Tank Blow out

[sphelps]

I can't tell you exactly what caused the failure but I highly doubt it was from over tightening a bulkhead. I do know that when you modify a standard type tank so the bottom looks like Swiss cheese you can no longer rely on support from the edges only, you need to reconfigure how the base is supported typically with foam and a solid base.

Reason being is typical DIY drilling procedures result in a large number of defects around the holes that act as stress concentrations. These stress concentrations greatly reduce safety factor, even a slight surface imperfection can double the local stress. These high local stresses can now easily spread into cracks with a decent load, adding additional support to the base reduces the load and therefore the resulting stress.

[Myka]

Originally Posted by sphelps

I can't tell you exactly what caused the failure but I highly doubt it was from over tightening a bulkhead. I do know that when you modify a standard type tank so the bottom looks like Swiss cheese you can no longer rely on support from the edges only, you need to reconfigure how the base is supported typically with foam and a solid base.

Reason being is typical DIY drilling procedures result in a large number of defects around the holes that act as stress concentrations. These stress concentrations greatly reduce safety factor, even a slight surface imperfection can double the local stress. These high local stresses can now easily spread into cracks with a decent load, adding additional support to the base reduces the load and therefore the resulting stress.

Agreed.

[mark]

feel for you, but never understood how over-tightening a bulkhead would crack a tank

[reefwars]

Originally Posted by mark

feel for you, but never understood how over-tightening a bulkhead would crack a tank

especially only a quarter turn past hand tight

[b_james]

The other possibility was that when i drilled the hole, it may have shelled in a way that it caused a small fracture - similar to a chip on a windsheild developing a crack.

Anyways thats all water under the bridge...

[b_james]

Originally Posted by sphelps

I can't tell you exactly what caused the failure but I highly doubt it was from over tightening a bulkhead. I do know that when you modify a standard type tank so the bottom looks like Swiss cheese you can no longer rely on support from the edges only, you need to reconfigure how the base is supported typically with foam and a solid base.

Reason being is typical DIY drilling procedures result in a large number of defects around the holes that act as stress concentrations. These stress concentrations greatly reduce safety factor, even a slight surface imperfection can double the local stress. These high local stresses can now easily spread into cracks with a decent load, adding additional support to the base reduces the load and therefore the resulting stress.

I would go as far saying it looked like swiss cheese having four 3/4" bulkheads on a 93 tank IMO is not excessive. There are many factors that may have contributed to the fracture. One possibility could have been the placement of a large 2 foot tall drift wood stump. one of the legs sat within inches of the bulkhead where the fracture occured. Personally I have a feeling it may have more to do with the quality of the hole i drilled. I didnt tape the underneath to minimize shelling.