Cracked Eurobrace

[wrightme43]

I know my dog isnt in this hunt. If it were mine I would give serious consideration to Don W.s way. To me it looks to be the easiest, and strongest way to move the stress that caused the fracture to a solid structural member. Just my opinion. Bummer on your tank Nikki.

 

[Scooterman]

You know drilling all those bulkhead holes in the bottom, back etc, I really don't see how a few smaller holes would hurt, & then strap the top in 316L stainless, make sure to get "L" Low carbon stainless, we use these offshore all the time, regular 316 will corrode faster although even the "L" rust some over time. Titanium would be as much as a new tank LOL probably, whatever I'm sure it is stressful enough, who wants a tub of water on the floor. Please keep us posted with your progress Nikki.

 

[kevinpo]

My background is 28 years in plastics so I thought I would share a little about acrylic so those interested will have a better sense why it does what it does.

Acrylic is a crystalline material. Picture small diamond shaped molecules packed together with their flat sides lined up as apposed to an amorphous material like Polycarbonate, which has molecules like spaghetti interwoven together. As a rule crystalline materials are more chemical resistant but can fracture along the flat planes of the crystals. Acrylic is also hydroscopic which means is absorbs water. When processing acrylic it is pre-dried for 4 hours to remove any water. Failure to fully dry it before processing ruins the material forever as the moisture boils when the material is heated to 480F destroying the molecular bonds.

Now that you have a little information about acrylic let’s talk about what can cause it to crack. The first thing is heat history. It is heated to about 180F for 4 hours to dry it before processing, then to 450F when processed. If the edges are flame polished then you are looking at 3 heat histories. Most companies also allow the use of regrind material of up to 10% (many times the operators sneak in 25-30%) so sometimes the material may have even more heat histories.

The next factor is chemical stress. Although acrylic is chemically resistant, when subjected to load stress, heat, and ultraviolet, along with chemicals it can, and often does fail over time.

IMO/IME the most critical factor is ultraviolet radiation. All of you have probably seen your trash cans crack over time. This can be caused by the factors listed above (except moisture, as polyethylene is non-hydroscopic) but mostly it is caused by UV. Although they seem to crack when it’s cold this is only the result of the hardening of the already weakened plastic. You can extend the life of your cans by 5 times if you keep them out of the direct sunlight. Black cans also last the longest (but that’s a whole other topic J ).

Another factor is stress failure under load over time.

Also notched izod impact failure (this is just the measure of plastic to fail when notched or at corners without a generous radius).

Usually acrylic, when subjected to one or two of the factors above performs quite well but if too many factors come together failure can occur.

First our tanks are put together using a chemical solvent at the joints and then are often flame polished.

Then we fill them with water putting the joints under constant load.

Next we add metal halides (even the ones with UV shielding) are still producing small amounts of UV. The metal halide light does not have to shine directly down on the acrylic to impact it. UV travels quite well bouncing off the surface of the water and traveling through the edge of the acrylic (it acts like a light pipe) to the joint, which was bonded chemically. If you are using metal halides the top surface of the acrylic stays relatively dry due to the evaporation by the lights while the underside absorbs water as it is constantly exposed to it. I do not believe that heat from the halides is an important factor unless you are exceeding 130F (you can only touch it for 2-3 seconds).

So it is the combination of factors coming together that causes the failure.
As James stated earlier it is better to leave the crack as the stress has been relieved but add a patch over it to strengthen the joint. My tank was patched in such a manner on 9/03 and is still doing fine today. The crack has not progressed. I have developed many fine micro cracks along the joints and edges (only on the top) that I suspect were flame polished. I have a fan blowing across the top of my tank that keeps the acrylic cool to the touch so heat is not an issue.

In conclusion, it is my opinion that you can expect a life of 10+ years if you do not use metal halide lighting or 5-7 years with halides.

Perhaps this post is way more information than you wanted but some of you may find it useful.

Regards,
Kevin

 

[Scooterman]

I like it Kevin thanks.

