Short Notes: Testing of Hardness and Impact Strength | Short Notes for Civil Engineering - Civil Engineering (CE) PDF Download

 Page 1


Testing of Hardness and Impact Strength 
Impact strength
The impact strength is the ability of a material to absorb shock and impact energy 
without breaking. The impact strength is calculated as the ratio of impact 
absorption to test specimen cross-section. •
• Impact strength is the resistance of a material to fracture under dynamic load.
• It is a complex characteristic which takes into account both the toughness 
and strength of a material.
• In S.l. units the impact strength is expressed in Mega Newton per m2 
(MN/m2).
• It is defined as the specific work required to fracture a test specimen with a 
stress concentration in the mid when broken by a single blow of striker in 
pendulum type impact testing machine.
• Impact strength is the ability of the material to absorb energy during plastic 
deformation.
• Brittleness of a material is an inverse function of its impact strength.
• Coarse grain structures and precipitation of brittle layers at the grain 
boundaries do not appreciably change the mechanical properties in static 
tension, but substantially reduce the impact strength.
• Impact strength is affected by the rate of loading, temperature and presence 
of stress raisers in the materials.
• It is also affected by variation in heat treatment, alloy content, sulphur and 
phosphorus content of the material.
• Impact strength is determined by using the notch-bar impact tests on a 
pendulum type impact testing machine.
Page 2


Testing of Hardness and Impact Strength 
Impact strength
The impact strength is the ability of a material to absorb shock and impact energy 
without breaking. The impact strength is calculated as the ratio of impact 
absorption to test specimen cross-section. •
• Impact strength is the resistance of a material to fracture under dynamic load.
• It is a complex characteristic which takes into account both the toughness 
and strength of a material.
• In S.l. units the impact strength is expressed in Mega Newton per m2 
(MN/m2).
• It is defined as the specific work required to fracture a test specimen with a 
stress concentration in the mid when broken by a single blow of striker in 
pendulum type impact testing machine.
• Impact strength is the ability of the material to absorb energy during plastic 
deformation.
• Brittleness of a material is an inverse function of its impact strength.
• Coarse grain structures and precipitation of brittle layers at the grain 
boundaries do not appreciably change the mechanical properties in static 
tension, but substantially reduce the impact strength.
• Impact strength is affected by the rate of loading, temperature and presence 
of stress raisers in the materials.
• It is also affected by variation in heat treatment, alloy content, sulphur and 
phosphorus content of the material.
• Impact strength is determined by using the notch-bar impact tests on a 
pendulum type impact testing machine.
This further helps to study the effect of stress concentration and high velocity 
load application.
Factors Affecting Impact Strength:
• If the dimensions of the specimen are increased, the impact strength also 
increases.
• When the sharpness of the notch increase, the impact strength required 
causing failure decreases.
• The temperature of the specimen under test gives an indication about the type 
of fractures like ductile, brittle or ductile to brittle transition.
• The angle of the notch also improves impact-strength after certain values.
• The velocity of impact also affects impact strength to some extent.
Hardness:
• Hardness is the resistance of material to permanent deformation of the 
surface. It is the property of a metal, which gives it the ability to resist being 
permanently deformed (bent, broken or shape change), when a load is 
applied.
• The hardness of a surface of the material is, a direct result of inter atomic 
forces acting on the surface of the material.
• Hardness is not a fundamental property of a material, but a combined effect 
of compressive, elastic and plastic properties relative to the mode of 
penetration, shape of penetration etc.
The main usefulness of hardness is, it has a constant relationship to the tensile
strength of a given material and so can be used as a practical non-destructive test.
Hardness measurement can be in Macro, Micro & Nano-scale according to the
forces applied and displacements obtained. •
• Macro-hardness measurement: Measurement of the Macro-hardness of 
materials is a quick and simple method of obtaining mechanical property 
data. The Macro-hardness measurement will be highly variable and will not 
identify individual surface features. It is here that micro-hardness 
measurements are appropriate.
• Micro hardness measurement: Micro hardness is the hardness of a material 
as determined by forcing an indenter into the surface of the material under 
load, usually the indentations are so small that they must be measured with a 
microscope. Micro hardness measurements are capable of determines the 
hardness of different micro constituent with in a structure.
• Nano hardness Measurement: Nano hardness tests measure hardness by 
using indenter, on the order of nano scale. These tests are based one new 
technology that allows precise measurement and control of the indenting 
forces and precise measurement of the indentation depth.
Hardness Measurement Methods:
• Methods of hardness testing depends on following factors:
• The direct thrust of some form of penetrator into the metal surface,
• The ploughing of the surface as a stylus is drawn across it under a controlled 
load.
Page 3


