10 DISTRIBUTION OF STRESS. Art. 13 



shearing stresses and pure shearing; stress from tensile and coin- 

 pressive stresses of equal intensities acting at right angles to 

 eaoh other 1 . 



Practically, it is seldom necessary to find the resultant of 

 several stresses, but in addition to the three simple stresses, three 

 others are distinguished, which are due to a non-uniform distri- 

 bution of tension, compression and shear. The different kinds 

 of stress are, 



1. Tensile. Molecules pulling against each other. 



2. Compressive. Molecules pushing against each other. 



3. Shearing. Molecules sliding past each other. 



4. Bending. A combustion of tension and compression 

 in different parts. 



5. Buckling. A combination of compression and bending. 



6. Torsional. " Shearing by twisting. 



Tensile stresses occur in trusses ; shearing and bending 

 stresses in simple beams and girders; buckling stresses in the 

 compression members of all beams and in columns ; and torsional 

 stresses are usually avoided. 



Combinations of the above stresses are of frequent occur- 

 rence and are usually treated as being simply superimposed, but 

 this is not always strictly correct. 



13. Distribution of Stress. The ordinary cases of the dis- 

 tribution of the intensity of stress are illustrated in Fig. 6. 

 Cases (e), (f), and (g) are combinations of the other cases. 

 Beyond the elastic limit the distribution of stress is different ex- 

 cept for cases (a} and (&). 



Practically, it is difficult to get these ideal distributions of 

 stress. The following are some of the variations from ideal con- 

 ditions. 



(a) The material may not be of homogeneous structure as 

 in wood, for example; indeed any material will vary somewhat 

 from perfect homogeneity. 



(b) There may be initial stresses due to unequal heat 

 treatment; this is particularly true of cast iron in which the outer 

 parts are in compression, due to the cooling of the inner parts 



'For investigations of interesting conditions of stress, see works on 

 nx chanics. 



