PSI - Issue 66

Aditya Khanna et al. / Procedia Structural Integrity 66 (2024) 370–380 Author name / Structural Integrity Procedia 00 (2025) 000–000 The specimens were pre-cracked until approximately 2 mm crack extension and the applied SIF range, Δ , during pre-cracking was around 14 MPa.m 1/2 . After pre-cracking, block loading was repeatedly applied to the specimens until failure. Over 1 million cycles were applied to both specimens after pre-cracking and before fracture failure occurred. The maximum load was kept constant for the entire duration of the test at max = 9.5 kN. The minimum load min was stepped between 0.095 kN ( = 0.01) and 4.75 kN ( = 0.5). The number of cycles applied per load block (constant ) was adjusted throughout the test to achieve a roughly uniform spacing of the data points on the logarithmic / versus ∆ curves (Sales et al., 2024). The compliance offset method described in ASTM E647 was used to estimate the crack tip opening load. This differential method is highly sensitive to external noise in the strain and load signals. Hence, a lowpass Butterworth filter was applied to the raw load and strain signals. The bandpass frequency was set to 10 times the loading frequency to filter out high-frequency noise without altering the nonlinear response generated by crack closure. The fatigue test was nominally performed at a loading frequency of 10 Hz. However, the test frequency was intermittently reduced to 2 Hz for crack tip opening load estimation. Approximately 100 cycles were applied at the lower test frequency (2 Hz) and the datapoints for the crack tip opening load ratio and residual back face strain presented in the following sections are the “average” values of those 100 cycles. 4. Experimentally measured crack tip opening loads Results for the crack tip opening load, op (or the corresponding crack tip opening SIF, op ) can be expressed in terms of the non-dimensional opening load ratio, defined as = max − op max − min = max − op max − min . (7) Plasticity-induced crack closure (PICC) is the dominant mechanism of crack closure for long cracks in ductile materials, such as the present test specimens (Pippan and Hohenwarter, 2017). During fatigue crack growth testing as per ASTM E647 recommendations, the monotonic plastic zone size at the crack tip is much smaller than other characteristic dimensions, i.e., small-scale yielding conditions prevail. Under small-scale yielding, the opening load ratio depends predominantly on the stress ratio (Newman Jr., 1984) and the dependance of the plasticity-induced crack tip closure the specimen geometry, remaining ligament width, and ratio of applied stress to the flow stress is relatively weak. Hence, at constant stress ratio , the opening load ratio is not expected to change significantly during a fatigue test. Figure 2 shows the change in opening load ratio at constant stress ratio with increasing normalised crack length, ⁄ . Regardless of the orientation of the crack path relative to the weld deposition direction, a strong spatial variation in the opening load ratio is observed. At any given stress ratio, relatively constant values of opening load ratio are only obtained at ⁄ >0.5 . It is hypothesised that the variation in the opening load ratio at constant stress ratio is predominantly due to the residual stress field in the additively manufactured test specimens. The stress ratio in Figure 2 corresponds to the “applied” load or SIF range, i.e., ap = min , ap max , ap ⁄ . However, the residual stress field is superimposed on the applied stress field. Hence, the “actual” stress ratio at the crack tip is act = min , ap + res max , ap + res . (8) In other words, the residual stress field at the crack tip modifies the mean stress and subsequently, the crack opening load. To calculate the actual stress ratio given by Eq. (8), the residual SIF, res must be obtained using the methods reviewed in Section 2. 375 6