The mechanical properties of steel

The alloys and the heat treatment used in the production of steel result in it having different values and strengths. Testing must be accurate to determine the properties of the steel and to ensure adherence to standards.

Different steels have different values of strength and toughness depending on the alloys made and the heat treatments used. Testing methods are important to determine values and to ensure that standards are adhered to. Methods of testing determine the yield strength, ductility and stiffness through tensile testing, toughness through impact testing, and hardness through resistance to penetration of the surface by a hard object.

Tensile testing is a method of evaluating the structural response of steel to applied loads, with the results expressed as a relationship between stress and strain.

Specially shaped specimens of steel are subjected to a tensile force which gradually elongates the steel. The force is increased and the extension of the steel carefully measured.

The stress on the metal is evaluated by dividing the value of the force applied by the cross-sectional area of the specimen.

Stress = LOAD in Newtons / CROSS-SECTIONAL AREA of specimen in mm2

Strain is measured by calculating the increase in length of the specimen as a proportion of the original length.

Strain = INCREASE IN LENGTH in mm / ORIGINAL LENGTH in mm

The relationship between stress and strain is a measure of the elasticity of the material.

If a load is applied to a material, and that material fully recovers, that is it returns to its original length once the load is removed, the elastic limit of the material has not been reached. However, if the material does not recover on removal of the load, it must have exceeded its elastic limit and will be said to have deformed permanently and started to behave plastically.

The elastic behaviour is characterised as the ratio of stress to strain, and is referred to as Young's modulus.

The elastic behaviour is characterised as a relationship between the stress applied and the resultant strain, and is expressed mathematically as:

STRESS / STRAIN

otherwise known as Young's modulus. This relationship allows comparisons to be made between different materials.

As the value of Young's modulus changes between materials, so then does their elastic behaviour, which is a real indicator of their stiffness. Steel has a high value of Young's modulus which is about 205 kN/mm2 (approximately three times the value for aluminium).

The distinction between elastic and plastic behaviour can be seen from the stress-strain curve.

To explain this behaviour more fully, stress is plotted against strain in the following figure.

Stress against strain

Stress against strain

 

The initial part of the curve shows a linear relationship between stress and strain. This is the elastic region. The slope of this initial part of the curve is a measure of the material stiffness (Young's modulus); the steeper the slope, the smaller the deformation for a given load.

Beyond the initial linear part of the stress-strain curve, further increase in load produces very large increases in strain. This is the PLASTIC region.

Although there is still a considerable reserve of strength beyond this level of loading, the yield stress is normally used as the failure criterion when assessing the strength of a structural member because of the very large deformations associated with plastic behaviour. Furthermore, strains within this range are associated with permanent deformations.

Beyond the plastic region, further increases in strain are associated with more pronounced increases in stress. This is due to strain hardening.

Eventually, the specimen will reach a maximum level of stress and fail. This is the ultimate tensile strength of the steel.

Typical stress strain curve and idealised stress strain curve

Typical stress strain curve and idealised stress strain curve

 

Strength is a very important characteristic of steel, but other properties must also be considered when considering structural performance.

The yield strength represents the point where behaviour changes from elastic to plastic. The ultimate tensile strength is the end point of the stress-strain curve representing rupture of the material. Both yield strength and ultimate tensile strength can be increased by altering the chemical composition of the steel - particularly the carbon content.

Yield strength can also be improved by mechanically working the steel, although this reduces its ductility - the ability of the steel to sustain high strains prior to failure

At very cold temperatures some steels may lose ductility.

Under some conditions steel may exhibit a brittle, rather than ductile, mode of failure. This is not normally a problem in building structures since modern steels are manufactured so as to avoid such behaviour. However, in extremely cold conditions - for instance exposed steelwork where temperatures start to fall well below 20°C, the brittleness of steel will increase rapidly.

Steels with high carbon contents are more vulnerable under stress, and stress concentrations due to imperfections or details should be avoided.

In these low temperature conditions steel alloys are carefully specified; typically the manganese-carbon ratio is increased and the nickel content reduced, and specialist advice must always be sought.

The behaviour of steel is also notch-sensitive. Fixing details which can impose defects on the steel, for instance from screw threads, must therefore be avoided.

Knowledge of failures under these conditions comes from experience in naval engineering where brittle fracture was a phenomenon studied in ships in the North Sea during the war, and brought to light particularly by the failure of the Liberty boats

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