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Some Important Mechanical Properties

 

Mechanical properties including elasticity, yield strength, ultimate tensile strength and ductility are usually part of material specifications and are obtained by tensile testing. These properties are described in more detail below as well as being shown on the diagram.

 

Elasticity/Stiffness

This is a measure of elastic deformation of a body under stress which is recovered when the stress is released. The ratio of stress to strain in the elastic region is known as stiffness or modulus of elasticity (Young’s Modulus). When the stress goes beyond the elastic limit the material will no longer return completely to its original dimension.

Yield (or Proof Strength)

Stress needed to produce a specified amount of plastic or permanent deformation. (Usually a 0.2 % change in length)

Ultimate Tensile Strength (UTS)

The maximum stress a material can withstand before fracture.

Ductility

The amount of plastic deformation that a material can withstand without fracture.

The relastionship between stress and strain (load and elongation)

 

The diagram shows the output from a tensile test. These tests are widely used for determining machanical properties of materials. A standard test specimen is axially loaded. The load is increased at a uniform rate and the specimen elongates and finally fractures. The load is recorded as well as the elongation. The results are usually plotted as a stress/strain graph where

stress = load/original cross sectional area
strain = increase in length under load / original length

Properties such as yield, UTS and ductility as well as the stiffness are obtained from the plot.

Hardness

The resistance to abrasion, deformation, scratching or to indentation by another hard body. This property is important for wear resistant applications.

Toughness

This is commonly associated with impact loading. It is defined as the energy required to fracture a unit volume of material. Generally, the combination of a high UTS and a high ductility results in a higher toughness.

Fatigue Strength and Endurance Limit

Fatigue failure results from a repeated cyclic application of stress which may be below the yield strength of the material. This is known to be the most common form of mechanical failure of all engineering components. The number of stress cycles needed to cause fatigue failure depends on the magnitude of the stress. Below a certain stress level material does not fail regardless to the number of cycles. This is known as endurance limit and is an important parameter in many design applications.

Creep Resistance

The plastic deformation of a material which occurs as a function of time when the material is subjected to constant stress below its yield strength. For metals this is associated with high temperature applications but polymers may exhibit creep at low temperatures.

 

 

Typical Tensile Testing Set Up

A Vickers Hardness Testing Machine