A new approach was implemented to confirm the start of ductile tearing relative to assessments by other methods in coupon specimens and pipe test data.&nbsp; The method develops on the Key-Curve methodology by Ernst/Joyce and is similar to the ASTM E-1820 Load Normalization procedure.&nbsp; It is consistent with Deformation Plasticity relationships for fully plastic behavior.&nbsp; Using this Experimental Key-Curve method, crack initiation can be determined directly from load versus CMOD or load versus load-line displacement data obtained from a fracture test without a need for additional instrumentation required for crack initiation detection. &nbsp;It is based on the fact that plastic deformation of homogeneous metals at the crack tip follows a power-law equation until the crack initiates.&nbsp; Crack initiation is determined at the point where the power-law fit to the load versus plastic part of CMOD curve in the region before crack-initiation deviates from the total experimental load versus CMOD curve. &nbsp;The procedure for power-law fitting of the data requires some care to be exercised such that the fitted data is above the elastic region and early small-scale plastic region, but include data before crack initiation.&nbsp; An iterative regression analysis was done to achieve this, which will be shown. The iterative fitting in this region typically results with R2 values that are greater than 0.9980.&nbsp; This method can be either used in conjunction with other methods such as d-c EP or unloading-compliance methods as a secondary (or primary) confirmation of crack initiation (and even crack growth); or can be used alone when other methods cannot be used. &nbsp;Furthermore, when using instrumentation methods for determining crack-initiation such as d-c EP or a-c EP in a fracture toughness test, it is good to have a secondary confirmation of the initiation point in case of instrumentation malfunction or high noise in the measured d-c or a-c EP signals. &nbsp;In addition, the Experimental Key-Curve procedure provides relatively smooth data for the fitting procedure, while unloading-compliance data to get small crack growth values frequently has significant variability; which is part of the reason that JIc by ASTM E1820 is determined using an offset with some growth past the very start of ductile tearing.&nbsp; In this work, the Experimental Key-Curve method had been successfully used to determine crack-initiation and demonstrate the applicability for several fracture toughness specimen geometries such as SEN(T), and C(T) specimens. This method was also used to determine crack initiation in flawed pipe tests under combined pressure and bending. &nbsp;In all the cases analyzed, the Experimental Key-Curve method gave consistent results that were in good agreement with other crack initiation measuring methods such as d-c EP, but seemed to result in less scatter.

Determination of Crack-Initiation in Fracture Toughness Testing Using an Experimental Key-Curve Methodology

Paper Proceedings: https://asmedigitalcollection.asme.org/PVP/PVP2021/volume/85314

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