experimental methods for material measurements at high strain-rate lorenzo peroni, massimiliano...

Experimental methods for material measurements at high strain-rate

Lorenzo Peroni, Massimiliano AvalleDipartimento di Meccanica, Politecnico di Torino

Workshop on Materials for Collimators and Beam Absorbers

2L. Peroni, M. Avalle – Politecnico di Torino

Contents

Introduction

Material behaviors, characterization

Experimental methods

Mechanical testing equipment

Conclusions


Dynamic effects on material behavior

Variation in yield strength

Variation in failure strength (ultimate tensile strength)

Variation in elongation at failure

Different work-hardening behaviour

Stress-strain characteristic and the effect of strain-rate on the mechanical behaviour of a material

For many materials, strain-rate has negligible effect on the elastic modulus

Quasi-static mechanical characteristic

Dynamic mechanical characteristic

s d

s s

r d

r s

r dr s

E

E


Strain-rate effect: some experimental results

Different classes of polymers tested: PP, PA6, TEEE, PS, PC, EVA…

0 20 40 60 80 100 1200

0.5

1

1.5

2

2.5

corsa (mm)

forz

a (

kN)

test 0.1 mm*s-1

test 0.8 mm*s-1

test 8 mm*s-1

test 80 mm*s-1

test 6000 mm*s-1

test 8800 mm*s-1

PP, mechanical characteristics

a

f

c

d

e

b

10-2

100

102

104

1

1.5

2

2.5

velocità (mm/s)

Co

effi

cie

nte

din

am

ico

k

PA66PPPSPCTEEE

staticy

dynamicyk

10-2

100

102

104

0

1000

2000

3000

4000

5000

6000

velocità (mm/s)

Mo

du

lo d

i Yo

un

g (

MP

A)

Dati sperimentali PPCowper-Symonds fit

10-2

100

102

104

0

1000

2000

3000

4000

5000

6000

velocità (mm/s)

Mo

du

lo d

i Yo

un

g (

MP

A)

Dati sperimentali PA66Cowper-Symonds fit


Multiaxial behavior (plastics, foams…)

Therefore, the plastic collapse condition cannot be characterized from the result of a single (uniaxial) test, having a given ratio of hydrostatic/deviatoric stress components, but it is necessary to perform several tests with different combinations of deviatoric and hydrostatic stress components

As it is well known plastics and cellular materials yield is not independent on the hydrostatic component of stress

hyd

hyd

Von Mises

Tresca


Testing methods


Uniaxial tension test

VCR

VAQ DAQ

Sample

VCR

VAQ DAQ

Sample


Shear: torsion test

Torsion loading test rig (shown with an aluminum foam sample mounted on it)


Shear: 4-point asymmetrical tests

xz

x

z

• Used for the mechanical characterization of ceramics, and ceramics composites (CfC’s) according to ASTM C1469 standard

Foamglas


Shear strength of joinings

Cu/CfC joint shear test

Offset single-lap

W, CFCPure Cu

CuCrZr

W, CFCPure Cu

CuCrZr

W, CFCPure Cu

CuCrZr

*CFC/Cu/CuCrZr and W/Cu/CuCrZr joints for ITER

Shear/compression

Torsion

Double-notch (ASTM C1292)

SiC joined by:• Silicon• Glass


Test chamber

Fluid

p

Axial rod

Hydrostatic tests

1

3

Uniaxial compression

Hydro-compression

dev

hyd

Hydrostatic compression

1

3


Hydro-compression

dev

hyd


A = R = p

1

3



Hydro-compression

dev

hyd

a

b

c

1

3



Hydro-compression

dev

hyd

a

b

c

A R (= p)

Fluid

p

A

Axial rod

Radial rod


Hydrostatic and hydro-compression test

Hydrostatic (compression)

Hydro-compression


Fatigue loading

102

103

104

105

106

107

10-4

10-3

10-2

10-1

Reversal to failureS

train

Strain-life curve AISI 1070

elastic strainplastic straintotal strainfit elastic strainfit plastic strainfit total strainrun out

-0.02 -0.015 -0.01 -0.005 0 0.005 0.01 0.015 0.02

-600

-400

-200

0

200

400

600

Strain

Stre

ss (M

Pa)

Experimental hysteresis loop

• Stress-life and strain-life approach• Rotating bending (metals), plane bending

(polymers, composites) high-cycle fatigue• Tension/compression low-cycle/high-cycle

fatigue (evaluation of the hysteresis of the material)


