on Maintenance Reliability

“Assumption 1: Safety is increased by increasing system or component reliability. If components or systems do not fail, then accidents will not occur. (p. 7)

This assumption is one of the most pervasive in engineering and other fields. The problem is that it is not true.

Safety is a system property, not a component property, and must be controlled at the system level, not the component level. 

New Assumption 1: High reliability is neither necessary nor sufficient for safety. (p.13)”

These statements were excerpted from Nancy Leveson’s “Engineering a Safer World“. 

This contradicts the common belief there is a direct correlation between Safety and Reliability. I personally, being in the Reliability field for 30+ years, believe there is a correlation between Reliability and Safety. But I would assert that it is not a direct correlation. 

In past articles, we have explored the basics of Erosion, Corrosion, Fatigue and Overload. Our emphasis has been on creating an awareness and appreciation for the value of failed components, to any investigation.

In this article we are going to delve into Fatigue a little bit more (because it is the most common fracture pattern) and see how we can use an evidence-based, deductive logic process to determine what elements of Fatigue may have been at play. We are trying to create an intellectual curiosity within the front lines about ‘making the call’…is it Fatigue, or some other fracture pattern?

I recently received an interesting LI inquiry that I felt others could learn from the answers that I was able to get. This is not my expertise so I sought out answers from some colleagues who were more familiar with fasteners.

Here is the original inquiry (translated from Portuguese so I hope Google Translator did a good job):

“I would like to know if you can help me clarify my doubts about:

1. The purpose of conducting a study of the coefficients of friction in screws, threads and nuts?
2. Is there an accurate standard for such testing?
3. What methods can be perfected in projects that target this type of trial and application?
4. Can surface treatments influence changes in results?
5. Who is responsible for such testing (the manufacturers and suppliers of raw materials, the assembler and/or its various final assembly applications)?

Lets start with a fresher on general Component Fatigue.

• Fatigue occurs when a material is subjected to repeated loading and unloading.
• When the loads are above a certain threshold, microscopic cracks will begin to form at a material’s surface.
• Cracks always begin in high stressed areas of a material.
• Eventually a crack will reach a critical size, and the structure will suddenly fracture.