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Why does a Piece of a Spark Plug work So Well at Breaking Car Glass?


A broken ceramic piece from a spark plug can easily break a car window. Why does it work so well when it weighs so little and is thrown at a slow speed?


" First of all lets talk about the glass on the car "

Tempered glass, which is used for the side windows of most vehicles, is manufactured with an extremely high surface compressive stress and high internal tensile stress, giving it strength and durability, but also leading it to abruptly shatter into thousands of tiny pieces when it breaks.

Why does it do this? Well let's understand the processing. This glass is cooled rapidly from liquid to solid. Glass that cools more quickly ultimately ends up at a lower density, hence a higher volume compared to glass that cools slowly. graph Well guess what, there is a thermal gradient in a pane of glass as it cools. Meaning the outside rapidly cools and the inside does not. This puts the outside of the glass in compression and in the inside in tension. This acts as a crack inhibition method, meaning that the stress necessary to propagate the crack must first overcome the compressive stress on the outside (since glass will fail in tension well before compression). So what do we get? A glass that is ultimately very strong, but has a massive amount of stored internal energy through the tempering process.

When we  use something small and hard to act as a stress concentrator. This can amplify the force applied and help penetrate this region. So in the case of the Spark plugs are made of aluminum oxide ceramic, which is exceptionally hard. The impact of the shard against the glass creates a tiny structural failing, which rapidly spreads the tension through the rest of the screen, causing it to break. Although effective, spark plug fragments will not work all the time. Spark plug fragments may not break glass if they are not sharp enough or thrown with too little force. Spark plug porcelain shards are also less effective on windshields because these come with a special laminate feature that reinforces their resistance to impact. That is also why you can buy punches (firefighters and other emergency responders also carry these) that are essentially hardened steel or diamond tipped and do the same thing.


In some states, broken shards of spark plugs are considered a weapon. This is because they are sometimes used by burglars to break into vehicles. In the state of California, carrying spark plug chips or pieces is considered a misdemeanor. Individuals caught carrying spark plug chips run the risk of paying a potential $1,000 fine or receiving a 6 month jail term.

Hardness of the glass compared to to impact material is definitely relevant since this interaction is very similar to a hardness test (Rockwell, Vickers, Knoop, etc. Indent tests). The material needs to be harder, or at least close to the same hardness as the glass. I highly discourage people from using the Mohs scale to get actual numbers.

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