Amit Rav

Measurement of spall strength of A12024-T4 and SS304 in plate impact experiments

Spall (dynamic fracture) strength of A12024-T4 and SS304 alloy have been determined at high strain rate. Gas gun experiments have been conducted on A12024-T4 and SS304 target plates in symmetric impact configuration. The spall in A12024-T4 and SS304 plates has been achieved by impacting them with parallel flying plates accelerated to velocities of 0.56 km/s and 0.6 km/s, respectively. The plate impacts in the two experiments introduced a shock wave of 4.38 GPa and 11.9 GPa, respectively in A12024 and SS304 target plates. In each experiment the interaction of tensile waves resulting from the reflection of shock wave from the target free surface and from the flyer free surface generated large tensile stress in the target which exceeded the spall strength causing spall fracture. The analysis of the velocity history of the target free surface recorded using VISAR reveals the dynamic yield strength of A12024-T4 to be 0.355 and spall strength of 1.46 GPa at strain rate of ~ 104 /s. These quantities in SS304 were determined to be 0.8 GPa and 2.69 GPa with strain rate of ~ 104 /s.

Shock induced spall fracture in polycrystalline copper

The plate impact experiments have been conducted on commercially available 99.99% pure polycrystalline samples of copper using single stage gas gun facility. The free surface velocity history of the sample plate measured using VISAR instrument is utilized to determine the dynamic yield strength and spall strength of copper. The dynamic yield strength and spall strength of polycrystalline copper sample has been determined to be 0.14 GPa and 1.32 GPa, respectively with corresponding strain rates of the order of 104/s.

Mechanical Properties of Shock Treated Aluminium Alloy Al 2024-T4

Plate impact experiment has been carried out on Al 2024-T4 alloy using single stage gas gun. The dynamic yield strength and spall strength of Al 2024-T4 sample has been determined to be 0.35 GPa and 1.43 GPa, respectively, from free surface velocity history measured using VISAR. The sample recovered after unloading from peak shock pressure of 4.4 GPa along with an unshocked sample is analyzed for mechanical properties using nano-indentation and scanning electron microscopy (SEM). The nano-indentation measurements reveal that the hardness and Youngs modulus for unshocked sample remains unchanged as a function of load (equivalently depth), however, the same for shocked sample decreases monotonically with increase of load up to ~40 mN and on further increase of load it remains unchanged, suggesting the (i) increase in hardness of shock loaded sample; (ii) the increase in hardness is limited to certain depth, which in our case is 845.12 ? 43.16 nm.

Shock induced spall fracture in aluminium alloy “Al2014-T4”

The plate impact experiments have been carried out on 8mm thick target plates of aluminium alloy Al2014-T4 at impact velocities of 180 m/s, 290 m/s and 500m/s, respectively, using single stage gas gun facility. In each experiment, the of free surface velocity history of the sample plate is measured using VISAR instrument and utilized to determine the spall strength and dynamic yield strength of this material. The spall strength of 0.87 GPa, 0.97 GPa and 1.11 GPa, respectively, measured for impact velocities of 180 m/s, 290 m/s and 500 m/s with corresponding average strain rates varying from 1.36×104/s to 2.41×14/s has been found to display nearly linear dependence upon the strain rates. The dynamic yield strength with its value ranging from 0.395 GPa to 0.400 GPa, though, is higher than the quasi static value of 0.355GPa, appears to be relatively independent of impact velocities up to at least 500 m/s or equivalently strain rates up to ∼ 9.4×104/s.

Fringe analysis software: Application to velocity measurement in dynamic compression experiment

Optical technique such as laser Doppler interferometry is quite popular for non contact measurement and has been applied in free surface velocity measurement in dynamic compression experiment. In this technique the information of measured physical quantity such as surface velocity is encoded in the phase of a fringe pattern. Fringe analysis software extracts the phase from the fringe pattern. The present work reports the development of robust fringe analysis software for complete time history of free surface velocity measurement from recorded fringe signals. The analysis code has been verified by generating free surface velocity history from different simulated fringe profiles. The effects of various measurement imperfections are also studied and corrective measures are coded in the software. Finally, software is validated by analyzing the plate impact experimental data of Al-2024T4 material. Good agreement was obtained between analysis result of experimental data and published data.

