ELASTIC PROPERTIES OF MATEKIALS 11 



of stopping this tendency to spread being by boring a small hole at 

 either end of the tissmv. 



The explanation of the above is that for stresses within the elastic 

 limit tlu' temperature of the body is not raised, and consequently all 

 the work of deformation is stored up in the body to be given out 

 again in the form of mechanical energy upon removal of the stivss. 

 the clastic limit is surpassed, the friction of the mole- 

 cules sliding on each other generates a certain amount of heat, and 

 the energy thus transformed into heat is not available for restoring 

 the body to its original configuration. 



15. Hardening effects of overstraining. AYhen such materials as 

 iron and steel are stressed beyond tin* Mastic limit, it is found upon 

 removal of the stress that the effect of this overstrain is a hardening 

 of the material, and that this hardening increases indefinitely with 



pie, if a plate of soft steel is cold punched, the 

 material surrounding the hole is severely strained. After an interval 

 of rest the effects -ted in a hardening of 



the material which continues to increase for months. If the plate is 

 subsequently stressed, the inability of the portion overstrained to 

 yield with the rest of tin- plate causes the stress to be concentrated 

 on the- I, and results in a serious weakening of the plate. 



;cal instil 'Hardening due to overstrain are found 



in plates subjected to shearing and planing, armor plates pierced by 

 cannon balls, plates and bare rolled, hammered, or bent when cold, 

 wire cold drawn, etc. 



16. Fragility. In the solidification of melted bodies different 

 parts are unequally contracted or expanded. This gives ri>e to in- 

 ternal stresses, or what is called latent molecular action, and puts the 

 body in a state of strain without the application of any external 

 forces. For instance, if a drop of melted glass is allowed to fall into 

 water, the outside drop is instantly cooled and consequently 



icted, while the inside still remains molten. Since the part 

 t contract while molten, the contraction of the outside 

 causes such large internal stresses that the glass is shattered. 



;es in which latent molecular aci fcfl have the character 



i explosive, in that they are capable of standing a large static 



stress but are easily broken by a blow, and for this reason they are 



