1. Experimental methods for material measurements at high strain-rate
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

2. Contents Introduction Material behaviors, characterization
Experimental methods
Mechanical testing equipment
Conclusions

3. Dynamic effects on material behavior
Stress-strain characteristic and the effect of strain-rate on the mechanical behaviour of a material 
r d
Variation in yield strength
Variation in failure strength (ultimate tensile strength)
Variation in elongation at failure
Different work-hardening behaviour
s d
r s 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

4. Strain-rate effect: some experimental results
PP, mechanical characteristics a f c d e b
In questa slide sono presentati alcuni dei risultati ottenuti dalle prove di caratterizzazione dinamica a trazione di cinque materiali plastici differenti, ovvero del polipropilene, del Nylon non caricato, un elastomero, del polistirene e del policarbonato. Come è facile osservare dai grafici proposti tali materiali presentano una notevole sensibilità alla velocità di deformazione sia per quanto riguarda il limite elastico che per il modulo di Young. Inoltre è significativo sottolineare come al variare della velocità di deformazione della prova cambi la forma della caratteristica deformazione-tensione e anche le modalità di collasso del materiale. Infatti se bassa velocità tutti i polimeri analizzati presentano una grande duttilità, a velocità di deformazione medio-alte dell’ordine di 100 s-1 risultano notevolmente più fragili o addirittura come nel caso del propilene presentano un comportamento lineare elastico fino a rottura.
Different classes of polymers tested: PP, PA6, TEEE, PS, PC, EVA…

5. Multiaxial behavior (plastics, foams…)
As it is well known plastics and cellular materials yield is not independent on the hydrostatic component of stress
shyd
Von Mises
shyd
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
Tresca

6. Testing methods

7. Uniaxial tension test

8. Shear: torsion test
Torsion loading test rig (shown with an aluminum foam sample mounted on it)

9. Shear: 4-point asymmetrical testsz
txz x
Used for the mechanical characterization of ceramics, and ceramics composites (CfC’s) according to ASTM C1469 standard
Foamglas

10. Shear strength of joinings
Offset single-lap
*CFC/Cu/CuCrZr and W/Cu/CuCrZr joints for ITER
Shear/compression
Torsion
Cu/CfC joint shear test
Double-notch (ASTM C1292)
SiC joined by:
Silicon
Glass

11. Hydrostatic tests sA = sR = p sA  sR (= p) Test chamber Fluid p
Axial rod
sA = sR = p
Fluid p sA
Axial rod
Radial rod
sA  sR (= p)

12. Hydrostatic and hydro-compression test
Hydrostatic (compression)

13. 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)

14. 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 (n12)
For unbalanced layered composites, bending tests are also required
Impact tests are also performed to measure dynamic properties energy absorption capability
GFRP sample with rosette to measure strain in different directions

15. 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 bar
Incident bar
Specimen

16. 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

17. Determination of the stress-strain characteristic
Time history measurement of:
Average stress (specimen)
Strain-rate (specimen)
Strain (specimen)
Signals synchronization
Evaluation of the stress-strain characteristic

18. 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).
Per coprire il range di velocità compreso tra 1-10 m/s, che separa i sistemi servo-idraulici convenzionali dalla SHPB, nel Laboratorio di Affidabilità e Sicurezza del Politecnico di Torino è stato appositamente sviluppato un sistema di trazione rapida a funzionamento pneumatico denominato FasTens. Lo schema di sinistra riportata il principio di funzionamento dell’attrezzatura e a destra potete osservare l’attrezzatura e alcuni suoi dettagli: in breve un cilindro pneumatico speciale di grande alesaggio viene caricato in pressione fino ad un massimo di 10 bar, e trattenuto nella posizione di inizio prova. Raggiunta la pressione voluta nella camera di alimentazione, il pistone viene rilasciato con un sistema di rilascio rapido, accelera e raggiunta la velocità di prova aggancia il provino e lo deforma ad una velocità variabile tra 1 e 10 m/s. Lo spostamento dell’estremità mobile del provino è rilevato da un trasduttore laser a triangolazione, mentre l’estremità fissa è direttamente collegata ad una cella di carico dinamica piezoelettrica o estensimetrica

19. 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

20. 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

21. 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

22. Experimental methods for material measurements at high strain-rate
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!