Simulation, modeling & verification of velocity interferometer in high strain rate experiments

Linear Time Invariant (LTI) model of optical velocity interferometer system used for measurement of time resolved velocity profile of moving object/surface has been developed. The working of this instrument is based on the measurement of the differential Doppler shift as a function of time, by beating two light signals reflected from moving surface of the target material, at two different instants of time separated by a small time interval of the order of a fraction of nanosecond to a few nanoseconds. The developed model is simulated and characterized in time as well as in frequency domain. The validation of the model has been carried out using correlation between the phase profile (equivalently fringe shift) measured in high strain rate (~104 s-1) experiments and that predicted by the model using velocity profile as input generated from the hydrodynamic simulations. The cross-correlation coefficient and magnitude squared spectral coherence coefficient values close to 1 indicate that the simulation is fairly close to the experiment.

Analysis of velocimetry data based on continuous wavelet transform: Application to shock wave physics experiments

Interferometry is well established technique for measuring particle velocity in shock wave physics experiments, where velocity information is encoded as the phase of a periodically varying intensity pattern. Conventional phase stepping technique by using a pair of quadrature signals cannot resolve rapid velocity changes and sub-fringe phenomena accurately in most of the velocimetry data. In this report a practical analysis method based on the Continuous Wavelet Transform (CWT) is presented to overcome these difficulties. CWT uses an adaptive time window to estimate the instantaneous frequency of signals. We begin by reviewing the CWT and the time-frequency localization properties of wavelets. The instantaneous frequencies of signal are accurately determined by finding the ridge in the scalogram of the CWT and then are converted to target velocity according to Doppler effects. A performance comparison between the CWT and phase stepping technique is demonstrated by a plate impact experimental data of Al-2024T4 material. Results illustrate that new method is superior in terms of accuracy and analysis simplicity.

Comparative Study of Indentation Size Effects in As-Sintered Alumina and Alumina Shock Deformed at 6.5 and 12 GPa

Nanohardness of alumina ceramics determines its performance in all contact-related applications because the issue of structural integrity gets determined at the nanoscale of contact. In spite of the wealth of the literature, however, it is not yet known in significant details how the high-strain rate flyer-plate impact at different pressure affects the nanohardness of dense, coarse grain alumina ceramics. Thus, the load controlled nanoindentation experiments were performed with a Berkovich indenter on an as-received coarse grain (~10 μm), high density (~3.98 gm·cc−1) alumina, and shock recovered tiny fragments of the same alumina obtained from gas gun experiments conducted at 6.5 GPa and 12 GPa shock pressures with stainless steel flyer plates. The nanohardness of the as-received alumina was much higher than that of the 6.5 GPa and 12 GPa shock-recovered alumina. The indentation size effect (ISE) was the strongest in alumina shocked at 12 GPa and strong in alumina shocked at 6.5 GPa, but it was mild in the as-received alumina sample. These results were rationalized by analysis of the experimental load depth data and evidences obtained from field emission scanning electron microscopy. In addition, a rational picture of the nanoindentation responses of the as-received and shocked alumina ceramics was provided by a qualitative model.

Spall fracture in aluminium alloy at high strain rates

Spall fracture strength and dynamic yield strength has been measured in 8mm thick target plates of aluminium alloy Al2024-T4 at high strain rates generated in three plate impact experiments carried out at impact velocities of 180 m/s, 370 m/s and 560m/s, respectively, using single stage gas gun facility. In each experiment, the free surface velocity history of the Al2024-T4 sample plate measured employing velocity interferometer system for any reflector (VISAR) is used to determine the spall strength and dynamic yield strength of this material. The spall strength of 1.11 GPa, 1.16 GPa and 1.43 GPa, determined from measured free surface velocity history of sample material in three experiments performed at impact velocity of 180 m/s, 370 m/s and 560 m/s, respectively, are higher than the quasi static value of 0.469 GPa and display almost linearly increasing trend with increasing impact velocity or equivalently with increasing strain rates. The average strain rates just ahead of the spall fracture are determ...

Wavelet transform based phase extraction for interferometer signal analysis

Phase or its associated derivative of signal carries very important information, particularly in the application of interferometry technique in non contact measurement, where measurand such as displacement, strain, velocity etc are encoded in the phase of a fringe signal. In most of the dynamic applications this interferometric signal becomes non stationary in nature. Fourier transform is not suitable for extracting the phase of a non stationary signal. Joint time frequency analysis tool such as STFT (Short Time Fourier Transform) may be applied but it has limitation of fixed resolution. On the other hand CWT (Continuous Wavelet Transform) is suitable as it uses adaptive time window which overcomes the fixed resolution problem in STFT. Present work reports the development of two different phase extraction algorithm namely phase estimation and frequency estimation technique based on CWT. Finally these algorithms are validated with simulated signal as well as experimental data and relative merits of these two approaches are discussed and results are presented.