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High Cycle Fatigue Tester Abstract

Metal fatigue is the fracture and failure of structural parts subject to cyclic loads that are much less than loads necessary to cause yielding or tensile fractures. High cycle fatigue failures (millions of cycles) can occur with surprisingly low loads. In many industries, equipment is subject to hundreds of millions or billions of cycles and fatigue failures are a constant problem. Demonstrating high cycle fatigue in the limits of the classroom is very difficult since the test of a single specimen can take hours. Demonstrating the fatigue limit requires a large sample size (lots of tests) and statistical analysis – nearly impossible to do in the classroom with current fatigue testers.

Faculty at Purdue University – College of Technology at New Albany (PUNA) developed a classroom fatigue tester to improve student learning of fatigue concepts. Their objective was to develop a table top, high cycle fatigue tester capable of demonstrating high cycle fatigue in a time period compatible with classroom instruction. The resulting prototype includes a fatigue tester that operates at around 30,000 cycles per minute, capable of achieving a million cycles in a little over 30 minutes, as well as the sensors and electronic circuits to count the cycles until the specimen breaks. The prototype was then tested in the classroom to show that it is capable of performing high cycle fatigue tests.

This paper describes the development and construction of a classroom ready fatigue tester and its associated electronics for a sophomore level mechanical engineering technology strength of materials course. It includes a discussion of the performance of the fatigue tester, and the assessment, evaluation and improvement planned for the project. Lastly, it describes the broader impact of this project to better educate engineering technology students in the implications of fatigue failures.

Introduction

In the limits of the classroom, both time and space, it is difficult for students to experience fatigue failures. Particularly difficult to demonstrate is high cycle fatigue failures. Loads in the millions of cycles take a considerable amount of time to complete, thus making it hard to complete within the time constraints of a class or lab period. Common fatigue testers operating at 1800 rpm would take over 9 hours to just to reach a million cycles with a single test specimen. Repeated tests (to get a large sample size) and statistical analysis are required in order to demonstrate the fatigue characteristics of a particular metal. Thus it is extremely difficult to fatigue test a metal in the classroom and students must rely on published test data to predict the fatigue properties of a metal.

In addition to the difficulty in demonstrating fatigue in the classroom, data on the fatigue characteristics of metals for more than million cycles is not widely available. Students are presented with the concept of a fatigue limit in carbon and low alloy steel – a level of stress below which the metal would not be expected to fail due to fatigue. For carbon steel, the fatigue limit transition occurs around a million cycles.1 Demonstrating this fatigue limit in the classroom is nearly impossible with a tester operating at 1800 rpm.

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Dues, J., & Le, N. (2006, June), High Cycle Fatigue Tester Paper presented at 2006 Annual Conference & Exposition, Chicago, Illinois. 10.18260/1-2--528