Evaluation of tensile properties using IIT

2018. 03. 26.

원종호

Evaluation of tensile properties using IIT

Contents

 Background

 POSCO project

- Evaluation of dynamic tensile property

 Yield properties of PE pipe

Introduction

-Suitability of new material -Degradation

-Accident & damage analysis

Need for nondestructive technique to evaluate material properties

at in-field

-Verification of feasibility

-Life prediction/safety assessment

-Construction of material D/B

Introduction

Deformation

Fracture

σ

_YS

, UTS, n, E , · · ·

K

_IC

, J

_IC

, δ

_IC

, · · ·

Destructive

How can I measure the mechanical properties?

I am working.

Do not touch !!!

Introduction

Specimens for tensile test Specimens for fracture test

Not applicable for small scale testing

Large scale testing!!!

Introduction

Convenient

In-situ & In-field System

Non-destructive & Local test

Simple & fast

Introduction

A c

Hardness,

Elastic modulus,

A

C

H = P

^max

C

eff A

E S

2

= π

Plastic deformation

Elastic deformation

Algorithm for strength evaluation

♦Step 1

Determining contact area taking into consideration plastic pile-up/sink-in

Spherical Indentation

Stress and Strain State in Material

 

 



= 

R , h n h f

h

_max

* IT c pile

♦Step 2

Defining stress and strain state

in materials underneath spherical indenter as representative stress and strain

c max

T A

F 1

σ = Ψ ε T = ξ tan θ

♦Step 3 & 4

Fitting to constitutive equation and evaluating tensile properties

True strain, ε

Τ

True stress, σΤ

σ=E(ε-0.002) σ=Kεⁿ Representative stress-strain points

E

Instrumented indentation test with a spherical indenter

Tensile properties Tensile properties σ

_{y, IT}

, σ

_{u, IT}

, n

_IT,

E

_IT

Force-depth curve of multiple unloadings

,

♦Step 1

Determining contact area taking into consideration plastic pile-up/sink-in

Spherical Indentation

Stress and Strain State in Material

 

 



= 

R , h n h f

h

_max

* IT c pile

♦Step 2

Defining stress and strain state

in materials underneath spherical indenter as representative stress and strain

c max

T A

F 1

σ = Ψ ε T = ξ tan θ

♦Step 3 & 4

Fitting to constitutive equation and evaluating tensile properties

True strain, ε

Τ

True stress, σΤ

σ=E(ε-0.002) σ=Kεⁿ Representative stress-strain points

E

Instrumented indentation test with a spherical indenter

Tensile properties Tensile properties σ

_{y, IT}

, σ

_{u, IT}

, n

_IT,

E

_IT

Force-depth curve of multiple unloadings

,

Step 1

Reference plane

Elastic deflection

h d

-

Plastic pile-up/sink-in

h pile

+

R h c

h

^d

h max

h

^pile

pile d

c h h h

h = max − +

S

h

_d

= ε L

^max

( ,

^max

) R n h f h

_pile

=

-W.C. Oliver & G.M. Pharr J. Mater. Res. (1992)

-S.H. Kim et al, Mater. Sci. Eng. A (2006)

Step 2

Indentation depth increases Stress and strain increase

γ γ

Representative Stress Definition

Ψ σ m R =

P

Ψ: Constraint Factor

(about 3)

2 max c

m

a

P L

= π

Representative Strain Definition

γ α α

ε tan

) / (

1

²

− =

= R

a R a

c c

R

-D.Tabor, The Hardness of Metals (1951)

-J.H. Ahn et al, JMR (2000)

Step 3

Kε n

= σ

h L

Loading

Unloading

σ

ε

Indentation load-depth curve Derived stress-strain points

Step 4

) 002 . 0 ( −

=

_y

n

y

E

K ε ε

T rue s tr es s

True strain

Yield strength

A L = σ

ε A d

dA d = − =

l l

σ σ d A

dA =

−

ε σ σ = d d

Tensile strength

u = n

ε

POSCO project

(Evaluation of dynamic tensile property)

background

Room Temperature Low Temperature

 Strain rate range : 0.001/s, 1/s, 10/s, 100/s, 200/s

 Schematic diagram of strain rate regimes

Dynamic strain rage : 10

⁰

~ 10

⁴

/s

Definition of strain rate

 Representative stress-strain definition (Expanding Cavity Model)

R h Rh R

a 2

²

2 . 0 2

.

0 = −

ε =

2

1 a

c

L ψ π σ =

Representative Stress Representative Strain

 indentation strain rate (Spherical indenter)

- Representative strain definition :

R h Rh R

a 2

²

2 . 0 2

.

