PhD Defense | Meng Li | Specimen miniaturization for fracture toughness determination of reactor pressure vessel steels - An experimental and modeling investigation

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

In the context of the structural integrity assessment of nuclear reactor pressure vessels (RPV), the mini-CT geometry is gaining worldwide attention to measure fracture toughness.

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.

Experimental investigations and finite element simulations are used as the two main analytical methods. In addition, micromechanics-based models are also introduced: the Rice-Tracey model and the Thomason model are used to analyze ductile fracture, and the particle distribution model is used to analyze brittle fracture.

The results show that the effect of constraint loss on fracture toughness can be compensated by applying an appropriate scaling factor in the ductile fracture regime and a new size correction in the brittle fracture regime. The effect of pre-crack non-uniformity can be considered limited, indicating that the requirements for pre-crack front curvature could be relaxed and therefore avoid unnecessary rejection of meaningful data from mini-CT specimens. The requirements for the initial crack length in mini-CT geometry should remain consistent with the ASTM standard. The results demonstrate that, by implementing the adjusted specifications, the mini-CT geometry is capable of producing reliable and accurate fracture toughness measurements.

Promotor:

• Thomas Pardoen (UCLouvain)

Co-Promotor:

• Ludovic Noels (ULiège)

SCK CEN mentors:

• Inge Uytdenhouwen (SCK CEN)

• Marlies Lambrecht (SCK CEN)

Click here for a list of obtained PhD degrees.