How a Tensil Stress Strain Curve Is Created

[MatthewMansfield]

Hello all

I was hoping someone could help me interpreting a Stress Strain Curve.

As part of my introduction to Civil Engineering I am trying to get my head around how a stress strain curve is created, for example I have a steel sample and wish to perform a tensile test.

While the steel sample is being stretched I want to know: -

Does the force applied to the steel sample continue to increase until the steel sample elongates at which point no additional load is applied until the steel stops elongating at which point more force is applied until the steel continues to elongate?

Or

Does the force applied to the steel sample continue to increase uniformly and the elongation measured at regular intervals?

Does this make sense?

Thank you.

 

[Ron]

Matthew....in its simplest form, a tensile stress-strain curve is a plot of stress on the y-axis and strain on the x-axis. As long as the sample is in its elastic range (prior to yield), if you remove the stress you should get a corresponding decrease in strain....in short, it is a linear relationship.

After the yield point is reached, the stress and strain continue to rise as load is applied up to the ultimate strength. This part of the curve is not elastic and if you removed the stress, you would not get a linear decrease in strain...in short, the strain would be plastic and permanent deformation would have occurred. As you pass ultimate strength, application of load results in no additional stress, but the strain continues....essentially the sample stretches more under the same load. It is also difficult to measure the strain accurately during this part of the test as the sample is stretching rapidly.

In times before, the strain was measured mechanically. Now with better techniques, the strain can be measured in a variety of ways and certainly with better accuracy.

 

[Compositepro]

Actually neither. Most tests are done with constant strain rate, but anything is possible. Screw machines will turn with a fairly constant rpm and the crosshead will move at a "constant" rate to failure while force is measured. Hydraulic machines control force but still usually adjust the force to maintain a constant strain rate. Using only force control would be dangerous because things will fly away after they break. Old machines used to use weights and levers to apply load. The advantage was that calibration was more robust and changes in gravity at different locations is automatically compensated. I have Mettler analytical balances that are over 80 years old that still accurately weight to a tenth of a milligram. These use counterweights that are selected by turning a knob.

To calculate modulus, the stretch of the specimen has to be measured accurately using strain gauges or extensometers. Crosshead travel can be calculated by turns of the screw but all machines deflect under load, and samples can slip slightly in grips, so crosshead travel is not an accurate measurement of sample stretch. This does not affect strength measurements, however.

Also, materials change diameter when they stretch in length. If you divide the measured load by the initial diameter of the test specimen you get "engineering stress". To get the true stress you have to measure the diameter as it changes under stress. There are lots of details to consider. In the real world, materials also change in cross-section as they stretch so engineering stress is usually more useful than true stress. But you have to be aware of what data you are looking at.

 

[oldestguy]

Too darn complicated answers. Think of the test as stretching a spring, up to a point where it is no longer elastic If this post is by a student, those posts are not allowed here. To delete it hit the red flag and you will get a message to reply to.