# How to Calculate Yield Strength

Written by joe butler
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Product designers must figure out how much 'give' something has, since it's a good bet users will put it through its paces, such as bending, squeezing or pulling it. How much something can be stretched and still return to its original form is called its elasticity. A related measurement, how much something like plastic or metal can stretch and stay stretched, or return to its original form but not break, is its yield strength. Determining this can take experimenting but is a vital part of the engineering process.

Skill level:
Easy

### Things you need

• Measurement tool (tension testing machine)
• Graphing/plotting tool (either software or graphing calculator)

## Instructions

1. 1

Begin your assessment of an object's possible yield strength by first determining whether it's ductile or brittle. According to Science Buddies.org, ductile means something can be torqued/bent, which changes its shape but doesn't damage it, like a paper clip. Or it can be brittle, which means it's easily permanently fractured or shattered, like glass.

2. 2

Find a standard to measure the yield strength. Engineers Edge says a common measurement of pressure in the United States is pounds per square inch. Irrigationtutorials.com said there are standard gauges and tools available to measure PSI, including more specific ones for measuring certain types, such as air, for which one would use a barometer.

3. 3

Measure stress and strain with something called a tension test machine. Sciencebuddies says these machines can apply varying amounts of controlled force for varying amounts of time.

4. 4

Repeat your torque/stress test on your ductile item, keeping track of the point where permanent damage or deformation occurs. In the paper clip example, the clip could be slightly bent to hold papers, and usually will return to its original state easily. But if you bend the clip back and forth enough times, it will finally and permanently snap. This would be its yield point.

5. 5

Add extra stresses if necessary and also keep track of those with your tension test machine, such as proportionally increasing the force used to bend or stretch your object or the amount of time the force is applied.

6. 6

Plot your experiment. Engineers Edge.com and Science Buddies suggest the best way to assess your product's yield strength and yield point is to make a graph. Make the y axis your applied stress and the measured strain in response to that stress the x axis.

#### Tips and warnings

• Units of pressure differ in countries that use the metric system and the Imperial system, used in the United States. You may need to convert your findings from PSI to Pascal, another accepted unit of pressure. Use an online conversion tool such as www.imperialtometric.com/conversion_en.htm#pressure.

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