jim page, 2007 chapter 8: engineering & material factors mina handbook
TRANSCRIPT
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Jim Page, 2007Jim Page, 2007
Chapter 8: Engineering & Material Factors
MINA Handbook
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Jim Page, 2007Jim Page, 2007
Why Study Engineering Factors?
• While the human element is present in virtually every mishap, one area that is often over-looked is the interaction between humans and their machines.
• We need to understand the concept of a system and how designers develop the machines we use and how those machines fail in order to be effective investigators.
• In modern systems, there is often a direct link between the design of the machine and the human error that precipitates the mishap.
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Jim Page, 2007Jim Page, 2007
Why Study Engineering Factors?
• Most materials in an industrial system contain stored energy.
• A mishap can be thought of as “ a spill of energy across some boundary”.
• When a material fails it generally releases some kind of stored energy by losing support, fragmenting, causing additional failure, or releasing toxic substance by burning or chemical reaction.
• Determining the cause of the initial failure requires some knowledge of the physical and chemical nature of industrial materials.
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Jim Page, 2007Jim Page, 2007
Properties of Materials• Humans have been very successful in adapting many of the natural materials
into tools to manipulate the environment.
• Where natural materials are inadequate, artificial ones have been invented and developed.
• In all cases, these materials are designed to achieve specific functions.
• To do these functions they must have structural and functional integrity throughout their useful life.
• When such integrity is lost and results in a mishap, the material failure becomes a primary concern of the investigator.
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Jim Page, 2007Jim Page, 2007
Load Carrying Ability
• Material structures are always under some degree of load. In general, there are three loads of interest:
Static Load – The weight of the material itself – this load factor is equal to 1
Dynamic Load – Here the material is loaded slowly (1-3 times the natural vibration period of the structure) – this load is 2 times the static load.
Impact Load – When the loading is more than 3 times the natural periodic vibration, the load effect depends on the speed of application:
* This load = ½ weight times speed of application squared.
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Jim Page, 2007Jim Page, 2007
Increase in Loading from Angle of Rigging
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Jim Page, 2007Jim Page, 2007
Types of Loads
Tension Compression
TorsionalShear
Bending
The Combination Stress
Compression
TensionShear
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Jim Page, 2007Jim Page, 2007 Wood
• Wood has intertwined fibers resulting in a degree of toughness.
• Wood also shows a tendency to creep.
• Wood will fail under excess static, dynamic, or impact loads.
• The most likely failure of wood structures is due to decay, rot or insect infestation.
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Jim Page, 2007Jim Page, 2007
Wood
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Jim Page, 2007Jim Page, 2007 Stone, Brick, Concrete
• Brick and stone are resistant to compression but weak in tension and shear.
• Brick and stone masonry owes structural integrity to the mortar binding not the material itself. Thus the greatest threat to masonry is failure of the adhesive binding.
• Since the mixture of the mortar or concrete is critical to its hardening and water is a key factor in the mix, water is the primary starting point for investigation.
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Jim Page, 2007Jim Page, 2007 Plastics
• Poor in tension and shear
• Good in bending
• Very susceptible to creep
• Toxic in fire
• Testing and life estimation is very difficult
• Poor in impact resistance unless specially designed.
