Investigation of the mechanical behaviour of superalloy bolted joints under extreme thermal conditions for aerospace applications

Abstract

The effort to increase thermal efficiency in modern aviation propulsion systems brings with it extremely aggressive operating conditions that push the structural limits of engine components. Bolted flange connections located in the hot regions of gas turbine engines are not merely simple connecting elements. They are dynamic thermodynamic interfaces that must maintain sealing and rigidity at critical temperatures such as 700°C. This thesis investigates in-depth the thermomechanical behavior of a two-flange bolted connection group made of Inconel 718 superalloy under demanding service conditions. The literature generally presents simplified strength approaches. This study utilizes analytical methods based on standards such as VDI 2230 and NASA-STD-5020B. Furthermore, a hybrid methodology providing cross-validation with high-precision 3D Finite Element Analysis (FEA) performed using ANSYS software has been adopted. The phenomenon of pre-stress relaxation, often overlooked in standard design processes, is one of the focal points of this research. The analyses revealed significant losses in pre-load strength during operation. This is largely due to the decrease in the elastic modulus of the materials along with different amounts of thermal expansion. FEA simulations predicted a pre-stress loss of 18.65% due to thermal effects alone. Furthermore, this value was calculated as 20.38% in analytical hand calculations, and these results largely coincide. This consistency supports the validity of the numerical model, but also reveals that a significant strength loss occurs in the design even before long-term creep effects come into play. The structural evaluation of the connection also highlights the risks of relying on uniaxial stress controls. The bolt appears to have a high safety margin of 117% under pure tensile load. However, when the bending moments created by flange rotation are taken into account, this margin drops to around 40% at the thread roots. This demonstrates how critical the "prying action" is in high-temperature designs. Furthermore, analyses determined that the flange net cross-section is the weakest link in the structure with a safety margin of 19.15%. While secondary alignment features such as rabbets were designed with an overly safe margin of over 1000%, indicating suitability for optimization studies in these areas. Finally, the fatigue life of the connection element was also calculated in this study. In this section, the strain-life approach was evaluated using the Smith-Watson-Topper (SWT) damage parameter. The material was assessed to exhibit local yielding or "shakedown" behavior during the initial assembly and warm-up cycles. In subsequent operating cycles, the stresses were found to stabilize within elastic limits. The calculated life of 73.4 million cycles confirms that the connection achieves a very long fatigue life, well beyond typical service requirements, as long as separation does not occur. The sealing function was further confirmed through a deliberately severe worst-case scenario, in which two of the twenty bolts were assumed lost and the remainder installed at the minimum permissible preload. Even then, the joint retained a separation margin of 1359%, showing that flange opening is effectively impossible within the operating envelope. A complementary transverse-load check showed that interface friction resists the maneuver-induced shear with a margin of 101.67%, confirming that the connection acts as a pure friction-grip joint in which the bolts are never exposed to direct shear. These findings provide a validated design methodology that bridges the gap between static theory and the dynamic reality of gas turbines.