Highspeed photography of compressive failure waves in glassesN. K. Bourne, Z. Rosenberg, and J. E. Field

Citation: Journal of Applied Physics 78, 3736 (1995); doi: 10.1063/1.360709 View online: http://dx.doi.org/10.1063/1.360709 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/78/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Highspeed photography with computer control Phys. Teach. 29, 356 (1991); 10.1119/1.2343347 Highspeed flash photography with sound triggers J. Acoust. Soc. Am. 87, S33 (1990); 10.1121/1.2028170 Highspeed flash photography with sound triggers Phys. Teach. 28, 12 (1990); 10.1119/1.2342920 Optimization of Frame Rates for HighSpeed Photography Rev. Sci. Instrum. 30, 902 (1959); 10.1063/1.1716378 HighSpeed Photography Phys. Today 9, 32 (1956); 10.1063/1.3059894

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High-speed photography of compressive failure waves in glasses N. K. Bourne,a) Z. Rosenberg,b) and J. E. Field Shock Physics, Physics and Chemistry of Solids Group, Cavendish Laboratory, Madingley Road, Cambridge CB3 OHE, United Kingdom

(Received 23 January 1995; accepted for publication 26 May 1995)

A mechanism for the compressive failure of soda-lime and borosilicate glasses is proposed based upon high-speed photography of impact on glasses. Shock loading was produced by the impact of a 50 mm diameter projectile so inducing shock states of one-dimensional strain in glass targets. The shock waves and failure fronts were visualised using the shadowgraph technique. The failure appeared to occur at discrete nucleation sites and propagated out to form a continuous front. The velocity of this front increased with higher impact stresses and varied with the glass composition. 0 1995 American Institute of Physics.

1. INTRODUCTION

Attention has recently been drawn to the possibility of brittle materials failing under compression in the region be- hind a shock front.im4 The plate impact experiment has been chosen as an idealised one-dimensional (1D) strain loading configuration in which a planar compression wave induces a uniform region of 1D strain behind a shock front in the target medium. In this configuration there is an axisymmetric stress system of which both longitudinal and transverse compo- nents may be measured independently allowing the devia- toric stress to be calculated. Two indirect classes of measure- ments have been used to infer the existence of a phase boundary travelling behind a shock front across. which the glass loses all or most of its spall strength and a portion of its shear strength. In these experiments waveform measure- ments of the longitudinal stress, free-surface velocity and/or the transverse stress histories were used to deduce existence of the wave.5-g In the first series of experiments, a longitu- dinal gauge sandwiched between the rear of the sample and a polymethylmethacrylate (PMMA) backing was usea to de- tect the pull-back signal from a spall plane generated within the target due to the interaction of release waves propagating from the gauge plane and the rear of the flyer plate. Adjust- ing the position of the spall plane (by varying the thickness of the flyer plate) would not be expected to vary the magni- tude of the spall strength through the thickness of the target. However, whilst this was found to be the case for metal or hard ceramic targets, glasses showed a dramatic loss of spall strength across a front deduced to be moving at -2 km s-‘. In the second series of experiments, a gauge mounted within the sample measuring the transverse stress showed an increase across a wave-front propagating at speeds commen- surate with those deduced from the spall strength measure- ments. This front causes a dramatic reduction in the shear strength of the material.”

Previous studies have concentrated on various composi- tions and non-metallic power glass composites6 as well as soda-lime glass5 In the present study it was decided to use float (soda-lime) and Pyrex (borosilicate) as examples of

a)Ele.ctronic mail: nkblO@phy.cam.ac.uk biAlso with RAFAEL, P.0. Box 2250, Haifa, Israel.

commonly used glasses used in composites. The use of high- speed photography allowed the possibility of both confirm- ing the existence of failure fronts and elucidating possible mechanisms by which the glass might fail under compres- sion.

II. EXPERIMENT

All the experiments described below used a polycarbon- ate sabot carrying a copper flyer plate of thickness 6 mm. This impact induces a pulse of duration -3 ,us. In spatial terms this gave a pulse length of - 18 mm from the shock front to the arrival of the longitudinal release from the rear of the flyer. The sabot was launched from the 50 mm single- stage gas gun located at the Cavendish laboratory. The ve- locity of the projectile was measured using a pin shorting technique to an accuracy of 0.5%.

The materials chosen were a borosilicate and a soda-lime float glass. Ultrasonic measurement of longitudinal and transverse wave speeds conducted on the samples studied provided an accuracy of -1%: material parameters are listed in Table I.

