Specimen miniaturization for fracture toughness determination of reactor pressure vessel steels - An experimental and modeling investigation by Meng LI


May 22, 2023




For the degree of Doctor of Engineering Sciences and Technology

In the context of the structural integrity assessment of nuclear reactor pressure vessels (RPV), the mini-CT specimen is gaining worldwide attention as one of the ideal miniaturized geometries for measuring fracture toughness. This geometry is particularly appealing for several reasons. Firstly, up to eight mini-CT specimens can be machined out of one broken Charpy specimen, thus avoiding the need to consume additional surveillance materials. Secondly, the geometry permits specimen reorientation, which facilitates the investigation of anisotropic effects. Another considerable advantage that enables this geometry to replace large ones is that the mini-CT geometry have shown potential to produce fracture toughness results equivalent to large samples.

However, the miniaturization of test geometries induces a loss of constraint, which may lead to deviation of fracture toughness values when compared to those measured from larger geometries. In addition, due to the reduction in size, this geometry does not fully satisfy the ASTM requirements with respect to the varies ratios of the specimen dimensions. In particular, the requirements relative to the pre-crack front curvature and initial crack length are challenging, leading to a large number of mini-CT specimens not fulfilling the standard requirements and therefore being considered invalid. Therefore, in this thesis, three main issues in using mini-CT geometries: 1) effect of constraint loss, 2) effect of pre-crack front non-uniformity, and 3) effect of initial crack length are investigated to promote the standardization of the mini-CT geometry in both brittle and ductile regimes. The results demonstrate that, by implementing the adjusted specifications given in this thesis, the mini-CT geometry is capable of producing reliable and accurate fracture toughness measurements.

Jury members :

  • Prof. Thomas Pardoen (UCLouvain, Belgium), supervisor
  • Prof. Ludovic Noels (ULiège, Belgium), supervisor
  • Prof. Eric Deleersnijder (UCLouvain, Belgium), chairperson
  • Prof. Aude Simar (UCLouvain, Belgium)
  • Dr. Benoit Tanguy (CEA, France)
  • Dr. Philippe Spatig (EPFL, Switzerland)
  • Dr. Inge Uytdenhouwen (SCK CEN, Belgium)
  • Dr. Rachid Chaouadi SCK CEN, Belgium)


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