Low Cycle And Finite Life Fatigue Experiment

An experiment for examination of fatigue failure theories is presented for potential adaptation at undergraduate mechanical and civil engineering programs. The focus of the experiment is placed on Low Cycle and Finite Life Fatigue. Design of the experiment and its associated apparatus allows for both symmetric (fully reversed) and non-symmetric reversed loading with different magnitudes applied to an array of cantilever beams. Several scenarios using beams with different lengths, sections, stress concentrations, and materials are proposed for destructive/fatigue failure testing. Other specimen with interesting features may be easily added to the package if desired. The time factor for conducting fatigue testing in an educational environment has been incorporated in the design process. Availability of the blueprints of all components of the robust apparatus, its cost effectiveness, ease of manufacture, and a proposed outline of the experiment make it an ideal addition to the archives of experiments in undergraduate engineering programs. IINTRODUCTION Laboratory experimentation is a critical final link for a thorough understanding and appreciation of scientific and engineering theories. Every possible effort should be made not to deprive the future engineers or educators from this vital component of their education [1]. It is therefore necessary to continue development of effective and efficient pedagogical methods and techniques for the engineering laboratory experience [2]. Laboratory apparatus is generally expensive due to low production levels, specialized features and significantly higher Design Costs built into the final cost. For example, the range of cost for a typical educational fatigue testing apparatus is from $28,500 to $32,500. These units are basically adaptations of the R. R. Moore Industrial Fatigue testing devices which cost in excess of $100,000. Such high costs may lead to lack of vital laboratory apparatus and in turn deprive the engineering students from being sufficiently exposed to important concepts such as verification of the theory through experimentation, interpretation and analysis of data and gaining sufficient background for designing experiments. However, if blueprints of the designs of a (desired) apparatus are available, and on site machining capabilities exists, a major cut may be expected in the final cost. Such designs and blueprints may be generated in-house in collaboration with undergraduate engineering students [3]. The authors hope that the colleagues in other engineering programs would find this effort worthy of potential adaptation in their program. P ge 10904.1 Proceedings of the 2005 American Society for Engineering Education Annual Conference & Exposition Copyright © 2005, American Society for Engineering Education IIOBJECTIVES OF THE PROJECT The following major objectives were set at the inception of the project; 1. To develop an experiment for examination of fatigue failure theories, 2. To create an opportunity for collaborative research and design efforts between engineering student(s) and faculty, 3. To generate a modular, cost-effective, reproducible apparatus with outstanding design characteristics, 4. To make all information necessary for fabrication of the apparatus and conducting the experiment available to engineering programs nationwide. The authors invited three Junior engineering students (Andrew Maulbeck, Mary Anne Bitetto and Greg Conway) to collaborate with them in materializing the above goals. The parameters in successful implementation of the processes involved for achieving the above goals were comprehensively discussed, outlined and a preliminary Gantt chart was generated. Through ten weekly scheduled meetings, alternative designs for each of the components, subsystems and the overall integrated system were evaluated, chosen and optimized. It took another two weeks to fabricate, modify, and test the reliability and capabilities of the apparatus. IIIBackground Roark and Young define Fatigue as “the fracture of a material under many repetitions of a stress at a level considerably less than the ultimate strength of the material” [4 ]. In a fatigue test, the specimen may be exposed to equal or unequal alternating stresses. When equal positive and negative stresses are applied, it is said that the loading is fully reversed. In this situation, a critical location of the specimen will experience equal levels of both tensile and compressive stresses in one full cycle. The benchmark for establishing the behavior of engineering materials under dynamic/fatigue loading is the “S-N” diagram. Here, “S” corresponds to the stress level and “N” to the number of cycles. Due to the uncertainties involved in material behavior and characteristics, a large number of specimens are tested at different stress levels for generating the “S log N” diagram. Ideally, the main objective in such tests is two-fold. First, to establish (for a given material), up to what stress levels the material will enjoy an infinite life (Endurance Limit); and second, to correlate the number of cycles at different stress levels that a material will be able to go through before coming to failure. The S-N diagrams for several engineering materials have been established as a result of comprehensive and highly time consuming tests. Generally, the results are more reliable for steel alloys compared to aluminum alloys. Low-cycle fatigue is defined on an S-N diagram as being approximately between zero and 1000 cycles. High-cycle fatigue is generally greater than 10 cycles. Finite life is assumed to be below 10 cycles [5]. A typical S-N diagram is shown in figure 1.