PSI - Issue 5

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

1021

3

Sulphur prints were prepared before the sample preparation to make visible existing segregation. The specimens were placed exclusively in the area of segregation, since the lowest material toughness had been expected due to the increased amount of impurity in these parts of the cross section. Based on the material properties to be determined and the associated test methods, for each material sample the following test specimens were produced and analyzed:

 6 cylindrical tensile specimens B5 according to DIN 50125,  12 Charpy impact test specimens according to EN ISO 148-1,  10 fracture mechanic compact tension specimens according to ASTM E1820-13.

All test specimens of one type were taken as far as possible one behind the other in the longitudinal direction of the profiles. That ensured that the provided material has approximately the same properties. Furthermore, it was taken care that during the preparation of the Charpy and fracture mechanic specimens the notch and fatigue crack has been located in an area of the cross section where rivet holes usually exist and thus cracks in the components are expected. Figure 1 shows the sulphur print of the cross section of an angle profile with the derived sectioning drawing for the test specimens.

Fig. 1 . Sulphur print and sectioning of material sample SGM21

2.2. Fracture toughness and reference temperature T 0 according to the Master-Curve-concept

The fracture behavior of structural steels corresponds to a temperature dependent transition of the material toughness from a ductile (upper shelf) to a brittle state (lower shelf). The transition region has a significantly higher variation than the upper and lower shelf. The wide range of the fracture toughness can be explained by the Weakest Link-Model of Landes and Shaffer (1980). That means that the weakest point of the microstructure at the crack front similar to the weakest link in a chain is responsible for the toughness of a specimen. At these weak points micro cracks will form which extend in an unstable manner and lead to a brittle failure. The reason for the wide scatter range is the stochastic distribution of the flaws of the microstructure in the ligament. The closer the weak point to the crack front, the lower the fracture toughness. With the length of the crack the probability increases that a flaw causes a cleavage fracture. For that reason the crack resistance is lower for thicker samples than for thinner ones. At the same time, the variations are less. For the brittle-ductile transition region they are captured within the Master-Curve-concept by Wallin (1998) by a Weibull distribution. For the failure probability P f a three-parameter distribution is assumed in which two parameters are permanently defined. The shape parameter m is 4, the threshold parameter K min restricts the lower bound of the fractural toughness of ferritic steels to 20 MPa√m.

   

   

   m

  

 K K K K  Jc

min

P

   1 exp

(1)

f

0

min