Fatigue crack growth on a straight pipe element under sodium thermal shocks

Accurate predictions of subcriticai fatigue crack growth are an important element in the leak-before-break demonstration of reactor piping systems. In fast breeder reactors several opposing factors affect the geometric evolution of the crack. Some are unfavorable (thermal shocks which induce bending and peak stresses; circumferential weld seams which also result in residual bending stresses; creep effects in high temperature loops) while others are favorable (thin gauge pipe walls; high fracture toughness of the 316 L Steel used). The authors first describe the FORTUNA test device used to apply cyclic thermal shocks on a straight pipe section submitted to a combination of primary membrane and bending stresses. The pipe element includes two circumferential cracks, one of which is located in a circumferential weld seam. Finite-element calculations of crack growth are then discussed, using linear elastic fracture mechanics and defect linespring type shell elements to allow low cost crack geometry updating. The analysis shows that thermal shocks resulting in equi-biaxial bending across the pipe wall should be one of the most severe loading configurations for leak-before-break demonstration. The experimental results will provide a basis for comparison.