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Executive Summary
ATA Engineering is an engineering consulting firm that provides analysis and test-driven design solutions. One of ATA’s test services includes fatigue testing. Fatigue testing revolves around the material science phenomena of fatigue in which localized structural damage was incurred from cyclic loading. ATA’s approach to fatigue testing is a method that utilizes test-measured strain based loads and finite element models. In order to validate analytical solutions and test new materials, ATA Engineering had a need to design and build a fatigue testing apparatus. This task can be achieved by means of designing and constructing a machine that will produce stress cycles by rotating a test article in four point bending.
Four point bending is a type of flexural test where the beam (or in this case a tube) is loaded at 4 points to create a span of constant bending stress in between the two loaded points closest to the center. See Figure 1 below for a visualization of the former concept.
Four Point Bending Shear and Moment Diagram
By rotating the test specimen, the span of constant moment starts oscillating and cyclic stress is induced. Eventually with enough reciprocating stress cycles, the test specimen will begin to form cracks and ultimately lead to fatigue failure. Note that the stress created in the test specimen is neither close to the yield strength nor able induce plastic deformation on the test article.
The fatigue tester should be able to produce 207 MPa (30 ksi) of stress over a span of 30.5 cm (1 foot). It also needs to be able to count the number of cycles the test article has revolved under cyclic stress. Depending on the material of the test article, the testing apparatus should produce results similar to the standard fatigue testing results of circular tubes. The dimensions of the test article was a 50.88 mm (2 inch) diameter and 3.175 mm (0.125 inch) wall circular tube within a reasonable length. Moreover, the fatigue testing apparatus should be safe to run because it will be left unattended for lengthy amounts of time.
Figure 1 – Overall Design
The final design of the fatigue testing apparatus consists of a self-reacting frame supporting a load frame and 4 mounted bearings. The mounted bearings held the test article in place while the motor drive system rotated a pulley system synced with the test article.
Figure 2 – Load Frame
An important aspect of the design is the load frame. The load frame takes all the load supplied by the cap screw and transmits it to the test article.A load frame was designed to transmit 207 MPa (30 ksi) of stress to the test article.
Numerous tests have been done on this machine. The tests were broken down into smaller components. The electrical system was first tested together. The motor controller was attached to the motor and the motor was able to be turned on and off at will and its RPM could be controlled as well. The bearings and the load frame were tested together. After the bulk of the structure was asssembled, the test article was inserted into the testing apparatus. This ensured that the bearings were lined up. Then we tested if the motor would be strong enough to rotate the test article once load was applied. The motor spun the test article without fail.
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