Workshop on Materials for
Collimators and Beam Absorbers
Experimental methods for material
measurements at high strain-rate
Lorenzo Peroni, Massimiliano Avalle
Dipartimento di Meccanica, Politecnico di Torino
Contents

Introduction

Material behaviors, characterization

Experimental methods

Mechanical testing equipment

Conclusions
L. Peroni, M. Avalle – Politecnico di Torino
2
Dynamic effects on material behavior
Stress-strain characteristic and the effect of strain-rate
on the mechanical behaviour of a material

r d
s d
r s

Variation in yield
strength

Variation in failure
strength (ultimate
tensile strength)

Variation in elongation
at failure

Different workhardening behaviour
E
s s
Quasi-static
mechanical
characteristic
E
Dynamic
mechanical
characteristic
r s
r d

For many materials, strain-rate has negligible
effect on the elastic modulus
L. Peroni, M. Avalle – Politecnico di Torino
3
Strain-rate effect: some experimental results
PP, mechanical characteristics
2.5
test 0.1 mm*s-1
forza (kN)
a
test 0.8 mm*s-1
2
b
c
test 8 mm*s-1
1.5
test 80 mm*s
-1
test 6000 mm*s -1
1
test 8800 mm*s -1
d
e
0.5
0
0
20
40
60
80
corsa (mm)
100
f
120
2
6000
PA66
PP
PS
PC
TEEE
 ydynamic
k  static
y
Modulo
Modulo di Young (MPA)
Coefficiente dinamico k
2.5
1.5
1 -2
10
10
0
10
velocità (mm/s)
2
10
4
5000
Dati
Datisperimentali
sperimentaliPA66
PP
Cowper-Symonds
fit
Cowper-Symonds fit
4000
3000
2000
1000
0 -2
-2
10
0
0
22
10
10
velocità (mm/s)
44
10
10
 Different classes of polymers tested: PP, PA6, TEEE, PS, PC, EVA…
L. Peroni, M. Avalle – Politecnico di Torino
4
Multiaxial behavior (plastics, foams…)
As it is well known plastics
and cellular materials yield
is not independent on the
hydrostatic component of
stress
hyd
dev
Pure shear
fracture
Uniaxial
tension
Von Mises
hyd
Tresca
Uniaxial compression
yield
Hydro-compression
Hydrostatic
compression
hyd
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
L. Peroni, M. Avalle – Politecnico di Torino
5
Testing methods
F
F
T
Adhesive
Adhesive
F
T
F
Uniaxial Compression
Uniaxial Tension
Torsion
F
p
dev
Pure shear
fracture
p
Hydrostatic
Compression
p
p
p
p
F
Hydro-Compression
Uniaxial
tension
Uniaxial compression
yield
Hydro-compression
Hydrostatic
compression
v
hyd
Split Hopkinson Pressure Bar
v
Dynamic
compression
Bending
L. Peroni, M. Avalle – Politecnico di Torino
6
Uniaxial tension test
Sample
VCR
VAQ
L. Peroni, M. Avalle – Politecnico di Torino
DAQ
7
Shear: torsion test
Torsion loading test rig (shown with an aluminum foam sample
mounted on it)
L. Peroni, M. Avalle – Politecnico di Torino
8
Shear: 4-point asymmetrical tests
z
txz
x
• Used for the mechanical
characterization of ceramics,
and ceramics composites (CfC’s)
according to ASTM C1469
standard
Foamglas
L. Peroni, M. Avalle – Politecnico di Torino
9
Shear strength of joinings
Offset single-lap
*CFC/Cu/CuCrZr and W/Cu/CuCrZr joints for ITER
Shear/compression
CuCrZr
Torsion
Cu/CfC joint shear test
Pure Cu
W, CFC
Double-notch (ASTM C1292)
SiC joined by:
• Silicon
• Glass
L. Peroni, M. Avalle – Politecnico di Torino
10
Hydrostatic tests
A = R = p
 dev
Test chamber
Fluid
Uniaxial compression
p
Hydro -compression
3
Axial rod
Hydrostatic compression
hyd
1
A  R (= p)
dev
Uniaxial compression
c
Hydro-compression
b
1
Radial
rod
p
Hydrostatic
compression
a
3
A
Fluid
hyd
Axial rod
L. Peroni, M. Avalle – Politecnico di Torino
11
Hydrostatic and hydro-compression test
Hydro-compression
Hydrostatic
(compression)
L. Peroni, M. Avalle – Politecnico di Torino
12
Fatigue loading
• 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)
