PSI - Issue 5

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

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The scale parameter K 0 is determined from the values of fracture toughness K Jc for unstable brittle failure. The values K Jc are assigned to a normalized toughness-temperature-curve. The Master-Curve describes the dependence of the median fracture toughness of 1T-samples with     0 ) ( 30 70 exp 0,019 T T K Jc med      . (2) The transition and reference temperature T 0 is defined by a median value of the fracture toughness of 100 MPa√m. The test standard ASTM E1921 is strictly only valid for macroscopically homogenous material. The multimodal Master-Curve approach by Wallin et al. (2004) allows the analysis of data sets which consist of several subsets, for example records of different batches of a material or non-homogeneous material. Therefore, this method is suitable for a joint evaluation of the fracture toughness of different mild steel samples. However, it is much more complex than the standard method according to ASTM E1921. The results of this evaluation for the analysed mild steels are shown in Figure 2 compared to the standard Master-Curve of ASTM E1921. As expected, the fracture toughness of the inhomogeneous samples are better represented by a multimodal Master Curve. Only two (2.4 %) of the totally determined 83 K Jc-1T -values are below the curve of 2 % fracture probability. However, it can be seen that (except the sample PA2) the fracture toughness of the old mild steels is also described sufficiently precise by the standard Master-Curve according to ASTM E1921. Based on experimental data and with the multimodal evaluation the reference temperature T 0 = – 30 °C is determined as well as a characteristic value of the fracture toughness (5 %-fractile) is obtained with     30 25,9 29,7 exp 0,0186 1 ,5%       T K MM Jc T . (3)

Fig. 2. Evaluation of all test series according to ASTM E1921 and the multimodal approach

2.3. Correlation between material toughness and grade of steel

To compare the fracture toughness of the analyzed old mild steels with those of higher mild steel grades, additional research was carried out by Sieber (2016) with four material samples from a welded steel truss structure (year of construction: 1937). All samples could be identified as Siemens-Martin steels. Similar to the research described above,