MECHANICAL PROPERTIES OF WOODS GROWN IN" UNITED STATES. 23 
SPRINGWOOD. 
The lighter and more porous layer of wood in the annual rings of 
a tree is known as the springwood or early wood. As the name 
implies, it is produced in spring growth, or in the earlier part of 
the growing season. 
STRAIN. 
The deformation or distortion produced by a stress or force is 
known as strain. 
STRENGTH. 
(Seep. 6.) 
The term "strength" as ordinarily used is a very indefinite one. 
It is usually thought of in connection with external loads or forces. 
Strength in its broad sense is a measure of the mechanical prop- 
erties, or of the ability of a timber to resist stress or deformation. 
Thus, strength in shear, strength as a beam, strength as a post, 
hardness, stiffness, toughness. These last three properties are not 
always thought of in connection with the term strength, but are 
unconsciously included whenever they are important in a specific 
use. See example of this as given on page 6. 
Seldom, if ever, do any two species contain all the various proper- 
ties in the same degree. This accounts for the special uses of the 
different species. 
Much confusion often arises from comparing species for a special 
use on the basis of properties or strength values not of first impor- 
tance in the specific instance. 
STRESS. 
Stress is distributed force. 
Fiber stress is the distributed force tending to compress, tear apart, 
or change the relative position of the wood fibers. 
Stress is measured by the force per unit area. Thus a short col- 
umn 2 inches square (4 square inches) and supporting a load of 2,000 
pounds will be under a stress or fiber stress of 500 pounds per square 
inch. 
SUMMERWOOD. 
(See p. 9.) 
Summerwood is that denser layer of wood in the annual rings of a 
tree which is put on in summer or the latter part of the growing 
season. 
TANGENTIAL. 
Tangential, as applied in this publication, means tangent to or par- 
allel to the curves of the annual rings in a cross section. Thus a tan- 
gential surface is a surface perpendicular to the radius of a tree. 
