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Schematic of the R‐curve depicting the initial ΔKthΔa = 0$\Delta K_{\text{th}}^{\Delta a \textrm{ } = \textrm{ } 0}$ under the hypothesis of an ideal closure‐free crack after compression precracking; with the development of the plastic wake, the ΔKth increases until the ΔKthstab$\Delta K_{\text{th}}^{\text{stab}}$ is reached.
Source publication
Fractures nucleated from defects and subjected to cyclic loading can experience propagation for a range of stress intensity factor ΔK well below the so‐called long crack threshold. This phenomenon is attributed to the development of crack closure mechanisms which may differ from those observed in laboratory tests conducted in accordance with curren...
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Citations
... The study of crack closure has been active for several decades from theoretical, experimental, and simulation perspectives [7][8][9][10][11]. Gradually, we could see these efforts paying off by providing prospective physical explanations for several phenomena common in the field of fatigue: mean stress sensitivity [11,12], physically short crack behavior [13], environmental effects [7,14] and notch fatigue [6,15,16], to name a few. However, widespread adaptation has yet to be realized, and phenomenological models are still commonly used. ...
... for positive stress ratios, and it should be corrected with a factor (1 − ) for negative stress ratios [12]. The terms th, and , ∈ [1, 2, … , ] denote the additional resistance development and saturation rate, respectively. ...
... However, reflections against the cyclic R-curves measured in [17] indicated that the baseline closure should be even lower (see Section 5.4). The data shows support for the modification of the intrinsic threshold with a factor of 1 − for negative stress ratios, as proposed by Patriarca et al. [12]. Given this input, the predicted KT-diagram could closely reproduce the observed El-Haddad type curve. ...
A model for the probabilistic cyclic R-curve has been derived. The model is based on the commonly used hypothesis of consecutive microstructural barrier fronts defining the erratic behavior of microstructurally short cracks and the transition to physically short cracks with declining importance of the microstructural features. The model can describe the linkage between the traditional cyclic R-curve analyses and the El-Haddad type Kitagawa-Takahashi diagrams with the asymptotic fatigue limit at small defect sizes. The model fit against the experimental non-propagating crack lengths perfectly matches the observed and predicted fatigue limit for several defect types and sizes. The presented framework can be used to analyze any geometry, loading history, or defect configuration, including defect interaction problems.
... The procedure adopted to fit the ℎ, values, the solid red line in Fig. 6(a), is the same followed by Beretta et al. [51] and by Barricelli et al. [52]. The crack growth properties of the present Ti6Al4V alloy are coherent with values from Polimi database on the same material, the same printing direction and with heat treatments at temperature that guarantee a similar microstructure [52,53], as well as with literature data [54,55]. ...
... The details of each step performed to define the cyclic R-curve are provided in the Appendix A. The analytical model used to fit the experimental data points, established by Maierhofer et al. [32], is based on Equation (5): The ΔK th defined by Equation (5) is divided into two components: an intrinsic one, ΔK Δa=0 th , and an extrinsic one, ΔK op th . The intrinsic component corresponds to the value of ΔK at zero crack extension and is defined as proposed by Patriarca et al. [57] in Equation (7): ...
Additive manufacturing (AM), particularly laser‐powder bed fusion (L‐PBF), has transformed the production of lightweight, high‐performance metallic components, with Scalmalloy emerging as a promising material due to its excellent strength‐to‐weight ratio, making it ideal for aerospace and automotive applications. However, process‐induced anomalies present challenges in achieving reliable fatigue performance, which requires robust methodologies for life prediction and defect‐tolerant design. This study investigates the fatigue behavior of L‐PBF Scalmalloy under various surface conditions (net‐shape and sandblasted) and orientations (vertical and inclined at 55°) using an advanced fracture mechanics framework. An experimental campaign evaluates fatigue strength under these varying conditions, with results compared between conventional models incorporating different assumptions regarding the long‐crack threshold (El‐Haddad model and NASGRO‐type equations) and R‐curve‐enhanced approaches. The analysis demonstrates the effectiveness of the R‐curve in improving fatigue predictions in all conditions tested, particularly in mitigating nonconservative results. This work advances the understanding of fatigue mechanisms in AM Scalmalloy, offering a framework for life prediction and defect‐tolerant design, ensuring more reliable applications of additively manufactured components in critical applications.
