Lecture 4 - What is reliability?

Question 1

When designing/developing an assest what questions are asked?

Answer 1

• What function does it need to provide?
• What is the required reliability?
• Who benefits from it?
• Who may be involved in its life?
• What risks are involved?
• What is the life cycle cost?
• Who pays for it?

Question 2

Define Reliability

Answer 2

There are two definitions:
- The probability that an item can perform its intended function for a specified interval under stated conditions.
- “Ability of an item to perform a required function under given conditions for a given time interval

Question 3

What is an item?

Answer 3

An “item” is used here to denote any component, subsystem or system (hardware and software)

Question 4

Why is it important to define the function of something?

Answer 4

• All items are designed to perform one or more required functions.
• Some of these functions are active and some functions are passive.
• To assess reliability we must first specify the required function(s) we are considering.

Question 5

What is the understanding of quality vs reliability?

Answer 5

• For a hardware item to be reliable, it must do MORE than meet an initial factory or quality specification –
• It must operate satisfactorily for a specified period of time in the actual application for which it was intended.
• Quality can be thought of as conformity of a product to its specification, while reliability is the ability to continue to comply with its specification over its useful life.

Question 6

What are the objectives of reliability engineering?

Answer 6

• To apply engineering knowledge and specialist techniques to prevent or to reduce the likelihood or frequency of failures.
• To identify and correct the causes of failures that do occur, despite the efforts to prevent them.
• To determine ways of coping with failures that do occur, if their causes have not been corrected.
• To apply methods for estimating the likely reliability of new designs, and for analysing reliability data.

Question 7

Why do engineering items fail?

Answer 7

• The design might be inherently incapable.
• The item might be overstressed.
• Failures might be caused by
  – variation
  – wear out
  – time-dependent mechanisms
  – sneaks
  – errors (incorrect specifications, designs, coding, assembly, maintenance, operation)