12 STEENGTH OF MATEKIALS 



called brittle or fragile. The explanation of fragility is that the vil ra- 

 tions caused by a blow are reinforced by the latent internal stresses 

 until rupture ensues. 



17. Initial internal stress. In certain bodies, such as cast iron, 

 stone, and cement, a state of internal stress may exist without the 

 application of any external force. This initial internal stress may be 

 the result of deformation caused by previously applied loads, or may 

 be occasioned by temperature changes, as mentioned in the preceding 

 article. The first load applied to such bodies gives them a slight 

 permanent deformation, but under subsequent loads their behavior 

 is completely elastic. The first load, in this case, serves to relit -\ v 

 the strain due to initial internal stress, and consequently tin- U'havior 

 of the body under subsequent loads is normal. A body which is free 

 from internal stress is said to be in a "state of ease," a term which 

 is due to Professor Karl Pearson. 



18. Annealing. The process of annealing metals consists in lu-at- 

 ing them to a cherry redness and then allowing them to cool slowly. 

 The effect of this process is to relieve any initial internal stress, or 

 stress due to overstrain, and put the material in a state of ease. 

 Hardening due to overstrain is of frequent occurrence in engines i 

 and the only certain remedy for it is annealing. If this i> im; 

 ticable, hardening can be practically avoided by substituting b'i 

 for punching, sawing for shearing, etc. 



19. Temperature stresses. A property especially characteristic of 

 metals is that of expansion with rise of temperature. The proportion 

 of its length which a bar expands when its temperature is raised one 

 degree is called the coefficient of linear expansion, and will IK- <lrii..i.-.l 

 by L. The following table gives the value of L for one 

 Fahrenheit for the substances named. 



Steel ' hard L = .00(H, 



" soft L = .000M;i 



Iron ' cast L = . 0000063 



g ht = .0000008 



i = . 0000028 

 L = . 0000047 







If a body is fixed to immovable supports so that when the tem: 

 ature of the body is raised these supports prevent it from expanding, 



