Impact test report

ObjectiveTo determine ductile to brittle transition temperature using V notch Charpy impact test.

Apparatus Charpy impact testing machines Steel samples Thermometer Boiling water for high temperature Ice + salt for sub-zero temperature

Theory and Background Background During World War II a great deal of attention was directed to the brittle failure of welded Liberty

ships and T-2 tankers Some of these ships broke completely

in two, while, in other instances, the fracture did not

completely disable the ship.

Most of the failure occurred during the winter months,

Failures occurred both when the ships were in heavy seas

and when they were anchored at dock These calamities

focused attention on the fact that normally ductile mild steel can become brittle under certain

conditions1.

The Titanic began its maiden voyage to New York just before noon on April 10, 1912, from

Southampton, England. Two days later at 11:40 p.m., Greenland time, it struck an iceberg that was

three to six times larger than its own mass, damaging the hull so that the six forward compartments

were ruptured. The flooding of these compartments was sufficient to cause the ship to sink within

two hours and 40 minutes, with a loss of more than 1,500 lives. The scope of the tragedy, coupled

with a detailed historical record, have fuelled endless fascination with the ship and debate over the

reasons as to why it did in fact sink. A frequently cited culprit is the quality of the steel used in the

ship's construction. A metallurgical analysis of hull steel recovered from the ship's wreckage

provides a clearer view of the issue.

Scientist observed several fractures in Second World War and predicted that

1. The fractures are of brittle type.

2. These occur below yield strength.

Analysis of these fractures showed that the reason behind is the notches.

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Figure 1: titanic ship

Impact test report

These may be due to

1. Design feature

2. Fabrication process (e.g. welding)

3. Flaws in the materials (e.g. porosity)

TheoryA broad research program was under taken to find the causes of these failures and to prescribe the

remedies for their future prevention, In addition to research designed to find answers to a pressing

problem, other research was aimed at gaining a better understanding of the mechanism of brittle

fracture and fracture in general1.

Fracture Fracture may be defined as the mechanical separation of a solid owing to the application of stress.Fracture toughnessFracture toughness is an indication of the amount of stress required to propagate a pre-existing

flaw. It is a very important material property since the occurrence of flaws is not completely

avoidable in the processing, fabrication, or service of a material/component. Flaws may appear as

cracks, voids, metallurgical inclusions, weld defects, design discontinuities, or some combination

thereof (http://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/Mechanical/

FractureToughness.htm).

Toughness Toughness is defined as the ability of a material to absorb energy. It is usually characterized by the

area under a stress-strain curve for a smooth (unnotched) tension specimen loaded slowly to

fracture (3).

Notch toughnessNotch toughness represents the ability of a material to absorb energy usually determined under

impact loading in the presence of a notch. Notch toughness is measured by using a variety of

specimens such as the Charpy V-notch impact specimen, the dynamic-tear specimen, and plane-

strain fracture-toughness specimens under static loading (KId) and under impact loading (KIc) (3asm).

Types of FractureThere are various types of fracture which are encountered.

1. Ductile fracture2. Brittle fracture3. Inter-crystalline4. Fatigue fracture

Apart from these there fracture may be both

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Brittle and ductile

Fracture dependenceDuctile-to-Brittle Fracture Transition. Traditionally, the notch-toughness characteristics of low- and

intermediate-strength steels have been described in terms of the transition from ductile to brittle

behavior as test temperature increases. Most structural steels can fail in either a ductile or a brittle

manner depending on several conditions such

1. Temperature2. Strain rate (ϵo)3. State of stress4. Section size5. Notch acuteness

These variables may change a ductile fracture to a brittle fracture in service leading.

Notched-Bar Impact TestsVarious types of notched-bar impact tests are used to determine the tendency of a material to

behave in a brittle manner. This type of test will detect differences between materials which are not

observable in a tension test. The results obtained from notched-bar tests are not readily expressed

in terms of design requirements, since it is not possible to measure the components of the triaxial

stress condition at the notch. Furthermore, there is no general agreement on the interpretation or

significance of results obtained with this type of test.

A large number of notched-bar test specimens of different design have been used by investigators

of the brittle fracture of metals. Two classes of specimens have been standardized for notched-

impact testing.

Charpay V-notch impact test (in USA)

Izod test (in UK)

Charpy V-notch impact testThe most widely used specimen for characterizing the ductile-to-brittle transition behavior of

steels. These specimens may be tested at different temperatures and the impact notch toughness

at each test temperature may be determined from the energy absorbed during fracture, the

percent shear (fibrous) fracture on the fracture surface, or the change in the width of the specimen

(lateral expansion).

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Impact test report

Figure 2: Charpy v-impact test

The notched-bar impact test is most meaningful when conducted over a range of temperature so

that the temperature at which the ductile-to-brittle transition takes place can be determined.

Note that the energy absorbed decreases with decreasing temperature but that for most cases the

decrease does not occur sharply at a certain temperature.

The principal advantage of the Charpy V-notch impact test is that it is a relatively simple test that

utilizes a relatively cheap, small test specimen. Tests can readily be carried out over a range of sub

ambient temperatures. Moreover, the design of the test specimen is well suited for measuring

differences· in notch toughness in low-strength materials such as structural steels. The test is used

for comparing the influence of alloy studies and heat treatment on notch toughness.

