Strength-Stress with Limited Information

Abstract

Chris and Fred discuss this thing called ‘stress-strength’ we hear a lot about in the world of reliability … especially when we need to use it to work out ‘reliability stuff.’

Key Points

Join Chris and Fred try to answer a question from one of our listeners regarding a ‘stress-strength’ problem where he was given a ‘maximum stress’ for a particular application and a ‘minimum strength’ for a particular component. So how do we work out its reliability?

Topics include:

Why not go to a lab? Not just a physical lab … but a simulated lab using Finite Element Analysis (FEA) or Finite Element Modelling (FEM)? There might be a chance you can go to a lab that someone else has gone to (i.e. … review articles that might cover this topic). Go get measurements.
Why not use information you have? Like expert judgment? You can extract information if you ask in a really clever way.
Why not ask the vendor? If they give you a number … it has to come from somewhere. It might come from a rigorous testing and analysis phase of development (their answer will let you know if this is the case pretty quickly). It might come from an unsubstantiated guess (again … this will become obvious from their answer). You might be surprised!
Can you work out if it is NOT close? Perhaps the conservative estimate of ‘maximum stress’ is a LONG way below the conservative estimate of ‘minimum stress.’ If this is the case – then you don’t really need to do excruciatingly detailed data analysis. Your device will almost certainly work! Conversely, you might be able to quickly work out that your device will almost certainly fail.
But what if it IS close? As in … the strength and stress are relatively close. You need to characterize the random nature of each. The normal distribution (bell curve) is usually pretty good at modelling both. You might be able to use some conservative assumptions to model each, and see what you get! If you have enough information to answer the question … great! If not … then you need more information.

Enjoy an episode of Speaking of Reliability. Where you can join friends as they discuss reliability topics. Join us as we discuss topics ranging from design for reliability techniques to field data analysis approaches.