STRENGTH OF MATERIALS 205 



If we make a list of the various ways in which mechanical force 

 can be applied in the human body, e.g., by pressing, pulling, twisting, 

 bending, etc., and examine them carefully, we find that they can 

 be resolved into two only, viz., pressing and pulling. Tu attempting 

 to bend something we apply simultaneously a compressing and a 

 stretching force. Torsion, too, mav be resolved into these two 

 forces applied tangcntially to one another. 



As every substance develops within itself a force equal in 

 intensity, but opposite in sign from the applied force, we may 

 state that all the stresses developed in a material are of two kinds, 

 e.g. tensile and compressive. 



It is obvious that substances may be so formed that they bear 

 up under compression, but are unable to maintain their integrity 

 of form or of structure when pulled, or vice versa. For example, a 

 chain will withstand a great pulling force, but crumples up when 

 compressed along its length. The stresses developed in a walking 

 stick are parallel to its length, i.e. it may be pressed or pulled 

 by forces acting axially without undue danger of fracture, but a 

 bending force easily produces distortion. The axial forces produce 

 stresses that lie wholly within the material and. with reasonable 

 force, no shear occurs. 



Shear stress exists between two parts of a body in contact when 

 the two parts exert equal and opposite forces on each other 

 laterally in a direction tangential to their surface of contact. For 

 example, there is a shear stress on a l3olt or rivet when the two 

 plates which it holds together are pulled or pushed in opposite 

 directions in a plane parallel to the plane of normal cross-section 

 of the bolt. 



On the other hand, force applied to a walking stick at both 

 ends so as to bend it, very easily produces fracture. The line 

 joining the points of application of the two forces in this case lies 

 almost wholly outside the stick. 



There are two factors to be considered when we are dealing with 

 the ability of structures to maintain their integrity under applied 

 force. These are (1) strength of material, and (2) nature of 

 structure. 



(1) Strength of Material. The ability of any material to with- 

 stand stress may be found from various elastic constants, the 

 so-called moduli, of which there is one for each type of stress and 

 strain. 



Strain is the alteration of shape or dimensions resulting from 

 stress, and one has a tensile strain resulting from a stretch, com- 

 pressive strain from a thrust, and shear strain from shear stress. 

 If after the stress causing a strain has been removed, the strain 



