Mechanical Engineering Design Notes

Design Contents

Preliminary Matters
Design Methodology
..brain storming
..evaluation matrix
Statistical Considerations
..variability in materials
..variability in dimensions
..variability in loading
..preferred sizes
Design Factor
Introduction to Failure
Failure Theories
Application of von Mises
..criterion in 2 D

Stress Concentration
..and notch sensitivity
Failure Under Combined Loading
..combined bending and torsion
Failure Under Cyclic Loading
..fracture mechanics
Instability - Buckling
Concentrically Loaded Strut
..slender columns
..Euler formula
..effective length
..short and intermediate columns
Eccentrically Loaded Strut
.. theory
Shock Loading

Failure - General Introduction

An important objective of any design is to prevent unexpected engineering failures. However when safety critical components and systems are being designed, particularly for applications in aerospace and the petrochemical industries, a detailed assessment of possible failures is needed and carried out. This will often include a Failure Mode and Effects Analysis (FMEA) which would be carried out on every component that could possibly have an effect on the functioning of the plant or system.

As an example, for a new petrochemical plant this would probably involve the assessment of every valve. Such an assessment is further complicated by the fact that a valve that controls the passage of fluid will have several possible modes of failure, and the possible outcomes of each of these modes of failure must be investigated. For example such a valve could fail by:

  • seizing fully open
  • seizing fully shut
  • seizing in an intermediate position
  • starting to leak
  • if fitted with a position sensor, this could fail giving incorrect indication of position, possibly leading to alarms and actions that made matters worse

The effects of each of these failures must be considered. One consequence of this is that unless a component is essential for the function of the plant or system it is omitted from the design as this not only reduces risk of problems, but it also saves the cost of purchasing, inspecting, installing and maintaining the component!
However systems will often contain many such components and control strategies / computer software (and emergency plans) must include capabilities for handling all modes of failure and their effects on the system operation.
In the case of modern aircraft this involves multiple sensors, backup computer systems, software, power supplies and actuators.
Detailed operating procedures and risk assessments will be produced for aircraft, petrochemical and nuclear power plants. In the latter two cases, risks to the environment will normally also have to be considered.

Dr David J Grieve, Revised: 25th January 2014, 25th January 2010, original: 8th June 2009.

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