How They Work
The effectiveness of the ceramic shards is a result of the hardness of the material relative to tempered glass and the structure of the glass. Tempered glass used in automobile windows is softer on the Mohs scale than spark plug ceramic (specifically aluminum oxide ceramic). Aluminum oxide ceramic rates a 9 on the Mohs scale; diamonds are 10, glass is 6.5. Additionally, tempered glass is manufactured with an extremely high surface compressive stress and high internal tensile stress, giving it strength and durability, but also causing it to abruptly shatter into thousands of tiny pieces when it breaks. The result is that the hard, sharp shard of ceramic creates a small scratch in the glass which then rapidly propagates across the entire piece of glass as the internal tension is released. The glass subsequently shatters into many small pieces.
Read more about this topic: Ninja Rocks
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