Again, a tough wood is one that will not rupture until it has deformed considerably under loads at or near its maximum strength, or one which still hangs together after it has been ruptured and may be bent back and forth without breaking apart.

Toughness includes flexibility and is the reverse of brittleness, in that tough woods break gradually and give warning of failure.

Tough woods offer great resistance to impact and will permit rougher treatment in manipulations attending manufacture and use.
Toughness is dependent upon the strength, cohesion, quality, length, and arrangement of fibre, and the pliability of the wood.

Coniferous woods as a rule are not as tough as hardwoods, of which hickory and elm are the best examples.
The torsion or twisting test is useful in determining the toughness of wood.

If the ends of a shaft are turned in opposite directions, or one end is turned and the other is fixed, all of the fibres except those at the axis tend to assume the form of helices.