Yield strength of the material
Using the offset method, 0.2% offset yield stress, select the point on the curve that corresponds to the yield strength of the material.
Select the yield strength on the diagram. Click on “Show ruler” if you would like to use a ruler as you would with pencil and paper. The ruler can be extended or shortened by clicking and dragging on either end of the ruler at the squares.
The Correct Answer and Explanation is :
The yield strength of a material signifies the stress at which it transitions from elastic deformation, where it returns to its original shape after the stress is removed, to plastic deformation, where permanent changes occur. Determining this point is crucial for designing components that can withstand specified loads without undergoing permanent deformation.
Understanding the Stress-Strain Curve:
A stress-strain curve plots the relationship between the applied stress and the resulting strain in a material. Key regions of this curve include:
- Elastic Region: Characterized by a linear relationship between stress and strain, where deformation is reversible.
- Yield Point: The stress level at which the material begins to deform plastically.
- Plastic Region: Beyond the yield point, where permanent deformation occurs.
In materials lacking a distinct yield point, such as certain steel grades, the 0.2% offset method is employed to define the yield strength.
0.2% Offset Yield Strength Method:
This method provides a consistent means to determine the yield strength, especially for materials without a clear yield point. The steps are:
- Plot the Stress-Strain Curve: Conduct a tensile test to obtain the stress-strain data and plot the curve.
- Draw a Parallel Line: From the origin, draw a line parallel to the initial linear (elastic) portion of the stress-strain curve.
- Offset the Line: Shift this line horizontally by 0.002 strain units (equivalent to 0.2% strain).
- Determine the Intersection: The point where this offset line intersects the stress-strain curve corresponds to the 0.2% offset yield strength.
This approach effectively accounts for the initial non-linear behavior and provides a reproducible measure of yield strength.
Practical Example:
Consider a material with the following stress-strain data:
- Stress (MPa): 0, 50, 100, 150, 200
- Strain (mm/mm): 0, 0.001, 0.002, 0.003, 0.004
Plotting these values yields a stress-strain curve. By applying the 0.2% offset method, a line parallel to the initial linear region is drawn and offset by 0.002 strain units. The intersection of this line with the stress-strain curve indicates the yield strength of the material.
Conclusion:
Determining the yield strength using the 0.2% offset method is essential for materials lacking a clear yield point. It ensures that designs incorporate appropriate safety margins, preventing permanent deformation under expected service loads.
For a visual demonstration of the 0.2% offset method, you can refer to the following video: