Fatigue Shaft Design Verification for Bending and Torsion
Research Area: | Volume 4 Issue 1, Jan. 2015 | Year: | 2015 |
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Type of Publication: | Article | Keywords: | Fatigue, Design, Verification, Gerber Model |
Authors: |
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Journal: | IJEIR | Volume: | 4 |
Number: | 1 | Pages: | 197-206 |
Month: | Jan.-Feb. | ||
ISSN: | 2277-5668 | ||
Abstract: | A design verification study of some shafts based on a probabilistic approach using the Gerber fatigue failure rule is presented. The shafts are subjected to combined bending and torsional loads. The design model parameters are considered as random variables characterized by mean values and coefficients of variation (covs). The coefficient of variation of a design parameter is obtained by using first order Taylor series expansion for strength and stress in a stress-based fatigue design. A reliability factor is defined and related to the covs of the design parameters and a failure probability.
This study shows that deterministic engineering models can be transformed into probabilistic models that can predict the failure risk in a design situation. From the design verification of example 1, it is shown that the popular modified Goodman model (MGM) for stress-life fatigue design is very conservative in that application. Only one design point in Example 2 appears to be inadequate. Generally, the design of the three shafts appears satisfactory, though mostly conservative.
Probabilistic design seems to be the most practical approach in product design due to the inherent variability associated with service loads, material properties, geomet-rical attributes, and mathematical design models. It can be used to avoid over- or under-design problems while ensuring that safety and quality levels are economically achieved. The design objective should be meeting a desired reliability target, not a reliability or safety factor. The probabilistic model approach presented here needs to be explored for other design applications, so as to the make probabilistic design a common practice. |
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