Mechanics of materials-permanent set

In summary, the conversation discussed the properties of an aluminum bar, including its length, diameter, and modulus of elasticity. The bar was loaded with a tensile force and then unloaded, resulting in an elastic recovery strain of 0.001. The conversation also mentioned the use of stress-strain curves to determine the permanent set of the bar and the importance of using consistent units in calculations.
  • #1
quyn123
3
0
An Al bar has length=4ft and diameter=1inch. Modulus of elascity=10x10^6psi. The bar is loaded by a tensile force 24k and then unloaded. what is the permanenet set of the bar.



welll stress=F/A=210.77MPa with this value I made a dot on the corresponding stress strain curve. Then I shifted the linear part of the curve to the right until it hit the point. A vertical line at the 210.77Mpa is drawn on the graph and the small gap between the shifted line to this vertical line represents the elastic recovery strain. (sorry if its hard to understand I tried my best). So this elastic recovery strain region is calculated e.g to be 0.001. I know permanent set= elastic recovery strain x intial length...wouldn't this then state that permanent set is a representation of contraction length?

thanks in advance.
 
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  • #2
I don't know if you're from the USA, but I like your units of psi and feet and kips, so please don't fool with Paschals if you're given stuff in the old fashioned imperial units that America has grown to know and love.
Are you given anything else in this problem like a graph or the yield stress of the aluminum? Deformations are fully recoverable within the elastic region, which i assume may be at strains less than .001. What you are looking for is the inelastic recovery that results in the permanent set. Draw a line parallel to the straight part of the stress strain curve, that passes through the point of max stress. Where that line hits the x axis, read off the value of strain. This is the number to use for your permanent set calculation.
 
  • #3
Cheers for the help. It all makes sense.
 

FAQ: Mechanics of materials-permanent set

What is the definition of permanent set in mechanics of materials?

Permanent set, also known as plastic deformation, is the permanent change in shape or size of a material after it has been subjected to a load and then unloaded. It occurs when the material has exceeded its elastic limit and cannot return to its original shape and size.

How is permanent set measured in mechanics of materials?

Permanent set is typically measured as a percentage of the original length or diameter of the material. It can also be measured by the change in strain or stress after the material has been unloaded.

What factors influence permanent set in mechanics of materials?

The amount of permanent set in a material is influenced by factors such as the magnitude and duration of the load, the type of material, and the temperature at which the load is applied. Materials with higher ductility and lower elastic limits are more prone to permanent set.

What are the consequences of permanent set in mechanics of materials?

Permanent set can lead to structural failure or malfunction of a material or component. It can also affect the overall performance and lifespan of the material. In some cases, permanent set may be desirable, such as in shaping and forming processes for manufacturing.

How can permanent set be prevented or minimized in mechanics of materials?

To prevent or minimize permanent set, materials can be designed and selected based on their properties and the expected load conditions. Proper heat treatment and stress-relieving processes can also help reduce the likelihood of permanent set. Additionally, regular maintenance and inspection can help identify and address any potential issues before they worsen.

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