 

[big t]

Well Kevin thanks for that information. That was a interesting read for me atleast so you didn't waste your time Also that makes a compelling argument for glass over acrylic. Who wants to tear down their highly established tank of 5 to 10 years? Not this guy.

 

[DonW]

Kevin,

Are you saying that we will all have to start over in 5 years or is just the top going to be bad?
How long do the siliconed glass tanks last?

Don

 

[RGibson]

All Glass told me that there tanks could last as long as15 years.

 

[Ed Hahn]

Ok, here is a question. I heard something about direct heat onto acrylic etc. How about shielding it with wood like from our Wooden Canopy? I would think this would extend the life by blocking the heat, Am I right or wrong?

 

[DonW]

Ok, here is a question. I heard something about direct heat onto acrylic etc. How about shielding it with wood like from our Wooden Canopy? I would think this would extend the life by blocking the heat, Am I right or wrong?

Probably the other way around Ed. No canopy, less heat. I think your talking about blocking the joint with the canopy lip.

Don

 

[big t]

I know of several glass tanks working good after 20 yrs. Paul hamby is one.

 

[Ed Hahn]

Probably the other way around Ed. No canopy, less heat. I think your talking about blocking the joint with the canopy lip.

Don

Yes, that was my thoughts. No bare exposure or sheilding.

 

[kevinpo]

Are you saying that we will all have to start over in 5 years or is just the top going to be bad?
How long do the siliconed glass tanks last?

Hi Don,
That's a difficult question. It really depends on a combination of factors. For instance what wattage bulbs and how far down the sides it shines. I do not think heat (except where flame polishing has been done) is an issue in most cases but rather exposure to UV radiation and the constant load stress.

All Glass says 10 years for the modern tanks. I believe for the same reason: because a top and bottom plastic frame holds the tanks together, which I think is made from ABS (acronitrile butadiene styrene). It is even less resistant to UV and chemicals.

Tanks fro 15-20 years ago were made from much thicker glass and without support frames (they were decorative).
I have one at the store that is 35 years old and still doing fine. 8 years ago I put new silicone over the top of the old.

Regards,
Kevin

 

[wrightme43]

thanks kevin. I have a very cool book called Engineer to Win. Its by a guy that engineered and built indy and cart cars. Really a smart fellow. Goes into great depth about stress, stress risers, heat cycles, stress cycles, the effects of surface scratches, and many other things that cause failure in material. Really a useful book. I knew that it applied to acrylic, just not that much. Acrylic sorta seems like it acts like metal just less forgiving. LOL

 

[JPG@CFI]

All I have to say is wow! I am really happy to see so many members coming togeather to talk about acrylic tanks. One of my favorite subjects! I am a new sponsor to the sight, and I am really impressed with the debth of knowledge. That is why you all are here, right. I'm sure a few of you have seen my work, and am glad to talk in debth with anyone of you about how I go about my tops.
Thanks, Jason Gregory

 

[NaH2O]

Jason - Welcome to Reef Frontiers! Its great to have you jump on this thread.

Here is a thought I had, and its OK, you can all laugh if this question is crazy because I'm clueless when it comes to this kind of thing. Let's say I install the angle iron, as Mike suggested (or even the carbon fiber cover)....wouldn't that put stress on the seams at the back and bottom of the tank? (my tank is one piece for the front and sides (rounded corners), one piece on the bottom, and one for the back, then the Eurobrace). Wouldn't it be better to have angle iron at the top and bottom?

Don - I might have some more questions for you when we get closer to doing the fix.

Thanks everyone!

 

[DonW]

Don - I might have some more questions for you when we get closer to doing the fix.

Thanks everyone!

If you decide to go with the last one I posted, let me know I have some 1.5" acrylic scraps.
As far as the metal frame goes. A angle Iron band is what your looking for, just stainless.

Don

 

[Scooterman]

I have to say it is nice to get so much input, I guarantee you that now I will have more to think about when I get my next tank.
Jason Gregory I would really like to see more about how you make your tops, even pictures, this sure is something I would of never thought about until now. Maybe we could come up with a UV tape or cover to glue on the top to help extend the life?

