You can measure material plasticity by testing how much it can deform permanently before breaking.
One common and effective method, as described in material testing practices, involves using a tensile testing machine. Take a sample of the material, place it in a tensile testing machine and stretch it till it fails. Determine what the percentage of elongation at failure. This value provides a key indicator of the material's ductility and plasticity.
The Tensile Testing Method
Tensile testing is a fundamental engineering test used to determine a material's strength and deformation characteristics under pulling (tensile) stress.
Here's a breakdown of the process based on the reference:
- Sample Preparation: A standardized sample of the material, often shaped like a dog bone with specific dimensions, is prepared.
- Machine Setup: The sample is securely clamped into the grips of a universal testing machine (often called a tensile testing machine).
- Applying Stress: The machine applies a controlled pulling force to the sample, gradually increasing the stress.
- Stretching to Failure: The test continues, stretching the material, until it breaks or fractures.
- Measurement: During the test, the machine records the applied force and the corresponding elongation (change in length) of the sample.
Understanding Elongation at Failure
As a material is stretched, it initially deforms elastically (it returns to its original shape when the force is removed). Beyond a certain point (the yield point), it begins to deform plastically (it undergoes permanent change in shape). Plasticity is the ability of a material to undergo this permanent deformation without fracturing.
- Elongation at Failure: This metric specifically measures how much the sample's length increased plastically before it failed, expressed as a percentage of its original length.
- Significance: A higher percentage of elongation at failure generally indicates greater plasticity and ductility. Materials with high elongation can be significantly stretched, bent, or shaped without breaking, while those with low elongation are often brittle.
Here's a simple illustration of how different materials might compare:
| Material | Typical Elongation at Failure (%) | Relative Plasticity |
|---|---|---|
| Brittle Metal | < 5 | Low |
| Polymer | 10 - 100+ | Moderate to High |
| Ductile Metal | 15 - 50 | High |
| Elastomer | > 300 | Very High |
(Note: These are general ranges and can vary significantly based on specific material grade and processing.)
Why Percentage Elongation is Key
Percentage elongation at failure is a crucial metric for engineers and designers because it indicates a material's tolerance for deformation before fracture. This is vital for applications where materials might be subjected to bending, shaping, or impacts. Materials with high elongation can absorb more energy before failing, making them suitable for structural components or applications requiring toughness.
