[Illustration:  FIG. 1.—­Stress-strain diagrams of two longleaf pine beams.  E.L. = elastic limit.  The areas of the triangles 0(EL)A and 0(EL)B represent the elastic resilience of the dry and green beams, respectively.]

There are three kinds of internal stresses, namely, (1) tensile, (2) compressive, and (3) shearing.  When external forces act upon a bar in a direction away from its ends or a direct pull, the stress is a tensile stress; when toward the ends or a direct push, compressive stress.  In the first instance the strain is an elongation; in the second a shortening.  Whenever the forces tend to cause one portion of the material to slide upon another adjacent to it the action is called a shear.  The action is that of an ordinary pair of shears.  When riveted plates slide on each other the rivets are sheared off.

These three simple stresses may act together, producing compound stresses, as in flexure.  When a bow is bent there is a compression of the fibres on the inner or concave side and an elongation of the fibres on the outer or convex side.  There is also a tendency of the various fibres to slide past one another in a longitudinal direction.  If the bow were made of two or more separate pieces of equal length it would be noted on bending that slipping occurred along the surfaces of contact, and that the ends would no longer be even.  If these pieces were securely glued together they would no longer slip, but the tendency to do so would exist just the same.  Moreover, it would be found in the latter case that the bow would be much harder to bend than where the pieces were not glued together—­in other words, the stiffness of the bow would be materially increased.

Stiffness is the property by means of which a body acted upon by external forces tends to retain its natural size and shape, or resists deformation.  Thus a material that is difficult to bend or otherwise deform is stiff; one that is easily bent or otherwise deformed is flexible.  Flexibility is not the exact counterpart of stiffness, as it also involves toughness and pliability.

If successively larger loads are applied to a body and then removed it will be found that at first the body completely regains its original form upon release from the stress—­in other words, the body is elastic.  No substance known is perfectly elastic, though many are practically so under small loads.  Eventually a point will be reached where the recovery of the specimen is incomplete.  This point is known as the elastic limit, which may be defined as the limit beyond which it is impossible to carry the distortion of a body without producing a permanent alteration in shape.  After this limit has been exceeded, the size and shape of the specimen after removal of the load will not be the same as before, and the difference or amount of change is known as the permanent set.