Fig. 6 presents the pressure histories from two strain gauges:
SG1 positioned at 80 mmfrom the base and SG2 at 25 mmfrom the
base. The SG2 history shows that the impact is followed by a sudden
pressure (strain) rise, reaching its maximum of 4.8 bar. The slope of
the trace represents the pressure rise rate. The strain rate can be
obtained using thin-wall assumption (derivative of Eq. (10)). Here,
the value of 12,500 bar/s is obtained, which corresponds to strain
rate of 14 s1. The positive pressure part of the trace (first 1.65 ms),
which corresponds to a half of the waterhammer period, is superimposed
with small higher frequency oscillations with a period
approximately equal to 0.9 ms. Similarly to PT history, these oscillations
are attributed to natural oscillations (see Table 1, TNO, Eq.
(7)). The duration of this part corresponds to the time in which the
pressure wave travels two water levels. According to values
measured, the wave speed in the system is 155 m/s, and corresponds
well to the theoretical value given in Table 1 (Eq. (2)). Fig. 6
also indicates that there is a time delay between the signals
recorded by the two strain gauges, thus proving the existence of
awave travelling through the bottle. The time delay at a pressure of
2 bar is around 0.35 ms, which corresponds well to a wave speed of
155 m/s predicted by waterhammer theory.
Fig. 7-left presents the pressure histories from the strain gauge
SG2 for four different drop heights. All traces follow the same
pattern, having almost the same period. This is expected according
to both waterhammer and mass-spring model theories, since in
both cases the period of the main oscillation does not depend on
the drop height. On the other hand, the maximum pressure
magnitude increases with drop height, and it agrees well with
waterhammer predictions as shown in Fig. 7-right.
4.1.2. Results for bottles with an originally manufactured base
Fig. 8-left shows pressure histories from all sensors when the
bottle with an originally manufactured base is tested. Again, the
pattern of all traces is very similar to each other, but noticeably
different than that observed in the tests on bottles with a flat base.
The pressure rise after the impact is not as fast as in the previous
tests, and a long low pressure part of the trace (pressure magnitude
around minus 1 bar of relative pressure) has practically disappeared.
In addition, many oscillations with significantly reduced