Failure Analysis of Aircraft Stringer
For MIE222, mechanics of solids, we were instructed to find common modes of failure of an aircraft stringer through various lab techniques. The types of testing performed are below.
Lab 1: Fatigue Testing
I found that the stringers on aircraft are most likely to experience fatigue from the increasing and decreasing pressure upon the ascent and descent of the plane in the form of cyclic loads. The fatigue testing machine produces a cyclic rotating bend with an applied load of 6kg and applies it to the specimen. My team then applied the results of the lab to reinforce the importance of material selection for the stringer to minimize manufacturing defects, shortening the life of the component.
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Material tested: cold-rolled steel 4340, U-notch, and V-notch specimens
Lab 2: Mechanical Testing
For this lab, we tested the mechanical properties of Aluminum 6061 through tensile testing and a strain gauge test. This lab was another demonstration of the importance of material selection and demonstrated the principles of mechanical testing.
Lab 3: Photoelasticity
In this lab, we used an acrylic copy of our stringer to utilize photoelasticity under our loading conditions. Results included isochromatic, isoclinic, and monocromatic patterns to determine principal stresses. We used a plane and circular polariscope to obtain results, then related fringe orders with stress magnitudes using the stress-optic law.
Lab 4: FEA
Done in the computer lab, we processed a 2D model of the stringer using ANSYS Workbench. I learned about the effect of different meshes and how to analyze results using the program. The results highlighted a general area of concern for where a fracture would initially propagate, and aligned with other methods of testing.
Lab 5: Strain Gauge
The last lab delved deeper into using strain gauges, testing a stringer specimen made of aluminum 6061-T6. The stringer had uniaxial and rosette gauges attached, and a Wheatstone bridge circuit was used to detect the resistance changes. This is a great non-destructive alternative to actively measuring stress and strain in a specimen; a preventative method for aircraft applications.