Other work by Rosenberg et aL5 has determined the Hugoniot elastic limits (HELs) for these materials to be 6.4 GPa for soda-lime float and 7.5 GPa for Pyrex. It was de- cided to test each material at three stress levels correspond- ing to a third, two-thirds and around the HEL. The impact velocities at each of these levels were chosen to be ‘250, 520 and 760 m s-l, respectively. Separate gauged experiments to determine points on the Hugoniot for each of the materials at the various velocities gave the corresponding stresses in the targets as indicated in Table II. The gauges used were Mi- cromeasurements manganin gauges (type LM-SS-125CH- 048) and the calibration of Rosenberg et al.” was used in reducing the voltage data collected.

Prior to conducting the shock loading experiment the glass was examined using optical microscopy at a magnifi- cation of 200 in an effort to observe any inhomogeneities. The only apparent structure observed was a layering within the Pyrex seen as variations in refractive index in planes perpendicular to that of the shock (ream); this feature is com- mon with glasses. Each of the blocks was cut from 25 mm thick sheets into 90X90 mm’ targets. The edges were pre- pared to the same surface finish except for opposite faces

3736 J. Appl. Phys. 78 (6), 15 September 1995 8 1995 American Institute of Physics [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

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TmLB I. Material parameters. COllldi~g

CL (km sL-‘) (20.5%) c‘y (km s-1) (20.5%) P (kg me31

Borosilicate (Pyrex)

6.05 3.69 2230

Soda-lime (Float)

5.84 3.46 2490

W A

Coaxial pin

which were polished to allow photography through the tar- get. The blocks were pierced with a hole into which a coaxial pin was inserted which was then polished back to the surface of the sample. This pin was short-circuited at impact and used as a trigger for the high-speed camera.

While conditions along the central impact axis are those of one-dimensional strain, lateral release waves propagate from the edges of the flyer plate, relieving the stress and returning the material behind the release front to a triaxial strain state. This is illustrated schematically in Figure l(a).

The released material forms a cylindrical pipe of increas- ing wall thickness through which the central region of uniaxial strain is viewed.Since the laterally released com- pression front travels more slowly than the unrelieved front, the edges of the wave lag back and the intersection of the wave with the free target surface will lie behind the main front. In order that surface interactions did not obscure the phenomena observed in the central region, the targets were made much larger than the flyers (see Figure 1).

The high-speed camera used in these experiments was the Ultranac FS50 1 lo used in framing mode. An advantage of using this new generation image-converter camera was that the exposure and interframe times could be controlled inde- pendently throughout the sequence. The exposure time for each frame was 50 ns, while the time at which each frame was taken is shown in the figure labelling. The sequences were back lit using a collimated beam of light from a Xenon discharge tube. (type QCAS) which delivers - 100 J in 100 ,us. The light was collimated before it passed through the target so that light refracted from the shock region was not collected in the camera lens [Figure l(b)]. The flash was triggered by the shorting of the velocity pins so that it gave maximum output before the camera triggered at impact.

III. RESULTS

Two sequences are presented for float and Pyrex, indi- cating typical behaviour observed at each of the stress levels since it was found that the qualitative appearance of the fronts observed did not change.

In Figure 2 the flyer impacts onto a float glass target at 250 m s -l. The flyer has impacted the target from the top of

TABLE II. Induced target stresses.

Impact velocity (LO.596) m s-l

250 520 760

Float (?5%) GPa

2.2 4.1 6.8

Pyrex (25%) GPa

2.1 4.5 6.4

Collimat‘ed light sollrce

FIG. 1. (a) Schematic of a plate impact on glass targets. A coaxial trigger pin is used to fire the camera. A shock, S, travels into the target and lateral releases, R. travel inward to destroy the region of 1D strain. The glass target is chosen to be much larger than the flyer diameter in order that intersections of the shock with the free surface, I, do not obscure details of the processes occurring in the bulk. (b) A schematic of the optical path of collimated light through the shock is shown. The lateral rarefactions are not included since their effect is difficult to interpret. I

the frame inducing a dark front S which travels downwards. Along the centre of the frame is a vertical fiducial line which represents the central impact axis and on which are placed two markers at distances of 15 and 20 mm from the impact face. The field of view of the frame is thus of the order 15 X 10 mm’. A longitudinal gauge trace obtained for an im- pact on a similar sample shows the stress wave ramping up to a peak value over -300 ns and thereafter remaining con- stant behind the shock. The dark areas in the photographs correspond to regions in which density changes giving rise to refraction of light out of the shock. Where the density (and thus refractive index) remains constant to the rear of the shock, light is not refracted and enters the lens. Calculating

PIG. 2. Soda lime glass impacted from the top at 250 m s--I. A shock S travels down through the frames leaving in frame 2. The scale markers are 5 mm apart and the first is 15 mm from the impact face. A failure front appears behind in frame 2 and a damage site A, nucleates and grows in frames 3 and 4. The reflected release R from the free surface enters the frame from below in frames 5 and 6. The exposure time for each frame is 50 ns.

