Cavendish Laboratory Fracture & Shock Physics

Size: px

Start display at page:

Download "Cavendish Laboratory Fracture & Shock Physics"

Griffin Simpson
5 years ago
Views:

1 Cavendish Laboratory Fracture & Shock Physics Split Hopkinson Pressure Bar Studies of a Nitrocellulose-based PBX System AWE Nitrocellulose Meeting April 2007 D.R. Drodge, D.M. Williamson, W.G. Proud 1

2 Overview Materials: EDC37 and NC-K10 Technique: Split-Hopkinson Pressure Bar with Pulse Shaping and Temperature Control. Results & Discussion 2

3 Sample Materials 3

4 Sample Materials EDC37: 91% HMX filler, 9% NC-K10 binder NC-K10: plasticised nitrocellulose Motivation: examine sub-t g behaviour to extend current data set. DMTA figure courtesy of Dr D. Williamson 4

5 Story So Far We already have EDC37 data for: Fixed T (room temp), varying strain rates Fixed strain rate (10-3 s -1 ), varying T (D.M. Williamson et. al., paper awaiting publication) ramp up strain rate, drop T, see what happens 5

6 Viscoelastic Response 6 Glassy Rubbery T g Transition Melt Temperature OR Loading Time Period Storage Modulus

7 Not-so-subtle differences Strain Rate vs. Loading Frequency Small strain vs. strained to failure Pure polymer vs. filled polymer THUD! 1T F 0 sin(wt) 7

8 Current Data 8 Glassy Rubbery T g Transition Melt Temperature OR Loading Time Period Storage Modulus

9 Plan Use high strain-rate apparatus to raise the transition temperature Use cooling chamber to induce glassy behaviour Find something out. 9

10 Apparatus 10

11 Split Hopkinson Pressure Bar Sample Striker Input Output Trapper Gauges measure strain in Bars 11

12 Split Hopkinson Pressure Bar Sample Striker Input Output Trapper Gauges measure strain in Bars Strain Particle Velocity Z E Stress A Force Sample Dim s SAMPLE STRAIN SAMPLE STRESS 12

13 Split Hopkinson Pressure Bar Input Gauge Output Gauge 0.3 Gauge Voltage / V Time / microseconds 13

14 Split Hopkinson Pressure Bar Incident Transmitted Input Gauge Output Gauge Gauge Voltage / V Time / microseconds Reflected 14

15 Split Hopkinson Pressure Bar Incident Input Gauge Output Gauge Gauge Voltage / V 0.3 Transmitted Time / microseconds Reflected v L v R F I F T F R L 15

16 Split Hopkinson Pressure Bar ε& = v R L v L σ = F A (assumes equilibrium) T S v L v R F I F T F R L 16

17 Split Hopkinson Pressure Bar σ σ L R = = F A T S F I + F A S R Do Front and Back stresses match? if not, we have a problem v L v R F I F T F R L 17

18 Split Hopkinson Pressure Bar Shallower Pulses = Reach Equilibrium Sooner Copper Shim (annealed at 450C for 2hrs) Place on end of input bar to cushion blow Strain Time / μs 18

19 Split Hopkinson Pressure Bar Smoothed Pulse (timeshifted) Reflected Force Transmitted Force Incident Force Two-Wave Force

20 Other considerations Friction. Mitigate by lubrication with appropriate substance (silicon grease) Inertia. Demonstrated* to be negligible for strain rates below 10 5 s -1 provided sample geometry is sensible. *G.T. Gray III (2000) ASM Handbook vol. 8 20

21 Heating / Cooling System Thermocouple on output bar. 21

22 Heating / Cooling System 22

23 EDC37 Samples Supplied in disc form by AWE 3mm Length 8mm Diameter 23

24 NC-K10 Samples Supplied as ~1mm thick sheets by AWE Make double-thickness sheet Punch out ~3mm diameter discs Place between lubricated bars and squeeze Measure dimensions using sophisticated multichromatic photometric array 24

25 NC-K10 Samples 25

26 NC-K10 Samples Obtain height and width. Thickness well defined Diameter less so hence stress is subject to error 26

27 Results 27

28 EDC37 Results 28

29 Failure Near-constant failure strain 29

30 EDC37 Results 30

31 EDC37 Strength 31

32 EDC37 Strength Glassy? Glass Transition Rubbery 32

33 EDC37 Modulus Estimate Usually, elastic behaviour occurs before sample equilibrium reached Pulse Shaping allows earlier equilibrium Stress-strain gradient offers estimate of elastic modulus 33

34 EDC37 Modulus Estimate 34

35 EDC37 Modulus Estimate? 35 Glassy T g Transition Rubbery

36 NC-K10 Results 36

37 NC-K10 Results Yield Stress taken as zero for melt Stresses have near 10% error from poorly defined sample diameter/area 37

38 Combined Strength Results 38

39 Combined Strength Results Lower T g?? Bizarre shape 39

40 Conclusions Hopkinson Bars allow detailed study of viscoelastic effects in high rate impact around the glass transition EDC37 Failure Stress continues to rise below T g High rate modulus estimates describe a near-ideal viscoelastic master curve at low strains NC-K10 Failure Stress peaks at T ~ -60C, then decreases Binder transition at lower T than EDC37, sharper 40

41 Extension Cold machining to produce better NC-K10 specimens from new material High-speed photography and softrecovery techniques to confirm failure modes Simultaneous diametric measurements to find Poisson s Ratio Investigate crystal behaviour at low temperatures 41

42 Acknowledgments D.R. Drodge and D.M. Williamson thank AWE W.G. Proud thanks QinetiQ 42

Using the split Hopkinson pressure bar to validate material models

rsta.royalsocietypublishing.org Downloaded from http://rsta.royalsocietypublishing.org/ on July 7, 218 Using the split Hopkinson pressure bar to validate material models Philip Church 1, Rory Cornish 1,