Session: MF-20-01 Material Quality and Failure Analysis-1

Paper Number: 122390

122390 - The Influence of Secondary Damage Mechanisms on the Suitability of the Single Specimen Unloading Compliance Technique to Assess the Fracture Toughness of SA508 Gr. 4N and A533 B Nuclear Pressure Vessel Steel 

High-integrity components for first-of-a-kind new nuclear reactors are subjected to extensive fracture toughness testing (FTT) qualification programmes to ascertain their resistance to ductile fracture and justify the toughness values used in safety case arguments. One cost-effective route to delivery of such programmes is the single-specimen FTT test method, which seeks to establish a complete J-integral resistance (J-R) curve from a single specimen by calculating the incremental J-integral with increasing crack extension. This is achieved by monitoring the change in compliance associated with incremental crack growth during testing. Importantly, it assumes that a single primary crack is generated from the tip of a pre-existing fatigue crack during fracture toughness testing, the extension of which can be directly correlated to the change in compliance of the test specimen.

The study described herein follows an investigation into the suitability of the single specimen unloading compliance technique for characterising the J-R fracture toughness behaviour of an A533B pressure vessel steel.  By comparing to data acquired from fracture toughness testing of SA 508 Gr. 4N, it is shown that high toughness, low yield strength structural materials can, despite meeting the minimum allowable specimen size requirements of ASTM E1820, result in secondary damage mechanisms that are thought to be driven by global plasticity and a loss of crack tip constraint. This work compliments previously reported work on linking fracture assessment methodology to fracture toughness testing procedures.

This secondary ductile damage, revealed through metallographic sectioning, extends below the fracture surface and is orientated in the primary crack plane. These secondary ductile cracks were observed to not only originate in the crack tip blunting region but also in the ductile tearing region; in some cases, secondary cracks were observed to be on the order of mm in length running parallel to the primary crack. This work shows how the presence of this secondary damage influences the compliance measurements obtained during single-specimen analysis techniques. The work also demonstrates how these secondary damage mechanisms may not be captured when using a conventional multi-specimen fracture toughness testing approach, which is often credited with being a more reliable testing approach over single specimen techniques.

Presenting Author: Ronnie Woodward Jacobs

Presenting Author Biography: TBC

Authors:

Adam Cooper Jacobs
Ronnie Woodward Rolls-Royce SMR
Andrew Harrison Jacobs
Andrew Wisbey Jacobs