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Jim Page, 2007Jim Page, 2007 Recognizing Metal Failures
• Primary concern – Why and how failure occurred
• Most failures are not from internal metallurgic defects but are from
– Improper installation
– Inadequate maintenance
– Unsatisfactory environment
– Excessive loading
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Jim Page, 2007Jim Page, 2007 Recognizing Metal Failures
• Shear Failure
– Smooth fracture surface
– Perpendicular to long axis of material
– Some deformation in ductile metals
– Buckling of panels will show direction of force
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Jim Page, 2007Jim Page, 2007 Areas to be Investigated• Geometry of the Part
– Notches, nicks, violent changes in surface can act to increase stress
• Loading speed– Speed of onset greatly affects applied force– Under tension a ductile metal may exhibit brittle fracture characteristics
• Temperature– Low temp – embrittlement– High temp – plastic flow
• Surface Treatments– Produce more resistant structure– Can result in surface cracks that propagate into the material
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Jim Page, 2007Jim Page, 2007 Recognizing Metal Failures
• Tension– Ductile fracture
• Elongation “necking down”• 45 degree slip plane fracture
– Rough granular appearance– Brittle fracture
• No elongation• 90 degree fracture plane
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Jim Page, 2007Jim Page, 2007Tension Failure
Highly Ductile Sheet or Thin Bar Stock
Rough Granulated Tension Type Zone
Smooth Shear Type Zones
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Jim Page, 2007Jim Page, 2007
Brittle Tension Failure
Extremely Small 45o Edges
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Jim Page, 2007Jim Page, 2007 Medium DuctilityTension Failure
Rough Granulated Tension Type Zone
“Castellated”
Smooth Shear Type Zone 45o Edges
Cup and Cone
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Jim Page, 2007Jim Page, 2007 Recognizing Metal Failures
• Shear Failure
– Smooth fracture surface
– Perpendicular to long axis of material
– Some deformation in ductile metals
– Buckling of panels will show direction of force
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Jim Page, 2007Jim Page, 2007 Compression Metal Failures
– Structure bulges under load
– Shear stress component at 450
– Tubular shows triangular dimples
– Local crippling and twisting
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Jim Page, 2007Jim Page, 2007 Compression FailureLocal Crippling of Channel Section
Local Instability ofFlanges. Occurs Prior to
Bowing of Column.
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Jim Page, 2007Jim Page, 2007 Compression FailureAngle Section Due to Torsional Instability
Loading is CompressionBut Failure Mode is Twist
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Jim Page, 2007Jim Page, 2007 Shear Failure in Metal
Smooth Surface
Straight Parallel Traces 45o Rough
Granular Tension Type Zone
b
DUCTILE
BRITTLE
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Jim Page, 2007Jim Page, 2007 Shear Loading of a Panel
Ductile Metal Panel Buckling Failure
A Line Drawn Betweenthe Large Arrow HeadsShows the Direction
of the Buckles
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Jim Page, 2007Jim Page, 2007 Shear Loading of a Panel
Tension Failure
Tension Failure OccursAfter Buckling is Completed.Tension Failure is 90o to the
Direction of Buckling.
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Jim Page, 2007Jim Page, 2007 Torsion Failures in Metal
Torsion Stress
– Twisting
– Causes both longitudinal and perpendicular forces
– Induces tension and compression loads to counter uneven internal loads
– Crinkling (local crippling) of hollow tubes
– Ductile materials show smooth fracture face
– Brittle show rough twisted fracture face
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Jim Page, 2007Jim Page, 2007 Torsion FailureDUCTILE
BRITTLE
Smooth Surface Shear
Type
Rough Granulated Tension Type
Zone
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Jim Page, 2007Jim Page, 2007 Torque Tubes
• A hollow torque tube is more efficient than a solid shaft.
• Maximum stress in the hollow tube is reduced by one-half compared with the solid shaft.
• Accomplished by moving material from the neutral axis to further out.
• Hollow tubes indicate failure under tension by buckling in the twisting direction.
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Jim Page, 2007Jim Page, 2007 Bending Load -Shear Failure
Solid Ductile Metal Shaft
45o Shear Surface
Shear Tension
Bending
The Combination Stress
Compression
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Jim Page, 2007Jim Page, 2007 Bending Load -Tension Failure
Solid Brittle Metal Shaft
90o Tension Surface
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Jim Page, 2007Jim Page, 2007 Bending Load -Shear Failure
Solid Ductile Metal Shaft
45o Shear Surface
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Jim Page, 2007Jim Page, 2007
Fatigue of Materials
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Jim Page, 2007Jim Page, 2007 Background
• Fatigue is present in all metallic parts
• Fatigue failures result from fatigue cracks
• Fatigue cracks almost always start from:
Stress Concentrations• Causes
– Design
– Manufacture
– Maintenance
– Operations
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Jim Page, 2007Jim Page, 2007
Stress and Strains
• Stress is the load acting on the part divided by the area of material supporting the load.
