PSI - Issue 5

Lars Sieber et al. / Procedia Structural Integrity 5 (2017) 1019–1026 Sieber, Stroetmann / Structural Integrity Procedia 00 (2017) 000 – 000

1024

6

For minimum Charpy impact energy of about 5 J the minimum fracture toughness K mat results in 25,5 MPa√m. This lower threshold value is below all crack toughness’s K Jc-1T of the analyzed mild steels und provides thereby a conservative estimation. The fracture toughness in the lower transition region of the toughness-temperature-curve can be estimated for a particular probability of failure P f as a function of the temperature T and the transition temperature T 27J with Equation (6). This approach is also applicable for old structural steels, but causes very conservative values of toughness for mild steels too.

0,25

0,25

   

   

  













K

T T

20 11 77 exp 0,019    



 

   

3 25



1 ln 1 

(6)

mat

J

27

B

P

f

The relationship between Charpy impact energy and fracture toughness (which is very often used in steel engineering) is the correlation of transition temperatures T 27J and T 0 by Sanz (1980). The modified type in Equation (7) is a feature of the steel grade selection in EN 1993-1-10. If one compares the toughness of the analyzed old mild steels with current structural steel grades (Figure 4), it is obvious that the correlation

T T J    18 27 0

C

(7)

is not valid for old mild steels. For comparable reference temperatures T 0 of the fracture toughness the transition region in the Charpy impact test is at significantly higher temperatures. This applies especially for rimmed steels.

Fig. 4. Toughness of old mild steels and modern structural steels compared to the modified SANZ correlation

3. Fracture behaviour of components made of old mild steels with punched holes

The danger of brittle fracture in riveted steel structures is primarily affeted by material-specific, construction specific and production-related influences. These have negative effects with regard to an enhanced tendency to ageing due to increased nitrogen concentration of the converter steels (Bessemer and Thomas procedure).