1. Introduction to Forensic Engineering
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Transcript of 1. Introduction to Forensic Engineering
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Forensic Engineering:
In Service Failure Analysis
Master MSC Materials Engineering
1. Introduction
Ph. D. Nuria Martn Piris
Aerospace Engineer
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1.1. Definition.
Forensis means Public in Latin;
Forensic has come to refer to legally sustainable documentation,usually applied to accidents, crimes, etc., belonging to courts of
law forensic engineering is the application of the art and
science of engineering in the jurisprudence (legal) system.
Forensic engineering may include: The investigation of the physical causes of accidents and
other sources of claims and litigation.
Preparation of engineering reports.
Testimony at hearings and trials in administrative or judicialproceedings.
Rendition of advisory opinions to assist the resolution of
disputes affecting life or property and investigation of
intellectual property claims, especially patents.
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Forensic engineering is the investigation of materials, products,
structures or components that fail or do not operate or function as
intended, causing personal injury or damage to property.
The consequences of failure are dealt with by the law of productliability.
Purpose of a forensic engineering investigation:
To determine cause or causes of failure with a view to:
Improve performance of a component.
Prevent product malfunctions.
Insure product life.
Prevent safety hazards while using the product.
Improve process development. Insure product quality.
Achieve process reliability.
Prevent customer dissatisfaction.
Prevent safety or environmental hazards. To assist a court in determining the facts of an accident.
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Types of objective Possible precipitating
situation
Product life cycle
Product development Demands of the market Prototyping
Product improvement Warranty costs Ongoing
Assignment of
responsibility
Reparations for
financial/physical damage or
bodily injury or death
After subject
event
Prevention of recurrence Any After subject
event
Objectives in failure analysis
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Forensic engineering aim is to explaining failures successfully.
We must have in mind the constant relationship between
forensic engineering and ethics.
The subject is applied most commonly in civil law cases, although
it may be of use in criminal law cases.
Early examples: investigation of bridge failures such as the Tay
rail bridge disaster of 1879. Many early rail accidents promptedthe invention of tensile testing of samples and fractography of
failed components.
With the prevalence of liability lawsuits in the late 1900s the use of
forensic engineering as a means to determine culpability spread inthe courts.
Edmond Locard (18771966), a pioneer in forensic science,
formulated the basic principle of forensic science: "Every contact
leaves a trace" Locard's "exchange principle".
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The Tay Bridge disaster occurred during a violent storm on 28
December 1879 (wind speed 130 km/h). The bridge collapsed while
a train was passing over it from Wormit to Dundee (Scotland), killing
all aboard. The bridge designed by Sir Thomas Bouch used lattice girders
supported by narrow iron piers, with cast iron columns and wrought
iron cross-bracing, ess extensive and robust than on previous
similar designs by Bouch. Bouch had not sought expert advice on "wind loading" when
designing the Tay Bridge. There were other flaws in detailed design,
in maintenance, and in quality control of castings.
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Forensic Engineering attempts to answer the following questions:
What element led to the failure?
Why criticality was reached?
How is the damage caused? How to avoid further collapses?
What is meant by Failure?
Failure: A term which infers that a component or assembly is in a stateof damage that renders it no longer capable of reliably performing its
intended function. For a more complete definition:
The component becomes completely inoperable; fracture need not
have occurred. The component operates but does so inefficiently.
The component operates but with excessive maintenance.
Operation of the part is unreliable.
Operation of the component presents a safety hazard.
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1.2. Classification of in service failures.
a. From the functional point of view. (Why the item fails to fullfil its
function).
Excessive deformation (distortion).
Plastic deformation.
Elastic instability (buckling).
Creep.
Fracture.
Full section.
Partial section.
Cracking (implies a lower stiffness).
Wear (material wastage). Changes in initial dimensions.
Changes in initial surface condition: corrosion (galvanic, pitting,
through wall perforation). Stresses are not required.
Degradation of the material by environmental agents or operatingconditions (i.e. melting, thermal shock).
