Strength - Elongation

General Information

• Strength measures the resistance of a material to failure, given by the applied stress (or load per unit area)
• The chart shows yield strength in tension for all materials, except for ceramics for which compressive strength is shown (their tensile strength being much lower)
• Elongation measures the percentage change in length before fracture
• Elongation to failure is a measure of ductility

Physical Insights

• Ceramics have very low elongations (<1%) because they can not plastically deform
• Metals have moderate elongation to failure (1-50%) with deformation occurring by plastic flow
• Thermoplastics have large elongations (>100%) because the molecules can stretch out and slide over one another
• Rubbers have long elastic elongations because the chains can coil/uncoil elastically
• Thermosets have low elongations (<5%) because the molecules are bonded together into a network so that they cannot slide over one another
• One way to strengthen a metal is to make plastic flow difficult – this reduces the ductility and elongation

Example Uses

• Few components are designed to elongate significantly - examples are impact protection like crash barriers.
• The chart helps identify materials which can be easily deformed into new shapes during manufacturing
• Designers like to use metals with significant elongations to failure because they are more forgiving of harsh treatment, e.g. car chassis, nails

Simple Questions

• What is the difference between elastic and plastic deformation?
• Why is lead used on old roofs and roof flashings?
• Select materials for a car bumper.
• Select materials for a bungee rope.
• Select materials for drawing pins.

Further Questions

• Why can’t rubbers be formed into shapes easily, even though they can be subjected to large deformations?

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