new guy new test lab!?

General Discussion
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After building my first (kite) surfboard, (thread here http://www.swaylocks.com/forums/new-guy-new-build-thread) I caught the itch that I’m sure many of you have and continue to scratch till you are raw and bleeding…,  that is how to make it better… 

My goal was to make it light. and strong as all hell.  I believe I accomplished this objective but it was merely light enough…better than most…above average…F’NG  ABOMINATION… 

I started last time with a bunch of panels and testing their integrity by jumping on them.  Ghetto but effective concept, boeing still drops shit onto airplane wings. and you can’t get more real world than jumping on a panel that you are designing to be jumped on in real life. It was cheap,  easy and weeded out some real loser constructions that up until I landed on them I thought were a good idea…The biggest weakness I found with this was that on a 1ft x 1ft panel, whether that panel creased or not  on impact seemed to be very dependent on where  you landed on the panel. Get too close to the edge and it’ll crease, nail it dead center, the same panel wouldn’t.  In a few cases my jumps were less than accurate and it was unclear if the panel was a turd or my jump just screwed it up.  needed something to control the jump and impact location  a little more.  But still be representative of a realistic of a jump.

So enter “the dentonator”,

A low energy impact machine…I used 1"1/4 pipe as it was heavy, cheap comes in long lengths and most importantly the cap for the end of it is almost an identical shape and size to a human heel bone.I glued on a chunk of EVA foam to the end in order to take the edge off the blow as my heels aren’t metal and have a good bit of padding.  

Previously I was jumping on my test panels from a height of 2ft heels first…a height that would produce  heel pain of a magnitude slightly higher than what I experience on my worst landings while riding,…this was my definition of tough enough. Having lots of eps foam kicking around I jumped on a raw chunk from the 2ft in hight  then added height and later weight to the dentonator (by filling it with lead shot)  until it would produce an equivalent dent depth as a 2 ft jump. 

However just a jump test doesn’t tell you everything, On a jump test the panels would dent or crease or delaminate, and in many cases the damage can be impossible to visually detect. Things like spectra did bitchin job taking ANY impact load…astounding really, however the material has virtually no bonding capabilities and as a result zero compressive properties, It was also clear that the plies had almost fully delaminated in absorbing the impact. Although it didn’t crease was the structure screwed up?  Was it good enough? …F’d if I know…

There is tons of data on impact properties of different materials and tons of data on flexural properties. However there is very little on Flexure AFTER impact. Particularly in structures like surfboards which have a deformable core…

For this I needed to build “a bendonator” 

 to test panels AFTER they had gone through “the dentonator”  I settled on 4 point bending test rig. I used 4 point bending as it is able to apply a constant stress across the dented area and will not just have peak stress in the middle that a 3 point bend will.   The damaged area in EPS from the dentonator was about 4" so I settled on 5" span for the supports on the center for the dentonator in order to load the whole dent.   I used a length of 24" as my foam comes in 4x8 sheets which I cut in half to build surfboards.  so the off cuts are 24" long meaning I can test scraps and not pay money for much material in the tests It was all build out of material that I cut off some scrap metal that I took from work and sloppily slammed together using my welder.  It should be good to about 760lbs of force,  Im aware strain rate is critical however I’ll just have to be consistent about how I apply the pressure.  

All in I think the 2 set ups cost me $40 as most off the stuff was scavenged from things I had laying around 

My goal is to find a panels that is lighter stronger and more impact resistant than my previous build… flex I can likely adjust with volume and thickness   but additional flex is would be a bonus.  

Comments, critcisms,  mentions of gross oversights or material combinations worth testing would be appreciated. I suspect some of you have been down this road already…

My current plan is to start with my last build

2 x 6oz  (1x 0-90 1 x 45) 

1/8" corecell

1 x 6oz @45

1lb EPS

1 x 6 oz @45

1/8 corecell

1 x 6 oz 0-90

I’ve got a good supply of nylon, spectra, kevlar and carbon to screw with as well but am still running calculations to rule out panels that will not be lighter than the above.  

