The Mechanical Properties of Metals

Before selecting an appropriate material for a particular application, the material’s behavior to loads or forces have to be investigated. For example, to choose the material for a shaft in a compressor, it is important to know what kind of forces it will be expected to bear. In other words the mechanical behavior or mechanical properties of the material will come to question. The mechanical properties of a material are carefully studied in experiments carried out under laboratory environment. Testing is based on various factors such as the magnitude of the load, nature of load (tensile, compression, shear), whether the stresses in play are constant over time or rise and fall over a length of time. Often materials are tested to failure to ascertain the maximum load compulsions. These mechanical characteristics are important to the manufacturers, researchers and the users of the material. ASTM standards are standardized techniques designed to test the mechanical properties. With everyone following a set standard, a lot more consistency and reliability can be achieved and the results of different materials (tested under same conditions) can be compared to each other to select the best material. That is why ASTM standards are commonly used to test the mechanical characteristics of materials. The ASTM standards for metals are different than those prescribed for other materials such as ceramics and polymers. This is partly due to the inherent nature of the metals and partly because metals are used in different conditions than these other materials.

Poisson’s Ratio

When tensile stress is applied on to a metal, it elongates with the strain increasing in the direction load/stress is applied. As long as the elongation is elastic, the strain or the deformation that the metal undergoes can be recovered; to put it simply the metal regains its original state once it is unconstrained. Now under these ‘elastic conditions’ as the metal stretches, the two parameters perpendicular to the applied stress (x and y) are compressed while the parameter parallel to applied stress (z) is stretched/elongated. The ratio of compression of the perpendicular parameter to the elongation of the parallel parameter is the Poisson’s ratio. For practical applications a negative sign is added to the ratio to get a positive Poisson’s Ratio. Possion’s Ratio= (-) x/z or (-) y/z.

Elastic/Young’s Modulus

This gives an account of the stiffness of the material. During the course of elastic deformation the stress on the metal is proportional to its strain. It is under these conditions the ratio of stress to strain is a measure of Young’s/Elastic Modulus. Modulus of Elasticity= Stress/Strain This is an important characteristic for design considerations. Metals with higher Elastic Modulus will be stiffer hence will bend less under load. As we know elastic deformation is not permanent and the material can regain its shape but plastic deformation is permanent. The strain achieved in plastic deformation cannot be recovered meaning the material loses its structural integrity.

Yield Point

Metals more or less are designed to work under elastic conditions, otherwise if permanent deformation takes place in the metal, it will not be able to work as effectively as intended. Hence it is important to know the point where further strain would lead to plastic deformation. The word yield point indicates the start of plastic deformation. The magnitude of stress at yield point (yield strength) shows how much stress could a material bear before it starts yielding. Steels and some other metals exhibit upper and lower yield points which will be discussed later.

Tensile Strength

Once in the plastic region, the metal continues elongation with increasing amounts of stress. The point at which the tensile strength in the metal reaches a maximum is called the tensile strength. This is the ‘highest amount of stress’ that the metal can bear. After this elongation proceeds albeit at lower stress values till fracture occurs.

Ductility

The ductility is the measure of plastic deformation until fracture or the amount of energy a material can absorb in plastic deformation. A ductile material will be able to accommodate a lot of strain before it fails. Ductility can be expressed as percentage elongation or percentage reduction in the area. Percentage elongation=

 Percentage reduction in area =

 A brittle material, on the other hand, shows very little deformation before fracture. That is why brittle failures occur abruptly because the material after yielding can sustain very little plastic deformation like ceramics and glasses. In ductile metals, the fracture is prolonged due to their ability to accommodate increased strain. We can actually calculate how much plastic deformation the material can sustain before it fails. Similarly the manufacturers can have an idea of the permissible deformation during fabrication processes.

Resilience

It is the measure of energy a material can put up with before it loses its elasticity. So we are talking about elastic region here so energy absorbed under stress by the material has to be recovered once the constraints on the materials are lifted.

Toughness

Unlike resilience, toughness is the energy absorbed by the metal till it fractures. So here we are talking about the area under whole of the stress-strain curve.

Creep

You can describe creep as a time induced permanent deformation of a metal. Circumstances for creep include constant load over a period of time. It is displayed by all materials but in metals creep comes in to play usually when the metal is subjected to a temperature of ‘0.4 times its melting point’ or higher.

Hardness

It is the resistance a metal can conjure up in response to localized plastic deformation. In other words the resistance to a scratch or indentation is hardness. Various testing methods for hardness exist each having their own ASTM standards. The resistance to localized plastic deformation can be an important design consideration for metals that have to endure cyclic stresses (stress fluctuating between a high and low point). For metals will become more susceptible to fatigue failure once localized plastic deformation has taken place as these regions become home to stress concentration which can ultimately lead to fatigue failure.

Conclusion

It should be noted that there is much more to actually selecting a material for a particular purpose than its mechanical characteristics. Availability, economic viability, government rules and regulations, processing capability and so much more, but mechanical characteristics are a major governing factor in selecting a material.