Professor M Neil James - New tutorial homepage: http://www.fatiguefracture.com
He has also developed an
extensive tutorial dealing with practical applications of linear elastic fracture mechanics.
The tutorial covers Griffiths energy approach, the stress intensity factor, compliance concepts, and characterisation
of fatigue crack growth and stress corrosion cracking.
Failure of an engineering component or structure can be regarded as arising from either incomplete, inaccurate, or inappropriate information on, or consideration/handling
of, one or more stages of the design process. These stages can be visualised using systems partitioning
and the 'waterfall' diagram (Overhead 2)
. These indicate that the concept of ‘concurrent integrated design’
is important in ensuring integrity in overall design of an engineering system.
The complexity of a typical engineering system design means that failure can manifest itself in a number of different ways,
the consequences of which vary from minor irritation to catastrophic collapse of the structure, product sales and/or the company. Some of the manifestations of failure are summarised below:
1. It may be structural in nature.
- Excessive deformation (elastic or plastic) (e.g. Tacoma Narrows bridge)
These problems are caused by mismatch between structural and/or material properties and the system (functional) requirements imposed by the environment and design envelope (perhaps imposed by inadequate understanding of the system requirements).
A particular sub-set of such failures are those where a design is using ‘new technology’ or is at the leading edge of current engineering knowledge – such components and structures are at higher risk from unforeseen interactions/loads than more mundane ones – examples include:
- Comet airliner
- Point Pleasant (Silver) bridge 
Structural integrity has been defined as "the science and technology of the margin between safety and disaster" and hence, on this definition, the design process has to explicitly consider material and structural failure in assessing this margin and achieving lean, light-weight engineering. Routine techniques for characterising material and structural response include:
- Mechanical property and proof testing
Note that mechanical property and proof
testing have a 'chequered' history. Pre-industrial engineering capability was
associated with a generally low level of risk to users, and the reliability of products
was acceptable as there were no alternatives. The situation was really one where the
buyer bewares and it is worth noting that even kings occasionally suffered
when leading edge technology has failed. For example, early cannon were prone
to exploding during firing and James II of Scotland was killed in 1460 when a large cannon
(firing a 300 lbf projectile) called the Lion burst. Risks to the maker/tester
could be high, and a macabre example of this was testing of Samurai
swords on convicted criminals in medieval Japan. This is illustrated with a typical sword testing diagram from the Yamada family , which is
labelled in terms of difficulty of achieving the cut in one stroke (1 indicating the most
difficult and 16 the easiest cut). The stoicism of the convicts is worth recording. One,
on hearing his sentence (to be cut in style 5) is reported as saying that he wished he had
known it in advance, because he would have swallowed several big stones to spoil the
sword. In the west, testing was usually less hazardous and this is illustrated by a bullet proof mark on a suit of armour for Louis XIV, which has
been made into part of the decoration as the centre of a flower .)
NDT to assess the defect state.
Fracture mechanics assessment of the criticality of cracks and defects.
Less routine characterisation may be via:
De-construction of ageing structures (e.g. Boeing Aircraft Company).
An interesting summary of the role of learning from failure in the context of engineering disasters, is provided at the
hosted by the Department of Materials Science and Engineering at the State University of New York at Stony Brook.
2. It may arise from human-system interactions which are not fail-safe.
Ad hoc and uncontrolled changes to design (e.g. Alexander Kielland accommodation platform [4, 5]).
Flawed decision making (relevant information/facts not known – Challenger space shuttle).
Potentially dangerous sequences of actions (e.g. Chernobyl).
Poor inspection procedures based on improper assessment of either critical locations and/or difficulty of detection (Alexander Kielland).
3. It may result from inadequate management of the design process.
Fundamental or extensive design changes late in the partitioning or integration stages of the overall design process (Alexander Kielland).
‘Visionary’ management leading to cost overrun, mismatch of expectations, friction between suppliers/partners in project (De Lorian Motor Company).
Inability to keep up with pace of technological change (‘static’ design rather than continuous change/replacement - e.g. slide rule manufacturers in early 1970’s made obsolescent by Hewlett-Packard - who plan on product obsolescence in 2 years).
4. It may result from inaccurate assessment of market wants/needs.
Designs which are ‘ahead of their time’, i.e. too advanced or too expensive to make (e.g. Concorde airliner , Hughes Spruce Goose flying boat).
Designs where the basic idea is useful, but the product emulates a ‘lead balloon’ because of poorly realised concepts (e.g. Sinclair C5 electric vehicle).
The net lesson from examination of such case studies is that consideration of failure modes and mechanisms is an integral part of successful design.
Good engineering practice applies the minimum necessary margin between safety and disaster (either structural or financial). An essential part of the training of an engineer should be practice in actively embracing the lessons of failure through detailed analysis of case studies, coupled with experience of the concepts of ‘concurrent integrated design’
and design system analysis.
Appreciation of the causes and ramifications of failure appears likely to assist in developing an ability for innovative and elegant design; in other words, facility in assessing the ‘margins’ in a design project.
This section of the 'Design as a Generic Tool'
module will consider some of the above case studies of engineering failure and highlight the lessons to be learnt in a, hopefully, informative and entertaining way. A project will be set in class to allow you to explore the issues surrounding other failures in a similar way, and to present the results as a group during scheduled lectures.
that the hyperlinks in the text are to images and video clips which illustrate aspects of the failures.
Bennett JA and Mindlin H (1973) Metallurgical aspects of the failure of the Point Pleasant bridge Journal of Testing and Evaluation JTEVA, Vol. 1 No. 2 March 1973, pp152-161.
Joli HL and Hojitaro I (1962) The Sword and Samé (Holland Press, reprinted by Latimer, Trend and Co. Ltd Whitstable, Kent).
Ffoulkes C (1988) The Armourer and His Craft (Reprinted by Dover Publications Inc, New York).
Easterling KE (1992) Introduction to the Physical Metallurgy of Welding 2nd edition. Butterworths, London. In library under accession number 671.52 EAS.
Lancaster J (1996) Engineering Catastrophes: causes and effects of major accidents, Abington Publishing, Cambridge, England. In library under accession number 363.34 LAN.