96 THE PLANT CELL WALL 



stronger natural fibers approach steel and common glasses. Ex- 

 perimental glass fibers ten-fold stronger than ordinary steel have 

 been produced, however. If tensile strength is divided by density, 

 the specific strength is obtained. This corrected figure may be 

 regarded as the maximum length of material which could be 

 hung vertically to bear its own weight. On this basis, wood, mild 

 steel, copper wire and many synthetics have similar values. Cotton 

 fibers, are comparable with strong steel wire, but ramie, flax, 

 nylon are far stronger. 



Compressive (crushing) strength is a measure of the maximum 

 stress which the material will withstand under longitudinal compres- 

 sion. It is expressed in the same manner as tensile strength. Wood 

 and synthetics such as nylon or phenol-formaldehyde resins are 

 comparable, and somewhat more resistant to compression than 

 concrete (and ordinary brick). Glass and metals are far stronger. 



The maximum stress developed at failure in the surface of 

 standard test beams supported at their ends is the modulus of 

 rupture, or flexural strength. This property is expressed by: 



flexural strength = 3/2W„, L /bd"-, 



Where W m = maximum load, L = length between supports, 

 b = width of beams, d = depth of beam. Woods, which range 

 from 6-14 kg/mm 2 , are similar or somewhat superior to flexural 

 strength to phenol-formaldehyde resins or nylons; superior to 

 some silicates; and from two- to three-fold lower than glass fiber 

 and cast iron. 



These comparisons taken together permit a reasonably favor- 

 able evaluation of woods for many purposes of construction. 

 In many instances, therefore, wood may compete with other struc- 

 tural materials. 



Other physical properties must also be considered as they 

 affect the utility of the various structural material. The specific 

 gravity of woods (with the exception of ebony) places them among 

 the lighter materials for their bulk. Until the advent of expanded 

 dolymers (e.g. polyurethane or polystyrene foams), woods were 

 almost uniquely in this class. The thermal conductivity of wood 

 is low when compared to most other materials, natural and synthe- 



