MATERIAL TESTING & FAILURES CLASS NOTES FOR MECHANICAL ENGINEERING

 

 MATERIALS TESTING & FAILURES

CLASS NOTES FOR MECHANICAL TESTING

Perform tests on materials to determine the followings:

1.Strength of the material

2.Internal / external defects

TESTING FOR STRENGTH

 Testing tells to use a material under a load in which it has the highest strength. Use a material under a tensile load  If  strong in tension. Continue testing till fracture takes place. These tests determine the strength of a material.

DESTRUCTIVE TESTS

 It means causing failure of a material under one type of load at one time. Such tests are the followings:

(a)    Tensile test to determine the tensile strength

(b)   Compressive test to determine the compressive strength

(c)    Shear test to determine the shear strength

(d)   Bending test to determine the bending strength

(e)    Fatigue test to determine the endurance strength

(f)    Impact test to determine the toughness strength

(g)   Hardness test to determine the hardness

APPLICATIONS OF DESTRUCTIVE TESTS

Use a material under a tensile load if strong in tension. In real applications, more than one type of load acts on a component. Do not use brittle materials in pure tension. Brittle materials are very weak in tension. Brittle materials fail in tension easily. Ductile materials are equally strong in tension and compression. These are relative weak in shear. A ductile material under a pure tensile load fails in shear. Under a pure tensile load, shear stresses exist on planes  at 450 to the planes of pure tension.

TENSION TEST

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It is one of the destructive tests. Do tension test  on ductile materials using standard specimens. Fix the specimen in the Universal Testing Machine. Apply tensile load gradually till the specimen fractures. Measure the increase in length with an extensometer fixed over a gage length. Calculate strains by increase in length divided by the original length. Calculate Stresses using divide load by the original area π d2/4.Then draw the stress strain curve. Various strengths measured from the curve are as under.

(a)    Elastic limit strength

Where straight line of stress strain ends

(b)   Yield strength

Where strain is more for lesser stress or where curve starts becoming steep

(c)    Ultimate tensile strength

Top point of the curve

(d)   Braking Strength

Corresponding to load at which the test piece breaks with a ‘THUD’ sound.

NON-DESTRUCTIVE TESTING

This testing is to know the internal defects. Fracture of the specimen is not there. Various non-destructive tests are as under:

(a) Magnetic Particle Testing (MT)

(b)   Ultrasonic Testing (UT)

(c) Electromagnetic Testing (ET)

(d) Laser Testing Method (LM)

(e) Leak Testing (LT)

(f) Liquid Penetrate Testing (PT)

(h) Radio-graphic Testing (RT)

FAILURE OF DUCTILE AND BRITTLE MATERIALS

Failure of Ductile Materials

A ductile material fails as soon as it crosses elastic limit. Elastic failure of ductile materials is not a physical failure. Components does not meet a breakdown.

(i)Mild steel under a tensile load fails by pure shear i.e.at 450 to the axis.

(ii) Ductile materials under fatigue loads fail like a brittle material (normal to the line of action of the load).

Failure of ductile materials is far from satisfactory by Guest theory. It agrees almost 100 % with the experimental results by maximum shear strain energy theory. Use Guest theory in design of  ductile material components as it is simple to use.

Failure of Brittle Materials

A brittle material fails on physical breaking.

(i)                  Breaking takes place by tensile fracture (normal to load) when tensile stress exceeds the ultimate tensile strength

(ii)                Brittle material fails by shear when compressive stress attains the maximum value. The fracture occurs on an inclined plane but not on the plane of maximum shear stress (450). Therefore this failure is not a pure shear failure.

Failure of Isotropic Brittle materials ( Glass and Bakelite) is satisfactory by Rankine Theory. However Maximum Principal Strain theory (St. Venant’s Theory) is more appropriate.

Non-isotropic Brittle Materials (Wood)  failure depends on the type of load. A single theory is not sufficient to explain the failure of the same anisotropic material under tensile/compressive/shear  loads.

Wood under compression fails by shear.

Cast iron under compression fails by shear at 500 to the axis of the load.

Cement under compressive load fails at 700 to the axis of the load.

Therefore, it is uncertain to know theory of elastic failure applicable for a non-isotropic material.