The Master Curve method utilizing very small C(T) specimen with 4 mm thickness (Mini-C(T)) has been recognized as a promising method for the fracture toughness evaluation in brittle-ductile transition temperature range. Further application of Mini-C(T) specimen on ductile fracture toughness evaluation in the upper shelf temperature range is demanded for a characterization of irradiated reactor pressure vessel (RPV) steels. Specimen size effect has not always been considered in ductile crack growth, however, recent researches report that smaller specimens may provide lower *J~Ic~* or *J-R* curve compared to larger specimens. Understanding the physical basis behind such specimen size effect and an establishment of correction method for the effects are essential.

To study the *J-*Δ*a* relationship for different specimen sizes, ductile crack growth tests on Mini-C(T) and 0.5T-C(T) specimens were conducted at room temperature for a RPV steel, SQV2A. The *J-R* curves between Mini-C(T) and 0.5T-C(T) specimens were comparable up to the crack extension Δ*a* of about 0.5 mm. Then, they deviated from each other in the region of further crack extension. The J-integral values of Mini-C(T) specimen were lower than those of 0.5T-C(T) specimen. Meanwhile, the critical stretch zone width (*SZW~c~*) of Mini-C(T) specimen was smaller than that of 0.5T-C(T) specimen. These results suggest that the ductile crack growth resistance is reduced due to the Mini-C(T) specimen geometry.

One of such specimen size effects can be brought if the plastic deformation is so large that the rotation center deviates from the one assumed in ASTM E1820. This deviation would affect both of *J* and Δ*a* estimations and may result in fracture toughness underestimation in case of smaller specimens with larger plastic deformation. Thus, detailed examination was carried out on Mini-C(T) specimens with two clip gauges at the load line and at the front face attached. The crack mouth opening angle *θ* and the distance from the load line to the rotation center *R* were calculated from the measurements of those clip gauges. Although the measured *θ* and *R* of some specimens show difference around 10% from the values ​​assumed by ASTM E1820, the *J*-Δ*a* estimation corrected by measured *θ* and *R* was comparable with the one following ASTM E1820 procedure. In addition, the *J*-Δ*a* estimations calculated from the load line displacement and from the front face displacement multiplied by 0.73 were almost identical. These results indicate that the measurement precision is sufficient and crack mouth opening behavior during the test is as expected as assumed model in ASTM E1820 even for the Mini-C(T) specimen.

Further investigation into the mechanism of the fracture toughness underestimation due to small specimen size is necessary to develop a correction method enabling to estimate intrinsic ductile fracture toughness of materials by Mini-C(T) specimen.


Ductile Crack Growth Resistance and Rotation Behavior of Miniature C(t) Specimen

Division / Sponsors: 
Pressure Vessels & Piping (PVP)


Paper Proceedings: https://asmedigitalcollection.asme.org/PVP/PVP2020/volume/83815

PVP2020 - Virtual Conference

Supporting the need among researchers worldwide for additional online and multimedia content, ASME now offers the ASME Conference Video Collection. This platform will collect and provide long-term access to conference presentation videos, encompassing all technology areas within ASME's technical conference and events program. About 30,000-40,000 engineers across all mechanical engineering and related disciplines participate in ASME conferences each year. This includes authors of approximately 6,000 to 8,000 technical papers published by ASME, the majority of whom also submit video presentations for our virtual conferences. The video collection will be available as an add-on to existing product packages or as a separate subscription.