S (psi) = F/A
• Strain is the deformation caused by the load.
e (%) = ΔL/L
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Jim Page, 2007Jim Page, 2007
Stress-Strain Diagram
Slope, Modulus of Elasticity
Strain, in/in
Str
ess,
lb/in
2
Ultimate Stress
Lower Yield Point
Upper Yield Point
Elastic Limit
Fracture
Plastic Range
Elastic Range
If the elastic limit is exceeded, the body will experience a permanent deformation.
Up to certain limiting loads, a solid will recover to its original dimensions.
Area within this portion of the curve equals
Modulus of Resilience
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Jim Page, 2007Jim Page, 2007
Stress-Strain Diagram
Slope, Modulus of Elasticity
Strain, in/in
Str
ess,
lb/in
2
Ultimate Stress
Lower Yield Point
Upper Yield Point
Elastic Limit
Fracture
Plastic Range
Elastic Range
If the elastic limit is exceeded, the body will experience a permanent deformation.
Up to certain limiting loads, a solid will recover to its original dimensions.
Area within this portion of the curve equals
Modulus of Resilience
Elastic range
(temp. deform.)
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Jim Page, 2007Jim Page, 2007
Stress-Strain Diagram
Slope, Modulus of Elasticity
Strain, in/in
Str
ess,
lb/in
2
Ultimate Stress
Lower Yield Point
Upper Yield Point
Elastic Limit
Fracture
Plastic Range
Elastic Range
If the elastic limit is exceeded, the body will experience a permanent deformation.
Up to certain limiting loads, a solid will recover to its original dimensions.
Area within this portion of the curve equals
Modulus of Resilience
Plastic range
(perm. deform.)
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Jim Page, 2007Jim Page, 2007 Fatigue
• Definition – Progressive localized structural damage. It occurs when material is subjected to repeated or fluctuating strains at stresses less than ultimate strength.
• General requirements for Fatigue
– Material prone to fatigue cracking
– Tension stress
– Local stress must reach plastic range
– Stress must vary cyclically in its intensity
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Jim Page, 2007Jim Page, 2007
RECOGNIZING FATIGUE
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Jim Page, 2007Jim Page, 2007
RECOGNIZINGFATIGUE
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Jim Page, 2007Jim Page, 2007 Corrosion
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Jim Page, 2007Jim Page, 2007 Corrosion
• Definition
– The disintegration of a metal that results from the interaction of metallic surfaces with one or more substances in the environment.
– This interaction is affected by such factors as temperature, stress, and fatigue loading.
– The result of this interaction is the transformation of the metal into chemical compounds.
– Moisture is the source of most compounds.
