A LOGNORMAL RELIABILITY DESIGN MODEL

Edward. E. Osakue, Lucky Anetor

Abstract: It is well known that design loads vary randomly during equipment operations. Similarly, material properties such as yield strength, tensile strength, fatigue strength, etc. are random variables. Design analytical models are approximations of reality and failure mode models are also approximations. Consequently, design solutions are not exact. Practical design must therefore, consider the random nature and statistical variability of design parameters. Reliability-based design models are developed to provide practical design methods. This paper develops a lognormal reliability-based design model that can be coded in Excel Spreedsheet. Two design examples are considered in demonstrating the application of the formulated model. In the first example, our result differs from the result from [7] by 0.9% on the conservative side. The weight of the beam in the second example differs from [9] by 14.53% positively for a reliability target of z = 3. This variance is largely due to differences in reliability targets. When the beam size is adjusted to closely match the reliability levels of [9], the weight of the beam becomes 1.4% lower for our model. Therefore the results from our reliability model give comparable but slightly conservative and realistic solutions.

Keywords: Lognormal, Reliability, Variability, Normal Variate

DOI: https://doi.org/10.15623/ijret.2016.0508043