IMPACT TEST

Introduction

Cast iron shows a tensile strength of about 12 tons per square inch. However, if it is

hammered with an ordinary hand hammer, it breaks. Based on this we say that cast iron is

brittle. There are many other materials which behave like cast iron. Even steel in a totally

hardened condition will be brittle. Many a times, an accidental fall of the material from

our hands results in its breaking into pieces. This type of brittle behaviour of materials is

most undesirable for the common engineering objects.

A simple tensile test does not reveal the brittle nature of the metals, and if only

the tensile test data are relied upon and the object put into use, failure is certain. It is,

therefore necessary to test the material under shock or sudden loading conditions.

Strength data are mostly academic. In actual use and practice, we rarely come

across the ideal case of gradually applied loads. When we sit in a chair, our weight of a

few kilograms is applied to the chair, suddenly. When an electric motor is put on, the

shaft takes a sudden torque of the full r.p.m., being at zero r.p.m. just before. Many

instances can be cited when loads are borne by engineering components suddenly.

IMPACT TESTS

There are two types of impact test available which are most commonly employed. They

are:

(i) Izod,

(ii) Charpy,

Principle

The principle employed in all impact-testing procedures is that a material absorbs a

certain amount of energy before it breaks. The quantity of energy thus absorbed is

characteristic of the physical nature of the material. If it is brittle, it breaks more readily,

i.e. absorbs a lesser quantity of energy, and if tough, it needs more energy in order to

fracture.

1

The methods of testing are also very similar. A swinging hammer is made to strike the

specimen held firmly in a vice. The hammer breaks the specimen on account of its

potential energy. The height of rise of the hammer on the other side indicates the residual

energy of the hammer. The energy actually absorbed by the material specimen in order to

fracture, is given by the difference between initial (MgH) and final energies (Mgh) of the

hammer.

To calculate the energy absorbed:

The energy absorbed is equal to the loss in potential energy of the system, according to

the equation:

Potential energy lost = MgH-Mgh

Or in terms of angle of fall (θ1) and angle of rise (θ2) the energy lost is given by

Potential energy lost = MgR (cosθ2 - cos θ1) Where R is the length of pendulum.

2

THE IZOD TEST

The Izod testing machine (see the Fig.) consists of a heavy triangular frame. At its apex is

a smooth bearing in which swings a heavy pendulum weight. The pendulum weight can

be lifted up to a height and clamped. When released, it carries an energy of 16.36 kg -m.

At the base there is a vice to fix the standard specimen. There is a pointer which moves

on a scale at the top to read the energy. The pointer is in turn moved by the swinging

pendulum weight. The scale is marked from centre to both the ends, so that the energy

absorbed by the specimen is read directly.

Izod impact testing machine

Specifications of test sample

The standard specimen for the Izod test is a square rod of 10 mm side. There is a 2 mm

deep, 45° notch made at a distance of 28 mm from the end of the specimen. The root of

the notch is finished with a 0.25 mm radius. A cylindrical specimen with a diameter of

11.4 mm and a 3.3 mm deep notch may also be used. The notch employed is also a 45°

V-notch, finished with a 0.25 mm root radius. The specimen is fixed in the machine in

3

such a way that the hammer strikes it at a point 6 mm from the top. The notch of the

specimen is fixed to be on level with the anvil, and faces the pendulum. Details of

dimensions, etc., are illustrated in the following figure.

All dimensions in mm

Test procedure

The testing procedure is as follows:

(1) The pendulum weight is brought up and clamped.

(2) The specimen is fixed in the vice.

(3) The scale is adjusted to read zero.

(4) The pendulum weight is released from the clamp.

(5) The energy absorbed by the specimen is read on the scale to give the impact strength

of the material.

The impact strength, as explained above, is the energy absorbed by the material. It is

expressed in kilogram force meters

4

The energy absorbed by the specimen can be found from the following equation:

Energy absorbed = MgR (cosθ2 - cos θ1) Where R is the length of pendulum.

(θ1) is the angle of fall and

(θ2) is the angle of rise

Representation of the Data

After an lzod test, the following particulars should be furnished, besides the energy

absorbed by the specimen:

(1) The temperature of the specimen, i.e., the room temperature,

(2) Model and capacity of the machine,

(3) Appearance of the fractured surface, and

(4) The number of specimens tested.

