Calculating percent elongation is a crucial step in understanding the mechanical properties of materials, particularly in the context of tensile testing. Percent elongation, often denoted as EL, is a measure of how much a material can be stretched before it breaks, relative to its original length. It’s an important parameter because it gives insight into the ductility of a material—the ability of a material to deform under tensile stress.
Formula for Percent Elongation
The formula to calculate percent elongation is straightforward and involves measuring the difference in length of the specimen before and after the tensile test, then dividing by the original length, and finally multiplying by 100 to convert the fraction to a percentage.
[ \text{Percent Elongation} = \left( \frac{\text{Change in Length}}{\text{Original Length}} \right) \times 100 ]
Or, in terms of the variables often used in engineering and materials science:
[ \text{Percent Elongation} = \left( \frac{L_f - L_0}{L_0} \right) \times 100 ]
Where: - (L_f) is the final length of the specimen after the tensile test (after it has broken or been stretched to a certain point). - (L_0) is the original length (gauge length) of the specimen before the tensile test.
Step-by-Step Process to Calculate Percent Elongation
Measure the Original Length ((L_0)): Before conducting the tensile test, measure the length of the specimen over the gauge section. This length serves as the reference point.
Conduct the Tensile Test: Perform the tensile test on the material specimen. This involves stretching the specimen until it breaks. Note the length at which the specimen breaks or the final length ((L_f)) after the test.
Measure the Final Length ((L_f)): After the tensile test, measure the length of the broken specimen. This includes the elongation (stretching) that occurred during the test.
Calculate the Change in Length: Subtract the original length from the final length to find the change in length ((L_f - L_0)).
Apply the Formula: Use the calculated change in length and the original length in the percent elongation formula to find the percentage.
Interpret the Result: The calculated percent elongation indicates how much the material can elongate when subjected to tensile stress. Higher percentages typically indicate greater ductility.
Example Calculation
Suppose a metal specimen has an original length ((L_0)) of 50 mm and, after a tensile test, it breaks at a final length ((L_f)) of 70 mm.
[ \text{Percent Elongation} = \left( \frac{70 - 50}{50} \right) \times 100 ] [ \text{Percent Elongation} = \left( \frac{20}{50} \right) \times 100 ] [ \text{Percent Elongation} = 0.4 \times 100 = 40\% ]
This means the metal specimen can elongate by 40% of its original length before breaking.
Practical Applications and Considerations
- Ductility: Materials with high percent elongation values are considered ductile, meaning they can undergo significant deformation without breaking. This property is crucial for materials that need to absorb energy (like in automotive crumple zones).
- Brittleness: Conversely, materials with low percent elongation values are brittle and tend to break without significant deformation. This brittleness can be problematic in applications where flexibility is required.
- Yield Strength: It’s also important to consider the yield strength of a material, which is the stress at which a material begins to deform plastically. Combining yield strength with percent elongation provides a comprehensive view of a material’s mechanical properties.
Understanding and calculating percent elongation is essential for materials scientists, engineers, and researchers to assess the suitability of materials for various applications, ensuring the reliability, safety, and performance of final products.
