Material Properties

Mechanical Properties of Materials: Explained

by Umair Ahmad Khan

Confused about which word describes what kind of material property? People often confused the words used to explain the properties of a material. With these words, you are able to describe pretty much any material. So, lets see what words are used to describe mechanical properties of materials and what do they mean?

These words are STIFFNESS, STRENGTH, DUCTILITY, BRITTLENESS, TOUGHNESS and HARDNESS.

With the understanding of these terms, you would also be able to understand why certain materials are used for certain applications.

Now, before we go into describing these terms, we need to understand some basic key terms which are STRESS, STRAIN and TENSILE TEST.

TENSILE TEST:

Tensile Test is a mechanical test which tells us the relationship between the FORCES and their EFFECTS on a sample of material.

It is performed by pulling an elongated sample of a material with tensile force on both sides. Sample first elongates and then succumbs to failure.

Mechanical properties of materials
TENSILE TEST

It gives the relationship between FORCES and EFFECTS in the form of STRESS-STRAIN Curve.

STRESS:

Stress is the FORCE applied to a material in terms of per unit surface area with SI units of meter/squared area. In tensile test, the stress is the elongating force applied on two ends of sample.

STRAIN:

The EFFECT of force on a material is STRAIN. It is the change in shape parameter whether it is length or volume of sample per initial shape parameter. In case of tensile test, that shape parameter is LENGTH. So, STRAIN then is “CHANGE IN LENGTH/INITIAL LENGTH”

STRESS-STRAIN CURVE:

Stress-Strain curve is the graphical representation of a material’s behavior to applied force. That it means it is a graph with STRESS on vertical axis and STRAIN on horizontal axis.

This graph explains the mechanical properties of materials.

The graph is obtained gradually by using the values of stress and strain from the tensile test. In below figures dots represents atoms.

  • Now, Initially up-to a magnitude of force dependent upon material nature the material does not change its shape. Its called the ELASTIC REGION and that magnitude to which this behavior occurs is called ELASTIC LIMIT. Permanent Shape change does not occur because the force is not enough to cause the atoms to be moved away from each and just when force is released, atoms come back to their original location because inter-atomic bonding.
  • The end of elastic limit is a point called YIELD POINT. After this point, the material deforms and changes its shape permanently. This happens because stress applied becomes much higher than the magnitude of inter-atomic or molecular bonding fails to take the material back to its original shape. The STRESS magnitude at yield point is called YIELD STRENGTH. Shape changes but still atomic-interaction exists but at a new level.
  • When the plastic deformation or permanent shape changing starts, sample in a tensile test elongates until a point called ULTIMATE STRENGTH. Ultimate strength means that most amount of strength a material can handle. After this point less and less stress can even result in huge strain. Now, material instead of elongating, it starts to decrease in cross section. This is called NECKING, leading to FRACTURE, where material sample breaks. Fracture is also called as ULTIMATE FAILURE.
  • So, as far as STRENGTH is considered, there are two kinds of it: YIELD STRENGTH and ULTIMATE STRENGTH.

STRESS-STRAIN Curve for this experiment is shown below:

Mechanical properties of materials

A lot of useful information about the mechanical properties of materials can be extracted from this graph. Lets see.

Mechanical Properties of Materials

Now, the regions on STRESS-STRESS strain represent a different properties. Lets explore one by one:

UP-TO YIELD STRENGTH

Material obeys famous Hooke’s Law up-to the yield strength or elastic limit. Hooke’s Law is given by “STRESS = K(STRAIN)“. Here, K is called YOUNG’S MODULUS or STIFFNESS MODULUS.

Value of Young’s modulus shows how the how much the material will temporarily change its shape with respect to applied stress.

Now, materials like Steel etc show much less temporary elastic deformation as compared to rubber on same level of stress.

So, that means, steel is much stiffer than rubber.

Meaning STIFFNESS is the ability of a material to resist change in shape.

YIELD STRENGTH TO FRACTURE

Ahead of yield strength, materials show permanent deformation until they fracture.

Now, this region highlights other two properties, DUCTILITY and BRITTLENESS.

Now, after yield strength, some materials show considerably higher magnitude of strain or change in shape as compared to others before fracture.

For example, a steel bar bends and shows much more change in shape as compared to glass which does not change much in shape.

Mechanical properties of materials
Steel can shaped by forging showing ductility-Image by Uwe Baumann from Pixabay

Steel is ductile and bends. Therefore, can be used in forging process.

Mechanical properties of materials
Crack formation in glass represents BRITTLENESS-Image by Zdeněk Tobiáš from Pixabay

Now, in STRESS-STRAIN curve, the area under the curve represents ENERGY ABSORBED until fracture.

Mechanical properties of materials
Energy Absorbed

Now, this energy absorbed gives us information about another property, TOUGHNESS.

Toughness is the amount of energy absorbed by the material until fracture. The more energy it is able to absorb more tough it is.

Diamond does not fracture easily and is considered to be one of toughest materials in existence. On the other aluminum is not so tough and cannot absorb too much energy.

HARDNESS

Hardness is one of few properties which cannot be calculated by Tensile Test. It is directly related to STIFFNESS and YIELD STRENGTH of a material.

Tensile test cannot show this property because hardness comes to play when compression force is applied to materials.

Hardness is the resistance to permanent shape change when a compression force is applied on a material”

It is used to describe how difficult it is to scratch or dent materials.

Usually, hardness is measured by Rockwell Hardness Test. It involves four steps.

  1. First, a minor load is applied with an indenter. This creates a reference point.
  2. Then, a major load is applied which indents the material.
  3. Finally, major load is removed while keeping the minor load.
  4. The difference in depth of cut, before and after removal of major load is then used to calculate the hardness of a material.

So, there you have it. Six basic mechanical properties of materials.

OTHER PROPERTIES

Apart from these six basic properties, there some other terms that are used to describe the mechanical properties of materials which are actually related to these properties. They are:

  • Malleability: Literal meaning is “being easy to shape”. It is the ability to be shaped permanently by forging, rolling or any other method of applying pressure.
  • Durability: Ability of a material to withstand wear and tear.
  • Stability: Ability of a material to resist shape change. A general term.

WRAPPING UP

So, lets summarize:

  • Higher stiffness means less change in shape.
  • Ductility means liable to be deformed plastically (permanently) before fracture, while Brittleness means no plastic deformation.
  • Toughness means the magnitude of absorbed energy before fracture.
  • Strength is of two types: Yield Strength means amount of stress that leads to plastic deformation and ultimate strength is amount that necking in the material.
  • Hardness, means the ability to withstand abrasiveness and scratches from compression forces.

So, which terms you had confused before reading this article? Comment Below

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