B –MATERIALS ASPECTS OF FATIGUE AND …taminmn/MMJ1133_Lecture slides-pdf...MMJ1133 –FATIGUE AND FRACTURE MECHANICS Definition of fatigue by ASTM [1] The process of progressive

MMJ1133 – FATIGUE AND FRACTURE MECHANICS

B – MATERIALS ASPECTS OF FATIGUE AND FRACTURE

MATERIALS ASPECTS OF FATIGUE AND FRACTURE M.N.Tamin, CSMLab, UTM 1


Course Content:

A - INTRODUCTION

Mechanical failure modes; Review of load and stress analysis –

equilibrium equations, complex stresses, stress transformation,

Mohr’s circle, stress-strain relations, stress concentration; Fatigue

design methods; Design strategies; Design criteria.

B – MATERIALS ASPECTS OF FATIGUE AND FRACTURE


Static fracture process; Fatigue fracture surfaces; Macroscopic features; Fracture mechanisms; Microscopic features.

C – FATIGUE: STRESS-LIFE APPROACH

Fatigue loading; Fatigue testing; S-N curve; Fatigue limit; Mean

stress effects; Factors affecting S-N behavior – microstructure, size

effect, surface finish, frequency.


Tensile Fracture


Cleavage (brittle) fracture Ductile fracture

Source: Vander Voort, G.F., Metals Engineering Quarterly, Vol. 16, No. 3,

1976, pp. 33.


Imperfections in SolidScrew dislocation


Fig. 4.3, Callister 7ed.

Burgers

vector b


neckingvoid nucleation

void growth and linkage

shearing at surface

fracture

σ

Fracture Process in Ductile Materials


Particles serve

as void

nucleation

sites.

50 µm

P. Thornton, J. Mater. Sci., Vol. 6, 1971, pp. 347-56.)

Slip by plastic

deformation


Ductile Fracture Surface

- Dimples in

• Transgranular fracture

(across grains)


- Dimples in

ductile fracture.

(Ref: Callister, W.D., Materials Science and Engineering – An Introduction, John Wiley 2002),

Mag 3500X


• Intergranular fracture

(between grains)

Brittle Fracture Surfaces


304 S. Steel (metal)Reprinted w/permission from "Metals

Handbook", 9th ed, Fig. 633, p. 650.

Copyright 1985, ASM International, Materials

Park, OH. (Micrograph by J.R. Keiser and

A.R. Olsen, Oak Ridge National Lab.)

4 mm


Ductile-to-Brittle Tanstition (DBT) temperature, TDBT

Energy absorbed BCC

FCC

ductile Transition Temperature

(ASTM specification)

The temperature at which

MATERIALS ASPECTS OF FATIGUE AND FRACTURE M.N.Tamin, CSMLab, UTM

TDBT

Energy absorbed

Temperature

FCC

Brittle

specimens show a fracture

of 50 pct. shear and 50 pct.

cleavage.

Nil-ductility temperature (NDT)

The ref. point in the transition

range giving the limiting condition

of temperature-stress combination

under which catastrophic fracture

can occur.

8


Ductile-to-Brittle Tansition (DBT) temperature, TDBT

BCC metals (e.g., iron at T < 914°C)

Impact Energy

polymers

FCC metals (e.g., Cu, Ni)


Impact Energy

Temperature

High strength materials (σσσσy > E/150)

More DuctileBrittle

Ductile-to-brittle transition temperature

(Charpy)

9


Impact properties of some materials

MATERIALS Impact energy

J (ft-lb)

AISI 1040 48.8


AISI 1040

steel

48.8

(36)

Gray cast iron

(class 20)

20

(15)

Ti-6Al-4V 22

(16)

10


Definition of fatigue by ASTM [1]

The process of progressive localized permanent structural

change occurring in a material subjected to conditions that

produce fluctuating stresses and strains at some point or

points and that may culminate in cracks or complete fracture

after sufficient number of fluctuations


after sufficient number of fluctuations

[1] “Standard Terminology Relating to fatigue and Fracture,” testing ASTM

Designation E1823, Vol. 03.01, ASTM, PA, 2000, pp. 1034


Fatigue fracture features

Square thread

column

Shaft under

torsion


Ref. Stephens et al., 2000

column

Bicycle seat

post bolt

Typical features:

• Distinct crack nucleation sites

• Beach marks indicative of crack growth

• Distinct final fracture region

12


Transition mode of fatigue crack growth in sheet metal



Single shear Double shear

13


Beach marks and fatigue striations

Observation:

• Involves crack nucleation and

growth.

• Size of crack at fracture vary from

1 to almost 100 % depending on

applied stress and fracture

toughness.

• fatigue crack region is signified

by beach marks, smoothness and

corrosion.


corrosion.

• fatigue crack usually nucleates at

surface.

• Microscopic fatigue cracks often

nucleate and grow on plane of

maximum shear

• Macroscopic fatigue cracks often

grow in plane of maximum tensile

stress


Slip due to external loads


PSB – persistent slip bands


Fracture Surface due to Fatigue



Microscopic edge view of fatigue crack growth process

Loading

direction


Shear

modeTensile

mode


Microscopic fatigue crack growth

Cleavage involves

fracture along specific

crystallographic planes

and is transcrystalline

in nature.


Striation and microvoid

coalescence in ductile material

Susceptible in BCC and

HCP crystalline

structure


Fatigue crack growth process



B –MATERIALS ASPECTS OF FATIGUE AND …taminmn/MMJ1133_Lecture slides-pdf...MMJ1133 –FATIGUE AND FRACTURE MECHANICS Definition of fatigue by ASTM [1] The process of progressive

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