PSI - Issue 75

Monisha Manjunatha et al. / Procedia Structural Integrity 75 (2025) 650–659 Monisha Manjunatha et al. / Structural Integrity Procedia (2025)

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Fig. 3. (a) Dimension of UFT sample; (b) Painted UFT sample ready for testing; (c) Surface roughness measurement; (d) Test sample in the Shimadzu USF-2000A. Fatigue crack propagation tests were conducted on Compact Tension (CT) samples in accordance with the ASTM E647 standard at room temperature. The study focused on three grades of steel: Grade 1, Grade 2, and Q355B. This study utilised compact tension (CT) specimens to analyse fatigue crack growth behaviour, following a procedure similar to that outlined in ASTM E647. Specimens are initially cut using waterjet technology and subsequently CNC machined to achieve the required CT test piece dimensions. The key dimensions of the specimens were W = 40mm , B = 10mm with a starter notch of a i = 10mm was introduced using electric discharge machining. The fatigue tests were conducted using an Instron 8801 servo-hydraulic testing machine with a 100 kN load frame configured as illustrated in Figure 4. A knife-edge fixture with a thickness of t=3.8 was mounted on the front face of the CT specimen to accommodate an Instron Crack Mouth Opening Displacement (CMOD) gauge with a gauge length of 10 mm. For Grade 1, the test was carried out with a stress ratio: R = 0.1, 0 and a loading frequency of 10 Hz. Before initiating the main test, the notch length was recorded and a sinusoidal cyclic load was applied to generate a pre-crack. Once the crack length reached approximately 12 mm, the pre-cracking phase was stopped. During this stage, the stress intensity factor range ΔK was monitored to comply with the standard. The test was terminated when the crack length reached 23 mm. Grade 2 was tested under the same conditions R = 0.1, frequency = 10 Hz. However, due to its higher ductility and microstructure composed of over-tempered martensite and ferrite, the crack growth behaviour differed from that of the fully martensitic Grade 1. Throughout the crack growth process, both side faces were monitored to ensure symmetric growth. If the difference in crack length between the crack growth faces exceeded 2 mm, the sample is discarded. For Q355B, a more ductile conventional structural steel, the test procedure remained largely the same, with the exception of the test frequency, which was increased to 20 Hz, and a stress ratio of R = 0. Additionally, a Back Face Strain (BFS) gauge with a resistance of 120Ω was affixed in a Quarter Bridge configuration to measure strain evolution on the back face of the specimen. While not a primary focus of this study, this setup was included with future corrosion-fatigue tests in mind. In corrosive environments, data will be extracted solely from the strain gauge; therefore, ambient condition data from the strain gauge will serve