High-speed photography of compressive failure waves in glasses

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<ul><li><p>Highspeed photography of compressive failure waves in glassesN. K. Bourne, Z. Rosenberg, and J. E. Field </p><p> 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 </p><p> [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:</p><p> On: Sat, 17 May 2014 14:26:23</p><p>http://scitation.aip.org/content/aip/journal/jap?ver=pdfcovhttp://oasc12039.247realmedia.com/RealMedia/ads/click_lx.ads/www.aip.org/pt/adcenter/pdfcover_test/L-37/1159287001/x01/AIP-PT/JAP_ArticleDL_051414/aipToCAlerts_Large.png/5532386d4f314a53757a6b4144615953?xhttp://scitation.aip.org/search?value1=N.+K.+Bourne&amp;option1=authorhttp://scitation.aip.org/search?value1=Z.+Rosenberg&amp;option1=authorhttp://scitation.aip.org/search?value1=J.+E.+Field&amp;option1=authorhttp://scitation.aip.org/content/aip/journal/jap?ver=pdfcovhttp://dx.doi.org/10.1063/1.360709http://scitation.aip.org/content/aip/journal/jap/78/6?ver=pdfcovhttp://scitation.aip.org/content/aip?ver=pdfcovhttp://scitation.aip.org/content/aapt/journal/tpt/29/6/10.1119/1.2343347?ver=pdfcovhttp://scitation.aip.org/content/asa/journal/jasa/87/S1/10.1121/1.2028170?ver=pdfcovhttp://scitation.aip.org/content/aapt/journal/tpt/28/1/10.1119/1.2342920?ver=pdfcovhttp://scitation.aip.org/content/aip/journal/rsi/30/10/10.1063/1.1716378?ver=pdfcovhttp://scitation.aip.org/content/aip/magazine/physicstoday/article/9/2/10.1063/1.3059894?ver=pdfcov</p></li><li><p>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 </p><p>(Received 23 January 1995; accepted for publication 26 May 1995) </p><p>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. </p><p>1. INTRODUCTION </p><p>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. </p><p>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 </p><p>a)Ele.ctronic mail: nkblO@phy.cam.ac.uk biAlso with RAFAEL, P.0. Box 2250, Haifa, Israel. </p><p>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. </p><p>II. EXPERIMENT </p><p>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%. </p><p>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. </p><p>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. </p><p>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 </p><p>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: On: Sat, 17 May 2014 14:26:23</p></li><li><p>TmLB I. Material parameters. COllldi~g </p><p>CL (km sL-) (20.5%) cy (km s-1) (20.5%) P (kg me31 </p><p>Borosilicate (Pyrex) </p><p>6.05 3.69 2230 </p><p>Soda-lime (Float) </p><p>5.84 3.46 2490 </p><p>W A </p><p>Coaxial pin </p><p>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. </p><p>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). </p><p>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). </p><p>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. </p><p>III. RESULTS </p><p>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. </p><p>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 </p><p>TABLE II. Induced target stresses. </p><p>Impact velocity (LO.596) m s-l </p><p>250 520 760 </p><p>Float (?5%) GPa </p><p>2.2 4.1 6.8 </p><p>Pyrex (25%) GPa </p><p>2.1 4.5 6.4 </p><p>Collimated light sollrce </p><p>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 </p><p>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 </p><p>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. </p><p>J. Appl. Phys., Vol. 78, No. 6, 15 September 1995 Bourne, Rosenberg, and Field 3737 [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: On: Sat, 17 May 2014 14:26:23</p></li><li><p>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&gt; 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. </p><p>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. </p><p>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&amp;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. </p><p>Figure 3 shows a sequence in which a Pyrex target has impacted under the same conditions as Figure 2. In the first frame, the s...</p></li></ul>


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