3D simulation of beneficial (compressive) residual stresses developed around a hole in a plate via passing a tapered mandrel through the hole
Holes are common feature of most components for fastening, lubrication, and routing of electrical wires. For example, the fuselage of a commercial airplane is assembled by riveting aluminum panels through thousands of rivets holes distributed over the entire surface of the airplane. As necessary as holes are for assembly purposes, they are also points of weakness due to decreased thickness of the material and unfavorable geometrical changes at the location of the holes. Fluctuating loads, such as those experienced by the fuselage of an airplane during flight, can result in the failure of fuselage structure by a mechanism known as “metal fatigue”, which starts by development of tiny cracks (micro-cracks) at the interior surface of the holes, which progressively grow in length with each application of fluctuating loads experienced during normal operation of the airplane. Most people have directly experimented with “Metal Fatigue”, by breaking a paperclip via back and forth bending. In order for micro-cracks to initially develop under the action of fluctuating loads, the material must experience forces that pull the material apart, i.e., tension. The reverse of this statement is also true, i.e., development of micro-cracks can be retarded if the material is subjected to compression. One can visualize the development of a crack in a material by separating index and middle fingers, where the resulting space between the two fingers would represent the development of a crack. This requires that the two fingers be pulled apart from one another, representing tension in the material. If the fingers are compressed together, there would be no resulting space between the finger, hence cracks in materials cannot develop under compressive loads.
The purpose of our research was to induce a state of “compression” that permanently resides around an existing hole, thereby retarding the evolution of micro-cracks that could later develop into full length cracks and the eventual failure of the material. This can be achieved through passing a short tapered rod (slightly larger on one side than the diameter of the hole), which would result in small (~1 - 3%) enlargement of the hole's diameter. Once the rod is removed from the hole, the elastic retraction of the material surrounding the cold-expanded hole towards the center results in compression of the hole, which in turn results in closing of any existing micro-cracks around the interior surface of the hole. Simulation results show the state of compression developed around a cold-expanded hole as represented by the red ring around the hole. To verify the simulation results, laboratory tests were conducted on aerospace-grade aluminum plates with a hole by subjecting the specimens to repeated fluctuating loads until failure. The untreated specimens failed at approximately 100,000 load cycles, while cold-expanded specimens were able to endure in excess of 2 million load cycles before failure.
This animation is part of the research conducted by Jahan Rasty's student Vipin Palande using HPCC resources.