Mechanical Properties of Metals Explained

June 7, 2021

In the world of manufacturing, whether you're still on the verge of putting your shop together or you're already an established manufacturer, it is of the utmost importance to thoroughly comprehend and fully grasp the mechanical properties of metals for better prediction of the manufacturing outcome.

Mechanical Properties of Metals
experimenting steel properties

Over the years, welded machines, tools and structures are usually subject to a serious amount of force. This is where the understanding of the mechanical properties of metals comes in handy.

It is very significant that the mechanical properties of metals are being considered while designing machine components to make sure that the range of usefulness is vast, guarantee a high performance and be able to establish the service that can be expected.

To produce the strongest and toughest welds possible, precision is a must! The weld should provide the same mechanical properties equal to or exceeding those of the base metals being joined.

Now, let's dig deeper and fully understand what you should know in the world of welding!

What are the Mechanical Properties of Metals? 

Strength

tensile strength

The ability of a metal to withstand deformation from an external force which is also known as mechanical load is called the strength of that material. 

The strength of the material is contingent upon the directions in which it is loaded which is quantified into four:

  • Tensile Strength: is the metal’s ability to counteract opposing forces attempting to pull it apart or the maximum load in tension a material will withstand before breaking. A tension test (which provides the elastic limit, elongation, yield point, yield strength, tensile strength, and the reduction in area) is used to determine the behavior of a metal under an actual stretch loading that develops the metal's maximum strength. The tensile strength is the most commonly used value for the strength of a material and is given in pounds per square inch (psi) (kiloPascals (kPa)).
  • Compressive Strength: The capability of a metal to withstand being crushed. Said another way the maximum attainable load in compression a material will withstand prior from a predetermined amount of deformation, or the ability of a material to withstand destruction acting in a given plane. When a material undergoes compression, external loads act on it in the direction of its centerpoint. 
  • Shear Strength: The capacity of a metal to tolerate forces of destruction trying to slice or cut it apart acting in a straight line but not in the same plane. Shear forces act parallel to one another in contradicting directions.
  • Torsional strength: The ability of a metal to withstand external forces attempting to twist it or the maximum load a material can withstand without failure during a large number of reversals of load.

The maximum stress that any material will withstand before deformation is called its ultimate strength. Therefore, the higher the ultimate strength of the material is, the greater the load it can carry. 

Elasticity

The ability of metal to return, resume or regain its normal shape or original size, shape, and dimensions after being deformed, compressed, stretched, or pulled out of shape is called elasticity. 

When a material has an external force applied to it, the external force causes the material to deform. The elasticity of a material is its power of returning back to its original form after deformation when the stress or load is released.

The point at which definite damage occurs with a small amount or no increase in load is called the yield point. 

The yield strength is the number of pounds per square inch (kiloPascals) it requires to produce destruction or deformation to the yield point.

At the point at which permanent damage starts, that's when the elastic limit is reached. 

Plasticity

plasticity

The quality of being easily shaped or molded is called plasticity which is closely related with ductility but the reverse of elasticity.

The plasticity of a material is its capacity to undergo some permanent deformation without rupture(brittle). It is the property of a metal that enables it to deform non-elastically; without fracture, they do not regain their original shape and size when the applied load is released.

In that case, the metal undergoes a certain degree of deformation without failure.

After the elastic range has been exceeded, only then will the plastic deformation take place. 

Plasticity is useful in a couple of processes like forming, shaping, extruding, and many other hot and cold working of metals, continuously increasing with increasing temperature, thus being a favorable property of metal for secondary forming processes.

Due to this property, various metals can now be transformed and converted into different products of required shape and size. This transformation into desired shape and size is affected either by the application of pressure, heat, or both.

Hardness

hardness

A material’s power to withstand a permanent change in shape when acted upon by an external force is known as hardness.

It is also defined as the metal's ability to resist to local penetration of an indenter of special shape and material under a given load, scratching, abrasion, or any other mechanical destruction by a harder substance.

Identifying a metal’s hardness is vital to help identify its strength and quality of heat treatment. 

The hardness of a metal is directly proportional to tensile strength and can be measured by the following testing machines:

  • The Brinell: one of the most popular types of machines to determine the hardness of the material where a hardened ball steel ball is forced into the surface of the specimen. It provides a Brinell Hardness Number (BHN) as the value of the metal's hardness.
  • Rockwell Hardness Tester: This test is computed upon the difference between the depth to which a test point is driven into a metal by a light load and the depth to which it is driven in by a heavy load.
  • Vickers hardness machine: directly reads when a diamond is pressed into the metal.
  • Shore scleroscope: this test utilizes a small diamond-tipped hammer fall by its own weight which will bounce from the surface of the specimen providing a hardness measure. 

Toughness

Toughness of a metal is the state of being strong enough not to break when a significant amount of external force is applied.

It is also the ability to absorb energy up to destruction and to resist shock, impact or any deformation forces like bending, twisting, torsion, etc. 

The toughness property is considered to be the combination of both high strength and medium ductility but the opposite of brittleness. It is measured by an impact test.

All ductile materials such as steel and steel alloys (manganese steel, wrought iron, mild steel, etc.) are tough materials that can withstand considerable stress, slowly or suddenly applied, and which will deform before failure.

