Dynamic Compression-shear Response of Brittle Materials with Specimen Recovery

-A new configuration for compression-shear soft-recovery experiments is presented. This technique is used to investigate various failure mechanisms during dynamic multiaxial loading of an AI203/SiC nanocomposite and TiB2. Velocity profiles of the target surface are measured with a variable sensitivity displacement interferometer, yielding normal and transverse velocity-time histories. A dynamic shear stress of approximately 280 MPa is obtained, in the AI203/SiC nanocomposite, for an imposed axial stress of about 3.45 GPa on a 540 Ixm thick sample. This dynamic shear stress is well below the value predicted by elastic wave propagation theory. This could be the result of stress-induced damage and inelasticity in the bulk of the sample or inelasticity on the sample surface due to frictional sliding. To gain further insight into the possible failure mechanisms, an investigation of compression-shear recovery techniques, with simultaneous trapping of longitudinal and lateral release waves, is conducted. KEY WORDSmFragmentation, damage, ceramics, impact, wave propagation, nanocomposites Normal impact soft-recovery experiments have been used in the past to study crystal plasticity 1 and microcracking in brittle materials. 2-5 In these experiments, a star-shaped flyer impacts a square specimen backed by another square plate called a momentum trap. Well-defined short duration axial stress pulses followed by a controlled release of waves were achieved. The main idea in this configuration is to keep a central octagonal region of the specimen plate relatively free from the effects of lateral unloading waves. Cylindrical, spherical and conical waves are generated at the flyer boundary (for a detailed discussion, see Ref. 1), directing most of the energy associated with the side rarefactions to regions at the periphery of the target plate and, hence, away from its center. Chang e t aI. 6 further investigated the effect of flyer plate thickness and shape on the stress history within the specimen. They observed that tensile cracks form, normal to the free edges of brittle square specimens, when a thick, 2 mm to 4 mm star-shaped flyer is used. By means of threedimensional numerical simulations, Chang e t al. showed H. D, Espinosa is an Associate Professor, A. Patanella is a Graduate Student and E Xu is a Graduate Student, School of Aeronautics and Astronautics, Purdue University, West Lafayette, IN 47907. Original manuscript submitted: February 18, I999. Final manuscript received: May 11, 2000. that in-plane tensile stresses are induced, on the back face of the sample, that could lead to the formation of cross-shaped cracks. This effect was attributed to the size mismatch between the flyer and the sample. As an improvement, Chang e t al. recommended the use of side momentum traps, following the ideas of Smith 7 and Hartman, 8 and a simple square flyer. Despite the additional complications in manufacturing specimens, with side momentum traps and small tolerances, this configuration could not prevent the formation of crossshaped cracks (see Fig. 12 in Ref. 6). Through three-dimensional numerical simulations of the dynamic event and the observation of crack patterns in alumina samples, the efficiency of the star-shaped flyer design was confirmed later by Espinosa e t aL 4 In contrast to the experiments reported by Chang et al. ,6 the recovered specimens did not exhibit cross-shaped cracks for pulse durations of about 300 ns and impact velocities up to 100 m/s. However, a highly cracked ceramic was observed on a ring connecting the inner corners of the star-shaped flyer. Based on three-dimensional calculations, Espinosa e t al. 4 attributed this damage to spherical release waves emanating from the flyer corners. More recently, wave trapping in the direction of wave propagation was extended to compression-shear experiments. 9-18 A multiplate flyer with a fluid lubricant was used by Yadav e t al. and Machcha and Nemat-Nasser, whereas a thin solid polymer layer, easy to manufacture, was used by Espinosa and colleagues. Mitigation of lateral release waves in compression-shear experiments was extensively investigated by Machcha and Nemat-Nasser. 13 Their results show that usage of the Kumar and Clifton concept results in lower tensile stresses within the specimen central region. Two types of star-shaped geometries and a square plate rotated 45 deg were investigated. However, the investigated multiplate flyer geometries made use of the star-shaped geometry on the second plate rather than the first plate. Moreover, Machcha and Nemat-Nasser conducted experiments only on round samples at impact velocities around or below 100 m/s. The ceramic samples were not recovered intact but rather as large fragments. However, cross-shaped cracks were observed when circular plates were employed for the first flyer plate, sample plate and back plates, and square plates were used as the second plate in multiplate flyers. We address the difficult problem of recovery of brittle specimens subjected to axial and shear stresses high enough to initiate damage but not necessarily leading to catastrophic failure. Typically, shear stresses of a few GPa are needed for initiating damage in confined brittle material such as A1203, Exper imental Mechanics = 321 SiC and TiB2. In this work, impact velocities between 100 m/s and 200 m/s are selected to achieve this goal. The upper limit is approximately twice as high as the impact velocities reported in the literature in similar studies. We also examine the use of the star-shaped flyer as a first plate in the multiplate flyer design. The response of square and round ceramic specimens is also investigated. In the latest case, a confining plate with a square shape is utilized. P r e s s u r e s h e a r R e c o v e r y E x p e r i m e n t s Plate impact experiments offer unique capabilities for the mechanical characterization of advanced materials under dynamic loading conditions. 3 These experiments allow high stresses, high pressures, high strain rates and finite deformations to be generated under well-characterized conditions. The testing techniques can be divided into two categories: (a) stress wave propagation tests (compression-shear impact and normal impact) and (b) nominally homogeneous deformation tests (high strain rate compression-shear configuration). All rely on the generation of one-dimensional waves in the central region of the specimen to allow a clear interpretation of the experimental results and the mathematical modeling of the material behavior. Compression-shear loading is attained by inclining the flyer, specimen and target plates with respect to the axis of the projectile. 19 By varying the angle of inclination c~, a variety of loading states may be achieved. The experimental setups for high strain rate compression-shear and wave propagation configurations, designed for specimen recovery, are shown in Figs. 1 and 2. Pressure shear recovery experiments offer several advantages over other experimental techniques in the study of damage and inelasticity in advanced materials. The stress amplitudes and deformation rates obtained in these experiments allow the identification of damage and material instabilities. Furthermore, if intact samples are recovered, the information gathered from these experiments can be substantially increased by correlation of real-time velocity profiles and microstructural features associated with mechanisms of inelasticity and damage. One of the problems of the compression-shear recovery experiment is the simultaneous trapping of the longitudinal and shear waves by a back plate. To solve this problem, the present investigation uses the multiplate flyer discussed in Refs. l0 and 14-17. Such a flyer consists of a thin solid film made of a material with a very low shear flow stress (e.g., a polymer), sandwiched between two thicker hard plates. Due to mismatch in impedances between the thin solid film and the bounding plates, a few reverberations within the polymer film are required to achieve the imposed normal stress at the impact face. This feature imposes another requirement in the design and manufacturing of the thin polymer film. The requirement is that the thickness of the thin film be minimized such that the time required to achieve a homogeneous stress state is only a small fraction of a microsecond. We have manufactured such a multiplate flyer by bonding two hard plates with a uniform 1 txm thick polymer layer (photoresist AZ 1350J-HOECHST CELANESE). We have observed that the uniformity of the thin film prevents tilt between the flyer plates that would otherwise perturb the interferometric measurements. To have a well-defined stress history within the sample, the flyer plate is backed by a low-impedance material and the projectile is stopped by a hard steel anvil. Fiberglass Tube Target 211v.!iii ++ +.+ Multiplete Specimen Ryer