In this section, the results from impact loading achieved by dropping a hardened steel ball from 2.25m will be presented. Height measurement was determined using laser measurement device.
Due to the large amount of kinetic energy, the object was seen to have significant bounce heights as a result. The corresponding bounce height is presented in the table for each measurement. The tables however, only include the first bounce height as the consecutive bounces are less than 0.5 cm and can therefore not be accurately measured using the footage at hand.
6.5.1 Solid Plate
For the solid plate, the scale for the x-axis was selected to display -0.05 sec before peak and showing 0.5 sec after impact. The y-scale limits were set to -600<y<1800 µm/m. Time scale was extended to capture the third impact from the steel ball.
6.5.1.1 Ball Drop Method 2.25m - Solid Plate in Air
Measurement for solid plate ball drop showed three characteristic peaks illustrated in Figure 93, with first at 1720 µm/m, and the second and third at 235.4 and 49.3 µm/m respectively. The second bounce being 13.7% of the first, and the third is 20.9% of the second impact.
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Figure 93: Impact response from ball drop test on a solid plate in air, measurement #8.
Little indications of significant plate to support rim collisions.
Table 13: Measurement, averages and standard deviations for strain gauge 1, center strain gauge for strain and bounce heights.
92 Based on all the results the average peak maximum was at 1730.77 µm/m with a standard deviation of 465.12 µm/m equivalent to 26.87% variation from the mean average. The bounce height from the first impact also showed an average of 16.23 cm with a standard deviation of 3.03 cm, equivalent to a variation of 18.67% from the mean average. The bounce height beyond the first was not accurately measurable as it was below 0.5 cm.
6.5.1.2 Ball Drop Method 2.25m – Solid Plate in Water
For the test done for solid plate in water, with total water level of 22 cm and temperature of 11.1 °C. The characteristic first and second impact of 1784 µm/m, with the second being 356.4 µm/m displayed in Figure 94. Second impact being equivalent to 19.97% of the initial impact.
Figure 94: Impact response from ball drop test on a solid plate in water, measurement #9.
Plate remains fully stable on the support ring for the first and second impact.
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Table 14: Measurement, averages and standard deviations for strain gauge 1, center strain gauge for strain and bounce heights.
Average first peak response based on the tests was 1778.46 µm/m with a standard deviation of 236 µm/m, being a variation of 13.26%. The bounce height resulting from the initial impact was equivalent to an average value of 6.61 cm, with a standard deviation of 1.67 cm, equivalent to a variation of 25.26% from the mean average.
6.5.2 Perforated Plate
For the perforated plate, the scale for the x-axis was selected to display -0.05 seconds before peak and showing 0.4 sec after impact. The y-scale limits were set to -600<y<1500 µm/m 6.5.2.1 Ball Drop Method 2.25m – Perforated Plate in Water
The results from the impact loading tests a large amount of high frequency vibrations as the perforated plate is impacted by the steel ball. From the slow-motion footage of the impact the plate can be seen to be elevated roughly 2 cm on average due to the initial impact. The value of the if these abrupt peaks are in the order of magnitude 982.1,389.5 and 263.9 µm/m for the first to third occurrence. This being illustrated in Figure 95.
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Figure 95: Result from ball drop tests in air from 2.25 m on the perforated steel plate, Measurement #4.
The number of conducted tests showed that the average first peak maximum is 1386.71 µm/m and standard deviation of 516.31 µm/m. Only a total of two bounces were visible in the results, which is clearly indicated in the graph.
Plate comes to rest Parts of plate rim colliding with
supporting rim
Ball colliding with plat a second time
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Table 15: Measurement, averages and standard deviations for strain gauge 1, center strain gauge for strain and bounce heights. standard deviation is roughly 37% of the average peak value. Average bounce height observed being 10.54 cm with a standard deviation of 3.91, being 37% of the average bounce height.
96 6.5.2.2 Ball Drop Method 2.25m – Perforated Plate in Water
Results from the perforated plates, where measurement nr. 9 was selected as a representative response for the average impact. Water level at 22 cm, with the water temperature at 25 °C
Figure 96: Result from ball drop tests in water from 2.25 m on the perforated steel plate (measurement #9).
In Figure 96 three visible peaks are shown in the results but the height of the second bounce being less than 1 cm meant that the third is even less feasibly detected. However, the measurement indicates that plate itself is not lifted of its supports, even for impact energy of 9.7 J. The peak impact response of 814.5, 489.5 and 96.1 µm/m from the first to the third peak.
Where the second peak is 60% of the first, and the third is 19.6% of the second peak. The deviation is 19.6% of the average value of the first peak maximum. It is important to note however that the third peak was not consistently present for all the measurements, as some only had two peaks.
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Table 16: Measurement, averages and standard deviations for strain gauge 1, center strain gauge for strain and bounce heights.
The results in water indicate non-significant variation in the average bounce height as the standard deviation is 19.6% of the average bounce height. Interestingly the results show that percentage wise the standard deviation is identical to the first maximum and bounce height but is most likely coincidental in this case.
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7 Discussion
In this chapter the results will be compared and discussed with regards to a multitude of factors, ranging from dampening effect in water, bounce & impact correlation, material properties &
behavior and more. Overall this chapter aims to give answers to what was discovered in chapter 6, in addition to raising questions pertaining to the nature of impact loading measurements.