Why Things Break (and how we try to make sure they don’t!)

Robert Love, Venkat Jayaraman

July 10, 2008SSTP Seminar – Lecture 6

Overview• Presentation

– Analysis• Statics• Mechanics of Materials• Finite Element Analysis (Method)

– Testing Methods– Production

• Industrial Quality Practice• Failure Mode and Effect Analysis (FEMA)• Statistical Process Control (SPC)• Six Sigma

– Post-Mortem• Failure Analysis

• Discussion• Activity

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Statics• Net force = 0• Free Body Diagrams– External Forces– Basic Structure

• Trusses, Members

– Constraints• Common: Fixed, Pin, Roller

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Loading and BC’s

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Mechanics of Materials

• External Loading Determines Internal Loading– Shear and Moment Diagrams

• Cross Section Characteristics– Centroid vs. Center of Mass– Moment of Inertia• MUST be given about an axis!

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Mechanics of Materials

• Internal Stresses– Shear vs. Axial Stress– Bending vs. Torsion– Represented by Stress Tensor

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Testing• Destructive (see below) vs. Non-Destructive (ex: dye penetrant)• Tensile Strength: Tensile Tester (also for fatigue)• Impact Strength: Charpy/Izod Testing (temp. variations)• Hardness: Vickers/Rockwell/Brinell Scale w/Indenter • Creep: Creep Test Rig• Fatigue: Fatigue Tester

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Finite Element Analysis• Types of Elements

– 1D: Bar, Beam– 2D: Quadrilateral, Triangle– 3D: Tetrahedral, Hexahedral– May use multiple Nodes

• Static or Dynamic Analysis– Static Load or Modal Analysis

• Common FEA Programs– Patran/Nastran, Abaqus, Pro-E

Mechanica, Ansys

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• Quality assurance is the set of planned and systematic actions necessary to provide appropriate confidence that a product or service will satisfy the requirements for quality.

• Quality assurance activities– Failure testing

» Operation of a product until it fails» Exposes many unanticipated weakness in the product

– Statistical Process Control» Effective method of monitoring a process through the use of tools

like control charts

Industrial Quality Practices

• FMEA – A tool used to comprehensively analyze and fix a failure of a product or process.

• Definitions– Failure mode - The way the failure occurs – Failure Effect – The immediate consequence of the failure

mode– Severity(1-10) – Degree of worst potential consequence– Occurrence(1-10) – Estimate of number of chances a failure

mode can occur – Detection(1-10) – Degree of ease of detection of the failure

mode– Risk Priority Number – Product of Severity, Occurrence and

Detection

Failure Mode and Effect Analysis (FMEA)

Failure Mode and Effect Analysis (FMEA)

FMEA Flow Chart

Sample FMEA Worksheet

• SPC - involves using statistical techniques to measure and analyze the variation in processes.

Statistical Process Control(SPC)

Control Chart

Process Capability

Voice of Customer

Voice of Process

This curve is the distribution of data from the process

The shaded areas represent the

percentage of off-spec

production

Ideally, you want to reduce the variability in the product until NO off-spec product is made.

• Six Sigma process is one that produces 3.4 defective parts per million opportunities

• Basic Methodology– Define Process Improvement– Measure key aspects – Analyze cause-effect relationships– Optimize the process– Control the process

• Implementation key roles– Executive leadership– Champions– Master black belts– Black belts– Green belts– Yellow belts

Six Sigma

Traditional

After Six Sigma

Failure Analysis• What was the “failure mode”? – Hint: Look at the surface (even under a

microscope)!– Visualize how the material broke– Use your knowledge of material bonding

and possible loading conditions to understand

– Look for evidence of other effects

• Know so that can prevent recurrence• Engineers as expert witnesses

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Ex: Impact

Failure Analysis• Some types of failures

– Fracture (brittle or ductile)– Impact– Buckling – Fatigue– Corrosion – High Temperature– Bending – Torsion– Shear– Vibration– Resonance/Flutter– Creep– Delamination– Weak Bonding/Joint/Weld

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Activity

• Finite Element Analysis Procedure– Define CAD Model– Define Boundary Conditions– Define Loading Conditions– Define Material Properties– Discretize model (make a mesh of finite elements)– Run Analysis– Visualize Analysis

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Activity

• 14.1GPa=Young’s Modulus,E (tensile)• 0.25=Poisson’s Ratio, nu• 1900kg/m3=density

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