Gaziantep universityGaziantep university Department of Civil Engineering Department of Civil Engineering

Impact Test on MetalsImpact Test on Metals Impact Test on MetalsImpact Test on Metals

Present By

Omar Adnan

What is Impact or Impact Force What is Impact or Impact Force ?? In mechanics, an impact is a high force or shock applied over a short time In mechanics, an impact is a high force or shock applied over a short time

periodperiod when two or more bodies collide. Such a force or acceleration when two or more bodies collide. Such a force or acceleration usually has a greater effect than a lower force applied over a usually has a greater effect than a lower force applied over a proportionally longer period. proportionally longer period.

At normal speeds, during a perfectly inelastic collision, an object struck At normal speeds, during a perfectly inelastic collision, an object struck by a projectile will deform, and this deformation will absorb most or all by a projectile will deform, and this deformation will absorb most or all of the force of the collisionof the force of the collision

The effect depends critically on the relative velocity of the bodies to one The effect depends critically on the relative velocity of the bodies to one another.another.

Example : Example : car crush, wind force, earthquakecar crush, wind force, earthquake etc.etc.

Impact test:- Is the test used for observing the mechanics that a material will exhibit when it experiences a shock loading that causes the specimen to immediately:- (1)deform,(2)fracture or rupture completely

Why impact test is necessary? Why impact test is necessary?

To measure (A):-To measure (A):-Impact energy Impact energy - the energy required to break a - the energy required to break a

standard sized sample with a sudden, sharp load.standard sized sample with a sudden, sharp load.

Joule is the unit used in measure the impact energyJoule is the unit used in measure the impact energy

Find (B)Find (B)ToughnessToughness - the impact test is used to find a material's - the impact test is used to find a material's

toughness , it's ability to absorb energy without breaking.toughness , it's ability to absorb energy without breaking.

Or the material's resistance to fracture when stressed Or the material's resistance to fracture when stressed

To determine (C)the tendency of a material in a To determine (C)the tendency of a material in a brittle manner brittle manner .It .It

detects difference between materials which are not in a tension test.detects difference between materials which are not in a tension test.

These values are important for the selection of materials that will be These values are important for the selection of materials that will be

used in applications that require the material to undergo very rapid used in applications that require the material to undergo very rapid

loading processes such as in vehicular collisionsloading processes such as in vehicular collisions

Types of materials for impact testing

Nearly all materials may benefit from impact testing, but the most

common types used are metals, plastics, woods, composites,

ceramics, and polymers. Generally these materials take the form of

sheets of varying thicknesses or short rods depending on the test.

However, most materials will experience either ductile or brittle

failure depending on the (1)type of test, (2)the rate of loading and

(3)the temperature of the sample.

Test Method & Standard SpecimenTest Method & Standard Specimen The most common methods of performing impact test are -The most common methods of performing impact test are -

1. Charpy Test Method . 1. Charpy Test Method .

2. Izod Test Method . 2. Izod Test Method .

These are three tests all essentially determine the same

characteristics of the material but differs in the orientation of

the test sample

Fig : V notch Specimen

Charpy test MethodCharpy test Method

The Charpy impact test, also known as the Charpy V-The Charpy impact test, also known as the Charpy V-notch test, is a standardized high strain-rate test which notch test, is a standardized high strain-rate test which determines the amount of energy absorbed by a material determines the amount of energy absorbed by a material during fracture.during fracture. This absorbed energy is a measure of a given material's notch toughness and acts as a tool to study

It is widely applied in industry, since it is easy to prepare and conduct and results can be obtained quickly and cheaply

Charpy Impact Test Specimen

Charpy test specimens normally measure 55x10x10mmwith 2mm deep notch at the middle with 2mm deep notch at the middle of a specified surface.. of a specified surface..

and have a notch machined across one of the larger faces. The notches may be:

•         V-notch – A V-shaped notch, 2mm deep, with 45° angle and 0.25mm radius along the base

•         U-notch or keyhole notch – A 5mm deep notch with 1mm radius at the base of the notch.

The specimen is set like a simply The specimen is set like a simply supported beam.supported beam.

Izod test MethodIzod test Method  Izod impact is defined

as the kinetic energy needed to initiate fracture and continue the fracture until the specimen is broken. This test can be used as a quick and easy quality control check to determine if a material meets specific impact properties or to compare materials for general toughness. 

