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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
3L. Peroni, M. Avalle – Politecnico di Torino
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
4L. Peroni, M. Avalle – Politecnico di Torino
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
5L. Peroni, M. Avalle – Politecnico di Torino
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
6L. Peroni, M. Avalle – Politecnico di Torino
Testing methods
7L. Peroni, M. Avalle – Politecnico di Torino
Uniaxial tension test
VCR
VAQ DAQ
Sample
VCR
VAQ DAQ
Sample
8L. Peroni, M. Avalle – Politecnico di Torino
Shear: torsion test
Torsion loading test rig (shown with an aluminum foam sample mounted on it)
9L. Peroni, M. Avalle – Politecnico di Torino
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
10L. Peroni, M. Avalle – Politecnico di Torino
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
11L. Peroni, M. Avalle – Politecnico di Torino
Test chamber
Fluid
p
Axial rod
Hydrostatic tests
1
3
Uniaxial compression
Hydro-compression
dev
hyd
Hydrostatic compression
1
3
Uniaxial compression
Hydro-compression
dev
hyd
Hydrostatic compression
A = R = p
1
3
Uniaxial compression
Hydrostatic compression
Hydro-compression
dev
hyd
a
b
c
1
3
Uniaxial compression
Hydrostatic compression
Hydro-compression
dev
hyd
a
b
c
A R (= p)
Fluid
p
A
Axial rod
Radial rod
12L. Peroni, M. Avalle – Politecnico di Torino
Hydrostatic and hydro-compression test
Hydrostatic (compression)
Hydro-compression
13L. Peroni, M. Avalle – Politecnico di Torino
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)
14L. Peroni, M. Avalle – Politecnico di Torino
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
15L. Peroni, M. Avalle – Politecnico di Torino
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
16L. Peroni, M. Avalle – Politecnico di Torino
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
17L. Peroni, M. Avalle – Politecnico di Torino
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
)
18L. Peroni, M. Avalle – Politecnico di Torino
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).
19L. Peroni, M. Avalle – Politecnico di Torino
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
20L. Peroni, M. Avalle – Politecnico di Torino
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
21L. Peroni, M. Avalle – Politecnico di Torino
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!