So to speak, the toughest metals are the hardest to break.

Brittleness

stiffness

The ability of a metal that can counteract mechanical load without plastic deformation is called Brittleness. It is also the probability that a material will fail or fracture under a fairly small shock, force, or impact.

Hardness and brittleness are directly related with each other as a metal’s hardness is increased so does its brittleness. On the other hand, it is the opposite of plasticity and ductility.

A brittle material cracks in a way that it could be put back together without any deformation, cannot be visibly deformed permanently or one that lacks plasticity.

Basically, metals that are brittle have high compressive strength but low in tensile strength.

Stiffness

Stiffness also known as rigidity is the mechanical property of metals that allows the material to resist elastic deformation or deflection within the elastic limit.

A metal which suffers slight deformation or not deformed at all under external force has a high degree of stiffness.

Steel is stiffer or more rigid than aluminum.

The stiffness of a structure is important in many manufacturing procedures. The level of stiffness is often one of the primary properties when selecting a material.

To determine the level of stiffness, the modulus of elasticity or young’s modulus which is the measure of stiffness for tensile and compressive loads will be calculated for the respective metal. 

The higher the value of Young’s modulus, the stiffer the material is.

Ductility

ductile

Have you tried bending a piece of aluminum foil? One thing that you can easily notice is its ability to remain that way. It's because it is ductile.

Ductility is the property of a metal that enables it to be stretched out into a thin wire and be permanently bent, twisted, rolled or extruded or otherwise manipulated by a tensile stress to change its shape without breaking or cracking and to retain the changed shape after the load has been released.

The ductility of a metal can be identified from the tensile test where the percent of elongation is determined.

Soft steel, copper, aluminum, and zinc are considered a good example of ductile metals.

Malleability

Malleability

Manufacturing companies need to push metals beyond their limits to form new shapes and become a new useful product. This is where malleability is of a good use. 

This property of metal indicates how easily a material can be maneuvered without breaking. This also refers to compressive stress, as in flattening, hammering and rolling of metals into thin plates and sheets of any other size and shape.

Malleability is another form of plasticity, and it allows a material to deform permanently under compression without breaking. 

Aluminum’s high malleability is why it’s so widely used to make thin foil.

Since a metal’s behavior is altered depending on the temperature, thus a metal might have good ductility or malleability at high temperatures, but poor ductility or malleability at room temperature.

This is why blacksmiths heat iron-based products until glowing before hammering them into shape.

Various metals such as gold, silver, tin, wrought iron, steel alloys, soft steel and lead are good examples of metals that possess high malleability. Gold has superior malleability that enables it to be rolled into sheets thin enough to transmit light.

Cohesion

It is a mechanical property of metal which they enable metal to resist from being broken into a fragment.

The cohesive strength of a material is the strength of bonding between the particles or surfaces that make up that material (Keary 1996). The cohesive strength is more specifically the inherent shear strength of a plane across which there is no normal stress.

Impact strength

inspection of iron pipe

The impact strength is the ability of a metal to resist suddenly applied external forces. The impact strength of a metal evaluates the ability of the metal to resist impacts.

Some metal may possess an adequate level of ductility under static forces but may fail under dynamic loads or impact.

Impact strength can be determined by measuring the energy absorbed in the fracture through the following tests:

  • The Charpy test: uses a specimen which is a beam supported at both ends and holds a notch in the center. The specimen is placed on supports and struck with a pendulum on the side opposite the notch. The accuracy and location of the notch is of extreme importance. There are several types of Charpy specimens; the V-notch type is the most popular.
  • Izode test. This test uses the pendulum-testing machine calculated equal to the weight of the pendulum times the height at which the pendulum is released and the height to which the pendulum swings after it has hit the specimen.

Fatigue

Fatigue represents the tendency to fracture under cyclic loading, or it is the lack of sufficient power to withstand repeated and/or continuous application and removal of stress. 

The machine tool components must resist such a significant amount of fatigue stress, and this must be considered while designing machine tool components, high-speed Aero engines, and turbines where they are expected to give long services under cyclic loads.

Creep

Creep is defined as the slow and progressive deformation with time at constant stress, or the failure or fracture of the material under constant stress at high temperature over a period of time.

The force for a specified rate of strain at constant temperature is the material's creep strength. The simplest type of creep deformation is viscous flow.

Some metals are usually displaying creep at high temperature, whereas plastic, rubber, and similar amorphous materials are very temperature sensitive to creep.

Conclusion

This knowledge of the mechanical properties of metals is indeed very vital in producing a top quality project where you can fully guarantee safety and stability. 

This will allow you as a welder to determine the range of usefulness of a metal and the quality service that can be expected from it. It will surely help you construct a safe, sound structure that meets engineering specifications.

As you plan on doing a project, thorough analysis of the environment in which the product will perform, including temperature, moisture, load requirements and finding a balance between each mechanical property is a must.

Another factor to consider is the product's intended use. 

Enlist the things that you want to expect from the project that you are planning to do. Make sure that all relevant factors have been considered before choosing an alloy to use. 

We really hope that we have been able to help you understand your doubts and questions about the mechanical properties of metals.

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