1

Structures Structures � Any structure is built for a particular purpose

� Aircraft , Ship, Bus, Train

� Oil Platforms

� Bridges and Buildings

� Towers for Wind energy, Electrical transmission etc.

Structures and MaterialsStructures and Materials

Structures are built with various materials

� Metallic Materials

� Aluminum alloys, Titanium alloys, Steels etc.

� Composites

� Metal matrix composites

� Ceramic matrix composites

� Polymer Matrix composites

� Non-Metals

� Wood

� Rubber

� Plastics etc.

Mechanical loads on structural materials and componentsMechanical loads on structural materials and components

� Structures are subjected to various types of mechanical loads inservice

� Tensile

� Compressive

� Shear

� Bending

� Fatigue

Structures and MaterialsStructures and Materials

� In addition, structures may be subjected to

� High Temperature (aero-engines, thermal power plants etc.)

� Creep

� Oxidation etc.

� Corrosion (coastal environment)

� Wear / Abrasion etc.

Structures should be designed in such a way that they do

not fail during their expected / predicted safe-life

2

Failure of Structural MaterialsFailure of Structural Materials

� In spite of good design, structures often fail during service

� Failure represents an adverse situation wherein a component

assembly fails to perform its intended function satisfactorily

� Failure is the gap between expectation and performance

Failure may be due to

• Excessive Elastic deformation

• Excessive Plastic deformation

• Fracture

Failure MechanismsFailure Mechanisms

Others / unknown

SCC / Corrosion fatigue / HE

Fatigue

High temp. corrosion / Oxdn.

Creep

Wear/abrasion/erosion

Failure ModesFailure Modes

Corrosion

% failures% failures

Engg. Comp. Aircraft Comp.Engg. Comp. Aircraft Comp.

2929 1616

2525 5555

0707 0202

0606 0707

0303 ----

0303 0606

1616 1414

Need for Mechanical Properties EvaluationNeed for Mechanical Properties Evaluation

To Determine Basic Design Data

To Determine Residual Strength

For Failure Analysis and Investigation

To Validate Design

To Characterize Newly Developed Materials etc.

Testing and Evaluation PhilosophyTesting and Evaluation Philosophy

Specimen level : large no. of tests

Component level : Few no. of tests

Full Scale level : One/two tests

Building block approachBuilding block approach

3

ServoServo--hydraulic Test Machine hydraulic Test Machine

Screw DrivenScrew Driven

ServoServo--hydraulichydraulic

ElectroElectro--servoservo--hydraulichydraulic

Specifications : Load capacitySpecifications : Load capacity

ServoServo--hydraulic Test Machine hydraulic Test Machine

PartsParts

Power PackPower Pack

Test FrameTest Frame

ControllerController

TransducersTransducers

Modes of operationModes of operation

Load ControlledLoad Controlled

Position/stroke controlledPosition/stroke controlled

Strain ControlledStrain Controlled

Orientation : Anisotropy Considerations During TestingOrientation : Anisotropy Considerations During Testing

Forging, Rolling, Forming etc. introduce textureForging, Rolling, Forming etc. introduce texture

L

T

LT

TL

L T

L

Tensile Testing Tensile Testing

Properties of Materials Evaluated

Elastic Modulus (E)

Yield Strength / Proof Strength (YS)

Ultimate Tensile strength (UTS)

Fracture Strength

Poisson’s Ratio (ν)

Ductility

% Elongation

% Reduction in C/S area

4

Tensile Testing Standard: ASTM E08M Tensile Testing Standard: ASTM E08M

Specimen

Threaded / straight ends

Miniature specimens

Tensile Testing ProcedureTensile Testing Procedure

Fix specimen in the grip

Zero all transducers

Set loading rate

Fix extensometer

Start / Acquire test data

Load/strain/position

Convert to stress/strain plot

Evaluate all properties

Tensile Testing Tensile Testing

0

200

400

600

800

1000

1200

1400

1600

1800

0 0.5 1 1.5 2 2.5

Strain %

Str

ess

, M

Pa

0

250

500

750

1000

1250

1500

1750

0 5 10 15

Strain %

Str

ess

, M

Pa

Ductile and Brittle FailuresDuctile and Brittle Failures

Ductile– Characterized by tearing

of metals accompanied by

gross plastic deformation

– Macroscopically they have a gray, fibrous appearance

Brittle– Characterized by rapid crack

propagation without

appreciable plastic

deformation

– Macroscopically they have a

bright, granular appearance

5

1. Failure of pins of an aero engine

Some examples of fatigue failures

Circumferential deformation

mark at 4.5 mm distance

from the pinhead

1mm

Transgranular fatigue

crack

Striations

Half moon shaped region

Failed roller shaft of an automobileFailed roller shaft of an automobile

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Fatigue failure Fatigue failure