0 −

= ε =

- indentation strain rate :

dt d ε

ε & = f ( h , R , V )

dt

d ε =

*hc : depth, R : indenter radius,

V : indentation speed

Definition of strain rate

4 6 8 10 12 14 16 18 20 22 24

0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

strain rate [ /s]

h [um]

Dynamic range (10⁰ ~ )

→ 압입 깊이 별 strain rate 감소

Change of strain rate

according to indentation depth

0 20 40 60 80 100 120 140 160

0.2 0.4 0.6 0.8 1.0 1.2

1.4 strain rate=constant strain rate=changed

V[mm/min]

Depth[um]

example : Indentation strain rate = 0.01

Result

0.00 0.05 0.10 0.15 0.20 0.25

0 100 200 300 400 500 600 700

Tensile_Static Tensile_0.1/s Tensile_0.2/s Tensile_1/s Indentation_Static Indentation_0.01/s Indentation_0.05/s

T rue s tr es s[ M P a]

True strain

[S-S curve comparison]

Confirming tendency of s-s curve according to strain rate

Equipment improvement

Minute control

AIS3000 Hardware

0 1 2 3 4 5 6 7 8 9 10

0.0 0.2 0.4 0.6 0.8 1.0

Indentation strain rate [/s]

indentation speed [mm/min]

Dynamic hardening factor

σ

^d

f(ε

_p

) = σ

^s

f(ε

_p

) · DHF *

*DHF : Dynamic Hardening Factor

Experimental constant (D)

[Joon mo Choung, Dynamic hardening behaviors of various marine structural steels considering dependencies on strain rate and temperature, 2013]

Change in DHF

1E-3 0.01 0.1 1 10

1.0 1.1 1.2 1.3

SM400 (σ₀=296.7MPa)

DHF

Strain Rate [/s]

1E-3 0.01 0.1 1 10

1.0 1.1 1.2 1.3

SM490 (σ₀=342.1MPa)

DHF

Strain Rate [/s]

SM400 SM490

Change in DHF with strain rate (tensile test)

0.1 1

1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07

SM490 σ₀ = 321.3 MPa

DHF

Strain Rate [/s]

0.1 1

1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07

SM400 σ₀= 266.4MPa

DHF

Strain Rate [/s]

SM400 SM490

Change in DHF with strain rate (IIT)

Yield properties of PE pipe

Introduction

Gas Chemicals Water Sewage

In Industrial In Metropolitan

• Corrosion & Chemical resistance

• Long life & Life cycle cost savings

• Leak free joint

• Light weight

• Flexibiility

• High ductility

• Easy installation

Advantages of PE

Indenter

* Background

* Issue (Application to PE)

R d1

R

d2

θ¹

θ²

d

Residual Indent (top view)

Ψ σ m R =

P

Ψ: Constraint Factor

(about 3)

γ ε

_T

= 0 . 25 sin

Representation

True strain, εΤ

True stress, σΤ

σ=E(ε-0.002) σ=Kεⁿ Representative stress-strain points

E

In order to Evaluate Strength of Polyethylene, Definition of strain rate on IIT is essential.

It is difficult to definite strain rate using spherical indenter. Because the contact area is consistently changing during indentation.

Indenter

* Approach

 Using flat-ended cylindrical indenter instead of spherical indenter

Characteristics

Indenter shape

Sharp

Spherical (Conventional representation)

Flat-ended cylindrical

No self-similarity : Not keeping resemblance

during indentation

X O O

Fixed contact area : constant contact area during

indentation

X X O

Closeness to

compression test

X △ O

Definition of strain

0 50 100 150 200 250 300 350

0 20 40 60 80 100

load (kgf)

depth (um)

a=250um a=500um a=1000um

* Load-depth curve

0.0 0.5 1.0 1.5 2.0 2.5 3.0

0.0 0.1 0.2 0.3 0.4

Pm (GPa)

h/R

a=250um a=500um a=1000um

Load ⇒ P

_m

h ⇒ h/R

* Normalization

h/R can be “Representative” strain

Strain rate

R h

r

• •

−

= χ

ε 1

From flat-ended indentation for creep test,

R q h

r 1 2

• •

⋅ ε =

A q h

r

• •

⋅

= 2 ε

※ q₁

& q

₂

are constant.

[P.M.Sargent, Mater. Sci. Technol., 1992]

[J.Lu, J Mech Phys Solids., 2003]

Experimental approach

In progress…

(Indentation strain rate definition by Doener & Nix )

Indentation strain rate Indentation test parameter

Plastic zone expansion rate of indentation