In fighter jets there are many different systems that make the plane fly, and one of these is the fuel system consisting of multiple parts including tanks and tubes. During flight, the pressure in these tubes can vary and occasionally pressure spikes can occur. Pressure spikes can be described as high increased pressure at a high rate and decreased pressure at an equally high rate during a limited time span. Depending on the peak pressure and duration of this pressure spike, damage or failure might occur in the tube structure. This is a problem that SAAB is analyzing and wants to find a less conservative approach of analyzing the effect of pressure spikes regarding the structural strength of tubes. In this master’s thesis this is explored. A method for applying pressure spikes to two different tube structures has been created. The pressure spike propagates through the liquid and applies the pressure spike to the tube through Mortar contact. The tube is modelled with shell elements and normal Lagrangian element formulation. The liquid is modelled with solidelements and Arbitrary Lagrangian Eulerian element formulation. The two different tube structures analyzed in this report have the same tube dimensions, a combination of the biggest radius and smallest wall thickness that occur in SAAB’s tubes. The difference between the two is that one structure is straight whilst the other is curved. A parametric study was carried out to analyze at what pressure peak different durations of the pressure spikes would result in a critical effective plastic strain. This critical effective plastic strain was chosen to be 0.08 for the aluminum tube. This is conservative since the fracture strain for the material is 0.12. Results froma static case was compared with the dynamic results from the described method to see how conservative this new method of pressure spike analysis is. The results from the comparison showed that the dynamic method allowed 65% higher pressure peak thanthe static solution. Using a pressure spike with 1ms duration and pressure peak at 19.5MP a in the straight tube compared with the internal pressure of 11.4MP a in the static method for the straight tube. For the curved tube, the dynamic method allowed 90% higher peak pressure for a pressure spike with 1ms duration and peak pressure of 18.5MP a. This pressure spike in the curved tube is compared with the static method for the curved tube which reached critical effective plastic strain at 9.4MP a. For pressure spikes with durations of 20ms in straight tubes, the dynamic and static results are similar. For pressure spikes with durations of 10ms in curved tubes, the dynamic and static results are similar. In these cases, it is reasonable to use the quicker static method instead of the new method of pressure spike analysis. The impulses in both the straight tube and the curved tube cases have a linear relationships with the duration of the pressure spike. For the longer durations the energy needed to reach critical deformation is higher since the affected area is larger. Shorter durations need less energy to reach critical deformation since the affected area is smaller.
| Identifer | oai:union.ndltd.org:UPSALLA1/oai:DiVA.org:ltu-106685 |
| Date | January 2024 |
| Creators | Roos, Arvid |
| Publisher | Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik |
| Source Sets | DiVA Archive at Upsalla University |
| Language | English |
| Detected Language | English |
| Type | Student thesis, info:eu-repo/semantics/bachelorThesis, text |
| Format | application/pdf |
| Rights | info:eu-repo/semantics/openAccess |
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