Multiple crack initiation Multiple crack initiation

Fracture surface appearance Fracture surface appearance Failure of LPTR blade of an aeroengine Failure of LPTR blade of an aeroengine

Failure of LPTR blade of an aeroengine Failure of LPTR blade of an aeroengine

Aloha aircraft accident: Hawaii, Apr. 28, 1988 Aloha aircraft accident: Hawaii, Apr. 28, 1988

More information available at www.aloha.netMore information available at www.aloha.net

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FatigueFatigue

� Failure of materials under cyclic load

� It involves

–Crack initiation

–Crack propagation

–Final fracture

� Total fatigue life : Life to ( initiate + propagate) a crack

Surface

� General observations

Stage I

Slip plane

crack

Stage II

Striation

mode

fatigue

Stage III

Superimposed

Static modes ;

Cleavage, void etc

� Macroscopic characteristics of a typical fatigue failure

Beach marks (macro)

Marks due to change in load amplitudes, delay, stop etc

Striations (micro)

Beach marks

� Microscopic characteristics of a typical fatigue failure

Striations (micro)

Marks on the fracture surface due to crack extension during every

cycle

Striations

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Mechanism of fatigueMechanism of fatigue

� In a smooth specimen

to and fro slip in 45 0 slip plane, formation of PSB s

Slip is not completely reversible

Leads to intrusion and extrusion

Intrusions act as a stress concentration

Crack initiates due to high stress concentration

Crack propagates under cyclic stress

Unstable crack when it reaches fracture toughness of the material

� Defects in materials such as grain boundary, ppts., cavities,

particles and engg. Shape such as bends, curves, holes etc act

as stress concentrators

� Some of the terms related to fatigue

Load

Time

P (max) ; σ (max) ; K (max)

P (min) ; σ (min) ; K (min)

∆P ; ∆σ; ∆K

One complete cycle

Load ratio = R = P (min) / P (max)

∆K = SIF range ; crack driving force

Stress concentration Stress concentration

� Linear Elastic Fracture Mechanics (LEFM) � Linear Elastic Fracture Mechanics (LEFM)

SIF = K = Y SIF = K = Y σσσσσσσσ √√√√√√√√((ππππππππa) (MPa a) (MPa √√√√√√√√m)m)

Fracture toughness = K Fracture toughness = K ICIC = Y = Y σσσσσσσσ √√√√√√√√((ππππππππa)a)

stre

ss

appσ

appσ

notchdistance

appσ

appσ

appσ

crack

stre

ss

distance

appσ

Stress Concentration Factor Stress Intensity Factor

SCF = KSCF = Kt t = = σσσσσσσσlocloc/ / σσσσσσσσnomnom

SIF provides a better representation of the crack tip stressesSIF provides a better representation of the crack tip stresses

KKt t = = 1 + 2 1 + 2 1 + 2 1 + 2 1 + 2 1 + 2 1 + 2 1 + 2 (a/(a/ρ) ρ) ρ) ρ) ρ) ρ) ρ) ρ)

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� Fracture Mechanics Answers to Queries

�What is the residual strength as a function of the crack size?

K = σσσσ ππππ a Y σσσσ =

KIc

π π π π a Y

σσσσ

a

for a range of crack length

Need to know:

•KIc for material

•Y-function for component

� Fracture Mechanics Answers to Queries

�What is the maximum permissible crack size under the service loading conditions?

K = σσσσ π π π π a Y

Need to know:

•KIc for material

•Y-function for component

• service stress, σservice

amax = 1

ππππ

KIc

Y σσσσservice

2

Representation of fatigue data Representation of fatigue data

� S-N curve

� ∆σ / 2 ∆σ / 2 ∆σ / 2 ∆σ / 2 = σσσσf’ (2 Nf ) b

∆σ∆σ∆σ∆σ∆σ / 2∆σ / 2∆σ / 2∆σ / 2

Endurance , fatigue limit

∆σ / 2∆σ / 2∆σ / 2∆σ / 2

No. of reversals to fail,

2Nf