Tensile Test, Part Two

Please read the previous part of this article here

The typical stress strain curve for ductile material is given here: 

The points to note from this picture are:

Elastic Limit

  The material is elastic till this point in the curve. The stress strain ratio is constant when the curve is linear within this zone. The material deforms in this zone but regains original shape and size once the load is removed. The working stress is always much below the Elastic Limit

Yield Point:

   Plastic deformation happens at this point. The deformation is permanent in nature and the original shape and size are not restored once the load is removed.

Creep:

   A small amount of creep may come into play due to sudden elongation of material. This effect of creep is not shown in this picture. Creep usually appears for negligible time and is not taken into account.

Ultimate Strength:

    Stress is necessary to obtain stain from the yield point onwards. Ultimate tensile strength (UTS), is the maximum stress that a material can withstand while being stretched or pulled before necking, which is when the specimen's cross-section starts to significantly contract. This is the highest point in the curve

Fracture Point:

    Once the Ultimate stress is crossed, the material starts necking (i.e. non-uniform reduction in area of cross section in specimen). The material then breaks apart.

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AllMyPosts

The picture shown is taken from http://invsee.asu.edu/. Please do contact them for more info

Tensile Test, Part One

Tensile Test:

    Tensile test is a simple test, wherein the specimen in question is subjected to uni-axial load (pulled apart) till failure. This test is used to plot the stress - strain curve there by coming to conclusion about

• Yield point
• Elasticity limit
• Point of  Proportionality
• and lot more factors including, true breaking stress, fracture point load, ...

 

   A sample of specimen is taken, and is pulled apart in apparatus known as Universal testing machine. The length and cross section area of sample are decided as per our needs. Nomenclature of the specimen is shown in figure.

    Once the equipment is set up, the load on the specimen is gradually increased noting down the stress and strain levels till the point of rupture (i.e. fracture). A typical curve for ductile materials is shown here:

In figure,

the points to be noted include: 

• Proportionality zone .. i.e the zone where HOOK's law is valid
• The region where elasticity is exhibited
• The zone where material yields and plastic deformation happens
• The ultimate tensile strength & the uniform elongation of specimen till then
• The fracture point and the local necking which happens before fracture

 with warm regards

AllmYPosts

PS:  Some info has been taken from Wikipedia and from http://invsee.asu.edu/. Please do refer to them for more info

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