Appearance of the fracture

It is advisable that the fracture surfaces of the specimen are also studied besides just

recording the energy absorbed in breaking the specimen. The study of the fracture

surfaces helps us in determining the impact strength in relation to the transition

temperature range of the material. We know that in the transition range, the fracture

changes from ductile to brittle. That is the fibrous fracture with deformation changes into

the crystalline fracture. The energy absorbed also decreases. In between these two

extremes, the fracture will be a mixed one (see the Fig.). It consists of a well-defined

crystalline or brittle portion, generally away from the region of the notch. Of course, the

extents of the brittle (or crystalline) region and the fibrous (or silky) regions directly

control the impact value. An estimate of the percentage crystallinity computed by the

naked eye can be a useful guide and is computed as follows:

Percentage Crystallinity

=Crystalline area in mm2

X 10080

5

Appearance of impact fracture

A. COMPLETELYFIBROUSB. CRYSTALLINEPLUS FIBROUS.C. COMPLETELY CRYSTALLINE.

THE CBARPY IMPACT TEST

The Charpy impact testing machine differs from the Izod machine in the method of

breaking the specimen. The specimen employed may be of any of the three types

illustrated in Figure. Specimen designs differ only in the shape of the notches. A V-notch

as in the case of the Izod specimen, a U-notch, or a keyhole notch is the three commonly

recommended for the Charpy specimens.

6

Specifications of test specimen

NOTE: ALL DIMENSIONSARE IN mm

The specimen is fixed in the machine as a simple beam (see the Figures). The notch in the

specimen does not face the hammer as in the Izod method: The opposite face of the notch

is fixed to receive the hammer blow. The hammer head, a pointed one of 8 mm radius,

strikes the specimen

7

Arrangement of the specimen in the Charpy machine

Test procedure

The testing procedure is as follows:

(1) The pendulum weight is brought up and clamped.

(2) The specimen is fixed in the vice.

(3) The scale is adjusted to read zero.

(4) The pendulum weight is released from the clamp.

(5) The energy absorbed by the specimen is read on the scale to give the impact strength

of the material.

The impact strength, as explained above, is the energy absorbed by the material. It is

expressed in kilogram force meters

The energy absorbed by the specimen can be found from the following equation:

Energy absorbed = MgR (cosθ2 - cos θ1) Where R is the length of pendulum.

(θ1) is the angle of fall and

(θ2) is the angle of rise

8

Representation of the Data

After an lzod test, the following particulars should be furnished, besides the energy

absorbed by the specimen:

(1) The temperature of the specimen, i.e., the room temperature,

(2) Model and capacity of the machine,

(3) Appearance of the fractured surface, and

(4) The number of specimens tested.

Appearance of the fracture

It is advisable that the fracture surfaces of the specimen are also studied besides just

recording the energy absorbed in breaking the specimen. The study of the fracture

surfaces helps us in determining the impact strength in relation to the transition

temperature range of the material. We know that in the transition range, the fracture

changes from ductile to brittle. That is the fibrous fracture with deformation changes into

the crystalline fracture. The energy absorbed also decreases. In between these two

extremes, the fracture will be a mixed one (see the Fig.). It consists of a well-defined

crystalline or brittle portion, generally away from the region of the notch. Of course, the

extents of the brittle (or crystalline) region and the fibrous (or silky) regions directly

control the impact value. An estimate of the percentage crystallinity computed by the

naked eye can be a useful guide and is computed as follows:

Percentage Crystallinity

=Crystalline area in mm2

X 10080

9

Appearance of impact fracture

A. COMPLETELYFIBROUSB. CRYSTALLINEPLUS FIBROUS.C. COMPLETELY CRYSTALLINE.

Differences between Izod and Charpy tests

Izod Test Charpy Test

1) Specimen length is 75 mm 1) Specimen length is 55 mm

2) notch is made at a distance of 8 mm

from top end

2) notch is made exactly in the middle of

the specimen

3) Specimen position in the vice is

cantilever beam

3) Specimen position in the vice is simply

supported beam

4) usually a V-notch is used 4) V or U or Key hole notches are used

5) The initial energy f hammer is less 5) The initial energy f hammer is high

Notches

The notch in the test specimen has two effects:

1. Stress concentration around the notch causes plastic deformation to occur in this

area. The decrease in area caused by the notch increases the stress to a value

above the yield stress for the material, while the rest of the specimen may still be

at a stress below the yield stress. This plastic hinge which develops at the notch

reduces the total amount of plastic deformation in the test specimen.

