jim page, 2007 chapter 8: engineering & material factors mina handbook

Jim Page, 2007Jim Page, 2007

Chapter 8: Engineering & Material Factors

MINA Handbook


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.


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.


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.


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.


Increase in Loading from Angle of Rigging


Types of Loads

Tension Compression

TorsionalShear

Bending

The Combination Stress

Compression

TensionShear

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.


Wood

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.

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.

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


• Shear Failure

– Smooth fracture surface

– Perpendicular to long axis of material

– Some deformation in ductile metals

– Buckling of panels will show direction of force

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


• Tension– Ductile fracture

• Elongation “necking down”• 45 degree slip plane fracture

– Rough granular appearance– Brittle fracture

• No elongation• 90 degree fracture plane

Jim Page, 2007Jim Page, 2007Tension Failure

Highly Ductile Sheet or Thin Bar Stock

Rough Granulated Tension Type Zone

Smooth Shear Type Zones


Brittle Tension Failure

Extremely Small 45o Edges

Jim Page, 2007Jim Page, 2007 Medium DuctilityTension Failure

Rough Granulated Tension Type Zone

“Castellated”

Smooth Shear Type Zone 45o Edges

Cup and Cone


• Shear Failure

– Smooth fracture surface

– Perpendicular to long axis of material

– Some deformation in ductile metals

– Buckling of panels will show direction of force

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

Jim Page, 2007Jim Page, 2007 Compression FailureLocal Crippling of Channel Section

Local Instability ofFlanges. Occurs Prior to

Bowing of Column.

Jim Page, 2007Jim Page, 2007 Compression FailureAngle Section Due to Torsional Instability

Loading is CompressionBut Failure Mode is Twist

Jim Page, 2007Jim Page, 2007 Shear Failure in Metal

Smooth Surface

Straight Parallel Traces 45o Rough

Granular Tension Type Zone

b

DUCTILE

BRITTLE

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

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.

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

Jim Page, 2007Jim Page, 2007 Torsion FailureDUCTILE

BRITTLE

Smooth Surface Shear

Type

Rough Granulated Tension Type

Zone

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.

Jim Page, 2007Jim Page, 2007 Bending Load -Shear Failure

Solid Ductile Metal Shaft

45o Shear Surface

Shear Tension

Bending

The Combination Stress

Compression

Jim Page, 2007Jim Page, 2007 Bending Load -Tension Failure

Solid Brittle Metal Shaft

90o Tension Surface

Jim Page, 2007Jim Page, 2007 Bending Load -Shear Failure

Solid Ductile Metal Shaft

45o Shear Surface


Fatigue of Materials

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


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


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




Strain, in/in

Str

ess,

lb/in

2

Ultimate Stress

Lower Yield Point

Upper Yield Point

Elastic Limit

Fracture

Plastic Range

Elastic Range





Elastic range

(temp. deform.)




Strain, in/in

Str

ess,

lb/in

2

Ultimate Stress

Lower Yield Point

Upper Yield Point

Elastic Limit

Fracture

Plastic Range

Elastic Range





Plastic range

(perm. deform.)

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


RECOGNIZING FATIGUE


RECOGNIZINGFATIGUE

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.


• Affected by temperature, stress, & fatigue

loading

• Transforms metal into chemical compounds

• Brittle, scaly, or powdery

• Little mechanical strength

• Moisture source of most aircraft corrosion

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)

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

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

Jim Page, 2007Jim Page, 2007 Electrochemical Corrosion

Electron Flow

Conductive PathMetal

+

Moisture

=

Corrosion

Anode CathodeElectrolyte

(Water + Ions)


Steel Fastener(cathode)

Moisture EntersHere

AluminumSheet

Corrosion of Aluminum(Anode Corrosion Location)

Corrosion Around a Rivet

Jim Page, 2007Jim Page, 2007 Intergranular Corrosion

Moisture

Grain – Crystal (cathode)

Corroded, Grain Boundary(Anode, Corrosion Location)

Grain Boundary

Jim Page, 2007Jim Page, 2007 Single OverloadDuctile Intergranular

Jim Page, 2007Jim Page, 2007 Exfoliation

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

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


Stress Corrosion Cracking (SCC)

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.

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.


Technical Assistance

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

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


How To Get Help

– Use local base resources

– Call convening authority

– AFSC

DO NOT INVITE EXPERTS ON YOUR OWN!

Jim Page, 2007Jim Page, 2007 How To Use Technical Assistance

– Telephone– On-site evaluation– Tear down reports– Exhibit security

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


Final Advisor Consideration

Access only to what enables


“Only critical components are ever lost during engineering investigations”

jim page, 2007 chapter 8: engineering & material factors mina handbook

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