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So the first test on the dentonator was a bit of a shit show.  my baseline panels creased and snapped,   I screwed up this test in a number of ways 

  1. the panel was 24 x 8" I believe this is too narrow which encouraged the impact energy to propagate to the edges earlier than I would have expected. 

  2. the panel was not on level ground and as a result the impact of the dentonator favoured one side over the other.  

 

Regardless I videotaped it, and caught something really cool I assumed the panels would bend in a circular manner, around the impact location  when infact they were bending in a straight line and buckling under the impact load…this is very consistent with many of the boards I’ve repaired which have creased straight across the deck…I always assumed the boards were dented under foot then broken and creased later as the damaged area was loaded…now I’m not so sure…It could  also be ONLY because of my narrow  rail-less panel in this test. but I’m seeing something I didn’t expect…at all…and that means I’m learing something…I just don’t know what it is yet…

 

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First I would say , brilliant , second I would question if you have a real life, sorry only joking . you are right your test does show the panel creases straight across , very interesting to be able to see it in real time , im sure one of the very bright engineering types on sways wll chime in with an explanation . The detonator and bendonator are simple but effective and will likely give simple answers to your questions and that is most likely all you need to achieve your objective of making a lighter stronger board . Good luck

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As a mechanical teacher i am a big fan of lab tests. A good help for my studdents to see what happened.

If you want a nice “volcano cratere” impact print you must use a spherical impactor and impact need to be perpendicular to surface.

What happen is: under impact, eps foam compress a lot, so locally your skin experiment a 3 point flex where the face in compression buckle. To increase resistance: use a denser eps foam (higher compressive strengh= less deformation) and or use stiffer skin (thicker more fiber, stiffer material, etc…)

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I too am a big fan of lab tests.  Seriously…   Bra-fuckin-o!  I’m paying close attention to this one.  I hope Benjamin Thompson sees this and can further elucidate, not to imply that you’re not holding your own on this.  He has done a lot in the way of testing fabrics and weave orientation, not to mention the bounce/flex test and other stuff.  

You might also check what Yvon Chouinard did several years back with panel testing… http://www.fcdsurfboards.com/about-fcd

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I was thinking about your panel creasing straight across , you do not show if your panels are glassed on both sides , in my opinion the test panels would need to be glassed on both sides to give a better representation of a surfboard  and the panel cores should all be the same thickness and the same density  . good luck.

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I really like the names of your test rigs and the whole idea of testing: a little proof to go with all of the ideas and experiences. Keep up the good work! -J

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Lemat thanks what you wrote makes perfect sense,  I was expecting the volcano type impact pattern as this is what I’ve seen in many simulations and test results as they all use hemisperical impactors…I will not be pursuing this type of standardized testing as the flat round pipe end is more representative of a heal and as such closer to real world scenerio.

That stated, the heal of the dentonator is scaled appropriately for real world but the panel is not…an 18" wide panel from a real surfboard would have 2.25 x the buckling resistance of an 8" panel due to the width component in the moment of inertia according to the Euler equations… It also explains why the board I built of the same construction as these panels has survivied RUTHELESS beatdowns over the last couple of months.

The lingering  question for me is the failure mode that I posted above,  buckling due to impact…Is this a legit concern in real life?..Ie does it/could it happen when a board is jumped on hard enough or was it just an anomally due to a big impactor on a small panel…

The subpart of this question is What impact will a typical rail play in this.  I wrap all my rails and they end up with with about 18-24oz of glass (considerably more than the deck) Would the rails prevent the impact buckling or would the deck have some type of localized buckling /creasing only around the impact zone…I guess I can always smash some shit to find out…but if someone already knows…love to hear it.  

More pressing however is I need to adjust my rig,  I broke my baseline test.  this is the “good enough” standard around which all others will be tested…and it failed out of the gate.  

 

a) do I decrease the weight/height of the detonator? 

b) Increase  the width of the panel. 

c) consider the fiberglass a fail and try other combos.  

