Professor Doug Perovic Materials Science and Engineering

“The entire physical world is most properly regarded as a great energy system: an enormous marketplace in which one form of energy is forever being traded for another form according to set rules and values. That which is energetically advantageous is that which will sooner or later happen. In one sense, a structure is a device which exists in order to delay some event which is energetically favored. It is energetically advantageous, for instance, for a weight to fall to the ground, for strain energy to be released and so on. Sooner or later the weight will fall to the ground and the strain energy will be released; but it is the business of a structure to delay such events for a season, for a lifetime, or for thousands of years. All structures will be broken or destroyed in the end, just as people will die in the end. It is the purpose of medicine and engineering to postpone these occurrences for a decent interval”

[Ref: J. Gordon, Structures: Or Why Things Don’t Fall Down, (New York, Penguin Books), 1978]

  Unacceptable difference between expected and observed performance

  Do not achieve expected quality of performance for the expected period of time

  Look beyond Catastrophic fracture problems…

  Excessive deformation   Serviceability problems   Excessive vibrations   Inadequate environmental control systems   Premature deterioration   Leaks

  Much of knowledge used to design, construct, manufacture and operate engineered facilities and products has been obtained through learning from failures

  Failure analysis improved engineering design practices

  Trial-and-error Trial-and-success   Development and evolution of design codes, standards

of practice, construction and manufacturing procedures   Dissemination of failure analysis information to design

professionals (c.f. medical pathology medical science)

  Usually refers to the determination of how a specific component or product has failed

  Consideration of materials selection, design, product usage, methods of production, mechanics of failure within component

  Investigations typically include:   Product Failure (e.g. fracture of critical part of a product)   Process Failure (e.g. a manufacturing process fails to achieve

the intended effect)   Design Failure (e.g. many products fail prematurely)

  Usually refers to managerial aspects of failure   General analysis of system failures rather than the failure of a

specific part   Analysis of how procedural and managerial techniques can be

improved to prevent reoccurrence of the problem   Used in association with large systems (e.g. power plants,

construction projects, manufacturing facilities etc.)   Emphasis on safety and quality assurance through formalized

procedures   “Reliability Engineering”   “Predictive Maintenance”   If these is no deep appreciation of product defects and their

criticality, no management or statistical method will solve product failure problems

  Objective 1: Preventing catastrophic failures of critical plant production systems   Objective 2: Avoiding deviations from acceptable performance levels that result in personal

injury, environmental impact, capacity loss or poor product quality   Purpose of RCFA is to resolve problems that effect plant or product performance (benefit

for plant or product improvement)   RCFA is normally NOT meant to fix blame for an incident (relieves self-preservation

attitudes and promotes positive teamwork to resolve the problem)   Effective RCFA requires discipline and consistency   Data gathering/interview process must separate fact from fiction by employing

investigating team to put aside preconceived notions and perceptions associated with conditioned human experience and nature

  Analysis based on fact and clearly stated assumptions that can be confirmed or proven   Extensive personnel requirements and substantial cost limit general use of RCFA   RCFA generally not applied on problems that are random or nonrecurring events   RCFA method typically used for problems involving: equipment, machinery or systems

failures; operating performance deviations; economic performance issues; safety; and regulatory compliance issues

[Ref: R.K. Mobley, Root Cause Failure Analysis, (Newnes, Boston, 1999]

Four common techniques:

1.  Failure Mode and Effects Analysis (FMEA) 2.  Fault-Tree Analysis 3.  Sequence of Event Analysis 4.  Cause and Effect Analysis

  Design-evaluation procedure to identify potential failure modes and determine effect of each on system performance

  Documentation of standard practice, generates historical record and serves as basis for future improvements

  Logical step sequence process starting at lower-level subsystems or components

  Assumes failure point of view by identifying potential modes of failure along with their failure mechanism

  Each failure mode effect is traced to system level   Each failure mode and resulting effect is given criticality rating based on

probability of occurrence (P), severity (S) and detectability (D)   Criticality threshold represented by Risk Priority Number (RPN)= P x S x D   High criticality threshold ratings lead to design changes   Advantages: (i) more reliable designs, (ii) improved reliability by anticipating

problems and instituting corrections (iii) improved validity of analytical method

  Disadvantages: (i) logic trees based on failure probabilities at component level for standard conditions and extrapolation techniques cannot be used to modify data for particular applications (ii) full application is very expensive

  Analyzing system reliability and safety   Objective basis for analyzing system designs, justifying system changes, performing

trade-off studies, analyzing common failure modes, and demonstrating compliance with safety and environment requirements

  Different from FMEA since restricted to identifying system elements and events that lead to one particular undesired event

  Detailed deductive analysis that requires considerable information about the system   Ensures all critical aspects of a system are identified and controlled   Qualitative and quantitative deductive reliability analysis providing insight into system

behaviour   Points out aspects of a system that are important with respect to failure of interest   Presents various combinations of possible events occurring in a system that lead to the

undesired top event   Event denotes a dynamic change of state that occurs in a system element, which

includes hardware, software, human and environmental factors   Fault Event represents an abnormal system state   Normal Event is expected to occur in the system   Top event and more basic fault events linked by event statements and logic gates

  Graphical approach to root cause failure analysis   Referred to as “Fishbone Graph” or “Ishekawa Diagram”   Minimum of 4 major classifications of potential causes

plotted: (i) human, (ii) machine, (iii) materials and (iv) method   Logical evaluation of actions or changes that lead to a specific

undesirable event   Does not isolate the specific factors that caused the event   Advantage: Displays all possible causes that may have

contributed to the event   Disadvantage: No clear sequence of events that leads to

failure

  Software programs used to generate sequence-of-events diagram for all investigated events

  Ideal for organization of information collected   Identifies missing or conflicting information   Improves understanding by showing relationship between events and the

incident   Highlights potential causes of the incident   Dynamic document that is continually modified until event is fully resolved   Logical order used to describe confirmed events in active rather than passive

terms   Each assumption and unconfirmed contributor to the event must be either

confirmed or discounted during course of investigation with sequence-of-events diagram modified accordingly

  Requires precise definition and qualification of each event, forcing function and qualifier

  Qualifiers should provide all confirmed background or support data needed to accurately define the event or forcing function