www.cranfield.ac.uk

Gustavo M. CastelluccioResearch Senior Lecturer in Manufacturing

castellg@cranfield.ac.uk

Fundamentals of Degradation

Through-life Engineering Services Summer School

©Cranfield University

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This lecture is about damage and degradation

• What is degradation?

• Different degradation mechanism.

• Factors driving damage.

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What is degradation?

“Loss of desired property or quality”

For instance, loss of:

• Structural resistance,

• Electrical conductivity,

• Light reflectivity,

• Chemical inertness,

• Information, etc.

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Types of structural degradation

• Fracture

• Fatigue

• Wear

• Corrosion

• Creep

• Aging

• ….

Localised

damage

Diffused

damage

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Localised Damage

Fracture Mechanics…

• Relates the nominal applied loads with crack propagation –

How do I apply this for a component?

• Assumes an initial defect size – What if I do not have one?

• Is based on crack similitude of homogeneous isotropic

materials – What if I have a weld?

Cracks correspond to a local discontinuity of the

material in which new surface was created.

1Anderson, T. L. Fracture Mechanics: Fundamentals and Applications. 3rd ed. CRC Press, 2004.

Fracture1

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Localised Damage

• KI is the driving force that can promotes crack growth.

• Unstable crack growth occurs when the applied KI is greater than

the critical stress intensity factor KIC.

• YI can be found in handbooks or computed with finite elements.

Stress

Intensity

Factor

Load appliedGeometrical attributes

Linear Elastic Fracture Mechanics (LEFM)

If there is extensive plasticity? Elasto-Plastic Fracture Mechanics (EPFM)

( , , )I IK Y a B W a

W

B

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• 1st mode: Opens the crack and it is usually

the most detrimental mode.

• 2nd mode: Slides the crack leaving a step on

the surface.

• 3rd mode: Tears the crack by shearing

parallel to the surface.

1st: tensile

2nd: sliding

3rd: tearing

Localised Damage

The amount of fracture damage depends on the loading conditions:

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Localised Damage

Fatigue damage corresponds to the accumulation of degradation

during cyclic loading.

2Suresh, S. Fatigue of Materials. 2nd ed. Cambridge University Press, 1998.

The range of stress and strains and the mean stress controll damage.

Fatigue2

Max

Min

2

Max

Min

2

PeriodPeriod

Zero mean stress (R=-1) Mean stress>0

Stress ratio

Min

Max

R

Max

Min

Random loading

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Under cyclic loading, the fatigue lifetime of a crack comprises.

•Crack Initiation (Nucleation)

•Crack Growth (Propagation)

•Crack Failure

Cycles N,

Time t

Cra

ck

dim

en

sio

n

Initiation

Propagation

Failure

Localised Damage

Fatigue

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Localised Damage

Fatigue crack propagation can be computed by following fracture

mechanics approaches

Paris’ Law mda

C KdN

( , , )I IK Y a B W a

Fatigue crack initiation is much more difficult to compute and is a

source of large uncertainty when predicting the remaining life of a

component.

Fatigue

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S-N curves: Stress vs number of cycles until failure

Localised Damage

Fatigue

Longer lives have

larger variability

Jha et al. Eng.

Frac. Mech. 2009

High cycle Fatigue (HCF)Low cycle Fatigue (LCF)

~104

Very / Ultra High cycle Fatigue

(VHCF/UHCF)

~106 - 107

Surface

dominated failure

Subsurface

dominated failure

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Localised Damage

Crack initiation:

• Pre-existing defects and geometrical stress

concentrators are typical locations where

cracks initiate.

• Accumulation of plastic deformation initiate

cracks if no defects are present. This

process is assisted by the environment

crack initiation in vacuum or air are different.

Pineau A. and Bathias C. Fatigue des Matériaux et des Structures 1. Lavoisier; 2008.

