Wood Shear Strength Inconsistency

[Agnaa]

Bump.

 

[KLOL506]

Bump.

Calc's been updated as per the above instructions.

Jim Hawkins cuts a tree

vsbattles.fandom.com

 

[Agnaa]

True, but I'd like more calc group input than just us two. Sadly, I already pinged everyone without a response....

 

[KLOL506]

@Amelia_Lonelyheart @Executor_N0 @Spinosaurus75DinosaurFan @Mr._Bambu @Therefir @DMUA @Damage3245 @DemonGodMitchAubin @Jasonsith @Wokistan @Migue79 @Armorchompy @Psychomaster35 @CloverDragon03 @Dark-Carioca @AbaddonTheDisappointment @Aguywhodoesthings @DontTalkDT What do you think of the following conclusions for wood-cutting feats?

It's because it depends on where the direction of force is coming from with respect to the grain/fibers of the wood.

Like, imagine you have a bunch of plastic drinking straws tied together with a rubber band, with all of the straw openings facing up/down. Squishing the bundle of straws perpendicular to the openings is going to be way easier than trying to squish them length-wise.

As a result, there are two strength values for compressive strength listed in @Arceus0x 's pdf-

(This also means that the compressive strengths of wood at angles between 0 and 90 degrees to the grain are gonna be somewhere between the two values, and can be determined by trigonometry)

The reason why shear strength is inconsistent is because the listed shear strength of wood is measured parallel to the grain, while the listed tensile strength is measured perpendicular to the grain, meaning multiplying the perpendicular tensile strength by 0.6 will (in theory) get the perpendicular shear strength, which is going to be different from the parallel shear strength.

Unlike compressive strength, shear strength of wood perpendicular to the grain is much higher than shear strength parallel to the grain. (Source)

Cutting feats are measured as pulverization or compressive strength of a thin slice. (Source- @KLOL506

Cutting wood from the side (think swinging axe at a tree) will use perpendicular to grain compressive strength

Cutting wood from top-down (think chopping logs on a splitting block) will use parallel to grain compressive strength

It's the equation for resolving forces, just reversed.

compressive strength at an angle = sqrt[(cos(cut angle)*perpendicular compressive strength)^2 + (sin(cut angle)*parallel compressive strength)^2]
So, using White Oak Compressive strengths-

Angle of Cut	Compressive Strength at Angle
0 degrees	9.1 MPA
15 degrees	15.64388146 MPA
30 degrees	26.21273545 MPA
45 degrees	35.93612389 MPA
60 degrees	43.53966582 MPA
75 degrees	48.35368624 MPA
90 degrees	~50 MPA

(At 0 degrees, it'll just be the perpendicular compressive strength, while at 90 degrees, it'll just be the parallel compressive strength)

To illustrate it-


(The harder part of cutting at an angle would be getting the area of the cut rather than the compressive strength)

And yeah this can be ignored for materials that don't have axis-specific strengths like wood.

 

[KLOL506]

Bump

 

[Agnaa]

@Amelia_Lonelyheart @Executor_N0 @Spinosaurus75DinosaurFan @Mr._Bambu @Therefir @DMUA @Damage3245 @DemonGodMitchAubin @Jasonsith @Wokistan @Migue79 @Armorchompy @Psychomaster35 @CloverDragon03 @Dark-Carioca @AbaddonTheDisappointment @Aguywhodoesthings @DontTalkDT What do you think of the following conclusions for wood-cutting feats?

Bump.

 

[KLOL506]

I already did what I had to do. Now we just wait for the other peeps.

 

[Agnaa]

I was hoping that quoting it would ping them again.

 

[Therefir]

@Amelia_Lonelyheart @Executor_N0 @Spinosaurus75DinosaurFan @Mr._Bambu @Therefir @DMUA @Damage3245 @DemonGodMitchAubin @Jasonsith @Wokistan @Migue79 @Armorchompy @Psychomaster35 @CloverDragon03 @Dark-Carioca @AbaddonTheDisappointment @Aguywhodoesthings @DontTalkDT What do you think of the following conclusions for wood-cutting feats?

I think it's fine and it makes sense.

 

[KLOL506]

@Amelia_Lonelyheart @Executor_N0 @Spinosaurus75DinosaurFan @Mr._Bambu @Therefir @DMUA @Damage3245 @DemonGodMitchAubin @Jasonsith @Wokistan @Migue79 @Armorchompy @Psychomaster35 @CloverDragon03 @Dark-Carioca @AbaddonTheDisappointment @Aguywhodoesthings @DontTalkDT What do you think of the following conclusions for wood-cutting feats?

Bump

Pls respond

 

[Second22]

compressive strength at an angle = sqrt[(cos(cut angle)*perpendicular compressive strength)^2 + (sin(cut angle)*parallel compressive strength)^2]
So, using White Oak Compressive strengths-

Little question, where do you find perpendicular compressive strength?

 

[ElajRuengies]

Little question, where do you find perpendicular compressive strength?

Typically it'll be listed alongside parallel to grain compressive strength

 

[Second22]

​

Another method is to use the formula:

• Compressive strength = (compressive strength parallel to grain) * (cos(θ))^2

Where compressive strength parallel to grain is the compressive strength of the wood when the load is applied parallel to the grain and θ is the angle between the load direction and the wood grain direction.

The formula compressive strength = (compressive strength parallel to grain) * (cos(θ))^2 is based on the principle of orthotropy, which assumes that the compressive strength of wood is dependent on the angle of the load direction relative to the grain. This formula is widely accepted because it is simple to use and provides an accurate estimate of the compressive strength of wood at an angle.

if i'm not mistaken that the formula​

compressive strength at an angle = sqrt[(cos(cut angle)*perpendicular compressive strength)^2 + (sin(cut angle)*parallel compressive strength)^2] is not widely accepted in the field of wood mechanics, and it's not a commonly used method to calculate the compressive strength of wood at an arbitrary angle.

Edit: that while the formula Compressive strength = (compressive strength parallel to grain) * (cos(θ))^2 is widely accepted and commonly used, it's not the only formula available and other formulas might be more accurate for specific cases

 

[Flashlight237]

We attribute different shear strengths to wood in different calculations.

Some of them, such as this calc, use a tensile strength of 70-140 MPa, multiplied by 0.6 to get shear strength,

Some of them, such as this calc, use a shear strength of 7.3774 MPa.

These values are 5.7-11.4x off from each other. Both of them are currently accepted.

We need to find a consistent value to apply to calculations, and we need to update old calcs accordingly.

This is part of the reason why, whenever I do any calc, I avoid any generalization (ex. 8 j/cc for stone) and try and match whatever type of stone or tree is in a scan with a real-world stone or tree.

 

[Second22]

However, the formula used by Ejel is more detailed. Should the compressive strength at an angle of other wood be calculated as well?