Composites

For orthotropic materials like most composites, tests at different loading angles are required to obtain the different moduli (in-plane E11, E22, G12) and Poisson’s coefficient (12)

For unbalanced layered composites, bending tests are also required

Impact tests are also performed to measure dynamic properties energy absorption capability

y = 0.030996x - 17.644203

y = 0.047850x - 10.837615

y = -0.090067x - 12.971095

0

20

40

60

80

100

120

140

-2000 -1000 0 1000 2000 3000 4000 5000

Strain (µm/m)S

tres

s (M

Pa)

SG1 (µm/m)SG2 (µm/m)SG3 (µm/m)

GFRP sample with rosette to measure strain in different directions


The Split Hopkinson Pressure Bar (SHPB)

Operating principle The projectile hits the incident bar generating a compressive wave train The wave train propagates at the speed of sound in the bars material

and reaches the specimen, then:► It is partly reflected► Partly crosses the specimen and goes through the transmission bar

The reflected and transmitted waves are measured By reconstruction based on the two signal the dynamic mechanical

characteristic is obtained

Proiectile

Transmission barIncident bar

Specimen


The Split Hopkinson Pressure Bar (SHPB)

Split Hopkinson Pressure Bar (SHPB, compression test) Split Hopkinson Tensile Bar (SHTB, tensile test)

Tensile specimen

Steel sheet

Bulk adhesiveCompression specimen

Bulk adhesive

Aluminum foam


7 8 9 10

x 10-4

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05Deformazione del provino

tempo ( s )

de

form

az

ion

e

Determination of the stress-strain characteristic

7 8 9 10 11

x 10-4

-1

-0.5

0

0.5

1

1.5x 10

-3 Deformazioni indotte alle interfacce provino-barre

tempo ( s )

de

form

az

ion

e Onda riflessaOnda trasmessa

dttL

ct wavereflected

projectilespecimen 02

tA

AEt wavedtransmitte

specimen

barsbarsaverage

Time history measurement of:

• Average stress (specimen)

• Strain-rate (specimen)

• Strain (specimen)

Signals synchronization

Evaluation of the stress-strain characteristic

0 0.05 0.1 0.15 0.2 0.250

200

400

600

800

1000

1200Caratteristica meccanica "ingegneristica"

deformazione

ten

sio

ne

( M

Pa

)

0 0.05 0.1 0.15 0.2 0.25 0.30

200

400

600

800

1000

1200Confronto tra caratteristica meccanica ingegneristica e vera

deformazione

ten

sio

ne

( M

Pa

)

Caratteristica meccanica veraCaratteristica meccanica ingegneristica

7 8 9 10

x 10-4

-2500

-2000

-1500

-1000

-500

0

500Velocità di deformazione del provino

tempo ( s )

ve

loc

ità

de

form

az

ion

e (

s -1

)

7 8 9 10

x 10-4

-400

-300

-200

-100

0

100Tensione del provino

tempo ( s )

ten

sio

ne

( M

Pa

)


Dynamic tensile equipment: FasTENS

To cover the speed range from 1 to 10 m/s, in tensile loading, in between the hydraulic systems and the SHPB, a special fast tensile equipment, pneumatically actuated, has been developed (FasTENS).


Dynamic compression: ComPULSE

Pneumatically actuated Maximum speed up to

15 m/s Maximum available

energy 3 kJ Load measurement with

piezoelectric load cells, maximum load 220 kN

Stroke measurement with laser transducer (Keyence)

Suitable also for tensile, bending, and other tests using special fixtures


Low/high temperature testing

A climatic chamber coupled with the ComPULSE equipment, was developed for dynamical tests down to -–40°C (will be further improved to be pushed down to -80°C) and up to 100°C

The sample (or component) can be conditioned but also tested at various controlled temperatures


Concluding remarks

The spectrum of mechanical tests available is very large, to cover many possibility of loading, even far beyond established standard

The mechanical characterization of materials is one of the first steps in the design of high performance structures

Custom testing solutions are routinely developed, and will be likely to be developed for innovative and advanced materials

In most cases a single type of test is not sufficient to describe in details the properties and behaviour of an advanced material or composite

Experimental methods for material measurements at high strain-rate

Lorenzo Peroni, Massimiliano AvalleDipartimento di Meccanica, Politecnico di Torino

Workshop on Materials for Collimators and Beam Absorbers

Thank you for your attention!

experimental methods for material measurements at high strain-rate lorenzo peroni, massimiliano...

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