How much force is needed to shatter safety glass?

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    Force Psi
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Discussion Overview

The discussion revolves around calculating the force required to shatter safety glass, particularly in the context of martial arts demonstrations and real-life incidents. Participants explore the relationship between force, pressure, and the properties of glass, including its breakage patterns and material thresholds.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant inquires about the calculation of force in pounds and its translation to psi, referencing a personal observation of safety glass shattering.
  • Another participant clarifies that psi measures pressure and emphasizes the importance of the size of the impacting object in determining breakage.
  • It is noted that breaking glass involves multiple factors, including pressure, force, direction, duration, and distance, making predictions complex.
  • A participant mentions that a small impact from a sharp object can break glass that would withstand larger impacts from softer objects.
  • A proposed formula for pressure is presented, suggesting that breakage depends on whether the pressure exceeds a material threshold, though it is acknowledged that this is a simplified view.
  • Some participants reiterate the complexity of predicting glass strength and the nuances involved in such calculations.

Areas of Agreement / Disagreement

Participants express differing views on the existence of a simple formula for calculating the force needed to break glass, with some arguing against the simplicity of such calculations while others attempt to provide a basic framework.

Contextual Notes

The discussion highlights limitations in the assumptions made about glass strength and the factors influencing breakage, indicating that a comprehensive understanding requires consideration of various conditions and material properties.

Halil Halil
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Hello, I am curious about how to calculate lbs of force like in those martial arts shows where they measure martial artists and professional mma fighters' punching power, and how it would translate to psi. This is because I saw a man at a youth hostel hit a safety glass on a door and shattered it. Its breakage pattern was that it broke around so I assume it was safety glass and not toughened glass. If the cheapest type of tempered glass is 10 000 psi( correct me if I'm wrong), then how much lbs of force does that translate into? And please could you show me your calculations and the formula for it!

Halil
 
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Psi standard for "pounds per square inch". This is a measure of pressure. So actually, breaking something depends on the size of the impacting object. This is why people can lay on beds of nails without getting hurt. The pounds are spread out, but if you try laying on just one nail, it's concentrated, and you will probably have to go to the hospital.
 
Halil Halil said:
And please could you show me your calculations and the formula for it!
There is no simple formula for that sort of thing. What is needed to break something is a combination of pressure, force, direction of force, duration of force and distance over which the force acts. In the case of a brittle material like glass, there is definitely no easy way to predict 'strength' under all circumstances. A small impact from a sharp tool like a metal worker's punch can shatter a car windscreen that could withstand the impact of a body, flying through the air at it.
This recent thread shows the way this sort of question can go. There is seldom a satisfactory answer. When it is necessary to have a glass sheet reliably 'strong enough', I think the technique is just to make it very thick with loads in hand.
 
sophiecentaur said:
In the case of a brittle material like glass, there is definitely no easy way to predict 'strength' under all circumstances.
The fact you can shatter a car window with a pebble sized bit of ceramic is a great example of this;
 
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The formula would be something like:
Pressure = (Force of impact)/(Size of impacting object's contact)

Then whether or not you get a break is dependent on some threshold of the material:
if Pressure < Threshold, then no break
if Pressure > Threshold, then break

This is the simplest case. There's a lot more nuance to this kind of thing.
 
DuckAmuck said:
The formula would be something like:
Pressure = (Force of impact)/(Size of impacting object's contact)
Re-read #3. There is no simple formula.
DuckAmuck said:
There's a lot more nuance to this kind of thing.
Indeed.
 
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