I always hear that, about the welds being stronger than the piece itself. I'm starting this think this is a myth. Maybe, in the best possible of all scenarios that might be true, but its an awful coincidence that every failed acrylic aquarium (short of someone bashing it with something) I ever saw was at the seams.
Question about DIY acrylic
- Thread starter CichlidDan
- Start date
depending on the type there is a difference in strength from slighty less than 50% (49.95%) to I beleive only 45% this is off the top of my head though and completly wrong possibly but thats what I remember.johnptc;2157067; said:
you always hear it because it's a fact, you see failures happen at the seams because that is where the highest stress loads are generally found as well as the seams are subjected to having to be PROPERLY WELDED, this is the key and why you see MOST failures happen, the edges are not properly prepared for welding it doesn't take much of a mistake to ruin a otherwise good seamspiff;2157091; said:
The seams are not stronger than the acrylic. You are using a solvent to dissolve part of each sheet, meld the two, and then let the solvent dissipate out through the acrylic. At best the seam will be exactly as though there was never a line there. If you try to break a properly seamed tank, usually the crack runs partway along the seam, and partway through the base acrylic. All of this is pretty moot though, because the 1/4" sheets would not be welded together in such a way that the weld would bear any stress.
It is common practice to make the top and bottom of the tank thinner acrylic than the sides... 1/4" is pushing it though, maybe 3/8" would be OK. two 1/4 sheets welded together along every edge including the cutouts would not allow crap to build up between the sheets and would probably be strong enough.
two 1/4 sheets welded together will not be as strong as 1/2 for several reasons, but they will definitely be a lot stronger than a single 1/4" sheet.
It is common practice to make the top and bottom of the tank thinner acrylic than the sides... 1/4" is pushing it though, maybe 3/8" would be OK. two 1/4 sheets welded together along every edge including the cutouts would not allow crap to build up between the sheets and would probably be strong enough.
two 1/4 sheets welded together will not be as strong as 1/2 for several reasons, but they will definitely be a lot stronger than a single 1/4" sheet.
This is going to be my first DIY and I am still scared to start, have any of you guys ever made your own and have any tips to make it much stronger. I read that making 1/2"x1/2" strips and welding them in along all of the load bearing stressed seams should help. Any of you guys have any tips, positive or negative experiences to share before I start this. I will get in a lot of trouble if this thing fails in my parents basement...hahaha.
johnptc;2157067; said:
In tension and shear, yes (omitting creep). In shear strain it too is also as simple as multiplying/dividing by two (or whatever other factor). In compression you have the problem of an increased tendency for buckling. In tensile strain, the thinner material will be disproportionally weaker at equal forces to the thicker material due to the creep or thinning cross sectionally while stretching. However, this is for extreme loads and designs that we would typically see in the hobby would not come close to seeing this happen.
My fear is that between the cross braces, as the tank bows out, it would cause the top sheet to buckle. If no braces were used, the top sheet would easily buckle and would then lend no support to the front and back panel. A single brace would greatly reduce this, and two would reduce it even further.
However, for using a 1/4 piece for a front panel compared to a 1/2" panel, my intuition says that their strength is not a simple factor of two. Since they are supported around the sides, the thicker panel should show better strength and less bowing proportionally . (I am thinking shear stress in a moment equation since they are equally supported around the edges.)
basslover34;2157045; said:This is the comment which I have targeted on....not about welding 2 together however if welded together you actually have a stronger piece since the weld itself is stronger than the acrylic that it's holding... I haven't run the math but I beleive that you would be looking at the weld making up any difference that the 2 pieces would have lost vs 1 and then some... I'll leave our resident nerd (Chompers
) to run the numbers
The word 'stronger' is a very loose term. In the materials data sheet that I had linked to, there is a whole list of big words all relating to different strengths and each has its own value with many of them varying greatly.
If the only forces were to pull a sheet length wise, then it will be very simple math. If you double the thickness, you will double the strenth. Two 1/4 sheets will be equal to one 1/2 sheet regardless if they are glued or not.
In a bending situation, two 1/4 sheets that are glued will be much stronger (resist bending) than two sheets that are allowed to slide against each other. The reason for this is the outside (convex side) glued sheet will have to stretch while the inside (concave) sheet will have to compress. In the unglued sheets they both bend without the linear deformations and therefor require less force to bend them. (Any strengths or weakness in glues will be so insignificant that they can be neglected.)
As for saying that a welded seam is stronger than a solid piece is very debatable. Properties of glues vary greatly. If a glue exceeds the properties of the material, the added tensile strength is actually limited to the thickness of the glue used. Since the thickness of the glue compared to the thickness of the material is nearly always insignificant, the added strength is also insignificant. However, to split hairs or for marketing purposes, yes the glued seam will be stronger.
If the glue is significantly weaker than the material, the tensile strength parallel to the weld will remain unchanged but the strength perpendicular to the weld will be limited to the strength of the glue.
In the corners of acrylic tanks, the forces are perpendicular to the seam so this is why failure sometimes tends to occur there.
spiff;2157091; said:
Yes, you are right. It is largely myth created by marketing firms blowing things out of proportion. I have tested some wood glues that had made this claim and all of the glues had failed the test. This isn't to take the measure of any acrylic or plastic glues, but to take the measure of the marketing firms.
Honestly, I have to agree that I also have seen more acrylic seam failures than panel failures. It is largely due to the experience and attention to detail of the tank assembler. Some manufacturers have the reputation of building bad tanks only because they don't have people that care or know the difference. I have an acrylic tank that has broken panels (1/2") but the seams are perfect. I can honestly say that the seams in that tank are not the weak link.
