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# Physics/Shearing force and the required material strength

Question

Shear question
Hello Steve,

As always, thanks for volunteering in this site and thank you for all your answers in the past - they have really helped me.

I am interested in calculating the elements involved when cutting an object using a shearing force, and I am interested specifically in the required material characteristics of the objects that perform the cutting action.
I have a design where a piece of plastic is sheared/cut when placed between two objects, above and below, and pressure is applied from the top.
As you can see in the attached image, the green section is held in place between the two red objects. The darker red object is pushed down, hopefully shearing the green object at around the point colored yellow. (This operation is performed only once).

Based on the material's shear strength and the geometry of the object to be sheared/cut, I have calculated the amount of pressure required to shear the green object. I now have to think about the two red objects that press against the green part  during this operation. I need to make sure they are strong enough, and I was wondering if you could help me start to understand what factors govern whether the red objects are capable of performing this operation.
I am trying to find out what type of material strength is key here - would this be the compression strength do you think? My thinking was that if the compression strength in the red objects is lower than the shear strength in the green object, there is a danger that the pressure applied from above would simply squash the red objects and not shear the green one - is this a logical line of reasoning?
I am guessing the red objects' geometry is also a factor. Actually, my goal is to find a way to calculate/estimate the minimum required thickness of these parts, as well as the required material strength (whatever type of strength that may be). Could you suggest any way of doing this?

Sorry if my question is a but vague. Any suggestions on where to go from here to put me on the right track would be much appreciated!

Thanks and regards,
Eddie

Hello Eddie,

Let me start by saying that these questions are among the specialties of a mechanical engineer. Not one of mine. So my intuition is playing a large part in my answer.

I think strength in compression is important for both of the red objects. Primarily right at the ends of the trenches where the shear is to occur. For example, thinking of the bottom red object, imagine that compression allows the right hand end of the "trench" to deepen. It seems that this would allow, and perhaps encourage, the green object to bend as opposed to shear. I agree that the red objects need to tolerate the stresses they see better than does the green object. You want the green object to fail in shear.

In your sketch I don't see any indication that there is a means of preventing the left hand end of the green object from lifting when the downward force is applied at the right end. If not, I suggest that you add that.

Regarding trying to calculate the minimum requirements of the red objects. I don't know. My thoughts suggest experimentation.

I apologize for this being late,
Steve
Questioner's Rating
 Rating(1-10) Knowledgeability = 10 Clarity of Response = 10 Politeness = 10 Comment Thanks Steve, I appreciate your help as always!

Physics

Volunteer

#### Steve Johnson

##### Expertise

I would be delighted to help with questions up through the first year of college Physics. Particularly Electricity, Electronics and Newtonian Mechanics (motion, acceleration etc.). I decline questions on relativity and Atomic Physics. I also could discuss the Space Shuttle and space flight in general.

##### Experience

I have a BS in Physics and an MS in Electrical Engineering. I am retired now. My professional career was in Electrical Engineering with considerable time spent working with accelerometers, gyroscopes and flight dynamics (Physics related topics) while working on the Space Shuttle. I gave formal classroom lessons to technical co-workers periodically over a several year period.

Education/Credentials
BS Physics, North Dakota State University
MS Electrical Engineering, North Dakota State University