# Important Mechanical Properties of Metals (PDF)

Contents

Before discussing about the different important mechanical properties, it is essential to first understand what is a mechanical property.

## What is a Mechanical Property?

The mechanical property of a metal refers to the material’s ability to withstand mechanical forces and loads. It includes properties such as strength, stiffness, elasticity, plasticity, ductility, brittleness, ductility, toughness, elasticity, creep, and hardness.

## Most Important Mechanical Properties

### Strength

It is the ability of a material to withstand external forces without breaking. The internal resistance that a component provides to external forces is called stress. Strength is tested by Universal Testing Machine.

The strength of a material depends on the direction of load and is quantified in four ways:

#### Tensile Strength

Tensile strength is the ability of a metal to resist an opposing force trying to pull it apart, or the maximum tensile load a material can withstand before it fails. Tensile testing (providing elastic limit, elongation, yield point, tensile strength, and area reduction) is used to determine the behavior of metals under real-world tensile loads. Tensile strength is the most commonly used measure of a material’s strength and is measured in pounds per square inch (psi) (kilopascals (kPa)).

#### Compressive Strength

Compressive strength is a metal’s ability to withstand shattering. It can also be defined as the maximum achievable stress that a material will withstand before a given amount of deformation, or the ability of a material to resist fracture applied in a given plane. When a material is compressed, an external load acts towards its center.

#### Shear Strength

The ability of a metal to withstand destructive forces intended to cut or slash, acting in a straight line but not in the same plane. Shear forces act in opposite directions parallel to each other.

#### Torsional Strength

The ability of a metal to withstand an external force tending to twist, or the maximum stress a material can withstand over many load cycles without failure.

### Elasticity

The ability of a metal to restore, resume, or recover to its original shape or original size, shape, and dimensions after being deformed, compressed, or stretched is called elasticity. When an external force acts on the material, the material is deformed by the external force. The elasticity of a material is its ability to return to its original shape after deformation when the stress or strain is released.

The point at which significant damage occurs with little or no increase in stress is called the yield point. Yield strength is the number of pounds per square inch (kilopascals) required to produce a fracture or deformation to yield strength. The elastic limit is reached at the point where the permanent damage begins.

### Elastic Deformation

It is defined as the shape change of material under low stresses that can be restored after the stress is removed. It can also be defines as temporary change in shape that reverses when the force is removed, allowing the object to return to its original shape.

### Plasticity

The ability to be easily shaped is called plasticity. It is closely related to ductility but is the opposite of elasticity. The plasticity of a material is its ability to deform permanently (brittle) without breaking. It is a characteristic of metals to deform inelastically. If it does not burst, it will not return to its original shape and size when the load is released. In this case, the metal is deformed instead of breaking. Plastic deformation occurs only after the elastic range is exceeded.

Plasticity is useful in some processes such as forming, shaping, extruding, and many other hot and cold working operations of metals. It increases continuously with increasing temperature, making it suitable for secondary forming processes. This property has made it possible to reshape various metals and transform them into various products of desired shapes and sizes. The transformation to the desired shape and size is accomplished by applying pressure, heat, or both.

### Plastic Deformation

Plastic deformation is the process by which sufficient stress causes permanent deformation. Sustained stress beyond the elastic limit results in a permanent change in the shape or size of a solid without rupture. No elastic recovery is possible after plastic deformation.

### Hardness

The ability of a material to resist permanent deformation when subjected to external forces is called hardness. Also defined as the ability of a metal to resist localized penetration by indenters of special geometries and materials under specified loads, scratches, abrasion, or another mechanical failure by harder substances. Determining the hardness of a metal is important for determining its strength and heat treatment quality. The hardness of the metal is directly proportional to its tensile strength.

### Toughness

The toughness of a metal is the strength with which it does not break even when a large external force is applied. It is also the ability to absorb energy to the point of destruction and to resist impact, shock, or deformation forces such as bending and torsion. It is the opposite of fragility. Toughness is usually measured by an impact test.  All ductile materials are robust materials capable of withstanding large loads. The hardest metal are the hardest to break.

### Brittleness

The ability of a metal to withstand mechanical stress without plastic deformation is called brittleness. It is also the probability that a fairly small force or impact will cause the material to break. Hardness and brittleness are interrelated to each other. This is because the harder the metal, the more brittle it becomes. On the other hand, plasticity and ductility are opposites. A brittle material that cracks sufficiently to restructure without deformation, is incapable of visible permanent deformation or lacks plasticity. Basically, brittle metals have high compressive strength but low tensile strength.

### Stiffness

Stiffness is the mechanical property of metals that allows the material to resist elastic deformation or deflection within its elastic range. Metals that do not deform easily or do not deform under external force have high rigidity. Structural rigidity is important in many manufacturing processes. Stiffness is often one of the most important properties when choosing a material. To determine stiffness, the elastic modulus of each metal is calculated as a measure of stiffness under tensile and compressive loads. The higher the value of Young’s modulus, the harder the material.

### Ductility

Have you ever tried bending aluminum foil? One thing you’ll easily notice is its ability to stay that way. Ductility is the property of metals that can be drawn into thin wires and permanently bent, twisted, rolled, extruded, or manipulated by tensile stress to change shape without breaking or cracking. The ductility of metals can be determined from tensile tests that measure elongation. Mild steel, copper, aluminum, and zinc are considered good examples of ductile metals.

### Malleability

This property of metals shows how easily the material can be manipulated without breaking. This also involves compressive stress, such as flattening, hammering, or rolling metal into thin plates or sheets of other sizes and shapes.

Malleability is another form of plasticity that allows a material to deform permanently under pressure without breaking. The high formability of aluminum is the reason why it is widely used in the production of thin foils. The behavior of metals changes with temperature, so metals exhibit good ductility or formability at elevated temperatures but become less ductile or formable at room temperature.

For this reason, blacksmiths heat their iron-based products until they glow before hammering them into a shape. Other metals such as gold, silver, tin, wrought iron, alloy steel, mild steel, and lead are good examples of highly malleable metals. Gold is highly malleable and can be rolled into sheets thin enough to let light through.

### Creep

Creep is defined as slow progressive deformation over time under constant load. It can also be defined as failure or rupture of a material under constant load at elevated temperature for a period of time. The force for a given strain rate at constant temperature is the creep strength of the material. The simplest type of creep deformation is viscous flow. Some metals typically exhibit creep at elevated temperatures, but plastics, rubbers, and similar amorphous materials are very sensitive to creep.

### Fatigue

When a material is repeatedly stressed, it will fail at stresses below the yield point stress. This type of material failure is called fatigue. Failures are caused by progressive cracks and are usually very fine and microscopic in size. This property is taken into account when designing shafts, connecting rods, springs, gears, etc.

### Machinability

It is the material property that refer to the relative cases in which the material can be cut. The machinability of a material can be measured in various ways. Compare tool life when cutting different materials, the thrust required to remove material at a specific rate, or the energy required to remove a unit volume of material. Note that brass is easier to machine than steel.

### Resilience

A material’s ability to absorb energy without changing its shape is called elasticity. e.g. steel. By calculating the area under the stress-strain curve up to the elastic point, the yield (elastic compliance) of the material is determined.