PSI - Issue 57

Lewis Milne et al. / Procedia Structural Integrity 57 (2024) 365–374 Lewis Milne et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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2. Materials The first material tested was the low-carbon ferritic-pearlitic structural steel Q355B, as defined in the standard GB/T 1591-2018 (2018). The testing of this material was also covered in a previous paper by the authors (Milne et al. 2022). The second material tested was S355JR structural steel, as specified in the standard EN 10025-2 (2019). This is the equivalent European grade to Q355B, with a similar prescribed yield strength, toughness and ductility. The chemical composition of the two steels are provided in Table 1. Table 1 - Chemical composition of the tested steels, according to the manufacturing certificates Material C Si Mn S P Cr Ni Cu Al N Q355B 0.193 0.200 0.88 0.005 0.021 0.020 0.010 0.010 0.030 0.004 S355JR 0.15 0.03 1.24 0.005 0.007 0.04 0.03 0.04 0.048 0.006 Representative micrographs of the two materials are presented in Figure 1. Comparing the two micrographs, it can be seen that both materials exhibit similar hot-rolled ferritic-pearlitic microstructures, however the Q355B contains larger, more equiaxed grains with significant banding, whereas the S355JR grains appear finer and more acicular. To evaluate the α -ferrite content of the two materials, a systematic point count procedure was carried out on the micrographs according to the procedures in ASTM E562-19 (2020), where it was observed that the Q355B and S355JR have similar ferrite contents of 79% and 76% respectively. In order to evaluate the mechanical properties of the materials, quasistatic tensile tests were carried out according to ASTM A370-22 (2022). Despite its finer, acicular grain structure, the S355JR actually exhibited a tensile and yield strength 8% lower than that of the Q355B. The evaluated material properties are presented in Table 2.

Table 2 - Key material properties of the tested steels Material Yield Strength, (MPa) Q355B 424 560 S355JR 388 513

Tensile Strength, (MPa)

Elastic Modulus, (GPa) 214 218

α -ferrite volume fraction

79% 76%

3. Test Specimens Hourglass UFT specimens with a standard gauge diameter of 3mm were designed using the equations described in the standard WES-1112 (2017). The resonant behaviour was verified using a harmonic analysis in ANSYS Workbench to ensure longitudinal deformation at 20kHz with no torsional or bending motion which would cause inconsistent loading throughout the cross section. As the mechanical properties of both tested materials are similar, the same UFT specimen geometry could be used for both materials. The gauge section of the specimens was polished to a mirror finish. The final specimen geometry, shown in Figure 2(a), resonated at 20.05kHz. The first tests to be carried out were the conventional frequency S355JR tests. As such, a standard cylindrical fatigue specimen geometry with a diameter of 6mm was used, as specified in BS ISO 1099 (2017). This geometry is given in Figure 2(b). In recent years, however, the importance of considering size effects when comparing fatigue tests across different frequencies has been highlighted. As discussed by Fitzka et al. (2021), it is important to ensure that the risk volume – the volume of the material which is experiencing at least 90% of the peak stress – is kept consistent between all test cases in order to avoid any size effects. The risk volume for this cylindrical sample is approximately 17x higher than that of the ultrasonic fatigue sample.

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Figure 2 - Dimensions of the (a) UFT specimen (b) S355JR 20Hz specimen and (c) Q355B 20Hz specimen