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Impact test report

It frequently is used for quality control and material acceptance purposes. The chief difficulty is that

the results of the Charpy test are difficult to use in design. Since there is no measurement in terms

of stress level, it is difficult to correlate Cv data with service performance. Moreover, there is no

correlation of Charpy data with flaw size. In addition, the large scatter inherent in the test may

make it difficult to determine well-defined transition-temperature curves.

Specimen in Charpay v-notch impact test The Charpy specimen has a square cross section (10 X

10 mm) and contains a 45° V notch, 2 mm deep with a

0.25-mm root radius. The specimen is supported as a

beam in a horizontal position and loaded behind the

notch by the impact of a heavy swinging pendulum

(the impact velocity is approximately 5 ms - 1). The

specimen is forced to bend and fracture at a high

strain rate on the order of 10 3 s -1.

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Figure 3: sample for charpy v-notch impact test

Impact test report

Figure 4: micrograph of charpy tested samples

INSTRUMENTED CHARPY TEST

The ordinary Charpy test measures the total energy absorbed in fracturing the specimen. Additional

information can be obtained if the impact tester is instrumented to provide a load-time 'history of

the specimen during the test. In figure an idealized

load-time curve for an instrumented Charpy test. With

this kind of record it is possible to determine the

energy required for initiating fracture and the energy

required for propagating fracture. It also yields

information on the load for general yielding, the

maximum load, and the fracture load.

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Impact test report

Transition temperatureAs mentioned previously, the absorbed energy of BCC metals changes drastically within the

transition region, we therefore have to identify a transition temperature, which can be used to

determine the suitable service temperature of particular materials in order to avoid metal failure in a

catastrophic manner. There are several criteria for the identification of the transition temperature.

Transition temperature is the temperature at which the test sample absorbs the most

fracture energy and possesses 100% fibrous fracture surfaces. This means brittle fracture is

neglected in this case and is considered to be the safest among other criteria. The transition

temperature is also called the fracture transition plastic or FTP.

Transition temperature is the temperature at which the percentage of cleavage and ductile

fractures are equal. This transition is also called fracture appearance transition temperature

or FATT because the fracture surface area is used as an indicator to determine the transition

temperature.

Transition temperature is the temperature correlating to an average absorbed energy value

of upper and lower shelf energy absorption. At or above this temperature, there is a

correlation that less than 70% of the brittle cleavage fracture that indicates a high

probability at which failure will not occur if the stress does not exceed about one-half of the

yield stress.

Transition temperature is the temperature at which the absorbed energy (C) equals 20J.

This criterion was introduced to determine toughness value of steels used during the World

War II. It is based on the idea that brittle fracture will not occur if the sample has the

absorbed energy above 20J. However this criterion might show no significant meanings for

other materials.

Transition temperature is the temperature at which there is none of the ductile dimples

appearing on the fracture surfaces. This temperature is also called nil ductility temperature

or NDT since there is no plastic deformation during fracture.

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Impact test report

Metallurgical Factors Affecting Transition TemperatureThe factors which affect the transition temperature are

Composition of steel Grain size Ageing phenomena Notched size and direction Tempering time Presence of martensite Lowest possible finish temperature of rolling

ProcedureRoom temperature test is first carried out by placing the Charpy impact specimen on the anvil and positioning it in the middle location using a positioning pin where the opposite site of the notch is destined for the pendulum impact. Raise the pendulum to a height corresponding to the maximum stored energy of 300J. Release the pendulum to allow specimen impact. Safely stop the movement of the pendulum after swinging back from the opposite side of the machine. When the pendulum is still, safely retrieve the broken specimen without damaging fracture surfaces. Repeat the test at the same test condition using another specimen to average out the obtained values. Charpy impact testing at temperatures other than room temperature is carried out following

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Impact test report

Prior to specimen impact, specimen is submerged in the medium for at least 5 minutes to ensure uniform temperature across the specimens. Specimen impact must be within 5 seconds after removing from the medium. Repeat the test at the same test condition using another specimen to average out the obtained values.

Data Sample Mild Steel

Angle during free falling 135o

Condition Room temperature Ice + salt Boil waterTemperature oC 10 0 98

CalculationHammer lift angle α = 139.5o

Distance from axis to centre of gravity D = 0.694m

Weight of hammer P = 40.98kg

E = PD (cosβ-cosα)

Case I

Room temperature

β = 96o

E = energy calculated = 18.77 Kgfm

Case II

Boiling water

β = 95o

E = 19.135 kgfm

Case III

Ice + salt

β = 112

E = 10.96 kgfm

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Impact test report

ResultAs we decrease temperature material fails with brittle manner. And absorbed energy also decrease. Because

material is much prone to brittle behaviour.

Reference Mechanical metallurgy 'g.e. dieter' 3rd edition

Mechanical Testing and Evaluation was published in 2000 as Volume 8 of the ASM Handbook. The

Volume was prepared under the direction of the ASM Handbook Committee.

Hand out lecture

http://www.saecanet.com/calculation_page/000380_000509_Charpy_impact_test.php

ASTM standards e 2

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