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Tensile Strength

Table of Contents

1. Definition: What is tensile strength?
2. Unit, Formula, and Calculation of Tensile Strength
3. Method for Determining Tensile Strength
4. The Tensile Strength of Various Materials
4.1 Tensile Strength Values of Typical Engineering Materials
4.2 Which metals have the highest tensile strengths?
5. Additional Facts About Tensile Strength
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1. Definition: What is tensile strength?

The material property known as tensile strength, also called tear strength, evaluates the strength behavior of materials. It indicates the maximum tensile stress that can be applied to a defined material specimen without failure. Conversely, this means that if a material’s maximum tensile stress is exceeded, it will break or tear. However, permanent (plastic) deformation can occur long before the maximum value is reached. Tensile strength is an important parameter for the design of structures. To obtain comparable results, the material property is determined using a standardized tensile test, such as ISO 6892 for metallic materials or ISO 527 for plastics.

2. Unit, Formula, and Calculation of Tensile Strength

Tensile strength is determined by means of a tensile test. It is calculated by dividing the maximum tensile force (Fmax) by the original cross-sectional area (Ao or So) of the tension member.

  • The formula is: Rm = Fz / Ao

In addition to the symbol Rm, the following symbols are also used for tensile stress: Rz, σ.

Since this is a stress value, the unit of tensile strength is MPa (megapascals) or N/mm² (newtons per square millimeter).

Furthermore, the tensile strength can be read directly from the stress-strain diagram. On the stress-strain curve, it corresponds to the highest point on the y-axis.

3. Method for Determining Tensile Strength

Tensile strength depends on the material. Therefore, it cannot be calculated directly but must be determined in a standardized tensile test using a material specimen. In practice, so-called proportional bars are used for this purpose; their length and diameter are specified in accordance with applicable standards for determining the yield strength, elongation at break, and tensile strength.

The specimen is clamped straight, tension-free, and without any bending in a tensile testing machine. The specimen is subjected to a controlled uniaxial tensile force. During the tensile test, the force increases slowly and proportionally until the bar breaks. During the test, the force F and the change in length of the specimen within the measurement range are continuously measured. Additionally, other important material properties such as the yield strength, necking, and elongation at break are recorded during the tensile test. The measured values are plotted as a so-called yield curve on a stress-strain diagram.

4. The Tensile Strength of Various Materials

When searching for a specific material, building material, or structural material, the respective material properties are of paramount importance, in addition to external differences. For example, a screw must be able to withstand a certain tensile stress without its head breaking off from the shank. Depending on the application, either a less expensive alloy steel screw is sufficient, or a more expensive titanium screw is required.

4.1 Tensile Strength Values of Typical Engineering Materials

Material Tensile strength in N/mm²
Wood 100 to 180
Concrete 4.5 to 5.2
Aluminum 70 to 600
Reinforcing steel 300 to 690
Case-hardened steel 500 to 1,200
Quenched and tempered steel 500 to 1,300
Titanium 400 to 1,200

 

Stone and concrete can withstand only low tensile loads, but can withstand high compressive loads. Steel, reinforced concrete, and wood can withstand both tensile and compressive loads.

4.2 Which metals have the highest tensile strengths?

Among structural metals, steel and titanium alloys have the highest values. With a tensile strength of up to 1,200 N/mm², titanium has the highest tensile strength.

5. Additional Facts About Tensile Strength

Tensile strength

The material property known as tensile strength indicates the tensile stress at which a component breaks. The yield strength defines the end of a material's elastic behavior. Once the yield strength is reached, the material transitions to the plastic state. It is therefore permanently deformed.

The test force is the load per unit of time to which the test specimen is subjected in the tensile testing machine. According to the standard, it is selected based on the material group to which the material under test belongs.

Materials with high tensile strength are often used when high strength is required and the workpieces or semi-finished products are subject to little or no mechanical stress. For parts that are, for example, still to be bent or otherwise machined, unhardened materials with lower tensile strength values are usually used. Hardening takes place only after machining.

Many semi-finished products, such as threaded rods or screws, are classified according to what is known as the strength class. Standards specify number combinations that allow for differentiation and comparison. Example: The screw has a strength class of 8.8. The number before the decimal point describes the tensile strength of the material. In this specific case, it is 800 N/mm². The number after the decimal point represents the yield strength. In the example, this would be 640 N/mm².

The maximum tensile force that a structural component can withstand can be calculated using the known cross-sectional area under tension and the tensile strength of the material used. To do this, the stress area is multiplied by the determined tensile strength (maximum tensile load: Fmax = Rm · A = force in N).

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