Elastic and plastic behaviour

Summary and Exam Tips for Elastic and Plastic Behaviour

Elastic and plastic behaviour is a subtopic of Deformation of Solids, which falls under the subject Physics in the Cambridge International A Levels curriculum.

Elastic Deformation occurs when a material temporarily changes shape under an external force but returns to its original form once the force is removed. This reversible process is governed by the elastic limit, beyond which the material does not revert to its original shape. In a force-extension graph, elastic deformation is represented by a straight line, indicating proportionality between force and extension.

Plastic Deformation is irreversible, occurring when a material permanently changes shape under force. On a force-extension graph, plastic deformation is shown above the elastic limit, where the graph curves, indicating non-proportional extension.

Brittle Materials, like glass, exhibit minimal plastic deformation and break suddenly, while ductile materials, such as rubber, can undergo significant plastic deformation, absorbing more energy before breaking.

The area under a force-extension graph represents the work done and the elastic potential energy stored in the material. This concept applies whether or not the material follows Hooke's Law. During loading and unloading, the force-extension graph shows different paths, with the unloading curve always below the loading curve. Elastic potential energy can be calculated using the formula $\text{EPE} = \frac{1}{2} k x^2$, where $k$ is the spring constant and $x$ is the extension.

Exam Tips

• Understand Key Concepts: Make sure you can differentiate between elastic and plastic deformation, and know the significance of the elastic limit.
• Graph Interpretation: Practice interpreting force-extension and stress-strain graphs, focusing on the differences between brittle and ductile materials.
• Hooke's Law: Be comfortable with applying Hooke's Law and calculating elastic potential energy using the formula $\text{EPE} = \frac{1}{2} k x^2$.
• Work Done and Energy: Remember that the area under the force-extension graph represents work done and elastic potential energy.
• Loading and Unloading: Understand how loading and unloading paths differ on a graph and what they signify in terms of energy dissipation.