Load Paths in a Catamaran

Discussion in 'Multihulls' started by AndrewK, Feb 24, 2009.

  1. Guest625101138

    Guest625101138 Previous Member

    Exactly right.

    But you stated earlier "flexing = fatigue". Fatigue is a change of state. If materials are operated below their fatigue limit there is no fatigue by definition. The attached link is for other's education - you are clearly beyond education because you think you already know everything there is to know:
    http://en.wikipedia.org/wiki/Fatigue_limit#cite_note-BandJ-0
    So providing the stress range is below the fatigue limit (or endurance limit) the material will tolerate it forever - barring of course other modes of failure such as corrosion.

    Would be sad situation if you had to replace every spring every few years. Spring designers ensure the material operates within its endurance/fatigue limit.

    Rick W
     
  2. Ad Hoc
    Joined: Oct 2008
    Posts: 6,379
    Likes: 503, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Rick

    You clearly do not understand the concept of fatigue, despite your protestations to the contrary by stating this "If materials are operated below their fatigue limit there is no fatigue by definition"

    You need to re-read the statement i posted. If you understand this then you will know that a repeated (cyclic or flexing -what ever words you care to select) action on a material will cause it to fail well BELOW its fatigue limit or yield strength.

    You stated if operating below there is no problem. This is 100% incorrect, as the quote provided and endless resources, including my own, can tell you.

    Very briefly, movement of the crystal lattice along well define planes in the crystalline structure, Miller Indices, are either easy, as in FCC materials like copper and aluminium, or rather difficult in others like CPH structure.

    This "movement" is along well define slip plans, these slip planes help to promote dislocations. These planes or line of dislocations once 'exposed' on the material surface cause extrusions and intrusion on the surface. These intrusions and extrusions initiate what is known as the first 3 stages of crack growth.

    1) Initiation
    2) Propogation
    3) failure.

    A single flex, no matter how small will cause a slip and hence a dislocation in the material. This leads to crack initiation and eventually failure. Hence the quote by Pook and numerous others...

    Your knowledge of, especially fatigue, is seriously lacking despite what you try and remonstrate to others about your knowdgle.
     
  3. drmiller100
    Joined: Feb 2009
    Posts: 92
    Likes: 0, Points: 0, Legacy Rep: 2
    Location: Idaho

    drmiller100 Junior Member

    How about we compromise. Everything fatigues, but some things have an infinite life.

    And I think the definition of fatigue limit suggests if we stay below it, the gizmo in question shouldn't fail.
     
  4. Guest625101138

    Guest625101138 Previous Member

    Ad Hoc
    As I said you are beyond education. Please read this carefully:

    "Fatigue limit" is DEFINED as the maximum fluctuating stress a material can endure for an INFINITE number of cycles.

    That is how it is DEFINED.

    INFINITE means forever.

    You just continue to dig a deeper hole so everyone can appreciate how little you really know. I have looked through other threads where you have displayed similar level of ignorance and I believe the forum has lost good contributors because of your pompous stance and unwillingness to learn.

    You have never bothered to acknowledge that you were dead wrong about there being zero moment in the load case I described earlier in the thread. You just brushed it off. No doubt you will just drop this issue on fatigue limit once you realise you are dead wrong.

    You diminish the value of the forum as a source of education where people can get good advice and learn from what others are doing. You continually offer ignorant opinions on subjects you clearly have no idea about. You pretend to be something you are not. You need to be challenged so you begin to think before you offer silly opinions that have their basis in nonsense.

    Rick W
     
  5. Ad Hoc
    Joined: Oct 2008
    Posts: 6,379
    Likes: 503, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Hmmm...let me just review this

    "...WRONG AGAIN. Stress cycling beyond fatigue limit causes fatigue.."

    so a cycle less than fatigue limit does not cause fatigue....so we can all flex away right up the limit and nothing will happen to my glass or ally structure, wonderful, great piece of advice, but wait there is more:

    "..Fatigue is a change of state.."

    Breath taking ignorance!

    Giving advice to others is by providing the theories for others to explore their own designs with. Giving someone numbers provides nothing.

