What is tensile strength Rm and how is it defined?
Definition of tensile strength in the tensile test
Tensile strength Rm describes the maximum tensile force per unit area that a material reaches before it fails. It is given in megapascals (MPa) or newtons per square millimetre (N/mm²). The value is determined in a standardised tensile test according to ISO 6892, in which a material sample is stretched under controlled conditions until it breaks.
Significance of Rm as a material characteristic
Tensile strength is a measure of the mechanical load-bearing capacity of a material. The higher the tensile strength, the more resistant the material is against tensile forces. In many design areas it is essential to select materials with sufficient tensile strength to ensure a safe and reliable design.
Difference between tensile strength and yield strength
Whereas yield strength (Rp) indicates the stress from which a material deforms plastically, tensile strength Rm designates the stress at which it finally fractures. Tensile strength is therefore always greater than yield strength.
How is tensile strength Rm calculated and measured?
Formula for calculating tensile strength
Tensile strength is calculated by the following formula: Rm = Fmax / A0, where Rm is the tensile strength in MPa or N/mm², Fmax is the maximum tensile force reached in newtons and A0 is the original cross-sectional area of the sample in square millimetres.
Execution of the tensile test according to ISO 6892
The tensile test is performed under controlled conditions. A standardised material sample is clamped into a testing machine and loaded with an increasing tensile force. The force rises until the material fails.
Unit of tensile strength: MPa or N/mm²
The results of the tensile test are given in megapascals (MPa) or newtons per square millimetre (N/mm²). Both units are equivalent: 1 MPa = 1 N/mm².
Which factors influence the tensile strength of a material?
Influence of material composition on Rm
The composition of a material has a direct influence on its tensile strength. Highly alloyed steels have, for example, higher tensile strength than pure metals. The fineness of the grain and the carbon content in steels also play a role.
Effects of heat treatment and deformation
Heat treatment of a material can significantly change its tensile strength:
- Quenched and tempered steels have higher tensile strength
- Annealed metals are softer and stretch more before they fail
Differences between metallic and non-metallic materials
Metals usually have higher tensile strength than plastics or ceramics. Typical values:
- Steel: over 1000 MPa
- Aluminium: 50 to 500 MPa
- Copper: 200 to 400 MPa
- Plastics: under 100 MPa
Why is tensile strength Rm important for design?
Significance for the load-bearing capacity of components
Components must be designed so that they can safely withstand the tensile forces occurring in operation. Tensile strength serves as a reference value for engineers to assess the load-bearing capacity of a material.
Rm as a limit for the maximum tensile stress
The maximum tensile stress must in no case exceed the tensile strength of a material, as otherwise sudden fracture or failure threatens.
Relationship between tensile strength and safety factors
Designs are always provided with safety factors in order to cover unforeseen loads. For example a safety factor of 1.5 to 2 is often chosen.
How does a material behave when tensile strength is reached?
Plastic deformation and necking
Before fracture, necking occurs in ductile material, i.e. the sample narrows at a specific location before it breaks.
Differences between brittle and ductile failure
- Ductile materials: stretch for a long time before they break.
- Brittle materials: fail abruptly without visible deformation.
Significance of elongation at break in the context of tensile strength
Elongation at break indicates how far a material stretches before it breaks. The greater the elongation at break, the better a material absorbs energy.
Which typical tensile strength Rm values do various materials have?
Typical tensile strength values for various materials:
| Material | Tensile strength Rm (MPa) | Material group |
|---|---|---|
| Structural steel (S235) | 360 to 510 | Steel |
| Quenched and tempered steel (42CrMo4) | 1000 to 1200 | Steel |
| Stainless steel (1.4301) | 500 to 700 | Steel |
| Aluminium (AlMgSi) | 200 to 350 | Light metal |
| Copper (Cu-ETP) | 200 to 360 | Non-ferrous metal |
| Titanium (Ti-6Al-4V) | 900 to 1200 | Light metal |
| Polyamide (PA 6.6) | 70 to 85 | Plastic |
| Polycarbonate (PC) | 55 to 75 | Plastic |
| GRP (glass fibre reinforced) | 200 to 500 | Composite material |
| CFRP (carbon fibre reinforced) | 500 to 1500 | Composite material |
How does nominal differ from true tensile strength?
Definition of nominal and true tensile strength
Nominal tensile strength only takes the original cross-sectional area into account, while true tensile strength includes the narrowing of the sample during the tensile test.
Significance of the change in cross section during the tensile test
Because the cross-sectional area of a material changes under load, true tensile strength can be up to 50 % higher than the nominal value.
Application areas for nominal and true tensile strength
- Nominal tensile strength: used in technical calculations and mechanical engineering.
- True tensile strength: applied in materials science and research.
GOBA Takeaway
Tensile strength Rm is one of the most important mechanical material properties and directly influences the design and safety of components. Its determination is carried out by the tensile test according to ISO 6892, with various factors such as material composition, heat treatment and deformation influencing the values. Engineers use tensile strength to determine the load-bearing capacity of materials and to secure their designs accordingly.