# Blunt force vs cutting force in penetration

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## Summary:

Are they fundamentally the same or different?

## Main Question or Discussion Point

My thought is that they both stem from PRESSURE. Sharp objects have more pressure than blunt objects given the same force and therefore can penetrate softer material better.

Some of my associates are suggesting that cutting by stabbing is entirely different than blunt force. But the definition of blunt force encompasses being stabbed or sliced. There is no minimum area or minimum force needed. Blunt is not well defined.

Am I right or is there another difference I'm not seeing?

Thinking of a bulletproof vest able to stop a bullet but not a knife stab. I did the math and it seems that the knife has multiple times more pressure than the bullet.

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jbriggs444
Homework Helper
2019 Award
As I understand your question, you are asking if the only essential difference between a sharp object and a blunt object is the concentration of stress, and shouldn’t every thing else be fundamentally the same.

I get where you are coming from, and, yes, the material properties don’t really change. However there are important differences. Most important is the direction of the local stress. As the sharp object begins to penetrate the sides act as a wedge pushing outward more than downward. This places the material right at the tip of the wedge under tension. Of course, the tip itself is locally compressing, but superposition applies. The tension being caused by the wedge allows the tip to slice through the material with much less resistance.

sophiecentaur
Gold Member
Possibly you could refine your thinking in terms of Stresses and Strains instead.
That's the way to go. Stress has the same units as pressure.
Furthermore, materials have a yield point, when the applied stress causes an extreme displacement. Materials have non-linear characteristics (nothing is linear in the end). A sharp point will produce extreme stress (N/m2) and the material can yield, where it would only stretch with a 'rounded' point with the same force.
Thinking of a bulletproof vest able to stop a bullet but not a knife stab.
Armour is designed against particular weapons . I would imagine that a stab vest would not be so good against bullets. Then there's the recent invention of bullet proof windows which resist incoming bullets but which allow bullets from within the room to get out. I have to apologise for the exuberance for guns in that video but it is certainly a smart bit of hybrid materials engineering.

Baluncore
2019 Award
Thinking of a bulletproof vest able to stop a bullet but not a knife stab. I did the math and it seems that the knife has multiple times more pressure than the bullet.
It is more involved than you might think. A good survey can be found in a book by: Tony Atkins. “The Science and Engineering of Cutting: The Patterns and Processes of Separating, Scratching and Puncturing Biomaterials, Metals and Non-metals”. 2009 – 2013.
It discusses slicing, penetration and blunt impact, Kevlar and chain mail, etc while avoiding unnecessary math. Chapter 9. Sharpness or Bluntness: Absolute or Relative?
Go to a library, or use https://www.bookfinder.com ; or find the .pdf copy.

sophiecentaur
Gold Member
It is more involved than you might think.
I have recently been using cutting tools on a lathe and the angles and materials involved all make a real difference to the speed of cutting, chatter and final finish etc.. I "might think" it is verrrry involved.