PSI - Issue 75

Lewis Milne et al. / Procedia Structural Integrity 75 (2025) 419–425

423

L.Milne et al. / Structural Integrity Procedia 00 (2025) 000–000

5

700

y = 500.95x 0.0325 R² = 0.7226

Y_min Y_pl

σ_UTS

650

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y = 363.69x 0.0589 R² = 0.8613

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Stress (MPa)

y = 289.54x 0.0631 R² = 0.8613

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Strain rate (s -1 )

Fig. 3. σ pl (b) σ min and (c) UTS values vs strain rate for S275JR, obtained from testing at high strain rate.

700

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b

c

700

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Y min (MPa)

Y pl (MPa)

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UTS (MPa)

High strain rate data HSE2 HSE1 Milella

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High strain rate data HSE2 HSE1 Milella

High strain rate data HSE2 HSE1 Milella

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0

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1.00E-04 1.00E-02 1.00E+00 1.00E+02 1.00E+04

1.00E-04 1.00E-02 1.00E+00 1.00E+02 1.00E+04

1.00E-04 1.00E-02 1.00E+00 1.00E+02 1.00E+04

Strain Rate (s -1 )

Strain Rate (s -1 )

Strain Rate (s -1 )

Fig. 4. Milella and HSE Models fitted to (a) σ pl (b) σ min and (c) UTS values from high strain rate tests for S275JR.

To account for these oscillations in the load measurements, a curve smoothing procedure was applied. To achieve this, the plots were split into 3 regions: (1) the elastic region with the elastic slope as and extracted parameter E ; (2) the yield plateau region with the yield strength σ pl ; (3) the strain hardening region ending with the necking described as UTS. A linear trendline was applied to the elastic and yield regions, and a 4th-order polynomial fit, as is suggested in ISO 26203-2 (2011), was applied to the plastic region. From this, two potential yield strength values were defined: σ min , where the plastic region trendline intercepts the elastic region trendline, and σ pl , which corresponding to the average stress in the “plateau region”. The UTS was determined using the highest value on the plastic region trendline. This process is shown in Fig. 2d for an S355JR specimen which was tested at 6m / s. The full set of high strain rate stress-strain curves, alongside the curve smoothing results, will be provided in further publications. Plots of the σ min , σ pl and σ UTS values against the strain rate measured from the DIC for all of the tested specimens are presented in Fig. 3 for S275JR grade, as an example. As expected, a clear increase in strength with strain rate was observed for all of the tested materials. Taking the σ pl power law trendline as an initial measure of the strain rate sensitivity, it can be seen that S275JR, S355JR and S275J2 all exhibited an increase of approximately 140 MPa between the 0.2 m / s test rate and the 18 m / s test rate. A similar response is observed for σ UTS . From this comparison, these steel grades all appear to exhibit a similar sensitivity to the strain rate of deformation.

4. Discussion

An example of the strength vs strain rate results for S275J2 from the high strain rate tensile tests is presented in Fig. 4. As expected, a clear increase in strength with strain rate was observed for all of the tested materials. For all 4 materials, a similar increase of UTS in the region of 80-120 MPa was observed across the test range. For S355JR,