previous section for room temperature tests. Variation of K = orz with the distance from

the crack tip is shown in Figure 3-19. Although the fracture loads decreases by 15% from

Specimen 1 to Specimen 3, fracture toughness increases by only 5%. The variation of

fracture toughness between the three specimens tested at cryogenic temperature is

insignificant. The result indicates that fracture toughness is not significantly affected by

the cryogenic conditions. But, the fracture load significantly decreases due to thermal

stresses present in the vicinity of the crack tip. For both cases at room and cryogenic

temperature, fracture toughness is estimated as 58 MPa. The result indicates fracture

toughness is a characteristic property not governed by temperature changes.

 It should be noted that the specimen dimensions and geometry are nominally the

same for room temperature and cryogenic temperature tests are the same. But, the

fracture load is significantly lower at cryogenic temperature. However the fracture

toughness, the critical stress intensity factor, seems to be the same although we input

lower loads in the finite element model. This is because there are significant thermal

stresses at cryogenic temperature which increases the stress intensity factor to a higher

value.