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Ball Drop: Failure Energy different than K.E.?

  1. Mar 4, 2010 #1
    Ball Drop: "Failure Energy" different than K.E.?

    I have a paper that describes some experiments done to study the failure of glass sheets of different materials and thicknesses. One test is the steel ball drop test (common in consumer electronics and elsewhere).

    In these tests or material specifications I usaully see that results are reported as the Joules at fracture or the drop height of the ball that causes fracture. Joules is reported to normallized the results (in case a different ball mass is used). Typically the Joules value is the Kinetic Energy of the ball before impact.

    In one chart in this paper I was reading, the authors have ball drop height on left y axis and "Failure Energy" in Joules on the right y axis. Both are linear scales. It is a little akward the way the two data sets are combined, but the average drop height at break and the Joules at break data more or less end up in the same place on the chart. The x axis has sample sets with different glass materials or thicknesses.

    The thing is the Joules values are about 13.53x too low if K.E. is being used to find the energy.

    13.53 is close to the conversion from Joules to ft-lb (off by order of magnitude). Did these guys make a conversion mistake or is there some way to measure the "Failure Energy" in a situation like this. I'm not looking for the force on the glass but rather the energy tranfered to the glass that makes it fracture. The authors could be measuring the bounce height of the ball rebound, but they don't mention it.

    I appreciate any thoughts.
  2. jcsd
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