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FIG. 3. Impact on Pyrex glass at 250 m SC’. The shock, S, can be seen entering from above in frame I. The region at the bottom of each frame is a PMMA backing block. The scale markers are 5 mm apart and the tirst is 1.5 mm from the impact face. Th> failure front enters in frame 2 and propagates through to frame 6. The white line entering from the right of frame 4 is believed to be lateral release wave illustrated in Figure 1. The grey mottled regions in this and the other photographic sequences are artefacts due to the image processing.

the thickness of the shock from a measurement of the ramp rise-time agrees well with the thickness of the dark band in the photographs.

In frame 2 of the sequence, taken 1.4 ,US later, a dark irregular front can be seen to the rear of the shock. It has the appearance of a series of bifurcating cracks propagating be- hind the compression front. Since the depth of field is lim- ited, several cracks can be seen in focus while others propa- gate out of plane and so out of focus. The distance between the shock and the failure front is -7 mm. An interesting feature of the failure mechanism can be seen in frames 3 and 4 taken 150 ns apart. A dark point a little to the left and below the label, A, can be seen forming in frame 3 which grows in frame 4. This is interpreted as the nucleation of failure at a local inhomogeneity within the glass. The failure front can be seen propagating further in frames 5 and 6. More points of nucleation can be seen growing in later frames. The velocity of the f&lure front as measured from the framing photographs was -2 km s-l in agreement with the velocities deduced by other workers.4 A reflected release wave, R, propagates from the free rear surface of the target and can be seen in frames 5 and 6. It should be noted that the thickness of the shock wave in the photographs may appear to be slightly thicker than in reality due to the parallax. The release wave R is thinner than the shock wave S which can be explained by noting that the ramping compression wave upon loading becomes a shock wave upon unloading due to the decreasing moduli with pressure.

Figure 3 shows a sequence in which a Pyrex target has impacted under the same conditions as Figure 2. In the first frame, the shock enters the target from above. The rear of the target was bonded to a PMMA block in which a gauge was embedded. In the second frame, the shock has travelled

i ii

0.2 0.4 0.6 0.8 Timc(,,s)

FIG. 4. Pyrex glass impacted at 320 m s-l. In (a) a gauge trace showing longitudinal stress versus time on the rear of a sample of Pyrex (dotted curve). Note the dual ramp on the rise of the trace. The solid curve is the differential of this trace. Dark features in the sequence correspond to regions in which the differential is non-zero which gives rise to an apparent double structnre on the rise. In (b) this double structure is seen in a frame from the sequence.

into the target and a dark crack front can be seen entering from above. The morphology of the cracks is very different from that observed in the previous sequence. The front can be seen -7 mm behind the shock and appears more regular than that seen in float glass and travelling at a higher velocity -3 km s-l.

At higher impact velocities the glasses behave in a simi- lar manner with further evidence of nucleation and growth of damage sites within the material. One interesting feature of the shock wave in Pyrex was the appearance of a dual struc- ture to the shock. Figure 4(a) shows a gauge record for an impact at 530 m s-’ on Pyrex glass. Remembering the pre- vious arguments concerning refraction of light from regions of changing density one would expect dark areas where the differential of the signal was non-zero. It can be seen that this would give rise to a dual structure to the shock front (dotted line) as is observed in a frame from the sequence taken at this velocity [Figure 4(b)]. This effect is observed at the two higher velocites but not at the lower as expected.

In order to obtain more quantitative information from the impacts, x-t diagrams were cons&ted by taking slices from each sequence along the impact axis at equal time steps. This procedure was used to produce a pseudo-streak photograph: an example is given in Figure 5(a) for an impact on float glass at 550 m s-l. Similar x-t diagrams are presented for the two materials at eacli of the velocities in Figures 5(b) and 5(c). The measured values of the failure wave and shock velocities are given in Table III.

Two immediate conclusions may be noted. Firstly, the failure wave velocity is higher in float than in Pyrex. Sec- ondly, the failure wave speed increases in both materials with increasing impact velocity. It should be noted that the failure front velocity is not constant as a consequence of the step- by-step propagation as damage is nucleated and thus the val- ues in the table are average values.