    You consistently want to talk numbers....those who cannot discuss or debate theories are lacking in such knowledge. Hence their input can only be from a practical point of view, such as don't use 6mm plate, (but still offer no 'technical' reasons why, so the comment is somewhat meaningless), or having stiffeners so close together the welder cannot weld them, is another type more in-line with a "numbers" person. This latter is more useful type.

    Numbers people consistently come out with..."how many designs have you done then...or show me your gallery.." All rather pathetic and unprofessional.

    The theory is being discussed for others to understand the mechanisms of failure.

    Those who cannot discuss such issues stick to their very very narrow band of "expertise" ie, their own numbers, and never stray form their "comfort level".

    I'm not hear like you or others to blow their own trumpet, I'm here to just offer advice. One can take it or leave, it doesn't bother me.

    But when there is such obvious and blatant misrepresentation to others it requires rectification.

    I don't need to show how many designs I have done nor my galley...my validation is simply by referring to well known and established references of such theories from any text book,when required if doubted. The 'student' can gauge for themselves if the theory is correct or not and how it may assist them.

    You just keep quoting numbers at everyone....for those who don't understand the mechanisms involved, it sounds like very good advice to them since they too are ignorant of such matters.
     
  6. drmiller100
    Joined: Feb 2009
    Posts: 92
    Likes: 0, Points: 0, Legacy Rep: 2
    Location: Idaho

    drmiller100 Junior Member

    >so a cycle less than fatigue limit does not cause fatigue....so we can all flex away right up the limit and nothing will happen to my glass or ally structure,




    Correct.
     
  7. Guest625101138

    Guest625101138 Previous Member

    Ad Hoc

    You should go away now if you are not prepared to admit you are wrong regarding fatigue limit. It is not sufficient to try to claim high ground because you operate in some la la land of higher science.

    Take a bit of time to look up definitions of fatigue or endurance limit in whatever engineering texts you care to look at and post them here.

    If you really do care about knowledge rather than silly claims to working in some sort of higher plane then post four or five references in your next post. They will support what I have been trying to explain to you.

    Rick W
     
  8. Ad Hoc
    Joined: Oct 2008
    Posts: 6,379
    Likes: 503, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Cutting and pasting links to very poor encyclopedias such as wiki doesn't do any thing for ones credibility.

    So, Mr Numbers, lets see how your theory stacks up.

    fatigue limit of steel 10^6.

    So a vessel at sea encounters a wave say once every 3 secs.
    So that is 10^6 x 3 cycles = > 8333 hours or 347 days.

    So any steel vessel after 1 years service NEVER experiences fatigue according to your theory. Increadible...!!!

    Wow..no steel vessels ever failing through fatigue...amazing!

    But quickly rush and Tell LR, DNV etc...they tend to disagree with you too.

    Oh and for those that have a grasp of reality and how to read a book, rather than just cut and paste, anyone can just peruse these books:

    Pook L.P., “The role of crack growth in metal fatigue”, The Metal Society London, 1983.
    Frost N.E., Marsh K.J., Pook L.P., “Metal Fatigue”, Oxford University Press, 1974, ISBN 019 8561148
    Cyclic Deformation and Fatigue of Metals, Edited by E. Bílý, Material Science Monographs 78, Elsevier 1993, ISBN 0 444 98790 8
    Anderson J.C., Leaver K.D., Rawlings R.D., Alexander J.M., “Materials Science, 3rd Edition”, 1985, Van Nostrand Reinhold (UK). ISBN 0-442-30626.
    Liebowitz H., “Fracture” – An Advanced Treatise, Volume 1, Academic Press Inc. New York, 1968.
    McGregor Tegart W.J., “Elements of Mechanical Metallurgy”, MacMillan Series in Materials Science, The MacMillan Company, New York, Second Printing 1967.
    Felbeck D.K., Atkins A.G., “Strength and Fracture of Engineering Solids”, Prentice-Hall Inc, N.J. USA, 1984, ISBN 0 13 851709 6
    Fuchs H.O., Stephens R.I., “Metal Fatigue in Engineering”, A Wiley-Interscience Publication, 1980, ISBN 0 471 05264 7
    Broek D., “Elementary Engineering Fracture Mechanics”, Noordhoff International Publishing, Leyden, 1974.