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b. According to the major or adjuvant factors involved.
Design errors.
Wrong assessment of service stresses and environmental
operating conditions. Wrong calculation or insufficiently precise.
Bad choice of materials (nature, condition and state) .
Insufficient attention to the design details.
Lack of foresight respect to periodic inspections.
Manufacturing errors.
Not suitable manufacturing process .
Machining errors.
Poor performing processes (welding, riveting, coatings).
Improper or insufficient handling or cleaning.
Assembly errors.
Fits and tolerances poorly chosen or made. Incorrect tightening torques.
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Defects inherent to the material (nature, condition, state)
Impurities or composition errors.
Inclusions or pores.
Inadequate fiber orientation.
Defective or incomplete heat treatment (local or general).
Decarburization.
Previous cracks (from quenching, grinding ...) .
Residual stresses (from heat treatment, welding).
Improper maintenance.
Poor performance or lack of periodic inspections.
Errors in replacing damaged parts.
Alteration of components in inspections or repairs.
Abnormal usage.
Utilization outside the limits imposed by the manufacturer.
Transgression of the regulations.
Unexpected incidental circumstances.
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Example: errors and stages leading to failures in structural
design and construction.
According to different studies:
Ignorance, carelessness, negligence 35%
Forgetfulness, errors, mistakes 9% Reliance upon others without sufficient control 6%
Underestimation of influences 13%
Insufficient knowledge 25%
Objectively unknown situations (unimagined?) 4% Others 8%
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Type of errors in design/planning
Conceptual errors 34% Structural analysis 34%
Drawings and specifications 19%
Work planning and preparation 9%
Combinations 4%
Primary causes of failure and how handled
No consideration 26%
Incorrect consideration 26%
Insufficient consideration 16%
Considered but risk accepted 22%
Consideration unknown 10%
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c. Based on the mode of application of the load and the type of
mechanical solicitation.
Static load.
Traction, compression, bending, torsion, shear, a combination.
Impact load.
Traction, compression, bending, torsion, shear, a combination.
Repeated varying loads fatigue.
A combination of load-time-temperature creep.
A combination of static load- time with conditions inherent to
material or environmental conditions.
Repeated contact actions:
Erosion.
Wear.
Fretting.
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d. Based on the circumstances in which the failure occurs.
Primary damage: referred to the item or component that has failed
to fulfill its function properly and produces the failure of thecomplete system.
Secondary damage: inadvertent changes to a component that
may or may not affect functionality.
Prior damage: damage that may have contributed to the failure.
Consequential damage: damage that resulted due to continued
operation post failure.
Mechanical/Environmental damage: subjected to the failedcomponent.
Handling.
Cleaning.
Storage.
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1.4. General procedure for Failure Analysis.
A method or procedure is:
a way of doing something,
in accordance with a definite plan.
The opposite of chance or luck.
A systematic sequence of activities: set of rules
Method and technique
Technique: the body of specialized procedures and methods
used in any specific field, esp. in an area of applied science.
The method is carried out through techniques.
How to proceed: three pillars.
Systematic assuring the quality of the work
Tools detect and measure damage
Time devoted enough, not excessive
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Stages of a Failure Analysis.
1. Collection of background data.
2. Selection of samples.
3. Preliminary (visual) examination.
4. Nondestructive testing (NDT).
5. Mechanical testing.
6. Selection, identification and preservation of critical specimens.
7. Macroscopic examination and analysis of surfaces.
8. Microscopic examination and analysis of fracture surfaces.
9. Selection, preparation, examination and analysis of metallographic
sections.
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10. Determination of the actual stress state of the failed component.
11. Determination of the failure mode.
12. Chemical analysis (deposits, coatings, surface corrosion products).
13. Application of fracture mechanics.
14. Testing undersimulated-service conditions.
15. Consulting with experts in other disciplines.
16. Synthesis of all the evidence and formulation of conclusions.
17. Writing a report (including recommendations).
18. Follow-up recommendations.
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