10 STRESS AND STRAIN. 



shape, and the force applied in any manner, still, a 

 deformation will follow. 



It is very important to notice that the deformation s<> 

 produced must oe of one of two kinds. If a deformation 

 follows a force applied, and if after the deforming 

 force has been removed the body returns to its original 

 dimensions the deformation is said to have been elastic. 

 On the other hand, if, after removing the deforming 

 force, the body still retains its deformed condition, it is said 

 to be to some extent plastic. In what follows here, the 

 behaviour of bodies under both conditions will be dealt with. 



The materials of construction all exhibit these two 

 properties of elasticity and plasticity to a more or less 

 marked degree. It may be stated that no material of 

 construction is perfectly elastic, nor is any substance 

 perfectly plastic. In most of the materials elasticity is 

 practically maintained as the v force continues to be increased 

 up to a certain point, called the elastic limit, and after 

 this point has been reached and passed the body becomes 

 partially plastic, and elasticity approximate^ ceases. 



In case of the other materials, such as cement and 

 masonry, the properties of elasticity and plasticity do not 

 exhibit themselves to anything like so marked a degree as 

 they do in the case of the metals, but still, by means of 

 delicate appliances, these properties may be detected, 

 though only in a small degree. When forces are applied 

 to such materials, the tendency is, if these forces are 

 sufficiently great, to cause rupture, crumbling, and dis- 

 integration of the substance. 



3. Stress and Strain. Where bodies and materials are 

 employed in the formation of works of construction they 

 are subjected to what are called stresses, the stress upon a 

 body arising from either a force or a system of forces. 

 Stresses may be of various magnitudes, and may be 

 produced by the Aveights of the structures themselves in 

 which they occur, or by causes which are external to the 

 structure. 



The result of the action of a stress upon a body is 

 always to produce a strain, which is a deformation of the 

 body. If the deformation only exists so long as the force is 

 acting, the strain is elastic ; if the deformation remains 

 after the removal of the stress, the strain is plastic. For 

 instance, if a piece of indiarubber be pulled by a longi- 

 tudinal stress, it will be elongated, and when the force is 

 removed the iudiarubber will return to its original 



