What is tensile strength?
Tensile strength is the highest tensile stress a material can withstand before it breaks. In tension a material is pulled apart, the opposite of compression. Tensile strength is expressed in newtons per square millimetre (N/mm²), which equals megapascals (MPa). It is determined with a tensile test: a specimen is stretched in a testing machine with increasing force until it fractures.
Tensile strength and yield strength
For metals such as steel, two values matter:
- Yield strength: the stress at which the material starts to deform permanently. Up to that point it springs back when unloaded. Structural engineers usually design with this value.
- Ultimate tensile strength: the highest stress reached in the test, after which the material necks and breaks.
For structural steel the number is in the grade name: S235 and S355 have a minimum yield strength of 235 and 355 N/mm² respectively. B500 reinforcing steel has a yield strength of 500 N/mm². The ultimate tensile strength of steel is always higher than its yield strength.
Tensile strength of common building materials
- Steel: very high tensile strength and ductile; it stretches considerably before breaking. Ideal for tie rods, cables and reinforcement.
- Concrete: strong in compression, but its tensile strength is only about a tenth of its compressive strength. Concrete therefore cracks easily in tension or bending, which is why it is reinforced: the steel bars carry the tensile forces.
- Timber: fairly strong in tension along the grain, but very weak across the grain. That is why wood splits easily when screws or nails are placed close to the end grain.
- Masonry: hardly any tensile strength. Walls crack when they bend or when foundations settle unevenly.
What it means in practice
- Use unreinforced concrete only where it is almost purely in compression, such as a blinding layer or the haunching behind kerbs. Slabs, beams and lintels need reinforcement.
- Do not place screws and nails too close to edges or end grain; pre-drilling prevents splitting.
- Always have structural elements designed by an engineer. A material’s tensile strength is just one value; connections, cross-section and safety factors count too.
Related terms
- Compressive strength
- Flexural strength
- Tensile splitting strength
- Modulus of elasticity
- Elasticity
- Reinforcement
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