers such as maching notches, scratches and pits. Surface finish affects the strength of a part subjected to fatigue loading because most machining operations leave a notch pattern and fatigue cracks usually originate in a notch. 125 is a a difficult machine finish on steel except on open flat or cylnd surfaces.
This finish I was thought is acceptable on
a) AL ALLOYS
B) STEEL HEAT TREATED OVER 200,000 PSI.
C) STEEL SURFACES IN CONTACT WITH AL ALLOYS EXCEPT WHEN IN CONTACT WITH HIGHLY STRESSED AREAS OF A 7075 PART.
A sharp edge can be compared to turbulence that exists around a rock in a stream (fluid flow analogy) therfore sharp edges are stress risers and usually the place where cracks begin. So if we use a radius, chamfer, and deburr holes we lower stress concentrations.
Well I tried!! lol
RE: A Mental Challenge for Aero Engineers
RE: A Mental Challenge for Aero Engineers
When I was taking history in high school, the teacher would walk around with a $20 bill in his hands and snap it. To this day I can still see this image and I can rememebr how 25 students watched in facination. He would then take a pin knife and knick the center edge and snap it while again circulating through the class room. As you can imagine, the bill eventually snapped in half. He had made his point. He had a lot of scotch tape too. Why a History teacher? The Industrial Revolution was our topic.
RE: A Mental Challenge for Aero Engineers
There will also be a deburring-surface finishing technical session at the WESTEC MANUFACTURING CONFERENCE, in Los Angeles, CA on Apr. 6. During this session, a paper will probably be presented, which deals with additional issues that have not been brought up yet by this thread.
RE: A Mental Challenge for Aero Engineers
On the mating surfaces fretting will be an issue with rough surfaces. The fretting will expose the grain inherently causing premature stress corrosion.
Also, a rough surface finish may cause problems with rejection indications during final NDI such as Dye Pen or Eddy Current.
RE: A Mental Challenge for Aero Engineers
Just under 2 cents worth!
RE: A Mental Challenge for Aero Engineers
During the war, aircraft manufacturing work was farmed out to many companies with no aerospace background and many practices were introduced to account for what these companies were actually capable of. By the time the war was over, these had magically become "standard" and we were then stuck with them until we started going metric (which means that a lot of us are still using them!)
If Beethoven had been killed in a plane crash at the age of 22, it would have changed the history of music...
and of aviation.
RE: A Mental Challenge for Aero Engineers
David A. Davidson
[DESC] Deburring Technical Group
SOCIETY OF MANUFACTURING ENGINEERS
www.deburringsolutions.com