ELASTIC PROPERTIES OF MATERIALS 9 



12. Classification of materials. Materials ordinarily used in engi- 

 neering construction may be divided into three classes, plastic, 

 supple, and elastic. 



Plastic materials are characterized by their inability to resist stress 

 without receiving permanent deformation. Examples of such mate- 

 rials are lead, wet clay, mortar before setting, etc. 



Supple bodies are characterized by their lack of stiffness. In other 

 words, supple bodies are capable of undergoing large amounts of 

 elastic deformation without receiving any plastic deformation. In 

 this respect plastic and supple bodies exhibit the two extremes of 

 physical behavior. Examples of supple bodies are rubber, copper, 

 rope, cables, textile fabrics, etc. 



Elastic bodies comprise all the hard and rigid substances, such 

 as iron, steel, wood, glass, stone, etc. For such bodies the plastic 

 deformation for any stress within the elastic limit is so small as 

 to be negligible ; but when the stress surpasses this limit the plastic 

 deformation becomes measurable and gradually increases until rup- 

 ture occurs. This permanent deformation is the outward manifes- 

 tation of a change in the molecular arrangement of the body. For 

 a stress within the elastic limit the forces of attraction between the 

 molecules are sufficiently great to hold the molecules in equilibrium ; 

 but when the stress surpasses the elastic limit, the molecular forces 

 can no longer maintain equilibrium and a change in the relation 

 between the molecules of the body takes place, which results in the 

 body taking a permanent set. 



Rigid bodies have the character of supple bodies when one of 

 their dimensions is very small as compared with the others. An 

 instance of this is the flexibility of an iron or steel wire whose 

 i is very great as compared with its diameter. Furthermore, 

 1 todies behave like plastic bodies when their temperature is 

 raised to a certain point. For example, when iron and steel are 

 heated to a cherry redness they become plastic and acquire the 

 prnj.criy of uniting by contact. 



13. Time effect. It has been found by experiment that elastic 

 deformation is manifested simultaneously with the application of a 



B, but that plastic deformation does not appear until much later. 

 Tli us if .1 constant load acts for a considerable time, the deformation 