    And yes, i ahve read these books..most are in my own library!
     
  9. Guest625101138

    Guest625101138 Previous Member

    Fatigue limit is a STRESS value not a number of cycles. So this conclusion is also wrong (sorry drmiller). It is the safe level of stress to cycle material without the risk of fatigue failure.

    It is the stress RANGE for the cyclic stress. So positive and negative cyclic stresses have to be added.

    If the material has steady preload then this will reduce the fatigue limit -there are published diagrams called Goodman Diagrams that give the fatigue limit under prestressed condition.

    The figures given are always for parent material at specified tempers. If the material has been welded the fatigue limit will usually be much lower. Heat treating after welding will increase fatigue limit but it is usually deemed indeterminant so cannot be relied on. For welded steel a safe endurance limit is around 10% of the parent metal UTS. The endurance limit for parent metal is usually around 40% of UTS.

    I hope this helps for the understanding of others who are bothering to follow this thread.

    So Ad Hoc there is a huge body of research on fatigue/endurance limit and you simply deny there is such a concept - your ignorance is beyond comprehension. How do think people design springs - cross their fingers and hope???? How do you think people design any structure for that matter?? There are very few where fatigue does not play an important role in the calculations. It is a major issue for alloy boats and knowledgeable competent designers are very well aware of the endurance limit of aluminium. Often it becomes the key determinant of weight.

    Rick W
     
  10. drmiller100
    Joined: Feb 2009
    Posts: 92
    Likes: 0, Points: 0, Legacy Rep: 2
    Location: Idaho

    drmiller100 Junior Member

    Lets try this.

    I have a 10 foot long piece of hardened spring steel, a half inch thick, and 2 inches wide.

    I put a one pound stress applied to the middle of it, repeatedly for 10 with 10 zeroes behind it.

    will it fail????
     
  11. drmiller100
    Joined: Feb 2009
    Posts: 92
    Likes: 0, Points: 0, Legacy Rep: 2
    Location: Idaho

    drmiller100 Junior Member

    My understanding was eventual failure would be a function of the stress and number of cycles. One limit would be failure on the first cycle, the other limit would be infinity.

    Does "stress" include the cycles when talking fatigue???
     
  12. Ad Hoc
    Joined: Oct 2008
    Posts: 6,379
    Likes: 503, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    what mechanical properties of the steel...any temper...welded or unwelded, how is it attached, how is the load applied, what kind of loading constant amplitude or constant stress etc etc??

    Need a bit more info than that....but hey ask mr numbers, he seems to to think he knows more than the current known laws of physics and their definitions.
     
  13. Guest625101138

    Guest625101138 Previous Member

    Ad Hoc

    I REPEAT - fatigue/endrance is a stress RANGE. You seem to think it is related to the number of cycles. The only number of cycles it relates to is INFINITY. You have not taken in anything I have stated above you are so keen to defend your ignorant statements.

    Go though the books you have referenced if you have any belief in what others write, look up the index for "fatigue limit" and tell us all how your beloved texts define fatigue limit.

    The fatigue limit for steel depends on the type of steel. For high strength spring steel I use, it can be cycled through 700MPa and survive forever. For good quality marine allow it can be cycled through 115MPa and will survive forever.

    These are not my numbers they are published material data. They are fundamental to structural design. Anyone ignorant of them should never get near commercial design.

    Rick W
     
  14. Guest625101138

    Guest625101138 Previous Member

    It will not fail - guaranteed. You can put another 10 zeroes and it will still not fail.

    Rick W
     

  15. Guest625101138

    Guest625101138 Previous Member

    You will often see endurance limit associated with a number of cycles (typically 10e8) but this is simply what it has been tested to. It would take a very long time to test it to infinite cycles if you know what I mean. So there is no valid test but usually if it will hang in for 10e8 it will go for 10E6 times that so testing beyond 10e8 does not add value. The endurance curves knee around 10e7.

    Rick W
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.