Lecture #2: Split Hopkinson Bar Systems

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1 Lecture #2: Split Hopkinson Bar Systems by Dirk Mohr ETH Zurich, Department of Mechanical and Process Engineering, Chair of Computational Modeling of Materials in Manufacturing

2 Uniaxial Compression Testing Useful experiment to characterize the flow behavior of materials at large strains Hydraulic actuator Fixed cross-head piston loading platen (top) load cell loading platen (bottom Base plate 2 2 2

3 Uniaxial Compression Testing undeformed deformed F c F F c = applied compression force l = current length L l L 0 = initial length l = current length F c L 0 = initial length Axial strain definitions l 1 (engineering or nominal strain) ln[ 1 ] L eng Axial stress definitions F F eng (engineering or nominal stress) 1 ] A [ eng eng 0 A eng Only valid for incompressible materials (true or logarithmic strain) 0 0 (true stress) 0 0 (extension) (shortening) (tension) (compression)

4 Applied force [kn] Applied force F [kn] Engineering stress [MPa] Engineering stress [MPa] True stress [MPa] F c F Uniaxial Compression Testing l F c Force-displacement curve Engineering stress-strain curve True stress-strain curve Change in in length l-l l-l 0 0 [mm] Engineering strain [-] [-] Logarithmic strain [-] 4 4 4

5 Uniaxial Compression Testing Numerical Simulation ( also topic of computer lab #2) Source:

6 Uniaxial Compression Testing Experimental challenges Recommended size for metals testing: H=15mm D=10mm Plastic buckling Shear buckling H/D too large Poor alignment anisotropy barreling Friction too high H/D too small optimal Excellent lubrication H/D ~

7 Strain rate in a compression experiment undeformed deformed L u l L l u u 0 0 (extension) (shortening) Nominal strain rate eng L l 1 eng l L u L True strain rate ln[ 1 eng ] eng 1 eng Equiv. strain rate eng 1 eng 7 7 7

8 Compression of a cylindrical specimen Principle of Quasi-static Equilibrium F in L initial F out Quasi-static equilibrium F in ( t) F ( t) out 8 8 8

9 Time scale #1 Strain at the end of the experiment: Average strain rate (over time): max av Duration of experiment T max av Note: T is independent of specimen dimensions! Example: 0.15 max av 500 / s T s 0.3ms 300s 9 9 9

10 Time scale #2 Wave propagation speed: c E Specimen length: L L c Wave travel time t L c Note: t does depend on specimen dimensions! Example: c 5000m / L 10mm s t s s

11 Principle of Quasi-static Equilibrium Long time scale: Duration of experiment T max av Short time scale: Wave travel time t L c L c Condition for quasi-static equilibrium: (when testing an elasto-plastic material) Fin( t) Fout( t) t T

12 Principle of Quasi-static Equilibrium Condition of quasi-static equilibrium t T L c max av Example for challenging experiments (w/ regards to quasi-static equilibrium) Brittle materials, e.g. ceramics max ~ 0.01 Soft materials, e.g. polymers c ~ 1000m / s Materials of coarse microstructure, e.g. metallic foams L 1mm

13 Dynamic testing of foams Example for challenging experiments (w/ regards to quasi-static equilibrium) Zhao et al. (1997)

14 LIMITATION OF UNIVERSAL TESTING MACHINES Vibration issues t s t 20 1 s t 2s Condition of validity max Texp max[ t av breaks often down at around 10/s i ]

15 LIMITATION OF UNIVERSAL TESTING MACHINES Ringing of conventional load cell

16 SEPARATION OF TIME SCALES Characteristic time scale of testing system t sys versus Duration of experiment T exp max av Universal testing machines t sys T exp SHPB t sys Texp [ s ]

17 Wave Equation Derived wave equation for bars under the assumption of uniaxial stress x, u dx dx x position axial displacement Differential equation for axial displacement u[x,t]: 2 2 u c 2 x 2 u 2 t 0 longitudinal elastic wave velocity: the particle velocity: c E v[ x, t] u [ x, t] u t

