What is The Difference Between Proportional and Non-proportional Clamping Force?

While there are many varieties of gripping technologies (wedge, screw, hydraulic, pneumatic, etc.), they all can be classified as proportional or non-proportional according to the way in which the clamping force is exerted on the specimen.

Proportional Clamping Force – The clamping force exerted on the specimen is proportional to the tensile load applied to the specimen.

Non-Proportional Clamping Force – The clamping force on the specimen is independent of the tensile load applied to the specimen.

 

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What is the Best Way to Check the Functionality of my ElectroPuls™ Load Cell?

Instron load cells are robust and built to last. Therefore, checking the load cell’s functionality on a daily basis is not usually necessary unless you suspect a real problem. That being said, it is always a good idea to be safe than sorry!

To safely assess the functionality of your load cell, you can simply place or hang a known mass from the load cell to verify that the actual reading corresponds to the value of the mass. A mass of 20Kg or more is usually the safest bet for an ElectroPuls E1000 and  E3000. You can always contact Instron support if you suspect you are having issues with your load cell. Remember: make sure you set appropriate limits whenever conducting any kind of test or load cell reading! Read more

What is n-value?

In one of our previous posts, we blogged about plastic strain ratio, r. Besides r-value, another parameter that is commonly measured in metals testing is the n-value. So what is n-value?

n-value, also known as the strain hardening exponent, is the measure of a metal’s response to cold working. Cold working is the plastic deformation of metal below its recrystallization temperature and this is used in many manufacturing processes, such as wire drawing, forging and rolling. As a ductile metal is plastically deformed during cold working, it becomes harder and stronger while its ductility decreases. The strain hardening exponent gives us an indication of how much the metal hardens or becomes stronger as it is plastically deformed.

It has been found that the relationship between true stress and true strain of a material during cold working can be represented by the equation:

 

where σ and ϵ represent true stress and true strain respectively and can be calculated using the equations below. The constant K is called the strength coefficient and varies from alloy to alloy. 

 

where S = engineering stress and e = engineering strain

Putting the first equation in logarithmic form:

The strain hardening exponent n can thus be determined from the slope of the logarithmic form of the true stress versus true strain curve within the plastic region.

A metal with large strain hardening exponent n responds well to cold working. The figure below illustrates the true stress versus true strain curve (in logarithmic scale) for metals with large and small n-values. You can see that for a given amount of strain, a metal with a larger n-value obtains a greater degree of strain hardening.

To measure engineering strain e for determination of n-value, we only need an axial extensometer. However, it is worth noting that even though n-value and r-value are two distinct parameters, the test for measuring them is usually conducted at the same time. This has led to the two tests being commonly referred to collectively as “r and n” testing. So although a transverse extensometer is not needed to measure n-value, most people will have it anyway because they are also measuring r-value at the same time.

To learn more about n-value, check out ASTM E646. 

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