| Work Detail |
Swiss researchers have studied the impact behavior of ice balls on an aluminum Hopkinson rod at different speeds, diameters, and temperatures. Researchers at the Southern Switzerland University of Applied Sciences and Arts have conducted a series of tests to evaluate the impact of ice balls on solar modules with the aim of improving standards for measuring their safety resistance to hail. In the article « An experimental investigation of ice ball impact behavior to improve PV panel hailstone safety », published in the International Journal of Impact Engineering , they explain that in their analysis they considered that natural hail balls can have diameters between 5 mm and 100 mm, with final speeds reaching between 10 m/s and 50 m/s. They also accounted for the varying velocities and temperatures of the ice balls using a modified version of the split Hopkinson pressure bar (SHPB), which is commonly used to test the dynamic stress-strain response of materials. Spherical ice samples were prepared with diameters ranging from 25 mm to 90 mm. A gas gun was used to achieve theoretical velocities of 25, 50, 75, and 100 m/s. Four different ice temperatures were tested: -4 °C, -10 °C, -20 °C, and -28 °C. Three setups in total were used to measure the load-time history signals from the ice ball impact, the scientists emphasize. The first consists of a 30 mm diameter, 1.5 m long aluminum rod, while the second and third consist of a 3 m long aluminum rod with 30 mm and 60 mm diameters, respectively. The longer setups were used to increase the pulse recording time, thus avoiding the superposition of reflected waves and allowing larger ice balls to be tested. The research group conducted more than 100 tests using various diameters, speeds, and temperatures, including 45° impact tests to examine the influence of impact angle. Analysis of the test results showed that the size, impact velocity, temperature, and collision angle of the ice balls significantly influenced the momentum measurement, as increasing size and impact velocity were directly related to increasing peak force and its associated time. “When the temperature is lower, the maximum force is greater and the peak time is shorter. When collision angles were varied, the maximum (peak) load was affected by the angle of impact, but not the time at which it occurred,” the academics explained. “A refined analysis, incorporating both dynamic effects, such as tensile strength under dynamic conditions, and the effects of impact angle, reveals a consistent trend. This is highlighted by the fact that test results across multiple impact angles can now be accurately represented within linear (log-log) graphs, highlighting the reliability of the trend.” The researchers concluded that dynamic and static characterization of the proposed tests is very difficult to achieve and emphasized that each variation in the test setup can lead to different results. To reproduce the effect on panels or gain a more complete understanding of the problem, it is necessary to examine the impact on less rigid surfaces, they added. Looking ahead, the research group plans to compare the load-time history signals generated by ice ball impacts on a rigid aluminum bar with those observed in photovoltaic modules equipped with strain sensors on their backs. |