18 General Solution General solution of wave equation t u x u c ] [ ] [ ], [ ct x g ct x f t x u ], [ ], [ ], [ ], [ t x t x c x g c x f c t x u t x v r l displacement: particle velocity [ ct] x f [ ct] x g Leftward traveling rightward traveling ], [ ], [ ], '[ ], [ t x t x x g x f t x u t x r l strain

19 Elastic Wave Velocities Particle velocity depends on applied loading, while the wave velocity is an intrinsic material property Material Density [g/cm 3 ] Young s modulus [GPa] Wave velocity [m/s] Air 340 Steel Aluminum Magnesium Tungsten Lead PMMA Concrete All values are rough estimates and may vary depending on the exact material composition and environmental conditions

20 Hopkinson Bar Experiment Typical system characteristics: Striker bar length: L s =1000 mm Input bar length: L i = 3000 mm Output bar length: L o = 3000 mm Bar diameter: D= 20 mm Specimen characteristics (for simplified theoretical analysis): Ideal plastic, constant force

21 rightward leftward total Hopkinson Bar Experiment

22 rightward leftward total Hopkinson Bar Experiment

23 rightward leftward total Hopkinson Bar Experiment

24 rightward leftward total Hopkinson Bar Experiment

25 rightward leftward total Hopkinson Bar Experiment

26 rightward leftward total Hopkinson Bar Experiment

27 rightward leftward total Hopkinson Bar Experiment

28 rightward leftward total Hopkinson Bar Experiment

29 rightward leftward total Hopkinson Bar Experiment

30 rightward leftward total Hopkinson Bar Experiment

31 rightward leftward total Hopkinson Bar Experiment

32 rightward leftward total Hopkinson Bar Experiment

33 rightward leftward total Hopkinson Bar Experiment

34 Force Strain HOPKINSON BAR TECHNIQUE Measuring force with a slender bar R (, t) x A applied 2( L x ) / A c ( t) A ( x R A measured, t) ( x L A, t) Time x A / c A R Time ( x, t) ( 2 L x ) / A c L (, t) x A F x A L L ( x, t)

35 Force Force Force HOPKINSON BAR TECHNIQUE Measuring force with a slender bar duration of valid measurement Time Time Time F A x A L x B R ( x, t) ( x, t) B

36 HOPKINSON BAR TECHNIQUE Direct impact experiment: One force measurement (output bar only) specimen output bar u out striker specimen output bar v 0 out L out Strategy: Experiment ends before leftward traveling wave in output bar reaches strain gage Length of output bar L out T 2c out duration of the experiment

37 Strain Strain Strain HOPKINSON BAR TECHNIQUE Split Hopkinson Pressure Bar (SHPB) experiment Two force measurements L L L 2L st / c R ( 0, t) R R Time Time Time ( 0, t) ( / 2, t) (, t) L in L in v 0 striker input bar specimen output bar L st L in

38 Strain HOPKINSON BAR TECHNIQUE Criterion to avoid wave superposition at strain gage position 3L in / 2c L 3Lin / 2c Lin / 2c 2Lst / c L in / 2c Lin 2L st 2L st / c R Time ( / 2, t) L in v 0 striker input bar specimen output bar L st L in

39 Strain Strain SPLIT HOPKINSON PRESSURE BAR (SHPB) SYSTEM Input force measurement: Kolsky (1949) measured in L calculated ( t) ( t L / 2c) re L in wave transport F in EA re inc Time in R Time inc in ( t) ( t L / 2c) R in F in v 0 striker input bar specimen output bar L in

40 SPLIT HOPKINSON PRESSURE BAR (SHPB) SYSTEM v 0 striker input bar specimen output bar Calculate forces and velocities at specimen boundaries Input bar/specimen interface: in R in L inc re tra F v Output bar/specimen interface: out R in in F v Specimen specific post-processing EA re c out out re EA c tra tra inc inc F in F out v in Verify quasi-static equilibrium Coupling with other measurements (e.g. high speed photography) Calculate stress, strain and strain rate v out

