How to Calculate Yield Point

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You can easily calculate the yield point of a material to find the point where the material "gives," or breaks away. The yield point is the point beyond which a material is damaged and will not go back to its previous form after the force has been applied to a certain point. It is important to find out the yield point of certain materials so that you have know what is the maximum amount of force or pressure you can apply to the object before it is permanently damaged.

Things You'll Need

  • Tension-compression testing machine
  • Plotting tool for graphs (calculator)
  • Metal specimen
  • Measurement gauges
  • Take an aluminum alloy of diameter 0.505 inches and a gauge length of 2 inches.

  • Use the tension-compression testing machine by placing the aluminum alloy inside it so that force of varying degrees can be applied on it.

  • Slowly increase the axial load, which is the force administered along the lines of an axis, on the aluminum alloy and note the elongation that keeps on taking place with the increase in load. When you increase the load from 2,310 pounds to 4,640 pounds, the elongation in the aluminum alloy increases from 0.00220 to 0.00440.

  • Repeat the third step for as long as permanent deformation of the aluminum alloy has not taken place. The fracture point for the aluminum alloy is when a load of 13,600 pounds is applied.

  • Record the cross sectional area and length of the aluminum alloy obtained before, in this example 0.505 inch and 2 inches, respectively, and determine the values of the normal strain and stress.

    Load (lb.) Elongation (inches) Strain (inches/inches) Stress (psi)

    0 0 0 0

    2 310 0.0022 0.0011 11 532.92

    4 640 0.0044 0.0022 23 165.70

    6 950 0.0066 0.0033 34 698.62

    9 290 0.0088 0.0044 46 381.32

    11 600 0.011 0.0055 57 914.24

    12 600 0.015 0.0075 62 906.85

    14 000 0.02 0.01 69 896.49

    14 400 0.025 0.0125 71 893.54

    14 500 0.06 0.03 72 392.80

    14 600 0.08 0.04 72 892.06

    14 800 0.1 0.05 73 890.58

    14 600 0.12 0.06 72 892.06

    13 600 Fracture 67 899.45

  • Plot a graph of the values of stress along the y-axis and strain along the x-axis.

  • Ensure that you have determined the yield point that lies at the point where the straight line of the graph begins to curve to the right on the x-axis (horizontal axis). The yield point could also be noted when the metal finally gives way. The maximum load for the aluminum alloy is thus, 13,600 pounds and the yield point is 69,896.49 psi.

Tips & Warnings

  • Test one brittle and one ductile material. The ductile material like a paper a clip will have some form of deformation, whereas a brittle material (glass) will just fracture at the yield point.
  • Use other loading conditions like compression and bending to see how the material performs apart from the tension loading technique.
  • The stress-strain graph diagram is different for different types of materials.
  • The measurement devices used for different materials are varied. For example, air is measured with a barometer.

References

  • Photo Credit Jupiterimages/Photos.com/Getty Images
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