PSI - Issue 42

S. Lindqvist et al. / Procedia Structural Integrity 42 (2022) 42–49 Author name / Structural Integrity Procedia 00 (2019) 000–000

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curves. The specimen is unloaded and reloaded at specific intervals. The data is used for calculating the J-integral and crack length at each unloading increment. In the basic method, the complete area under the load-displacement record is used for calculating the final J integral. The crack length is measured from the fracture surface. This method results only to one J-integral and crack data length point, whereas with the unloading compliance method one obtains several points that form the J-R curve. If the basic method is used, several specimens are required to form the J-R curve. Normalization method is in a sense a combination of the basic method and the unloading compliance method. No unloadings are required in the load-displacement record, but the data is normalized and from the normalized data the corresponding J-integral and crack length value can be estimated for each load-displacement data point. The downside is that the method is only applicable as long as the final measured crack size corresponds to a crack extension of not more than 4 mm or 15 % of the initial uncracked ligament, whichever is less. If this crack extension is exceeded, this specimen cannot be analyzed according to the normalization method as described in ASTM E1820-15.

Liquid nitrogen

Specimen

Copper coil

Figure 1. a) Cooling of the specimen. b) The location of the thermocouples. The depth of the holes varied from 12.5 mm to 4 mm.

3. Results and discussion Figure 2 shows the J-R curves for the material at room temperature, 100, 200, and 300 ° C [5]. The fracture toughness is lowest at 200 ° C. at room temperature the material tends to split (delamination of the ligament). Splitting is typical for rolled steels. The origin of splitting is related to the stress triaxiality ahead of the crack-tip and the low toughness of the materials in the rolling plane, which can cause delamination in the ligament in a plane perpendicular to the main crack. Splitting results into pop-ins in load-displacement records of fracture tests. Splitting is related to some metallurgical characteristics of the material like crystallographic texture, elongated inclusions, banded microstructure, and/or central segregation. Splitting does not affect the intrinsic fracture toughness in the original crack growth direction. [6]