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Jim Page, 2007Jim Page, 2007 Corrosion
• Affected by temperature, stress, & fatigue
loading
• Transforms metal into chemical compounds
• Brittle, scaly, or powdery
• Little mechanical strength
• Moisture source of most aircraft corrosion
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Jim Page, 2007Jim Page, 2007 Corrosion Forms
• Uniform attack
– Over whole surface
– Usually chemical
– Rust, Aluminum oxide, Stainless steel
• Highly localized pitting
• Intergranular
– Negates chemical bonds
– Only mechanical grains interaction left
– Exfoliation (proceeds parallel to surface)
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Jim Page, 2007Jim Page, 2007 General Types of Corrosion
• Direct Chemical Attack– Nearly even rate over entire surface– Corrosive agents
• High-Temperature Oxidation– Reaction of metals with oxygen at high temperatures
• Electrochemical Corrosion– Something to corrode (anodic metal)– Cause for corrosion (cathodic metal)– Continuous liquid path– Conductor to carry flow of electrons
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Jim Page, 2007Jim Page, 2007 Galvanic Series of MetalsMost Anodic, Corroded End
MagnesiumMagnesium Alloys
ZincAluminum Alloys (Low Strength)
CadmiumSteel or Iron
LeadChromium
Brass and BronzeCopper
Stainless SteelsTitanium
Copper-nickel alloysSilver
Nickel (passive)Graphite
GoldPlatinum
Most Cathodic, Protected End
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Jim Page, 2007Jim Page, 2007 Electrochemical Corrosion
Electron Flow
Conductive PathMetal
+
Moisture
=
Corrosion
Anode CathodeElectrolyte
(Water + Ions)
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Jim Page, 2007Jim Page, 2007
Steel Fastener(cathode)
Moisture EntersHere
AluminumSheet
Corrosion of Aluminum(Anode Corrosion Location)
Corrosion Around a Rivet
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Jim Page, 2007Jim Page, 2007 Intergranular Corrosion
Moisture
Grain – Crystal (cathode)
Corroded, Grain Boundary(Anode, Corrosion Location)
Grain Boundary
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Jim Page, 2007Jim Page, 2007 Single OverloadDuctile Intergranular
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Jim Page, 2007Jim Page, 2007 Exfoliation
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Jim Page, 2007Jim Page, 2007 Stress Corrosion Cracking (SCC)
• Environmentally induced, sustained stress
• Exacerbated by residual tensile stresses remaining from material heat treatment or fit-up
• Also triggered by operation loads and forces from buildup of corrosion by-products
• Mitigated in design by aligning principal grain direction with primary load path
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Jim Page, 2007Jim Page, 2007 Stress Corrosion Cracking
• Combination of Tension Loads and Corrosive Attacks
– Crack Initiation – Physical breakdown of protective films and subsequent corrosive attack
– Crack Propagation – Electrochemical attack on surfaces of crack, particularly at crack apex, point of highest stress
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Jim Page, 2007Jim Page, 2007
Stress Corrosion Cracking (SCC)
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Jim Page, 2007Jim Page, 2007
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Jim Page, 2007Jim Page, 2007 Hydrogen Embrittlement
• High strength steels are susceptible to cracking when hydrogen enters the metal.
• Hydrogen may come from corrosion reaction with water or during electroplating.
• Typically, the failure to adequately bake a part after electroplating is the cause of hydrogen embrittlement.
• Hydrogen embrittlement is a form of SCC.
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Jim Page, 2007Jim Page, 2007 Wear
• The slow removal of material from the surface of a component by mechanical action. Generally undesired.
• Sometimes wear is a necessary ingredient (break-in) on new or overhauled equipment.
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Jim Page, 2007Jim Page, 2007
Technical Assistance
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Jim Page, 2007Jim Page, 2007 Requirements
• What is needed/necessary?
• Who is capable/available?– Manufacturer’s representative– Contractors– Laboratories– Military experts
• At investigation location/laboratory
• Advisors work for investigator
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Jim Page, 2007Jim Page, 2007 Typical Types of Assistance• Engines • Airframe• Instruments/light bulbs/switches• Systems• Metallurgy• Performance• Human factors• Fire patterns• Contractors• Government labs
– NTSB– FAA– FBI
• Commercial labs
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Jim Page, 2007Jim Page, 2007
How To Get Help
– Use local base resources
– Call convening authority
– AFSC
DO NOT INVITE EXPERTS ON YOUR OWN!
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Jim Page, 2007Jim Page, 2007 How To Use Technical Assistance
– Telephone– On-site evaluation– Tear down reports– Exhibit security
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Jim Page, 2007Jim Page, 2007 Involved Advisor Privilege
– Investigation disclosure restrictions
– Team understanding of:• Avoiding conflict of interest• Protecting proprietary information• Potential litigation
– Personal copy of report/data risks
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Jim Page, 2007Jim Page, 2007
Final Advisor Consideration
Access only to what enables
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Jim Page, 2007Jim Page, 2007
“Only critical components are ever lost during engineering investigations”