SN curve for Natural Rubber
SN curve for Natural Rubber
(OP)
Hi,
Are there any established SN curves or alike for natural rubber.
Are there any established SN curves or alike for natural rubber.
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS Come Join Us!Are you an
Engineering professional? Join Eng-Tips Forums!
*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail. Posting GuidelinesJobs |
SN curve for Natural Rubber
|
SN curve for Natural RubberSN curve for Natural Rubber(OP)
Hi,
Are there any established SN curves or alike for natural rubber. Red Flag SubmittedThank you for helping keep Eng-Tips Forums free from inappropriate posts. Reply To This ThreadPosting in the Eng-Tips forums is a member-only feature.Click Here to join Eng-Tips and talk with other members! |
ResourcesWhat is rapid injection molding? For engineers working with tight product design timelines, rapid injection molding can be a critical tool for prototyping and testing functional models. Download Now
The world has changed considerably since the 1980s, when CAD first started displacing drafting tables. Download Now
Prototyping has always been a critical part of product development. Download Now
As the cloud is increasingly adopted for product development, questions remain as to just how cloud software tools compare to on-premise solutions. Download Now
|
RE: SN curve for Natural Rubber
RE: SN curve for Natural Rubber
No, there are no SN curves for rubber. Standard fatigue theory deals with granular crystalline structures (metals). Although some people have tried to apply such theories to polymeric and elastomeric materials, the general consensus is that such an approach is not very fruitful.
There is a whole area of research trying to develop fatigue theories that are more appropriate to chained structures, but this is very much still in the theoretical/research stages.
Brad
RE: SN curve for Natural Rubber
Brad
RE: SN curve for Natural Rubber
There's an interesting couple of curves in P.B.Lindley showing the fatigue life of NR subjected to tensile deformations at 2Hz. One at 0% minimum strain, maximum strain varying. The other at 250% maximum strain, minimum strain varying. The latter showing an increase in fatigue life as the minimum strain increases. I wonder if you could substitute the strains for stress from a stress/strain tensile test curve, so constructing an approximate SN curve.
RE: SN curve for Natural Rubber
RE: SN curve for Natural Rubber
Oscar
RE: SN curve for Natural Rubber
The same holds true for any material in fatigue--fatigue properties are far more varied than "mechanical" properties such as moduli, density, etc.
But again, this is somewhat of a moot point as many have tried to apply conventional metal fatigue theories to rubbers and seen little success.
Brad
RE: SN curve for Natural Rubber
SN curves for rubber are usually referred to as Wohler (pronounced 'verler') curves. These are plotted with log cycles on x axis, against stress (n/mm2) on the Y axis.
These are experimentally derived for different polymers, by performing a mumber ( 6-10)of cyclic fatigue tests at different stess levels.
Most manufacturers of rubber antivibration mountings ( including the one I work for) routinely produce these curves, so the data should be available. The book 'Rubber Springs Design' by E.F. Gobel ( probably put of print) discusses this type of S/N curve.
Best regards
Tom
RE: SN curve for Natural Rubber
Tom,
Are you the same Tom Aspin that worked in Silentbloc Technical Dept.
And yes, we do have the book you mentioned, silly of me to overlook it.
Regards,
Conrad.
RE: SN curve for Natural Rubber
RE: SN curve for Natural Rubber
Despite of these limitations I would suggest the following procedure to follow in order to receive a prognoses for service life of the o-ring in question:
Perform a non-linear finite element analysis of your sealing problem, where a typical cyclic loading should be simulated. Detect the areas of the cross section where strains are large. Get the full information on the strain tensor in those areas. Determine the components of the strain tensor which are in tension, because they may be the most dangerous ones. If you have the impression that one of the tension-strain components is much greater than the other ones (which often appears to be)you may be justified to handle that deformation mode for further considerations only. Now correlate the strain energy density found in the considered area of the cross section with the tear energy appropriate for the given deformation mode (see for example "How to design rubber components" by A. Gent). Thus you will get a relation dealing with a virtual crack you think about to be present in the considered area of the cross section of the o-ring. Then you may assume you would know the critical tearing energy ( ... and indeed one can assume a "most-lower" value - to be on the safe side). With all those assumption in mind you can state whether the current dynamical loading may be critical concerning a possible growth of some virtual cracks (flaws etc of certain crack-length). If you thus come to the conclusion that initial cracks ( always be present, independently of compound quality which may have a typical length of the order of several tens of micro-meters)subjected to the given loading condition would most probably grow, then you should be justified to claim that the seal may not survive. Otherwise you may of course claim the opposite.
Well, that procedure works well only if the assumptions made concerning the distribution of crack-length of pre-existing flaws and other imperfections in the bulk rubber is rather good.
But even if you do not know ( ...as is usually the case)the mentioned "distribution" you can get benefit out of the above described "calculus" when you for example state that you have in your rubber component a rather big number of small cracks with a crack-length of 50 micro-meters or so.
RE: SN curve for Natural Rubber
RE: SN curve for Natural Rubber
RE: SN curve for Natural Rubber
Gotcha. Thanks again.