ASPECTS OF MATERIALS FAILURE - Faculty of Mechanical ...taminmn/SKMM4133_Module_3_Aspects_failure.… · •Creep Rupture •Buckling •Static Delayed Fracture SKMM 4133 Failure

ASPECTS OF MATERIALS FAILURE M.N. Tamin, UTM

SME 4133 Failure of Engineering Components and Structures

1

MODULE 3

ASPECTS OF MATERIALS FAILURE

SKMM 4133 Failure of Engineering Components and Structures



2

FAILURE OF MATERIALS

Modes of failure

Mechanical properties and behavior

Failure (yield) theories

Factor of safety

Overview of fracture mechanics

Stress states at crack tip

Stress intensity factor

Fracture toughness

Ductile-to-brittle transition behavior




3

Failure Versus Fracture

Failure

Inability of a component to perform according to its intended function.

Fracture

Separation of a component into two or more parts.




4

Modes of Failure

• Gross Yielding

• Fatigue Fracture

• Creep Rupture

• Buckling

• Static Delayed Fracture




5

Fracture of a Specimen in a Tension Test

Specimen

Load cell

Specimen grips

Crosshead

Data acquisition

system

Extensometer




6

Terikan

0.0 0.1 0.2 0.3 0.4 0.5

Teg

asa

n (

MP

a)

0

200

400

600

Ujikaji A

Ujikaji B

Stress-strain curves for Type 316 SS

Properties Values

y0.2% (MPa) 429

u (MPa) 604

E (GPa ) 208

n 0.0935

K ( MPa ) 682.65

r2 0.9857

Mechanical Properties and Behavior




7

Mechanical Properties of Some Materials

MATERIALS E

GPa (106psi)

So

MPa(ksi)

SU

MPa(ksi)

AISI 1040

steel

207

(30)

413

(60)

620

(90)

Stainless

Steel 314

193

(28)

205

(30)

515

(75)

7075

Al alloy

72

(10.5)

105

(15)

230

(33)

Gray cast iron - - 152

(22)




8

Yield Criterion

The material of a component subjected to complex loading will start

yielding when the (parametric stress) reaches the (characteristic stress)

in an identical material during a tensile test.

f() =

Other parameters:

Strain

Energy

Specific stress component (shear stress, maximum principal stress)

Theory of Failure




9

Theory of Failure Maximum-distortion-energy theory

22

13

2

32

2

21 2 Y

22

221

2

1 Y




10

Theory of Failure

Maximum-normal-stress theory

ult

ult

2

1




11

Ffail is determined from experimental testing of the material

Fallow is the allowable or working load

F.S. ≥ 1.0 to avoid failure

Factor of Safety

allow

fail

F

FSF ..




12

Fracture Mechanics - Overview

WWII Liberty Ships

Photo by Neil Boenzi, The New York Times.




13

A branch of mechanics that studies the relationships

between external loads applied to a deformable

body and the intensity of internal forces acting

within the body.

Mechanics of Materials

The mechanics that describes the response of

materials to loading in the presence of crack or

crack-like defects.

Fracture Mechanics




14

Scope of Fracture Mechanics




15

Linear Elastic Fracture Mechanics (LEFM)

Fracture mechanics within the confines of the theory of linear elasticity.

Analytical procedure that relates the stress magnitude and distribution in the neighborhood of a crack to:

the nominal applied stress

crack geometry (size, shape) and orientation

material properties

An underlying principle is that unstable fracture occurs when the stress-intensity factor at the crack tip reaches a critical value.




16

Basic Loading of Cracked Bodies




17

Stress Field Ahead of Crack Tip

Complex state of

stress exists in the

vicinity of a crack tip

yy

I

yy fr

k

2




18

Crack-tip Stress

Stress magnitude at the

crack tip approaches

(mathematically) an

infinite value

yy

I

yy fr

k

2




19

Crack-tip Plasticity

There is always a

plastic zone at the

crack tip

2

2

2

2

*

22 yield

a

yield

I

p

kr




20

The stress intensity factor,

KI describes the crack tip

stresses.

2a

Crack

τxy

σyy

σxy

θr

σ

σ

(or Y) – geometry factor

aKI

Stress Intensity Factor




21

SIF – Finite Width Correction

For 2a<<W,

aKI




22

Values of KI for different loading conditions and geometries

aa

K a K 1.1 a

units of K :

MPa m

or ksi in

Adapted from Fig. 8.8,

Callister 6e.




23





24





25

Based on data in Table B5,

Callister 6e.

Fracture toughness

represents the resistance of

materials to resist cracking.

Fracture toughness values

are determined from

fracture toughness tests.

Fracture Toughness of Some Materials




26

Fracture Toughness of Some Materials

MATERIALS KIC

MPa√m

Aluminum

2024

7075

7178

26

24

35

Steel

BS 816M40

BS816 M40

BS 535 A99

99

60

14

Titanium

IMI 318

IMI 318

115

55

SY

(MPa)

455

495

490

860

1515

2070

910

1035




27

Condition for Fracture

aa

K 1.1 a

Fracture occurs when the applied stress intensity

factor, KI reaches the value of the fracture toughness,

KIC of the material

IC

ICI

KπaYσ

KK




28

Impact Test

(Charpy)




29

Impact Energies of Some Materials

MATERIALS Impact energy

J (ft-lb)

AISI 1040

steel

48.8

(36)

Gray cast iron

(class 20)

20

(15)

Ti-6Al-4V 22

(16)




30

Fracture Surfaces of Impact Test Specimens




31

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

TDBT

Energ

y a

bso

rbed BCC

Temperature

FCC

Brittle

ductile Transition Temperature

(ASTM specification)

The temperature at which

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.


Documents

ASPECTS OF MATERIALS FAILURE - Faculty of Mechanical ...taminmn/SKMM4133_Module_3_Aspects_failure.… · •Creep Rupture •Buckling •Static Delayed Fracture SKMM 4133 Failure