10

2. The plastic deformation at the notch is constrained by the surrounding material

and this increases the tensile stress in the plastic zone. The degree of constraint

depends on the severity of the notch (depth and sharpness). The increased tensile

stress encourages fracture and reduces the work done by plastic deformation

before fracture occurs.

Some materials are more sensitive to notches than others and a standard notch tip

radius and notch depth are therefore used to enable comparison between different

materials.

Notch sensitivity

The extent to which the sensitivity of a material to fracture is increased by the presence of

a surface defect, such as a notch, a crack, or a scratch. Low notch sensitivity is usually

associated with ductile materials, and high notch sensitivity is usually associated with

brittle materials.

Notch toughness

Notch toughness is the ability that a material possesses to absorb energy in the presence

of a flaw.

The presence of a flaw, such as a notch or crack, a material will likely exhibit a lower

level of toughness. When a flaw is present in a material, loading induces a triaxial tension

stress state adjacent to the flaw. The material develops plastic strains as the yield stress is

exceeded in the region near the crack tip. However, the amount of plastic deformation is

restricted by the surrounding material, which remains elastic. When a material is

prevented from deforming plastically, it fails in a brittle manner.

Notch-toughness is measured with a variety of specimens of which the widely used one is

the Charpy V-notch impact specimen.

11

DUCTILE-TO-BRITTLE TRANSITION

The notched-bar impact test can be used to determine whether or not a material

experiences a ductile-to-brittle transition as the temperature is decreased. In such a

transition, at higher temperatures the impact energy is relatively large since the fracture is

ductile. As the temperature is lowered, the impact energy drops over a narrow

temperature range as the fracture becomes more brittle.

The transition can also be observed from the fracture surfaces, which appear fibrous or

dull for totally ductile fracture, and granular and shiny for totally brittle fracture. Over the

ductile-to-brittle transition features of both types will exist.

While for pure materials the transition may occur very suddenly at a particular

temperature, for many materials the transition occurs over a range of temperatures. This

causes difficulties when trying to define a single transition temperature and no specific

criterion has been established.

Definition

The temperature at which a material experiences a ductile-to-brittle transition is called its

Ductile to brittle transition temperature or shortly DBTT.

The transition from ductile brittle is shown in the following figure.

12

The ductile-brittle transition is exhibited in bcc metals, such as low carbon steel, which

become brittle at low temperature or at very high strain rates. FCC metals, however,

generally remain ductile at low temperatures.

Importance of DBTT

The chief engineering use of the transition-temperature curves is in selecting materials

which are resistant to brittle fracture. The design philosophy is to select a material which

has sufficient notch toughness when subjected to severe service conditions so that the

load-carrying ability of the structural member can be calculated by standard strength of

materials methods without considering the fracture properties of the material or stress

concentration effects of cracks or flaws.

METALLURGICAL FACTORS AFFECTING TRANSITION TEMPERATURE

The largest changes in transition temperature result from changes in the amount of carbon

and manganese. In mild steel for instance the transition temperature is increased 14°C for

each increase of 0.1 percent of carbon. This transition temperature is lowered about 5.5°C

(10°F) for each increase of 0.1 percent manganese. Increasing the carbon content also has

a pronounced effect on the maximum energy and the shape of the energy transition-

temperature curves.

The Mn/C ratio should be at least 3/1 for satisfactory notch toughness. A maximum

decrease of about 55°C (100°F) in transition temperature appears possible by going to

higher Mn/C ratios.

Phosphorus also has a strong effect in raising the transition temperature. The role of

nitrogen is difficult to assess because of its interaction with other elements. It is,

however, generally considered to be detrimental to notch toughness.

Nickel is generally accepted to be beneficial to notch toughness in amounts up to 2

percent and seems to be particularly effective in lowering the ductility transition

13

temperature. Silicon, in amounts over 0.25 percent, appears to raise the transition

temperature. Molybdenum raises the transition almost as rapidly as carbon, while

chromium has little effect.

Notch toughness is particularly influenced by oxygen. For high-purity iron it was found

that oxygen contents above 0.003 percent produced intergranular fracture and

corresponding low energy absorption.

Grain size has a strong effect on transition temperature. A decrease of grain size results

in a decrease in transition temperature.

The energy absorbed in the impact test of an alloy steel at a given test temperature

generally increases with increasing tempering temperature.

14