41 DESIGN OF A STEEL SHPB SYSTEM Duration of the experiment: Minimum output bar length: T max av s L out Tc / s 5000m / s / m Minimum striker bar length: Minimum input bar length: L st Tc / m Lin 2Lst 2. 5m v 0 striker input bar specimen output bar The bar diameters need to be chosen in accordance with the forces required to deform the specimen (force associated with incident wave should be much higher than the specimen resistance)

42 EXAMPLE EXPERIMENT v 0 striker input bar specimen output bar

43 Kolsky bar system Requirements: Striker, input and output bar made from the same bar stock (i.e. same material, same diameter) Length of input and output bars identical Striker bar length less then half the input bar length Strain gages positioned at the center of the input and output bars Launching system striker bar L/2 L/2 L/2 L/2 input bar specimen output bar strain gage strain gage

44 Kolsky bar formulas s l s s re Stress in specimen: EA s( t) tra( t) A Strain rate in specimen: 2c s( t) re( t) l s s tra inc Launching system striker bar L/2 L/2 L/2 L/2 input bar specimen output bar strain gage strain gage

45 compressive strain Wave Dispersion Effects compressive strain v [t] recorded by strain gage 1D THEORY EXPERIMENT Pochhammer-Chree oscillations v 2c long rise time time time

46 Wave Dispersion Effects Simplified model: axial compression only: Reality: axial compression & radial expansion: In reality, the wave propagation in a bar is a 3D problem and lateral inertia effects come into play due to the Poisson s effect! D ( 1 )D Inertia forces along the radial direction delay the radial expansion upon axial compression x ( 1 )x

47 Geometric Wave Dispersion Consider a rightward traveling sinusoidal wave train of wave length L in an infinite bar of radius a raveling at wave speed [ x, t*] c L Theoretical wave speed (1D analysis): c E L x Theoretical wave speed (3D analysis): 1.0 c / L c D Pochhammer-Chree 3D analysis The wave propagation speed depends on wave length! The 1D theory only true for very long wave lengths (or very thin bars) High frequency waves propagate more slowly than low frequency waves D L

48 Geometric Wave Dispersion Example: Rightward propagating wave in a steel bar [t] 2000mm [t] 20mm 2 1 L 40mm 1 L mm high frequency D L D L c 1 3.1km / s c km / s 2 1 f 1 78kHz f 2 25kHz high frequency T s T s low frequency 2 1 low frequency superposition 4 2 superposition

49 Example Geometric Wave Dispersion v [t] recorded by strain gage Chen & Song (2010)

50 Modern Hopkinson Bar Systems Launching system striker bar L input bar High speed camera L output bar specimen strain gage Features: Striker and input typically made from the same bar stock (i.e. same material, same diameter) Small diameter output bar for accurate force measurement Similar length of all bars Output bar strain gages positioned near specimen end Wave propagation modeled with dispersion Strains are measured directly on specimen surface using Digital Image Correlation (DIC)

51 ADVANCED TOPICS related to SHPB technique Accurate wave transport taking geometric wave dispersion into account Use of visco-elastic bars (slower wave propagation than in metallic bars, more sensitive for soft materials) Torsion and tension Hopkinson bar systems Lateral inertia at the specimen level Friction at the bar/specimen interfaces Dynamic testing of materials (where quasi-static equilibrium cannot be achieved) Pulse shaping Intermediate strain rate testing Infrared temperature measurements Experiments to characterize brittle fracture Multi-axial ductile fracture experiments Experiments under lateral confinement and many others

52 Reading Materials for Lecture #2 George T. Gray, High-Strain-Rate Testing of Materials: The Split-Hopkinson Pressure Bar : M.A. Meyers, Dynamic behavior of Materials (chapter 2): W. Chen, B. Song, Split Hopkinson (Kolsky) Bar :

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