PSI - Issue 10

D.G. Ntritsos et al. / Procedia Structural Integrity 10 (2018) 288–294

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D.G. Ntritsos et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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machine, steel shafts are subjected to dynamic loading cycles of complex nature. Common types of loadings that steel shafts are routinely subjected to are torsion and bending. More often than not, the steel shafts drive a cog wheel that is in turn connected to other cog wheels. The way the torque is transmitted to the cog when is through a mechanical link age, the common key. In order to successfully mate the shaft to the cog, the key must be in physical contact with both. According to Pedersen and Leergaard (2010), despite the fact that keys are the predominant way of transmitting torque, very little work has been done to fully assess the keyway design and the impact it has into the machine fatigue life. The design of the keyway is dependent to the shaft diameter and no other functioning condition is taken into account. On an experimental level the stress concentration of the keyway is studied in a number of papers, like Madayag (1969), Fessler et al. (1969), Eissa and Fessler (1983), using the method of photoelasticity, in order to assess the effect of the keyway design to the fatigue life of the machinery. In real life applications, the keyway is just one of the stress concentration areas present in a machine. Frequently one comes across machinery parts that present sharp geometric changes such as shaft steps, holes, keyways and threads. Although work has been put into developing stress concentration factors for a single stress concentration area by Peterson (1953), the case where two or more areas are adjacent remains and any work towards that remains non systemized and unstandardized. In this paper, the interaction between two adjacent stress concentration areas in a steel shaft is studied using dif ferent methods, in order to establish if the coexistence of them affect the overall fatigue life behavior of the part and to determine which one preponderate which. The first method is conducting actual fatigue tests using a testing apparatus originally designed for fatigue strength testing on two distinct sets of specimens. The second method is calculating the stress using the stress concentration factors found in Peterson (1953). The third method is the numerical one. Due to the lack of a mathematical model that adequately models the behavior of a part with multiple stress concentration areas, we created a parametric 3s model and then inserted in into to Autodesk Nastran Solver. We then run FEM analyses.

2. Experimental approach

2.1. The experimental protocol

The effect of the adjacent stress concentration areas in DIN CK45 specimen fatigue life is studied using the reverse bending fatigue tests. Tangible specimens were manufactured and tested using a conventional testing machine. The geometry of the specimens is adjusted to the machine used. In this test two different sets of five specimens each are compared. The first set includes single notched specimens without a keyway but with a single stress concentration area in the form of a shaft step. The second set includes specimens with a keyway adjacent to the shaft step. For the test procedure the ISO 1143:2010 standard has been taken into consideration.

2.2. Apparatus and specimens

For the experimental method a testing apparatus originally designed for material fatigue testing was used. The machine we used for the tests is a TERCO brand, MT 3012-E model. The testing apparatus uses an asynchronous single phase electric motor in order to rotate the specimen and a force application system capable of applying a maximum value of 300 N. A schematic of the working principle of the machine used in the test is shown in Fig.1.

Fig. 1. TERCO MT-3012-E working principle.