52
9.2 Experimental Details
The three types of gold samples were first characterized by measuring ambient density and longitudinal sound speed. The
measured values of ambient density and longitudinal sound speed for CED, PED, and wrought gold samples are 19.06 g/cm
3
and 3.284 mm/μs, 19.11 g/cm
3
and 3.292 mm/μs, and 19.08 g/cm
3
and 3.358 mm/μs, respectively. The density values given
above are the geometric density of the samples and the measured sound speed values have an uncertainty of ~3 m/s.
The experimental configuration used in the spall study is shown in Fig. 9.1. A CED gold sample, a PED gold sample, and
a wrought gold sample were bonded to a 1 mm thick annealed copper (Cu) buffer. The gold samples used in our experiment
were 10 mm in diameter and 1 mm thick. The target consisting of the Cu buffer and three gold samples were impacted at
1.307 mm/μs with a 0.75 mm thick annealed Cu impactor, which was launched using a 40 mm powder gun. The annealed Cu
used as the impactor and buffer material was produced by annealing OFHC Cu under vacuum at 600 °C for an hour [5].
As shown in Fig. 9.1, 11 photon Doppler velocimetry (PDV) [6] probes were fielded to measure the wave profile at the
back of each gold sample (probes 1–3), record the shock arrival time at the back of the buffer (probes 4–10), and to measure
the impact velocity (probe 11). Probes 1–3 and 5–7 were positioned on the 8 mm radius bolt circle (BC) and were spaced
every 60°. Probes 8–11 were positioned on the 16 mm radius BC. Measured shock arrival times at the back of the buffer were
used to calculate impact tilt (~2.3 mrad) and shock arrival times at the front of the three gold samples. Since the shock arrival
times at the front and back of the gold samples are known, the shock velocity through the gold sample can be determined
from the known sample thickness. Values of measured shock velocities for all three gold samples are listed in Table 9.1.
Shock velocities measured in this work are within about 2% of those calculated using the Hugoniot relation given in Ref. [7].
9.3 Result and Discussion
An upshifted, multiplexed PDV system was used to measure wave profiles (particle velocity histories) at the rear free surface
of the gold samples. Measured wave profiles for the three gold samples, exhibiting velocity “pull back” characteristic of
spall, are shown in Fig. 9.2. The large noise seen in the measured wave profiles was introduced by the use of an amplified
channel. Measured shock and peak particle velocities were used to calculate the peak longitudinal stress values listed in
Table 9.1 using the free surface approximation [8]. Spall strengths (σ sp ) of the gold samples were calculated from the measured velocity pull backs using the following equation [9–11]:
Fig. 9.1 Schematic of the experimental configuration for the gold spall experiment
A. Mandal et al.
9.2 Experimental Details
The three types of gold samples were first characterized by measuring ambient density and longitudinal sound speed. The
measured values of ambient density and longitudinal sound speed for CED, PED, and wrought gold samples are 19.06 g/cm
3
and 3.284 mm/μs, 19.11 g/cm
3
and 3.292 mm/μs, and 19.08 g/cm
3
and 3.358 mm/μs, respectively. The density values given
above are the geometric density of the samples and the measured sound speed values have an uncertainty of ~3 m/s.
The experimental configuration used in the spall study is shown in Fig. 9.1. A CED gold sample, a PED gold sample, and
a wrought gold sample were bonded to a 1 mm thick annealed copper (Cu) buffer. The gold samples used in our experiment
were 10 mm in diameter and 1 mm thick. The target consisting of the Cu buffer and three gold samples were impacted at
1.307 mm/μs with a 0.75 mm thick annealed Cu impactor, which was launched using a 40 mm powder gun. The annealed Cu
used as the impactor and buffer material was produced by annealing OFHC Cu under vacuum at 600 °C for an hour [5].
As shown in Fig. 9.1, 11 photon Doppler velocimetry (PDV) [6] probes were fielded to measure the wave profile at the
back of each gold sample (probes 1–3), record the shock arrival time at the back of the buffer (probes 4–10), and to measure
the impact velocity (probe 11). Probes 1–3 and 5–7 were positioned on the 8 mm radius bolt circle (BC) and were spaced
every 60°. Probes 8–11 were positioned on the 16 mm radius BC. Measured shock arrival times at the back of the buffer were
used to calculate impact tilt (~2.3 mrad) and shock arrival times at the front of the three gold samples. Since the shock arrival
times at the front and back of the gold samples are known, the shock velocity through the gold sample can be determined
from the known sample thickness. Values of measured shock velocities for all three gold samples are listed in Table 9.1.
Shock velocities measured in this work are within about 2% of those calculated using the Hugoniot relation given in Ref. [7].
9.3 Result and Discussion
An upshifted, multiplexed PDV system was used to measure wave profiles (particle velocity histories) at the rear free surface
of the gold samples. Measured wave profiles for the three gold samples, exhibiting velocity “pull back” characteristic of
spall, are shown in Fig. 9.2. The large noise seen in the measured wave profiles was introduced by the use of an amplified
channel. Measured shock and peak particle velocities were used to calculate the peak longitudinal stress values listed in
Table 9.1 using the free surface approximation [8]. Spall strengths (σ sp ) of the gold samples were calculated from the measured velocity pull backs using the following equation [9–11]:
Fig. 9.1 Schematic of the experimental configuration for the gold spall experiment
A. Mandal et al.