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2 lecture 4 - Failure Predictors and Safety Factors - slide

2 lecture 4 - Failure Predictors and Safety Factors - slide...

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MCEN30014 Mechanical Design Failure Predictors and Safety Factors Based on: Andrew Samuel and John Weir, Introduction to Engineering Design – Modeling, Synthesis and Problem Solving Strategies Oxford: Butterworth Heinemann, 1999 ISBN 07506 42823
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Lecture Outline The designer’s responsibility in design for structural integrity (revision) key concepts (strength, failure, loads) and relevant issues Modes of failure Summary of some key modes of structural failure Failure modes exercise (bicycle wheel & bicycle spoke) Failure prediction Why do engineers need failure predictors (“theories of failure”)? • example of a spoke (uni-axial stress) • example of a shaft (bi-axial stress) • example of a pressure vessel (tri-axial stress) • at what stress will each component yield? • material tests in the laboratory: uniaxial tension • the need for failure predictors Examples of failure predictors maximum direct stress, maximum shear stress, maximum distortion energy Design factors of safety What is a safety factor, and why do we use it? Systematic estimation of F d Material selection (this is covered in detail elsewhere) Mechanical Design
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Modes of failure Failure predictors (sometimes called “theories of failure”) Why engineering designers need failure predictors Main failure predictors Design factors of safety How are they used? How are they estimated? Mechanical Design
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“Design for structural integrity” Definition: It is the designer’s responsibility to choose the material and dimensions of an element to ensure that it has sufficient STRENGTH not to FAIL under the LOADS that it experiences in service (or to reduce the probability of the failure to an acceptably low level) Mechanical Design
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“Design for structural integrity” Definition: It is the designer’s responsibility to choose the material and dimensions of an element to ensure that it has sufficient STRENGTH not to FAIL under the LOADS that it experiences in service (or to reduce the probability of the failure to an acceptably low level) The designer needs to: specify/predict the actual LOADS specify likely mode(s) of FAILURE , and hence predict STRENGTH (i.e. loads that would cause failure) Today's lecture: "specify likely mode(s) of FAILURE” Mechanical Design
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“Design for structural integrity” Relevant issues in design for structural integrity: Load Material Environment Geometry Stability (e.g. sensitivity to small changes in geometry or load) Duty Life Failure Mode Mechanical Design
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“Design for structural integrity” Failure Modes examples: FRACTURE static loading brittle or ductile failure dynamic loading impact loading stress corrosion creep EXCESSIVE DEFLECTION elastic (linear) plastic (non-linear) INSTABILITY wear surface damage Mechanical Design
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“Design for structural integrity” Failure modes – example #1: BICYCLE WHEEL Question: How can it fail?
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