Mechanical properties of aluminium

What are mechanical properties?

Mechanical properties of aluminium, like other structural materials, are properties that are associated with the elastic and inelastic response of the material to the application of a load to it. These properies include the relationship between stress and strain.

Examples of mechanical properties are:

• modulus of elasticity (tensile, compressive, shear)
• tensile strength (tensile, compressive, shear)
• yield stress
• fatigue limit
• elongation (relative) at break
• hardness.

The mechanical properties of materials, including aluminium and its alloys, that are obtained by tensile testing of the material, such as tensile modulus, tensile strength, tensile yield strength and elongation, are called tensile mechanical properties.

Elastic modulus

The modulus of elasticity, often called Young’s modulus, is the ratio of the stress that is applied to a material to the corresponding strain in the range where they are directly proportional to each other.

There are three types of stresses and, accordingly, three types of elastic moduli for any material, including aluminium:

• tensile modulus of elasticity
• compressive modulus of elasticity
• shear modulus of elasticity (shear modulus of elasticity).

Figure 1 – Tensile curves of aluminum and low-carbon steel [1]

Figure 2 – The effect of alloying elements in aluminium alloys on their density and elastic modulus [1]

Tensile strength

The ratio of the maximum load before failure of a sample when testing it in tension to the original cross-sectional area of the sample.

Figure 3 – Tensile curves of aluminium in comparison with various metals and alloys [1]

Yield strength

The stress required to achieve a specified small plastic deformation in aluminium or other material under uniaxial tensile or compressive load. If the plastic deformation under tensile load is specified as 0,2%, then the term “yield strength 0,2%” (Rp0,2) is used.

Figure 4 – Typical stress-strain diagram for aluminium alloys [2]

Elongation

Often called “relative elongation”. An increase in the distance between two marks on a test specimen that occurs as a result of the specimen deforming under tension until it breaks between the marks.

The amount of elongation depends on the cross-sectional dimensions of the sample. For example, the amount of elongation that is obtained when testing an aluminium sheet specimen will be lower for a thin sheet than for a thick sheet. The same applies to extruded aluminium profiles.

Figure 5 – The effect of alloying elements on strength properties and relative elongation [1]

Elongation A

Percentage elongation after specimen rupture at initial mark distance 5.65 · √ S0, where S0 is the original cross-sectional area of the test specimen. The outdated designation of this quantity A5 is currently not used.

It is easy to check that for round samples this distance between the original marks is calculated as 5·d.

Elongation A50mm

The percentage elongation after specimen rupture, relative to the original length between the 50 mm marks and the constant original width of the test specimen (typically 12.5 mm). In the USA, a distance between marks of 2 inches is used, that is, 50.8 mm.

Shear strength

The maximum specific stress, that is, the maximum load divided by the original cross-sectional area that a material can withstand when tested in shear. Shear strength is usually about 60% of tensile strength. Shear strength is an important quality characteristic of rivets, including aluminium ones.

Figure 6 – Compressive strength, shear strength, load-bearing strength and
hardness of various aluminium alloys [1]

Poisson’s ratio

The relationship between longitudinal elongation and transverse shortening of a section in a uniaxial test. For aluminium and all aluminium alloys in all states, Poisson’s ratio is typically 0.33 [2].

Hardness

The resistance of a metal to plastic deformation, usually measured by making an impression.

Brinell Hardness (HB)

Penetration resistance of a spherical indenter under standardized conditions.

For aluminium and aluminium alloys, the hardness of NV is approximately equal to 0.3 Rm, where Rm is the tensile strength expressed in MPa [2].

If a tungsten carbide indenter is used, the designation HBW is used.

Vickers Hardness (HV)

Penetration resistance of a square pyramid diamond indenter under standardized conditions. Hardness HV is approximately equal to 1.10·HB [2].

Fatigue

The tendency of a metal to fail under prolonged cyclic stress that is well below its tensile strength.

Figure 7 – Difference in fatigue behavior of low-carbon steel and aluminium alloys [3]

Fatigue strength

The maximum stress amplitude that a product can withstand for a given number of loading cycles. Typically expressed as the stress amplitude that gives a 50% probability of failure after a given number of loading cycles [2].

Fatigue endurance

The limiting stress below which a material will withstand a specified number of stress cycles [2].

Sources:

1. TALAT 1501
2. Mechanical Properties of Metals /Amit Bhaduri – Springer, 2018