3738 J. Appl. Phys., Vol. 78, No. 6, 15 September 1995 Bourne, Rosenberg; and Field [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

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a)

;5rnrn

FIG. 5. (a) Pseudo-streak photograph for an impact on soda-lime glass at 530 m s”‘. Length and distance scales are shown by the picture. The shock front can be seen travelling ahead of the failure front. The shock and failure fronts for each velocity for (b) soda-lime and (c) Pyrex are shown. The solid line represents shots at 250, the light dotted line represents 530 and the heavy dotted line, 760 m s-l.

IV. CONCLUSIONS

This paper has presented the results of a high-speed pho- tographic investigation of the propagation of compressive failure fronts through borosilicate and soda-lime glasses. The two materials both show failure fronts which travel behind the shock. The morphology of the fronts is very different with a sinuous front for float glass apparently composed of a series of bifurcating cracks and a more regular front for Pyrex consisting of a much larger number of fine cracks. It is apparent from the sequences that the failure appears to nucle- ate at a few sites ahead of the front and that damage then spreads from these regions until coalescence occurs and the front moves onwards. There is no evidence that the failure

TABLE III. Measured shock (Usj and failure wave (Um) velocities.

Velocity (2 0.5%)

ms *’

Pyrex Float

us kv k?J (r.O.2) (20.2) (22) ( t 0.2) km s-’ kills-’ km s-’ km s-’

250 6.0 3.4 5.9 1.8 530 6.2 3.7 5.9 2.1 760 6.9 3.8 6.8 3.6

front is connected to the main shock, but it should be noted that only cracks opened greater than half a wavelength of the light used would be visible. Material failure ahead of the visible front is therefore possible.

The failure front velocity is found to be less in the case of float glass than for Pyrex, and is a function of the stress level applied. The wave velocity varies in the case of the float glass from - half to one times the shear wave speed, while in Pyrex the velocity is found to be around the shear wave within the accuracy of the measurement. It should be noted that the maximum stresses applied in this study were at or around the HEL of the material indicating that the entire range of behaviour studied is within the elastic range of the materials.

The precise nature of the failure is, as yet, uncertain but is possibly associated with the shear stresses in the shock front. Clearly a large amount of stored energy is released on failure which separates the failure surfaces sufficiently for the cracks to be visible despite the large superimposed com- pressive stresses. Defects appear to act as nucleation sit& but whether these are at an atomic or more macroscopic level has yet to be determined.

It has been demonstrated that failure waves can be suc- cessfully studied using the combined techniques of high- speed photography and stress gauges. We hope to extend this work to other materials, stress ranges and experimental tech- niques in future projects.

ACKNOWLEDGMENTS NKB thanks EPSRC for an Advanced fellowship. We

thank the DRA for their support of the gun facility, the EPSRC for a grant to purchase the high-speed camera and Pilkingtons PLC for the provision of samples. We thank Dr. Y. Mebar and Dr. T. Obara for their comments and encour- agement.

‘J. Wackerle, J. Appl. Phys.. 33, 922 (1962). ‘A. N. Dremin and G. Adadurov, Sov. Physics Solid State 6, 1379 (1964). “R. V. Gibbons and T. J. Ahrens, J. Geophys. Res. 76, 5489 (1970). 4G. L Kanel, A. M. Molodets, and A. N. Dremin, Combust. Explos. Shock

Waves 13,722 (1977). ‘2. Rosenberg, S. J. Bless, and D. Yaziv. J. Appl. Phys. 58, 3249 (1985). 6G. I. Kanel, S. V. Rasorenov, and V. E. Fortov, Shock Waves in Condensed Matter. edited by S. C. Schmidt, J. N. Johnson, and L. W. Davison (Elsevier, New York, 1990), pp. 451-454.

‘N. S. Brar, 2. Rosenberg, and S. J. Bless, J. de Physique N, Coll. C3,639 (1991).

*Z. Rosenberg, D. Yaziv, and Y. Partom, J. Appl. Phys. 51, 3702 (1980). ‘G. F. Raiser, J. L. Wise, R. I. Clifton, D. E. Grady, and D. E. Cox, J. Appl.

Phys. 75, 3862 (1994). “B. R. Garfield and M. J. Riches, Proceedings of the 19th International

Congress on High-speed Photography and Photo&s, edited by P. W. Fuller, B. R. Garfield, and I. Rendell @PIE, Bellingham, WA, 1991), pp. 1358.290-299.

“Z. Rosenberg, D. Yaziv, and Y. Partom, J. Appl. Phys. 51, 3702 (1980).

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