"In chapters 1 and 2 we discussed the behavior of the simplest type of structural member-namely, a straight bar subjected to axial loads. Now we consider a slightly more complex type of behavior known as torsion. Torsion refers to the twisting of a straight bar when it is loaded by moments (or torques) that tend to produce rotation about the longitudinal axis of the bar. For instance, when you turn a screwdriver, your hand applies a torque T to the handle and twists the shank of the screwdriver. Other examples of bars in torsion are drive shaft axles, propeller shafts, steering rods, and drill bits."

ANGLE
A MEASURE OF A CHANGE IN DIRECTION THAT COMMONLY USES UNITS OF DEGREES OR RADIANS

ANGLE OF TWIST
THE ANGLE THAT AN OBJECT IS TWISTED OR ROTATED

MOMENT
A FORCE APPLIED TO AN OBJECT AT SOME DISTANCE FROM THE ROTATION AXIS THAT MAY CAUSE THE OBJECT TO ROTATE. THIS IS THE SAME AS A TORQUE. THE MOMENT IS CALCULATED AS THE PRODUCT OF A FORCE TIMES THE DIRSTANCE OF ITS POINT OF APPLICATION FROM THE AXIS OF ROTATION.

PURE SHEAR
THE FLATTENING THAT SOMETIMES OCCURS WHEN AN OBJECT IS ELONGATED IN ONE DIRECTION AND REDUCED IN SIZE AT RIGHT ANGLES TO THIS DIRECTION.

RATE OF TWIST
ANGULAR MEASURE OF THE TWIST PER UNIT LENGTH OR TIME

SHAFTS

SHEAR

TORSION

TWISTING MOMENTS