[PSUBS-MAILIST] Fw:

[Sean T. Stevenson via Personal_Submersibles]

Hank, the little f you are referring to I presume is the lb_f notation? This is an artifact of Imperial / US standard units, which in common usage do not differentiate between mass and force.  In SI, the mass unit is the kilogram, and the force unit is the Newton. In Imperial units, the mass unit is actually the slug, but almost nobody uses slugs on a daily basis. Thus, the US standard adopted the pound as a unit of mass, differentiating it from the force unit with the subscripts lb_m and lb_f. That they are equivalent only holds true in standard earth gravity, so in any other gravity field you need to divide the lb_f by 32.2 ft/s^2 to get the mass in slugs, and then multiply by the actual local gravitational acceleration to get the force in lb_f. All in all, it's a mess, which is why I prefer to stick with SI / MKS units for everything.  In any case, 1 lb_m is the mass that weighs 1 lb_f in standard earth gravity.

That out of the way, getting to your actual question, be careful using published material property data, because it can be specific to shape or loading direction. Natural wood is an anisotropic material - it exhibits different mechanical properties in the grain direction than it does cross-grain. Dimensional lumber is usually milled longitudinally with the grain, so this may be what is quoted, but it is ambiguous. Also, while softwood lumber may have mechanical properties similar to that  of epoxy foams, the water absorption properties will differ, as I'm sure you're aware from your log salvage experience. Any coating you apply to encapsulate a block of wood will have different mechanical properties than the wood itself, which could lead to a breach of the waterproof integrity, and subsequent water absorption by the wood.  I would be very hesitant to consider this, unless you test a few samples experimentally, and also employ routine condition monitoring / density checks of such
parts in service.

Sean



On August 22, 2016 10:31:10 AM MDT, hank pronk via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>Hi guys, I have forwarded a page with mechanical properties of Douglas
>Fir wood in this email.  There is a rating for compressive strength and
>it has a little f in the description.  Not sure how to interpret that.
> Can I get an opinion on that.  I need to know the compressive strength
>in psi.  This could be a cheap form of buoyancy.  I would make a glulam
> then fibreglass the exterior to waterproof the block.  Of coarse as
>always i would make a sample and put it in my large pressure
>chamber.Thank you in advanceHank
>
>   On Monday, August 22, 2016 10:24 AM, xxx xxxxx <mp13 at live.ca> wrote:
> 
>
>  Douglas-Fir | The Wood Database - Lumber Identification ...
>http://www.wood-database.com/lumber-identification/softwoods/douglas-fir/
>
>
>   
>
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