I’m going with b) as it’s closer to real life.  I’m just hoping IF the panel survives the dentonator it will break with less than 760 lbs of force on the bendonator… the maximum width the bendonator can handle is 12" So I’ll run with that, the previous panels I jumped on were 12" and survived my jump test.  So if I go 12" carefully level and ensure a perpendicular impact all will be good. Bit more material cost per test but not crazy…will also speed up the FPS of my camera…can double the FPS with lowered resolution. 

@Gbzusa. The panels were glassed top and bottom in the construction I used  on my board, defintely need top and bottom or the flexure testing after impact is not possible.  Good point on the thickness. I’m shooting for a 2" panel in order to keep the flex numbers comparable. the last sample I planed down 1/8" both sides before adding the corecell.in order to get ~2"…core crush will be a challenge to control under the vacuum bag though…

as to the density I WILL change the core density. My goal is to make a panel which is lighter AND stronger. I’m not interested in JUST the skins, So it may be more efficient on a weight basis to decrease glass and increase core density, or eliminate sandwhich and increase glass or increase sandwhich thickness and decrease glass etc etc.

Anyway thanks for the encouragement guys,  I hope I find something we can all use.  If not I’ll at least find something I can use…guarantee I will be slower than shit doing any of this as all my building happens in the 1 hour a night after I’m done work and put my kids to bed…(yes I do have a life and it’s a great one) :o)  

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Width of panel will not change strengh a lot here because it’s localize flexural, where foam under compress. For those kind of stress stiffer (denser) foam is a good improvement because it spread load on greater surface and flex less so less chance of buckling crease. An other way is to keep light soft foam under and to increase skin stiffness. First solution will still dent, but less, and the panel overall stiffness will increase slightly, second solution will not dent (if no destruction) but panel will be really stiffer.

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A Third Idea would be to do a build that you layer the two foams making a sandwich of an inner core of less dense foam capped top and bottom with a dense foam 
Just a thought. Keep up the good work.
You Might want to look into some of the work being done by the Aerospace industry. They have already worked out most of this stuff. There is some good information out there that is no longer classified or considered industrial secrets

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Nice lab work Massive Swell. I’ve worked in a civil engineering testing lab, your processes are equal. Please try 6 oz. Carbon over 2 oz. Kevlar. A fellow i know has good luck with that combo but he is very light on his feet.

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Wanted to update you guys on where I got with this.

Tested the bendonator  using the semi buckled panel I smashed earlier in the pictures. .  Wanted to sort out any problems and also ballpark the magnitude of forces I’m dealing with.  As expected it completed the buckling along the previous buckled portion and this required a force of about 400lbs.  I’m quite concerened that I will max out my scales on panels that survive the denting. In addition uncovered some minor concerns about needing some funky camera mounts so  I can record the stress and strain at the same time. (panel blocks the view)  

After breaking the panel,  I wanted to sort out the dentonator a bit more using the virgin sections of the panel that had not been damaged by any of the testing.   I leveled the panel, dropped the weight and it SURVIVED! with merely a dent  I then moved to the last virgin section dropped the weight again and it failed in the same manner as the 1st…1/2 of it buckled.

I video taped them again using a camera with higher FPS.  And took the video from the side of the panel (instead of the front)  so that I could see things more closely. (albeit only one side) sadly the buckling occured on the OTHER side of the panel so I didn’t catch a close up of the panel buckling.  Regardless will post the pics when I can edit them out of my camera.

Ultimately I’m attibuting the inconsistent result to the massive wobble that is present in the dentonator.  the leverage of the tube on the wood allows it to float almost an inch.  I’ve sinced boxed the structure in so it’s straight, level and stays that way. Regardless going through the exercise made me realize that doing 3 impact tests is probably the best  way to compare panels and it’s been added to my process.

For future tests I’m going to widen the panel… doing testing that is on the edge of variability sucks…some of the videos showed the impact crater is reaching the unrestrained edges of the panel which makes the panel more prone to folding and this is why I think it’s buckling so unexpectedly early…I totally stomped the the CRAP out of a panel of this construction that was 12" x 12" and messed up my heels for a good week trying to break it and failed to do so.   So i’m gunna push it to 12" and see if I can still break it on the bendonator afterward… For reference the dentonator is a 22lb drop weight from 2 ft.   

