Effect of biaxial loading on fracture toughness of RPV steel

Reactor pressure vessels under some special regimes (i.e.pressurized thermal shock) are loaded by a strongly biaxial tensile stresses that ratio can reach even an opposite value in comparison with normal operating conditions. Special biaxial tests on cruciform type specimens with thickness up to 90 mm were tested in NRI Rez - special testing equipment and testing methods including measurements have been developed and realized. Testing equipment with maximum loading up to 1.5 MN allows to reach different biaxial loading ratios between 0 and 2. In Nuclear Research Institute in Rez comparative experimental tests on cruciform and beam specimens were performed. The aim of these tests was to examine the effect of crack depth and biaxial loading on fracture toughness for reactor pressure vessel material 15Kh2MFA. For evaluating the tests, the FEM (program SYSTUS) was used. The performed tests confirm shallow crack effect, i.e. increase of fracture toughness for shallow cracks compared to that one of deep cracks. Quantitatively, this increase was 53 - 93 %. Further, the performed experiments show decrease in fracture toughness of shallow cracks loaded biaxially compared to uniaxial loading of shallow cracks. Quantitatively, the fracture toughness decrease was about 20 %.