Testing of Hardness and Impact Strength 
Impact strength
The impact strength is the ability of a material to absorb shock and impact energy 
without breaking. The impact strength is calculated as the ratio of impact 
absorption to test specimen cross-section. •
• Impact strength is the resistance of a material to fracture under dynamic load.
• It is a complex characteristic which takes into account both the toughness 
and strength of a material.
• In S.l. units the impact strength is expressed in Mega Newton per m2 
(MN/m2).
• It is defined as the specific work required to fracture a test specimen with a 
stress concentration in the mid when broken by a single blow of striker in 
pendulum type impact testing machine.
• Impact strength is the ability of the material to absorb energy during plastic 
deformation.
• Brittleness of a material is an inverse function of its impact strength.
• Coarse grain structures and precipitation of brittle layers at the grain 
boundaries do not appreciably change the mechanical properties in static 
tension, but substantially reduce the impact strength.
• Impact strength is affected by the rate of loading, temperature and presence 
of stress raisers in the materials.
• It is also affected by variation in heat treatment, alloy content, sulphur and 
phosphorus content of the material.
• Impact strength is determined by using the notch-bar impact tests on a 
pendulum type impact testing machine.
This further helps to study the effect of stress concentration and high velocity 
load application.
Factors Affecting Impact Strength:
• If the dimensions of the specimen are increased, the impact strength also 
increases.
• When the sharpness of the notch increase, the impact strength required 
causing failure decreases.
• The temperature of the specimen under test gives an indication about the type 
of fractures like ductile, brittle or ductile to brittle transition.
• The angle of the notch also improves impact-strength after certain values.
• The velocity of impact also affects impact strength to some extent.
Hardness:
• Hardness is the resistance of material to permanent deformation of the 
surface. It is the property of a metal, which gives it the ability to resist being 
permanently deformed (bent, broken or shape change), when a load is 
applied.
• The hardness of a surface of the material is, a direct result of inter atomic 
forces acting on the surface of the material.
• Hardness is not a fundamental property of a material, but a combined effect 
of compressive, elastic and plastic properties relative to the mode of 
penetration, shape of penetration etc.
The main usefulness of hardness is, it has a constant relationship to the tensile
strength of a given material and so can be used as a practical non-destructive test.
Hardness measurement can be in Macro, Micro & Nano-scale according to the
forces applied and displacements obtained. •
• Macro-hardness measurement: Measurement of the Macro-hardness of 
materials is a quick and simple method of obtaining mechanical property 
data. The Macro-hardness measurement will be highly variable and will not 
identify individual surface features. It is here that micro-hardness 
measurements are appropriate.
• Micro hardness measurement: Micro hardness is the hardness of a material 
as determined by forcing an indenter into the surface of the material under 
load, usually the indentations are so small that they must be measured with a 
microscope. Micro hardness measurements are capable of determines the 
hardness of different micro constituent with in a structure.
• Nano hardness Measurement: Nano hardness tests measure hardness by 
using indenter, on the order of nano scale. These tests are based one new 
technology that allows precise measurement and control of the indenting 
forces and precise measurement of the indentation depth.
Hardness Measurement Methods:
• Methods of hardness testing depends on following factors:
• The direct thrust of some form of penetrator into the metal surface,
• The ploughing of the surface as a stylus is drawn across it under a controlled 
load.
Measurements of hardness are the easiest to make and are widely used for 
industrial design and in research. As compared to other mechanical tests, where 
the bulk of the material is involved in testing, all hardness tests are made on the 
surface or close to it. The following are the hardness test methods: •
• Rockwell hardness test
• Brinell hardness
• Vickers
• Knoop hardness
• Shore