ELASTIC PRECURSOR DECAY IN CERAMICS AS DETERMINED WITH MANGANIN STRESS GAUGES

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1 ELASTIC PRECURSOR DECAY IN CERAMICS AS DETERMINED WITH MANGANIN STRESS GAUGES Z. Rosenberg, N. Brar, S. Bless To cite this version: Z. Rosenberg, N. Brar, S. Bless. ELASTIC PRECURSOR DECAY IN CERAMICS AS DETER- MINED WITH MANGANIN STRESS GAUGES. Journal de Physique Colloques, 1988, 49 (C3), pp.c3-707-c < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1988 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSIQUE Colloque C3, Supplkment au n09, Tome 49, septembre 1988 ELASTIC PRECURSOR DECAY IN CERAMICS AS DETERMINED WITH MANGANIN STRESS GAUGES Z. ROSENBERG'~), N.S. BRAR and S.J. BLESS Impact Physics Laboratory, University of Dayton Research Institute, 300, College Park, Dayton, OH 45469, U.S.A. RESUME : Un des parametres les plus importants pour caracteriser le comportement dynamique des materiaux ceramiques est la limite Blastique d1hugoniot (LEH) en conditions quasi permanentes. Certains chercheurs ont indique une decroissance de la LEH dans les ceramiques tandis que d'autres n'ont observe aucune decroissance meme pour des eprouvettes relativement Qpaisses. Nous avons realisit des essais de choc de plaques avec deux alumines en utilisant des jauges de contrainte au manganin pour enregistrer 1'6volution de la contrainte A differentes distances du plan d'impact. Toutes les cibles ont Qte preparees en usinant des tuiles epaisses pour assurer l'uniformite du materiau. Nous avons trouve une decroissance du precurseur elastique en fonction de l'bpaisseur pour les deux ceramiques. Abstract - One ot the most islportant pardnieters tor characterizing the dynamic behavior of ceramic materials is the steady state Hugoniot Elastic Limit (HEL). Some workers have reported HEL decay in ceramics while others observe no decay even in relatively thick samples. We performed plate impact experiments on two a1 umi na ceramics using mangani n stress gauges to monitor the stress histories at different distances away from the impact pla.ne. All targets were prepared by machining thick parent tiles to assure material uniforniity. We find that elastic precursor decays with thicknesses in both of the ceramics. Plate impact experiments provide a means tu measure the dynamic compressive strength (e.g. Hugoniot elastic limit) and tensile strength (spall strength) for brittle materials. Ceramics, in general, are found to have very high Hugoniot elastic limits (HEL's) ranging between kbar, and relatively low spall strengths of 1-5 kbar /1-5/. One of the imporrant issues concerning dynamic properties of ceramics is the precise determination of HEL values (ghel). This is a difficult measurement because unlike.metals, ceramics do not always exhibit a clear two-wave shock structure. For example, for peak stresses <1.5 ohel in both fully dense /2/ and 85% alumina /3/ the rise of the stress signal is composed of a sharp jump (the elastic wave) foll owed by a more gradual, asymptotic-1 i ke "plastic wave". For higher shock wave amplitudes, a two wave structure is observed (see /3/). Precise determination of the HEL depends on the ability to resolve the elastic jump accurately. It was shown in /3/ that with manganin stress gauges, the most accurate results are obtained with the "back (l'~emanent address : RAFAEL. Haifa. Israel Article published online by EDP Sciences and available at