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Localised Damage

Fatigue

Surface failure Subsurface failureCrack initiationOriginally smooth

surface

Ma & Laird. Acta Met. 1989

Crack growth direction

Crack

initiates and

grow inside

the bar

Surface failure Subsurface failure

Filgueiras et al. 2013

http://www.aeronavlabs.com/failures_of_metals.htm

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• Thermomechanical fatigue: Thermal and

load cycles combined

• Corrosion fatigue: Corrosion enhances

damage and initiates cracks in materials that

would not otherwise fail under cyclic loading.

• Fretting fatigue: Surface wear and cyclic

loading combined.

Mixed fatigue damage

Localised Damage

In-phase

Out-of-phase

Temperature

Force

Temperature

Force

http://www.lambdatechs.com/fretting-fatigue.html

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Material Size

affect

Geometrical

Discontinuities

Surface

Conditions

Heat

treatment

Residual

Stresses

Operating

Temperature

Environment

Deformation

rate Stress-Time

History

Drivers of Localised Damage

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• Three primary types of wear:

• Adhesive,

• Abrasive,

• Fretting,

• Corrosive.

• Wear can lead to surface roughening

and cracks

Wear Damage

Degradation of a material due to the mechanical

interaction of surfaces

Wear

Kato and Adashi, 2001

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Lo

ad

Fre

qu

en

cy (

of

rub

bin

g/im

pa

ct)

Ma

teria

l P

rop

ert

ies

(Ha

rdn

ess)

Ma

teria

l P

rop

ert

ies

(Ela

stic M

od

ulu

s)

Co

rrosiv

e a

tmosphe

re

Hu

mid

ity

Tem

pera

ture

Asp

eri

tie

s/Im

pin

g

Part

icle

s S

ize a

nd

Sh

ap

e

Atta

ck A

ng

le (

of

imp

ing

ing

pa

rtic

le)

Wear + + - + + o + - +

Wear Damage

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• Principle of protective layers: use of interface-layers to serve as

surface protection, such as lubricants, surface film, paint,

plating, phosphate, chemical, flame-sprayed, etc.

• Principle of conversion: use better combinations of material

pairs as well as better surface finish and contact pressure

converting wear destructive effects to permissible levels.

• Principle of diversion: in which the component subject to wear is

replaced to a more economical element.

Controlling Wear Damage

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Corrosion Damage

Degradation of a material due to chemical reactions

Corrosion

• Pit corrosion is a localized process in

which the degradation penetrate into

metals at relatively small hot spots.

• Bulk corrosion is the active dissolution

of the entire surface and does not

require localisation.

Corrosion (Davis, 2000).

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Diffused Damage

Accumulation of plastic deformation with time.

Creep

• Creep refers to the accumulation

of irreversible deformation with

time. It is typically associated

with high temperature (T>Tm/2),

but stress can induce low

temperature creep.

Time

Length

Constant forceForce

increase

http://www.mee-inc.com/case-studies-list/fastener-failure/

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Diffused Damage

Creep

Examples in which creep is important:

• Turbines, furnaces, and heat

exchangers.

• Solder in electronics

• Polymers

ATSB Transport Safety Report AO-2011-062

http://www.tms.org/pubs/journals/JOM/0106/Frear-0106.html

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Diffused Damage

Transformation of material attributes with

time.

Aging

• Aging typically refers to a change in microstructural

attributes that evolve into low energy state configurations.

• Changes can be induced by external sources (e.g.,

radiation) or by internal metaestability.

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Diffused Damage

Examples in which aging is important:

• Satellites (radiation damage).

• Nuclear and chemical reactors.

• Medical implants.

Aging

http://spaceflight101.com/juno/mission-updates/

https://energy.gov/ne/materials-aging-and-degradation

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• Damage reduces performance and shortens the life of

components.

• Several degradation mechanisms exist and coexists.

Degradation assessments should consider all mechanisms that

are active.

• Life prognosis depends on understanding the degree of

degradation. Non-destructive inspection in service is key to

evaluate remaining component life.

Take-Home Message