 

[DonW]

Heck with it. I'm just going to let the coraline take over. Cover the sides, back and a front edges. Natural uv filter.

Don

 

[Acrylics]

Kevin,

I'm not so sure I'd agree with ya with regard to UV radiation on today's acrylic though I think we are in fair agreement on most of your points. Years ago, yep I would have to agree with it but today's acrylic is much better in this regard and due to reformulations in resins and changes to molecular binders, etc., most acrylic brands are guaranteed to show no ill effect from naturally occuring UV radiation for 5-10 yrs. The reformulations coupled with the fact that acrylic doesn't transmit any UV below 380-390 nm, and certainly no UVC - the "light tube" doesn't make any sense to me. While it's true that UVC can cause photochemical reactions in acrylic, it's not exactly a naturally occuring spectral range and I haven't been able to find any data regarding UVC and MH lighting.
The lifespan of an acrylic tank, with or without MH has far too many variables IMO to warrant a 5-7 yr term though I do respect the opinion. I just don't agree with it and while anecdotal, my experience differs as I have many tanks out there that we built in the '80's using MH and still holding strong. Factors that would affect the lifespan of the tank are too numerous to list (briefly) but if you were to say, the 5-7yr conclusion was an "average", I might be more inclined to agree, but 5-7yrs implies 7 yrs as the top end that one can expect which I would not agree with.
BTW I hope this comes accross as intended, I don't mean to sound defensive at all, not the case, I just enjoy the discussion

Nikki,

You'd want the angle iron on the front & back, ideally all 4 sides but... If you can imagine a guy pulling on each panel of your tank, if they pull hard enough - they can cause a crack, usually in the corners where the stresses are concentrated. This is essentially what is happening in your tank but water is pushing on the panels. This is why the crossbracing is important as it's much harder to shear acrylic than to crack out a corner.

James

 

[kevinpo]

Hello James and welcome to Reef Frontiers! Your comments are welcome

I have 8 acrylic tanks 1 built in 2001, 6 built March-June 2003, and one in 2004. I don’t know if acrylic sheet formulations have changed much since then but it is possible. I will contact the local supplier to obtain the brand and grade so I can contact the manufacturer for test data. 3 of them have 400W metal halides on them and all show small cracks and crazing. 4 others have VHO fluorescent lighting and show no signs. I suppose I could have you build me another 4’x4’ cube for the store and let you know how it does in 2-3 years .

The UV radiation I am referring to is UV-A (320-400nm) and UV-B (280-320nm). Metal halide can produce both of these in much higher amounts than occur in nature (Riddle 1998) both at bulb strike and during operation. Reflectors can distribute the UV allowing it to reach greater areas than the bulb burning alone. The maximum UV-A output has been measured at almost 5,000 µW – 2.5 times the amount the amount of maximum temperate latitude UV (with a reflector). The glitter lines we see with halide lighting are the result of light being focused much the way a magnifying glass does. Although they occur for only 5-20 milliseconds the intensity can be up to 5 times that at the surface (Stramski and Legendre 1992), which could equal 25,000 µW.

Most weather testing on plastics is done using .125 test plates in an environmental chamber. Normally no testing is done on the plastic under load at that time (it’s normally done as a separate test on a new plate).

In regards to the light pipe, acrylic can act much like an optic fiber. Fiber optic strands are polished in a radius on the end and carry light waves very efficiently long distances. When acrylic has a radius cut on the edge and then is flame polished it also transmits and captures the light (including UV) easily the 6-10” from the edge to the joint.

I would certainly agree that to say the lifespan of an acrylic tank is 5-7 years would be unjust and perhaps inaccurate. There are so many variations between bulbs of the same wattages and manufacturer not even considering all the other variables of reflector types and wattages that it would be almost impossible to establish an exact age for tank replacement. Fluorescent lighting also produces UV but in much less amounts.

My intention is not to bash acrylic but rather to provide information that might stimulate further thought, research, and discussion to enable other to make the best decisions for their application.

Here is an article that has test data and references:
http://www.aquarium-design.com/reef/uvlighting.html

Regards,
Kevin