The master curve methodology at elevated loading rates: A detailed interlaboratory study

Abstract

The Master Curve Methodology (ASTM E1921) provides an experimental approach to assess a material’s temperature-dependent fracture toughness, generally under quasi-static loading rates. For handling elevated loading rates, guidelines are outlined in annex A1 of ASTM E1921 and annex A14 of ASTM E1820. It has been observed in the literature, however, that the Master Curve concept can fail to adequately assess fracture behavior for elevated loading rates. Specifically, experimental data suggest that the shape factor p = 0.019 /°C and threshold K_min increase, and the implied Weibull distribution for K_Jc values can be unsuitable for elevated loading rates up to $\dot{K}$ = 10⁵ MPa√m/s. The aim of this paper is to address this topic with an interlaboratory study, allowing a systematic analysis of precision and accuracy of the methodology for moderately elevated loading rates of $\dot{K}$ = 10³ MPa√m/s. The work includes the participation of 6 labs with 3 materials (22NiMoCr3-7 or A508 Grade 2, its weld, and a high-strength steel, S690QL1) with 2 specimen types (C(T) and SE(B) specimens). The 22NiMoCr3-7 steel and its weld were additionally tested with small SE(B)10x10 specimens (pre-cracked Charpy).

The following results and conclusions were made. The Master Curve methodology works well for homogeneous materials, such as the RPV steel base metal at moderately elevated loading rates. The scatter between individual labs lies within the expected standard deviation of 10 °C. Testing is recommended near T₀ in order to avoid the effect of heat generation and crack arrest, which can affect T₀. For S690SL1, the Master Curve methodology appears adequate for moderate loading rates despite an observed influence of extraction location. The RPV steel weld can be adequately described by the inhomogeneity analysis of the Master Curve. The requirements for data sampling in ASTM E1820 A14 appear conservative. A violation does not strongly impact T₀. Liquid cooling mediums should be avoided for testing at elevated loading rates due to possible inertia effects of the clip gauge. The use of larger specimens appears advisable, since they allow a more reproducible cleavage fracture assessment, even for homogeneous materials.