Effects of crack tunneling and plasticity on the elastic unloading compliance technique for SE(B) - Current limitations and proposals
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2018
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De Andrade L.G.F.
Bolognesi Donato G.H.
Bolognesi Donato G.H.
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Procedia Structural Integrity
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DE ANDRADE, LEONARDO GIANGIULIO FERREIRA; DONATO, Gustavo. Effects of crack tunneling and plasticity on the elastic unloading compliance technique for SE(B) - current limitations and proposals. Procedia Structural Integrity, v. 13, p. 1908-1914, 2018.
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© 2018 The Authors.Understanding how cracks behave in high toughness structural materials is critical for high responsibility applications. Design and fitness-for-service activities in this scenario are highly dependent on accurate resistance curves (J-R curves) and fatigue crack growth data (da/dN-ΔK). Such mechanical properties correlate crack driving forces with instantaneous crack size a, determined using real time techniques such as the Elastic Unloading Compliance (EUC). In this method, load P and displacement V (CMOD) allow real-time compliance (V/P) computation, whose increase can be used to predict increasing crack size a. Despite usually accurate, EUC predictions sometimes deviate from post-mortem analyses (errors above 10% were found by the research group) or present spurious effects such as apparent negative crack growth. Several phenomena may affect EUC accuracy, including: stress triaxiality, side-grooves, specimen rotation, closure, crack tip plasticity and crack tip tunneling. This paper investigates the effects of tunneling and plasticity on the EUC technique applied to SE(B) specimens of varying thicknesses and geometrical features. Very refined numerical simulations were conducted considering varying thicknesses, levels of crack depths and five levels of crack curvatures. Regarding tunneling, results show that for the same equivalent ASTM-E1820 straight crack, elastic compliance decreases with the increase of crack front curvature, leading to a predicted crack smaller than ASTM equivalent. No significant deviations were detected within ASTM limits, however, when such limits are violated, significant deviations occur. As a step to improve size predictions for tunneled cracks, a new proposal for determining the equivalent straight crack was developed and expands the applicability of current methods. In terms of plasticity, results revealed that near-tip plasticity causes a decrease followed by an increase in specimen's compliance, whose deviation varies depending on its geometry and material properties. The effects of varying plasticity levels on crack size estimation could be identified and discussed.