Tensile strength of plastic materials

Tensile strength is one of the basic mechanical properties of a material. It tells us how much force a plastic can withstand before it breaks when stretched. This is extremely important in engineering and industrial design – especially for products that need to withstand certain loads, such as automotive parts, electronics housings, packaging or medical components.

What is tensile strength?

When plastic is stretched, the material first begins to resist, and then breaks at a certain force. This maximum force on the test specimen is called the tensile strength (denoted by σₘ or σb). It is measured in megapascals (MPa).

Tensile strength depends on:
– chemical structure (is the material crystalline, amorphous, fiber-reinforced, etc.),
– processing (e.g. injection molding, extrusion),
– and environment (temperature, humidity, etc.).

Why is it important?

Tensile strength helps in deciding:
– which material to choose for mechanically stressed parts,
– how thick the product should be,
– whether the plastic will withstand a certain application,
– and also how the material will behave at break (brittle or elastic?).

What do tensile property graphs look like?

Tensile properties are usually displayed in the form of a stress-strain curve. This shows how much the material resists stretching (vertical axis – stress) and how much it stretches (horizontal axis – elongation).

Example of typical stress-strain curves (Figure 1.8):

– 1: brittle materials (e.g. PMMA) – high strength, low elongation.
– 2: materials with a yield point – after initial yielding, the strength decreases.
– 3: materials with reinforcements – the strength increases slowly.
– 4: very elastic materials – high elongation, low strength.

Examples of real materials

In this more advanced picture we see the behavior of specific plastic materials, such as:
– PA66, POM, PMMA, PC: high strength, moderate elongation,
– ABS, PP, HDPE: lower strength, but more elongation,
– LDPE: very soft and elastic, with low strength.

Conclusion

Tensile strength is a simple but extremely useful parameter. By knowing these curves, we can quickly assess how a material will behave in real-world conditions – whether it will withstand a force, bend, or immediately break. Graphs are an indispensable tool in this.