Mechanical Design Uncertainty Calculator
What is design uncertainty?
Engineers must accommodate uncertainty. Uncertainty always accompanies change. Material properties, load variability, fabrication fidelity, and validity of mathematical models are among concerns to designers.
There are mathematical methods to address uncertainties. The primary techniques are the deterministic and stochastic methods. The deterministic method establishes a design factor based on the absolute uncertainties of a loss-of-function parameter and a maximum allowable parameter. Here the parameter can be load, stress, deflection, etc.
Equation of design uncertainty?
Design factor:
nd = loss-of-function parameter / maximum allowable parameter (1)
For structural analysis, the typical parameter is the load, then the equation becomes:
nd = loss-of-function Strength / allowable stress (1.1)
And,
Maximum allowable load = loss-of-function load / nd (2)
Or,
allowable stress = loss-of-function strength / nd (2.1)
Example calculation
Assume that the maximum load on a component is estimated with an uncertainty of ±25 percent, and the load that causes failure is estimated within ±10 percent. If the failure-inducing load is nominally 2500 lbf, calculate the design factor and the maximum allowable load that compensates for the absolute uncertainties.
To account for these uncertainties, the failure-inducing load should increase to 1/0.90, while the maximum allowable load should decrease to 1/1.25. Therefore, to offset these absolute uncertainties, the design factor, using Eq. (1), is calculated as:
nd= (1/0.90)/(1/1.25) = 1.39
Using Eq. (1), the maximum allowable load is then:
Maximum allowable load = 2500/1.39 = 1799 lbf
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TensorConnect project 2024 by pttensor.com Author: Caesar Wiratama