To this point we have used simple, semi-fabricated examples to illustrate concepts. Now we revert to a more realistic, real-world example. The model is still simple but the loading is complex. This example describes a typical multiple load case fatigue analysis as applied to a safety-critical component. Because it is safety critical, both a Total Life method (to ensure that it will survive its design life) and a defect tolerant approach (to ensure that a crack will not grow to failure too rapidly) are employed.

The component is a titanium alloy aircraft rear engine mounting lug. It is mounted across the rear of the aircraft wing and is used to locate a pin which constrains the rear of the engine in the x-y plane. The engine is restrained in the z-direction (direction of travel) by the front engine mounting. Thrust results in a downward distribution of pressure and is simulated by a cos2t pressure distribution over a 90 degree angle, amounting to a resultant force of around 1000 Newtons. The model is composed of 2D quadratic elements. The lug is mounted very stiffly along its top edge, simulated by constraining this edge for all degrees-of-freedom.

Loading around the lug hole is simulated by applying 8 different load cases. Each load is a separate FE analysis and spaced every 45 degrees. A loading spectrum is available consisting of horizontal and vertical loads that represent 1000 flights. The design life is approximately 30,000 flights.

As you can see, each load case gives a different stress distribution. It is not clear from these stress plots where the critical location will be when combined. To begin, start Pre & Post or MSC Patran and import the model and results using the MSC Nastran results file mounting_lug.op2 into a new database called mounting_lug.