Benny Glam

Fibre Bragg Grating sensor for shock wave diagnostics

We measured the response of short FBGs to a weak planar shock wave. The combined effect of the Photo-Elastic effect and the FBG strain was estimated theoretically depending on its orientation with respect to shock front (for 1550 nm FBG, parallel: 0.9 nm/kbar, perpendicular: -1.4 nm/kbar). The experimental results imply that the FBG/fibre survives for more than 1 μs at 5 kbar shock stress, and that our assumptions about the FBG behaviour under dynamic loading are valid, though more work is needed to fully quantify the effect.

Equation of state for aluminum containing helium bubbles

A theoretical model for equation of state (EOS) of aluminum with helium bubbles is presented. Based on this EOS, the influence of helium bubbles on shock loading is examined. The Hugoniot curve (temperature versus pressure as well as shock velocity versus particle velocity) for aluminum containing bubbles is calculated for various bubbles mass, bubbles percentage, and helium EOS models. The bubble mass and concentration seem to affect the measurably Hugoniot curve. The EOS model, implied for the helium in the bubbles, has minor significance, which means our model is not sensitive to the details of the helium EOS. Our findings are consistent with experiments available in the literature.

On the dynamic strength of 304l stainless steel under impact

Uniaxial strain plane impact (300-1700 m/s), loading and reloading experiments carried out on SS304L are reported. The aim of these experiments was to measure the material strength properties under shock compression. Most of the experiments reported here show a viscous type elastic precursor. The experimental results are compared to numerical simulations performed using a 1D code. The input physics to the simulations are the Steinberg equation of state and Johnson-Cook strength model. This model has been previously calibrated under uniaxial stress conditions in the rangee e =1-5×103 s-1. Our experiments extended the data into the regione e =105 -106 s-1. In spite of this extrapolation, there is a general agreement between simulations and experiments. However, differences in some details still exist.

Comparison of FBG responses to static and dynamic pressures

FBGs respond to external pressures in ways that reflect both the strain-optic effect and the geometrical variations, both induced by the applied pressure. While the response to static isotropic pressure is quite straight forward and intuitive, the response to anisotropic shock waves is much more complex and depends also on the relative orientation between the fiber and the shock propagation direction. We describe and explain experimental results for both cases.

Noninterferometric topography measurements of fast moving surfaces.

The topography of moving surfaces is recovered by noninterferometric measurements. The phase reconstruction is derived by measuring the intensities of a backscattered pulsed laser light and solving the transport intensity equation (TIE). The TIE is solved by expanding the phase into a series of Zernike polynomials, leading to a set of appropriate algebraic equations. This technique, which enables us to make a direct connection between experiments and the TIE, has been successfully tested in gas gun experiments. In particular, the topographies of a moving projectile and the free surface of a shocked target were recovered.

IMPACT EXPERIMENTS WITH ALUMINUM‐HELIUM BUBBLES TARGETS

The dynamic behavior of aluminum targets with helium bubbles was investigated in plane impact experiments. From the free surface velocity measurements the spall strength was calculated and analyzed. Theoretical comparison between spall creation due to voids growth and bubbles growth was made. The impacted targets were collected after the impact experiments and examined by TEM. These targets were compared to TEM pictures before the impact. The number of helium atoms in the bubbles was calculated from the electron energy lose spectrum (EELS). Comparison of bubble radiuses and concentration before and after the impact demonstrated bubbles coalescence and EELS measurements showed a reduction of the helium atoms concentration in the bubbles from ∼1028 m−3 before the impact to ∼1027 m−3 after the impact.

Study of dislocation walls evolution during spall in pure aluminum

The micro-structure of the spall plane of pure aluminum (99.9999%) was investigated in symmetric plate impact experiments. The aluminum targets were first heated at 450°C for 22 h for grain growth. The impacted targets were softly caught and collected for metallurgical analysis. It was found that at weak impacts with partial spall, voids with average size of 50±10 im were developed along the grain boundaries. The grain sizes in the vicinity of the voids are 50-250 μm, in comparison with grain sizes of 400-1000 μm in other areas of the specimen, revealing grains splitting under dynamic tension. Transmission Electron Microscopy (TEM) and electron diffraction show parallel dislocation walls, that create a sub-grain micro-structure inside grains with (011) orientation to the beam. In grains with other orientations no dislocation walls or sub-grain structure were found. We also found out pile up of the dislocations by glide mechanism along directions during the spall process.