Strain-life curve AISI 1070
-1
10
Experimental hysteresis loop
600
-2
10
200
Strain
Stress (MPa)
400
0
-200
-3
10
elastic strain
plastic strain
total strain
fit elastic strain
fit plastic strain
fit total strain
run out
-4
10
-400
-600
-0.02
-0.015
-0.01
-0.005
0
Strain
0.005
0.01
0.015
0.02
2
10
3
10
4
10
5
10
6
10
7
10
Reversal to failure
L. Peroni, M. Avalle – Politecnico di Torino
13
Composites
140
y = 0.047850x - 10.837615
120
y = -0.090067x - 12.971095
100
Stress (MPa)
 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
(n12)
 For unbalanced layered
composites, bending tests
are also required
 Impact tests are also
performed to measure
dynamic properties
energy absorption
capability
80
y = 0.030996x - 17.644203
60
40
SG1 (µm/m)
SG2 (µm/m)
SG3 (µm/m)
20
0
-2000
-1000
0
1000
2000
3000
4000
5000
Strain (µm/m)
GFRP sample with rosette to measure
strain in different directions
L. Peroni, M. Avalle – Politecnico di Torino
14
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
Incident bar
Transmission bar
Specimen
L. Peroni, M. Avalle – Politecnico di Torino
15
The Split Hopkinson Pressure Bar (SHPB)
 Split Hopkinson Pressure Bar (SHPB, compression test)
 Split Hopkinson Tensile Bar (SHTB, tensile test)
Tensile specimen
Steel sheet
Compression specimen
Bulk adhesive
Bulk adhesive
Aluminum foam
L. Peroni, M. Avalle – Politecnico di Torino
16
Determination of the stress-strain characteristic
 Time history measurement of:
Deformazioni
indotte
alle
provino-barre
Tensione
del interfacce
provino
Caratteristica
meccanica
"ingegneristica"
Confronto
tra caratteristica
meccanica
ingegneristica
e vera
x 10
-3
(( MPa
tensione
tensione
MPa )
deformaz
ione
1200
100
1.5
• Average stress (specimen)
1000
01
600
-2000
400
• Strain (specimen)
Caratteristica meccanica vera
Caratteristica meccanica ingegneristica
-300
-0.5
200
-400
0
-1
• Strain-rate (specimen)
Onda riflessa
Onda trasmessa
800
-100
0.5
0
7
7
0.05 8
0.05
8 0.1
0.1
0.15
9
0.159 0.2
tempo
deformazione
tempo (( ss ))
 Signals synchronization
10 0.2
0.25
10
0.25
0.3
11
-4
-4
x x10
10
 Evaluation of the stress-strain
characteristic
Velocità di deformaz ione del provino
Deformazione del provino
500
deformaz ione
0
 specimen t   
-0.05
-0.1
-0.15
2c0
Lprojectile
  reflected wave t dt
-0.2
A
 average t   Ebars bars  transmitted wave t 
A (s)
tempospecimen
-0.25
7
8
9
10
-4
x 10
velocità deformazione ( s
-1
)
0.05
0
-500
-1000
-1500
-2000
-2500
7
8
9
tempo ( s )
10
-4
x 10
L. Peroni, M. Avalle – Politecnico di Torino
17
Dynamic tensile equipment: FasTENS
Sensore di
spostamento
laser
Cella di
carico
dinamica
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).
Provino
Sistema
di rilascio
rapido
Aria
Smorzatore
L. Peroni, M. Avalle – Politecnico di Torino
18
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
L. Peroni, M. Avalle – Politecnico di Torino
19
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
L. Peroni, M. Avalle – Politecnico di Torino
20
Concluding remarks
 The mechanical characterization of materials is one of
the first steps in the design of high performance
structures
 The spectrum of mechanical tests available is very
large, to cover many possibility of loading, even far
beyond established standard
 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
L. Peroni, M. Avalle – Politecnico di Torino
21
Workshop on Materials for
Collimators and Beam Absorbers
Experimental methods for material
measurements at high strain-rate
Lorenzo Peroni, Massimiliano Avalle
Dipartimento di Meccanica, Politecnico di Torino
Thank you for your
attention!