rapidly applied, as in the Charpy impact test, the amount of energy absorbed 

 during fracture decreases gradually as testing is performed at progressively 

 lower temperatures until, at some temperature, the absorbed energy drops 

 dramatically. This temperature is known as the nil ductility transformation 

 (NDT) temperature, the temperature at which the specimen exhibits little 

 ductility before fracture. The NDT temperature can be defined by Charpy V- 

 notch testing as (1) the temperature at which a certain absorbed energy is 

 attained, (2) the temperature at which 50 percent shear fracture is attained 

 on the broken specimen, or (3) the temperature at which a certain lateral 

 expansion is attained on the specimen opposite the notch. A common value 

 for minimum absorbed energy at the NDT temperature for ordinary constructional 

 steels is 20 newton meters (15 foot-pounds); however, acceptable impact 

 values are often stated in ASTM or other material specifications. Complete 

 procedures for conducting many mechanical tests on metals including impact 

 tests are given in ASTM A370. Figure 48 presents a representative plot of 

 absorbed energy versus temperature for Charpy V-notch tests on a typical 

 carbon steel. 



Figure 48. Plot of Charpy V-notch test 

 on a low carbon steel. 



Values obtained from Charpy V-notch testing cannot be directly used in 

 engineering calculations for design. Notch toughness values become signifi- 

 cant only when correlated with a particular type of structure in a particular 

 service. These values are useful to compare different materials. The NDT 

 temperature determined by Charpy V-notch testing has correlated rather well 

 with temperatures at which service failures have occurred for components of 

 the same steel . 



Factors affecting the notch toughness of a metal are as follows: 



(a) Chemical composition, 



(b) gas content, 



219 