Further interesting testing is on hold as I’m  waiting for a corecell order, 

In the meantime, here is an interesting post from Ben thompson (someone mentioned him earlier)  along the same lines.  http://www.swaylocks.com/forums/tests-results-13-unique-constructions  I have since been stalking his posts on swaylocks. ALOT of REALLY good stuff there…

Will probably try one of the more successful nylon/glass  layups on 1.3 lb  (16psi compression strength)  foam and see how it smashes. while I’m waiting for the corecell.

I’m interested in some higher density EPS foams and where I can get them without shipping a blank up to Canada, if people have any leads lemmee know.  

also,  I should give wood veneers another kick at this.  what’s a good thickness to test?  and where can you guys get the stuff?

I’ve been running through a lot of calculations to figure out the weights of different constructions and whether they are lighter than the current one I was using…This type of exercise would lend itself well to a to board weight calculator  where the user selects the volume, surface area, materials, core weight,  number of plies, fabric weight,  volume fraction of laminate (or epoxy weight)  sandwhich type/thickness  etc. I may end up making this to save myself the repetitive screwing around…If there is  interest in this I can probably make something with a good user interface that everyone here can easily use and it wouldn’t bee too  much additional work. 

Or better yet maybe someone has done this already?

 

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Great thread!

Boardcad has a weight calculator with some of the features you mentionned, you may want to have a look, it’s free and a very good software.

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Here’s some animations of my last impact tests.

the first is one which did not survive (buckled on opposite side of camera)  

The second is one that did,

Same panel same layup as the one above Just different locations on the panel

I didn’t take much from these animations…maybe someone else can

 


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Apologize for my pace.

So I finished all my weight calculations and got my hands on a bunch of corecell off cuts for dirt cheap.

After doing the calcs and understanding the failure modes of my last build this is the one that in theory is likley  the lightest and most robust build I could make given the collection of materials I have (which is pretty extensive at this point)  So if this one just got destroyed, there may not be much that I can do that’ll be off the charts. 

…so far so good…Something really awesome happened here…I’m curious to see what happens to it on the bendonator it’s what’s INSIDE that counts… :slight_smile:  

 

 

 

 

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Here are the changes I made to the dentonator such that It will be more consistent, I had to box it in to prevent wobble leveled it and then screwed it down to the desk to keep it stable. I’ll build another baseline panel for comparison testing shortly.  

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Great testing but I have a few suggestions:

  1. Your panels do need to be wider, a minimum edge distance is 2.5 x hole, maybe make the panels 6x impactor diameter.
  2. Edge constraints have a very significant effect on specimen response. Impact specimens are normally held in a window frame type jig and lightly clamped along the long edge.
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…the integrity will be different if you round the rails and wrap the fibers like a surfboard. Fatigue is one thing, resistance to a spot impact is another; in surfboards better first.

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Meagain.   SPOT ON thanks!  I’m using a 1.5-2" impactor and have moved up to a 12" panel. I’ll see how my known good board contruction works in this panel layout but based on the video and stress whitened areas it doesn’t look like the circle of impact is reaching the edges (on the latest construciton) 

Will also add some form of window style clamp to the process to better control the edge criteria. looked at some videos from instron and  you’re right on with the edge supports too.

As for progress  dented panel above maxed out my scales at 800 lbs on the bendonator and didn’t crease buckle or fail

although it is a short wide span and I have yet to gain some perspective by testing other known panel constructions that have stood the test of time on a surfboard…my feeling is this is  an awful lot of force for a pool toy and likely more than sufficient construction method.   

TENTATIVE PASS.    

Please accept the following excuse for my slow progress. 

and yes of COURSE I  rode out of it  ;)

 

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Main problem with composits is the no regularrity of mechanical specs that come from heterogeneity of those materials. Need a statistic process.