PSI - Issue 48

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

76 4

characterized by exceptionally low strain rates typically below 0.001 /s (Paik et al., 2017; Park et al., 2015). Figure 2 presents a graphical representation of various tensile test results obtained for DH36 steel.

1000

0.70

Experiment:

Experiment:

Paik et al., 2017 (6.00mm) Park et al., 2015 (6.00mm) Cho et al., 2014 (10.20mm) Cerik et al., 2020 (12.50mm) COV (%): 9.29

Paik et al., 2017. (6.00mm) Park et al., 2015 (6.00mm) Cho et al., 2014 (10.20mm) Cerik et al., 2020 (12.50mm) COV (%): 3.98

0.65

900

0.60

0.55

800

0.50

700

0.45

0.40

600

Ultimate Tensile (MPa)

0.35

Fracture Strain (mm/mm)

0.30

500

0.25

400

0.20

-180 -160 -140 -120 -100 -80 -60 -40 -20

0

20

40

-180 -160 -140 -120 -100 -80 -60 -40 -20

0

20

40

Temperature ( o C)

Temperature ( o C)

(a) (b) Fig. 2. Tensile test results of the past studies using DH36 high-strength steel (with various specimen thickness): (a) ultimate tensile strength, and (b) fracture strain (Cerik and Choung, 2020; Cho et al., 2014; Paik et al., 2017; Park et al., 2015). Moreover, the influence of strain rate on the hardening behavior of the steel is an important factor to consider. A previous study conducted by Paik et al. (2017) examined the effect of strain rate on various steel grades and aluminum alloys across a temperature range of 20°C to -60°C. The findings revealed that as the temperature decreased, both yield and ultimate tensile strengths increased, while fracture strain and ductility decreased, indicating a more brittle nature of the steel. Specifically, the study reported a yield strength increase of 10.52%, 13.52%, and 14.44% when transitioning from quasi-static to dynamic (2 /s) loadings at temperatures of 20°C, -20°C, and -60°C, respectively (Park et al., 2015). Additionally, the ultimate tensile strength exhibited a minor increase in hardening, namely 6.87%, 4.10%, and 3.60%, at temperatures of 20°C, -20°C, and -60°C, respectively (Park et al., 2015). The effect of strain rate is typically observed in dynamic loading scenarios, such as impact in drop tests. To account for strain rate hardening, the Cowper-Symonds plastic constitutive equation can be utilized, incorporating the strain rate (/s) and two coefficients ( C and q ) (Cowper and Symonds, 1957; Hughes and Paik, 2010; Sohn et al., 2017; Sohn and Jung, 2022). The Cowper-Symonds equation can be expressed as follows: σ Yd =1+( C ε̇ ) q 1 σ Y (1) where σ Yd is the scaled stress (MPa), σ Y is the initial stress (MPa), ̇ is the strain rate (1/s), C (1/s) and q are coefficients. Several experiments have been generated the coefficients depending on the material type. Table 1 shows the Cowper Symonds coefficient.

Table 1. Coefficients in Cowper-Symonds plastic constitutive equation of various steels. Material C ( /s) q

Reference

Mild steel

40.4

5.0 5.0

(Cowper and Symonds, 1957)

High-strength steel

3200.0

(Paik and Chung, 1999)

100.0 5000.0 39,033.0

10.0 5.3 5,13

(Langdon and Schleyer, 2005) (Forrestal and Sagartz, 1978) (Hsu and Jones, 2004)

Stainless steel

4. Cryogenic Charpy V-Notch Test Slowing down the fracture progression of materials, particularly metals intended for general purposes is imperative to avert sudden failures. It is beneficial, at this juncture, to revisit the various fracture types, as it can facilitate further