Mechanical Properties of Metals

Its probably safe to say that metals are used in over 95% of all engineering designs in at least one location. Metals literally shape the engineering world and i reckon that its mainly due to its diversity.
A metal or alloy can be found with the required characteristics for any design task.
In this article I deal with a few of the main mechanical properties of metals, there are others but these remain the most important.

Elasticity

The ability of a metal to elongate when a force is applied and then return to its original size when that force is removed is its elasticity.
It’s important to remember that there is no permanent deformation unless the loading is taken beyond the elastic limit of the metal.
Thus metals or materials that undergo large non-permanent elongations are deemed extremely elastic.
The relationship between simple stress and strain signifies elasticity where the stress component refers to the Force applied and the strain the elongation produced.

How do we test Elasticity?

Tensile Test

Ductility

Ductility and Elasticity are closely linked with the only difference being when they are measured.
Dealing with permanent deformations, ductility gets measured after the elastic limit of the metal has been exceeded.
It measures how much permanent deformation a metal can take before failure.

How do we test Ductility?

Tensile Test

Malleability

Malleability is a measure of ductility but only when the force applied is compressive.
The best example of this is the age old image of iron smiths working on weaponry by pounding the life out of the metal with a hammer.

How do we test Malleability?

Tensile Test

Fracture Toughness

Most metal components are not perfect, inclusions in the metal and tiny cracks are common.
For companies that mass produce, this becomes a problem to monitor and control.
Fracture toughness provides a safety net for designers and manufacturers as it measures the metal’s resistance to crack propagation under extreme loads.
Cracks can also lead to failure at normal operating loads as the component is weakened when cracks are formed.
These localized high stress areas are typically where failure occurs.
Fracture toughness cannot be measured mathematically thus values given are only to be used as guidelines.

How do we test Fracture Toughness?

Charpy V-Notch Impact Test results are used as a benchmark although not entirely accurate.

Tensile Strength

Tensile strength is one of the most common values used in engineering.
It simply measures the maximum amount of tensile stress (pulling) a material can take up until it breaks. This occurs after the elastic limit has been reached in the material.
Tensile strength also gets referred to as the Ultimate Tensile Strength.

How do we test Tensile Strength?

Tensile Test

Compressive Strength

Where tensile strength is pulling, compressive strength is pushing. That is probably the only difference between these two properties for most metals.
Compressive strength will then measure the maximum amount of compressive stress (pushing) a material can take until it breaks after the elastic limit.
The term for this being the Ultimate Compressive Strength.

How do we test Compressive Strength?

Compression Test

Hardness

The hardness of a metal is a measure of how easily it can be indented.
This at first glance may not seem very useful but its close relationship to tensile strength and ductility is priceless.
Testing for hardness is very easy these days and tensile strength values can be derived using standard formulae.
It’s been found in practice that harder steels have high tensile strength but lower ductility and vice versa.

How do we test Hardness?

Rockwell Test
Brinell Hardness Test
Vickers Hardness Test

Impact Strength

Impact strength or impact energy measures the amount of energy required for failure to occur in a metal after sustaining heavy shock loads.
Unlike fracture toughness we can find actual values for impact strength.
Impact strength is measured in energy due to the testing method used.

How do we test Impact Strength?

Charpy V-Notch Impact Test

Fatigue Strength

Fatigue strength is extremely important to engineers and must be fully understood if any design work is to done.
The essence of fatigue strength is that metals will fail at lower stresses when the repeated loading changes in intensity or type. Where loading remains constant we can pretty much assume our tensile or compressive limits are safe.
Fatigue strength is also termed the fatigue limit or endurance limit. It is usually expressed as a stress limit tied to a cycle or period.

How do we test Fatigue Strength?

Fatigue Test

Summary

All the properties listed above require much more attention and research if a complete understanding of common metals is to be established.
Material selection for designs or replacement parts will be much easier if these basics are understood.