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Normally for structural timber, two values for compressive and tension strength are given. One value is given for timber in direction of the grain, and other is given at 90 degrees (perpendicular) to the grain. Common notation for these strengths is $f_{c,0}$, $f_{c,90}$, $f_{t,0}$ and $f_{t,90}$ where $t$ subscript refers to tension and $c$ to compression. Picture from here illustrates the idea:

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Some sources talk about timber strength in tangential and radial direction. I read this post for example, which states:

Wood is very strong in longitudinal tension and compression, much weaker in radial tension, and weaker still in tangential tension.

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This got me thinking: if timber is weaker in tangential loading than radial loading, why aren't there usually two values reported for the strength perpendicular to the grain? Aren't both radial and tangential loads perpendicular to the grain? Why is only one perpendicular strength value necessary to describe the strength of timber in perpendicular loading?

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  • $\begingroup$ Think about how timber is used in structures... and why it is used with the types of joints. $\endgroup$
    – Solar Mike
    Jun 20 at 19:12

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Consider the trunk of a tree as a set of concentric tubes, perhaps it helps distinguish between the following 3 cases:

What your 2nd source refers to as longitudinal tension or compression is indeed normal stress applied to the trunk, or a bar made from it, in the direction of the length of the trunk.

The radial tension and compression of your source refer to torques that bend the trunk, or a bar made from it, such that the trunk/bar buckle.

Finally, what your source refers to as tangential tension and compression is in fact shear stress that is created when trying to twist the trunk/bar.

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