Izod Impact Test Specimen

The Standard Size of IThe Standard Size of I  zod zod impact test Specimen is (64mm x impact test Specimen is (64mm x 12.7mm x 3.2mm) . The V notch 12.7mm x 3.2mm) . The V notch Shaped bar is also used . Shaped bar is also used . the Izod specimen is held rigidly in a vice type fixture with the notched side facing the direction of impact. The centerline of the notch must be in the plane of the vice top within .125 mm. Once the specimen is in place the hammer is released from a preset height and allowed to strike the specimen thus fracturing it at the vee notch

Izod test MethodIzod test Method The test piece is a cantilever, clamped upright in an anvil, with a V The test piece is a cantilever, clamped upright in an anvil, with a V 

notch at the level of the top of the clamp. notch at the level of the top of the clamp.  The  test  piece  is  hit  by  a  striker  carried  on  a  pendulum  which  is The  test  piece  is  hit  by  a  striker  carried  on  a  pendulum  which  is 

allowed to fall freely from a fixed height, to give a blow of 120 ft lb allowed to fall freely from a fixed height, to give a blow of 120 ft lb energy….energy….

ProcedureProcedure

Preparation ofof testtest piecepiece

ProcedureProcedure

Place the sample in positionPlace the sample in position

Calculation Calculation Impact  energy  Impact  energy   = mgH-mghmgH-mgh ; Where, m=mass of pendulum. g=acceleration due to gravity.=9.81ms-2

 The absorb energy in Joule Unit.

Fig : Impact energy

Difference between Charpy & Izod testDifference between Charpy & Izod test In Charpy method the Specimen set is like a In Charpy method the Specimen set is like a simply Supported simply Supported

beam beam On other hand in Izod method the Specimen set is like a On other hand in Izod method the Specimen set is like a CantileverCantilever. .

Thus Why in Charpy method gives a Thus Why in Charpy method gives a higher reading higher reading than the Izod than the Izod method.method.

The notch in the izod test is facing the striker, fastened in pendulum, while in the charpy test; the notch is positioned away from the striker.

In the Charpy method, there are two kinds of notches, the V-notch and the U-notch, while in the Izod method; there is only one kind of notch.

Factors Affecting Charpy Impact Energy

Factors that affect the Charpy impact energy ofa specimen as below:

1. Fracture mechanism

2. Notches

3.Yield strength and ductility

4.Temperature and strain rate

Notches

The notch serves as a stress concentration zone and some materials are more sensitive towards notches than others. The notch depth and tip radius are therefore very important.

Yield strength and ductility

For a given material the impact energy will be seen to decrease if the yield strength is increased, i.e. if the material undergoes some process that makes it more brittle and less able to undergo plastic deformation. Such processes may include cold working or precipitation hardening

The area under the stress-strain curve gives the fracture energy of the material. A ductile material has a greater fracture energy

Temperature and Strain Rate

Most of the impact energy is absorbed by means of plastic

deformation during the yielding of the specimen.

Therefore, factors that affect the yield behavior and hence

ductility of the material such as temperature and strain rate

will affect the impact energy.

This type of behavior is more prominent in materials with a

body centered cubic structure, where lowering the

temperature reduces ductility more markedly than face

centred cubic materials

The curve represents a change in fracture behaviour from ductile at high temperature to brittle at lower temperature

Brittle materials :- A material is said to be brittle, when subjected to stress, it breaks without significant deformation (strain) for examples are glasses and ceramics materials

Ductility materials:-  is a solid material's ability to deform under tensile stress .or those which could show plastic deformation for examples are Carbon steel.

Ductile to Brittle Transition

Lower shelf

Transition – mixed mode

Upper shelf

Some materials such as carbon steels undergo what is known as a ‘ductile to brittle transition’. This behaviour is obvious when impact energy is plotted as a function of temperature. The resultant curve will show a rapid dropping off of impact energy as the temperature decreases. If the impact energy drops off very sharply, a transition temperature can be determined. This is often a good indicator of the minimum recommended service temperature for a material.The increase in yield stress associated with low temperature or high strain rates can results in a material changing its mode of fracture from ductile to brittle and this is very important when selecting materials for engineering purposes.

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