PSI - Issue 48

Haris Nubli et al. / Procedia Structural Integrity 48 (2023) 73–80 Nubli et al. / Structural Integrity Procedia 00 (2023) 000 – 000

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investigations into the Ductile-to-Brittle Transition Temperature (DBTT). Acquiring knowledge regarding the DBTT of a specific metal or material can help prevent potentially catastrophic incidents (Braun et al., 2020). One approach to assessing the energy absorption capability required for material failure is through the utilization of a Charpy V notch test. The ASTM E23 standard test methods for notched bar impact testing of metallic materials serve as the accepted procedure for conducting Charpy V-notch tests (ASTM, 2019). In this test, an impact load is applied to the specimen by means of a striker, while the specimen is positioned on an anvil featuring an 80o slope perpendicular to the direction of impact from the striker (ASTM, 2019). To provide a visual representation, Figure 3 depicts the Charpy V-notch test schematic diagram per the guidelines outlined in ASTM E23. A cryogenic Charpy V-notch test was performed by (Kim et al., 2014) using SS400 and A-grade mild steels. Similarly, to cryogenic tensile testing, the temperature range spanned from 25°C to -196°C. To achieve the temperature of -196°C, liquid nitrogen was utilized as a safer cooling agent for the specimens. The authors reported the temperature transitions of A-grade and SS400 mild steels to be -50°C and -25°C, respectively. At the transition temperature, the absorbed energy ranged from 160J to 40J for A-grade steel and from 60J to 20J for SS400 mild steel. Consequently, it can be inferred that A-grade steel exhibits favorable characteristics for ship structures, particularly since such structures are predominantly subjected to dynamic loads, such as waves. Moreover, the findings of this study (Kim et al., 2014) indicate that A-grade steel remains reliable for ship structures deployed in arctic regions.

Fig. 3. Charpy V-Notch bar impact test scheme of ASTM E23 (ASTM, 2019).

Furthermore, structures susceptible to accidental loads, such as the accidental release of cryogenic substances, necessitate the utilization of higher-strength materials, including AH to FH grade or HSLA (High-Strength Low Alloy) steels. (Majzoobi et al., 2016) demonstrated that HSLA steel exhibits a temperature transition down to -80°C, assuming a moderate strain rate level. Similarly, FH32 steel displayed a comparable transition temperature, averaging at -70°C (Noh et al., 2018). FH32 grade steel exhibited the ability to withstand absorbed energy of up to 150 J at its transition temperature. Conversely, according to the experiment conducted (Noh et al., 2018), FH32 steel fractured with an absorbed energy of 25 J at -80°C, indicating its transition into a brittle state. EH32 steel, as reported by (Noh et al., 2018) displayed an identical transition temperature and absorbed energy capability. Table 2 provides the Ductile to-Brittle Transition Temperature (DBTT) values for various materials. In summary, high-strength steel exhibits excellent capacity to absorb accidental energy based on Charpy V-notch test results, making it suitable for applications in LNG-carrying vessels, such as LNG bunkering or LNG-fueled ships. According to (Nubli et al., 2022b), cryogenic gas exposure resulted in an impact of -18°C under severe conditions, assuming a release duration of 15 seconds, which aligns with the general response time of the emergency shutdown valve in LNG carriers.

Table 2. Transition temperatures of various steels. Material

Transition Temperature ( o C)

Reference

A-grade mild steel

-50

(Kim et al., 2014)