Failure as a Design      Criterion

   Fracture Mechanics

   Failure Analysis

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Wire Rope Failure

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Undercarriage Leg Failure

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Aircraft Towbar Failure

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Hail Damage

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Insulator Caps

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Fractography Resource




Professor M Neil James - New tutorial homepage: http://www.fatiguefracture.com

The development of these case studies was made possible with a grant from the UK Centre for Materials Education (http://www.materials.ac.uk), which is affiliated to the Learning and Teaching Support Network.

These pages deal with industrially-derived case studies in forensic engineering and failure analysis, which are firmly grounded in the context of their legal, insurance and design consequences. The development of this interactive learning package follows on from existing web-based projects in learning support via on-line tutorials, focussed internet resources and detailed analysis and discussion of case studies in failure.

The learning package illustrates a number of applications of some techniques widely used in industrial failure analysis, e.g. metallography, fractography and simple fracture mechanics. The examples are chosen and presented so as to guide the learner through the analytical steps and thought processes used in solving such problems. The learning interface is designed to achieve:
  • Focussed and hyperlinked background theory and information regarding use of techniques.
  • Guidance on the interpretation of information.
  • Multi-choice intermediate steps in the analyses.
  • Opportunities for the learner to explore other decisions in the analysis tree, and their possible consequences.
These particular examples have been chosen from real industrial consulting work, undertaken over the last decade or so, to illustrate unusual, interesting or potentially misleading aspects of failure investigation. The context of these aspects is related to the impact of engineering decisions on litigation, insurance claims, assessment of responsibility, and design/manufacturing modifications.

Some of these case studies have already been presented in a preliminary version of this type of format both to undergraduates and to practising engineers in seminars and lectures, and were well received. They have been refined, extended, and enhanced in content and number, and made available on the internet for student-centred interactive learning with appropriate multi-media support.

This learning package therefore provides an opportunity:
  • To improve and extend student learning of lecture material and laboratory classes in such areas as mechanical property testing and SEM practical work.
  • To develop transferable skills in failure analysis via 'real' industrial case studies in their original engineering context. These skills include interpretation of metallography and fractography, the application of simple fracture mechanics, and the use of deductive reasoning.
  • To allow larger larger numbers of students effective access to high quality self-paced learning resources relevant to the important area of failure analysis.
  • To improve awareness of real materials and engineering design/fabrication problems in undergraduate engineering programmes and in schools.
There is an associated fairly comprehensive FRACTOGRAPHY RESOURCE, which can be accessed directly from the site outline, given below, or from the relevant activity pages in the case studies. Other web sites exist that also consider failure analysis, and which can be used to support the information contained in this resource. See, for example, the Failure Analysis module at California Polytechnic State University.

The complete web site is outlined below and the various pages are linked off the outline. To gain benefit from the structured learning experience provided by this website, it is strongly recommended that a particular case study is followed in sequence, and that the overall site guide is used only to go back to particular pages to reinforce learning, or to check understanding.

The Case Studies

1. Failure of a Wire Rope on a Crane

The first case study illustrates some issues around determining whether a rope failure was primarily due to an overload event, or to fatigue. It deals with assessing whether insurance cover existed, and with design versus maintenance issues.  It centres around fractography of wires and relating the failure to tensile testing of rope sections.
First section - Background and visual observations
Second section - Tensile testing and fractography
Third section - Summary and conclusions, recommendations
Activity 1 - Rope bounce
Activity 2 - Original rope diameter
Activity 3 - Cause of reduction in rope strength
Activity 4 - Fractography of wires

2. Failure of a Main Undercarriage Leg on a Crop Duster Aircraft
This case study outlines the details of a more unusual and interesting example of the use of fractography in failure analysis, and indicates how fractographic and other scanning electron microscopy evidence may assist in obtaining an out-of-court settlement, on terms that are favourable to the clients.  It includes use of elementary fracture mechanics to support failure analysis.
First section - Background, initial examination and basis of litigation
Second section - Fractography
Third section - Estimation of tensile and shear loads at failure
Fourth section - Summary and conclusions
Activity 1 - Implication of initial observations
Activity 2 - Implications of SEM fractography
Activity 3 - Calculation of fracture stress for saddle clamp bracket

3. Failure of an Aircraft Towbar
This case study highlights potential errors in failure analysis that may arise when too much importance is placed on relative positions of failed components, rather than assessing the full implications of every fact in the case.  It also illustrates compounding of errors to produce a worse accident than expected from failure of the engineering component, and hence introduces the relevance of fault tree analysis and hazard assessment procedures to ensuring engineering reliability.  The issues surrounding 'sudden and unexpected' failure or 'progressive deterioration' are brought out.  It demonstrates the use of simple solid mechanics and fracture mechanics analysis to assist with selecting the correct failure scenario.
First section - Background
Second section - Initial failure investigation
Third section - Second failure investigation, properties and metallography of the shear bolt
Fourth section - Summary and conclusions
Activity 1 - Conclusions of first investigation
Activity 2 - Calculation of bending stress and comparison with estimated failure stress

4. Proving Damage to Polycarbonate Roofing
The main issue in this case study is proving that observed damage to polycarbonate roofing sheets, which was detrimental to their use in flower growing and which occurred at an unknown time, was the result of hail impact during a thunderstorm. It illustrates the use of fractography in a polymer, and the application of notch toughness principles, to counter an erroneous damage hypothesis, based on thermal expansion and contraction of the sheets.  It also demonstrates the detailed evidence required to convince some insurance companies that the damage is insured!
First section - Background
Second section - Further investigations, impact testing
Third section - Fractographic investigation
Fourth section - FE stress analysis
Fifth section - Summary and conclusions
Activity 1 - The diurnal heating hypothesis
Activity 2 - The Charpy impact results

5. Failure of Power Line Insulator Caps
Live high voltage power transmission cables fell onto the ground in Namibia when failure occurred in cast iron caps on the insulator string. This threatened lives, as well as disrupting power supplies over a large area. The issues here relate to explaining how a relatively low energy arc-over between adjacent caps could occur and cause the cap to 'burst', allowing the insulator string to fall to the ground with the live cable. This centers around crack initiation, microstructure and toughness in ferritic malleable iron.
First section - Background
Second section - Investigation, metallography, mechanical properties
Third section - Summary and conclusions
Activity 1 - Effect of multiple arc strikes on fracture toughness

6. Fractography Resource
Fractography Resource Part 1 - Fatigue Macrofeatures
Fractography Resource Part 2 - Fatigue Microfeatures
Fractography Resource Part 3 - Fast Fracture - Macrofeatures
Fractography Resource Part 4 - Fast Fracture - Microfeatures (Cleavage, MVC)
Fractography Resource Part 5 - Fast Fracture - Microfeatures (IG)
Compendium of Fractographs - Steels, aluminium alloys, WC-Co, stainless steel and polycarbonate

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