Rubber Tensile tests

[Griff]

hi spearos

I have been interested in the 16 vs 20mm rubber debate, so i did a little R&D.
I got hold of some rubber samples, 3 16mm, and 3 20mm, and stretched them using the testing equipment in our varsity lab.
the results are prety interesting. for just a straight pull to failure we got the following results

1 x rubber failure 16mm -30kg
3 x constrictor know pulling off-2 16mm and 1 20mm (all around 60kg)
1 x knot in dyneema wishbone comming undone -20mm 75kg
1 x wishbone plug pulling out 20mm (40kg)

these tests were done on juts 1 lenght of rubber, so to get a spearo's equivalent, multiply by 2.

the interesting thing was the small difference between the 16mm and the 20mm rubber. both rubbers were well within the linear elastic zone at normal usage force (say 35kg) the 20mm was still linear at 75kg!
the 16's curve steepened at around 55kg, which explained as follows-
when the rubber is relaxed it is a mass of jumbled up elastic fibers. as they are loaded, these rearrange themselves. as the load increases to the point described above (55kg) are now all lying parralell to the direction of load, and they start to break or the bonds start to break, hence the steeper loading curve. the 16mm actually pulls harder than the 20mm at this range.

the next thing i want to check out is the loss in elsaticity over time, by stretching two pieces of rubber of equivalent usage load and length for my gun (110), so i know whether the durability of the 16mm is a problem, because the load isnt.

i'll post more detail on the results when i get a chance to.

cheers
mark

 

[shaneshac]

Are you saying that a 16mm rubber pulled to 90%-100% of its elastic/plastic limit pulls harder than a 20mm rubber pulled to say 65% of its elastic limit?

How would you find out the limit on a set if bands in order to maximise performance?

 

[Kabullet]

Mark real interesting stuff, would be good to let the guys at the club hear it when you finish your tests. Any chance of heading to ballito for a braai, beer and a talk at one of our meetings?

Tony.

 

[Memo]

Shane in his case, he just pull the rubbers untill they break but I guess the limit shows itself when you feel the elastic margin finishes...

 

[rcerdena]

Nice R&D!!
It would be also interesting to now the impact and the speed that you get on the spear with what these rubbers deliver.

RCC

 

[shaneshac]

Memo,

The bands can stretch a bit more than their elastic limit without breaking. After this point has been reached the bands will not generate any more power.

If you are able to find this exact point you can get the most out of each set of bands you are using.

Does this make sense?

 

[Memo]

Sure I got what you are telling but in practice, (as far as I know) we can feel that point. You have the opportunity to adjust your rubber lenght so you can make a trial and error band and shorten it by 1cm each time, and could stop when you feel some edge...

 

[Griff]

Tony, thanks for the offer, are you part of the new salt rock club?
I dived tiffanies on saturday late afternoon. big surf, bad viz and plenty bluebottles made it a very short dive. i did see a brusher off the backline hotspot though. pretty unusual hey?

shane, to get the best repeatable performance, you shouldn't exceed the plastic limit. after this the material is irreversibly damaged. the assumtion made is that the steepening of the curve is when the rubber is "plastic" or whatever this behavior is called. its not like the behaviro of steel for example, where the curve flattens out after the material goes plastic.

to answer your question, the 16mm rubbers pulled harder at the equvilalent elongation than the 20mm, after the curve steepened. so its not really an applicable range, unless you want to fire your gun once.

basically, the limit in terms of band power is the strength of the spearo. i think there are very few people out there who can load a rubber with 100+ kg's of pull.

 

[Kabullet]

Yes Griff, the club was started by Chris and Warren. Meetings last friday of the month. Check out the website from google seach type salt rock spearfishing club. Dived uglies 10 days ago before i left for work. about 8m viz saw a +- 20 kg tunny and missed a couta +-10. Come check out the club..fees are cheep lots of comps and good bunch of guys.