3 C3-708 JOURNAL DE PHYSIQUE surface gauge" configuration in which the gauge is embedded at the interface of the specimen and a thick PMMA disc. Using this technique, it was determined that the ohel of 85% commercial alumina (AD-85 manufactured by Coors Ceramics Company) is 60 kbar for 6-10 mm thick specimens. In a later work /4/ on thicker specimens (to 25 mm), it was found that the amplitude of the elastic wave was lower. A similar result was obtained by Gust and Royce /1/ for higher purity specimens varying in thickness to 6 mm using free surface velocity measurements. On the other hand, Cagnoux and Longy /5/, made measurements on an alumina sample using a free surface measurement and found no decay in the amplitude of the elastic wave. These were relatively pure specimens, percent alumina, compared to the 85% of the specimens studied in Ref. /4/. The objective of the present work is to resolve the question of whether the elastic wave decay is characteristic only of AD-85 by making measurements in alumina ceramics containing different percentages of A EXPERIMENTAL Plate impact experiments were conducted using a 50 mm gaslpowder gun at the University of Dayton Research Institute. To ensure that the precursor decay is not the result of differences in mechanical properties of different source materials, specimens that were (1) ground from a thick ceramic plate (Ceramic 1) and (2) sliced from a thick plate and ground to the desired thickness of 5, 10, or 15 mm (Ceramic 2). Two types of sintered alumina, manufactured by Babcock and Wilcox, were used in this study. Their properties are listed in Table 1 below: (CL i s longitudinal sound speed). Percentage Alumina, TABLE 1 Density, and Sound Speed of Ceramic Samples Type % Alumina Density (g/cc) C~(mm/,/s) Microscopic observation made by using SEM/EDS technique reveal that microstructure of ceramic I is very similar to that of AD-85 supplied by Coors. Ceramic I1 is pure A1203 and the grain structure is bimodal. We used the "back surface gauge" configuration for monitoring the stress-time profile, shown in Figure 1. We used manganin gauges manufactured by Micro-Measurements type LM-SS-125CH-048. The calibration of the gauges under uniaxial strain loading is given in I Copper Impactors were used in all the experiments.

4 The amplitude of the elastic wave in the ceramic is determined from the measured stress-time profile of the manganin gauge in PMMA and the we1 1 known re1 at i on: u - '1 + '2 HEL z2. DpMHA where Z1 and Z2 are the shock impedances of the ceramic and PMMA, respectively, and the gauge. upmma i s the amplitude of the first wave arrival I M PACTOR -1 SPECIMEN 7 PMMA BACKING Fig. 1 - Schematic representation of the impactor and the target assmebly. 3 - RESULTS AND DISCUSSION Table 2 lists the relevant data for the different experiments. Experiments were designed for peak stresses near 200 kbar to assure that a clearly defined two-wave structure would be evident. Fig. 2 illustrates a typical gauge record which clearly shows the expected behavior of the "plastic" wave. The elastic part of the signals are composed of two steps instead of a single one. The second step is presumably due to the reverberation of the reflected elastic wave at the ceramic-pmma interface and the oncoming plastic wave as shown in Fig. 3. Values of ohel were computed from the first step. TABLE 2 Summary of Experiments and HEL Data Impactor Impact Ceramic type Experiment Thickness Velocity Thickness HEL No. (mm) (m/s) (mm (kbar) - - -

5 C JOURNAL DE PHYSIQUE Fig. 2 - Oscilloscope records from shots (Ceramic I), 0.2 V/div., 0.5 ps/div., and (Ceramic 11), 0.5 Vldiv., and 0.5 ps/div. and POSITION (XI Fig. 3 - Sche~natic representation of the origin of wave Ep

6 The data reveal a decay in the HEL stress for both ceramics. The decay is similar to the decay in AD-85 specimens (see /4/). The fact that the specimens were from the same lot and parent tile thickness excludes the possibilities of differences in mechanical properties due to different starting powders or densification processes. The HEL stress appeared to reach the steady state value in target of thickness - > 10 mm. Thus, HEL determination on alumina ceramics should employ targets at least this thick; otherwise, the compressive strength may be underestimated. ACKNOWLEDGEMENTS This work was sponsored in part by the United States Defense Advanced Research Project Agency (DARPA) under DARPA Order No (to - DuPont), -- Mark Laber is thanked for his contribution in performing the experiments. REFERENCES /I/ W.H. Gust and E.B. Royce, J. Appl. Phys., 42, 278, (1971). /2/ D.E. Munson and R.J. Lawrence, J. Appl. Phys., 50, 6722, (1929). /3/ Z. Rosenberg and Y. Yeshurun, J. Ap 1. Phys., 58; 3077 (1985). /4/ D. Yaziv, Y. Yeshurun, Y. Partom an: Z. RosenbFg, "In Shock Waves i n Condensed Matter 1987", S.C. Schmidt and N.C. Holmes, pp , (1985). /5/ r ~ a g n o u x and F. Longy, in Shock Waves in Condensed Matter 1987, S.C. Schmidt and N.C. Holmes, pp , (1988). /6/ Z. Rosenberg, D. Yaziv and Y. Partom, J. Appl. Phys., 51, 3702, (1980).