Reliability of the longitudinal welds of LHC main dipoles

The LHC main dipoles are assembled relying heavily on welding technology. In particular, two 15 m long longitudinal MAG welds along the shrinking cylinder, manufactured from AISI 316 LN stainless steel plates, close the cold mass and are submitted to tensile stress. The welds have to feature a design stress of 350MPa at 1.8 K and to guarantee the leak tightness to superfluid helium. Starting from available experimental data, the reliability of the cold mass welds in terms of fracture mechanics is studied under different hypotheses (i.e., presence of surface or embedded flaws). The analysis has been carried out using a wide range of different approaches, such as leak before break, LCF (low cycle fatigue), LEFM (linear elastic fracture mechanics) and established standards (WES 2805); also different approaches of propagation under fatigue are compared. The results of this study are meant to prove the robustness and reliability of the design at the working temperature of the magnets.