– Determination of Ductile-Brittle Transition Temperature Using the Charpy Impact Test

 

 

– Determination of Ductile-Brittle Transition Temperature Using the Charpy Impact Test
Objective: In this experiment, the effect of temperature on the fracture properties of steel at high rates of loading is investigated.
Apparatus: InstronCharpy Impact Testing Machine Ice (to chill specimens) Oven Tongs
Materials: Standard Charpy V-notch Specimens of 1045 annealed steel
References: Lab manual Van Vlack, Ch. 6 (reserve) Dieter, Ch. 7 & 14 (reserve) ASTM Standard E23
Background: Ductile and brittle failures of metals always happen during the service. Brittle fracture in metals may occur without warning (i.e. without prior
yielding), and can endanger life and property. The prevention of brittle fractures is an important responsibility of an engineer.
There are many variables that may affect the amount of plastic deformation for a material experiences prior to fracture.  The ambient temperature has a strong
influence on the deformation of BCC materials (especially iron and steel).  Below the so-called Ductile-Brittle Transition Temperature (DBTT), fracture occurs with
minimal plastic flow, and therefore little energy absorption.  Cracks, once started, may become unstable and grow even if the load on a material decreases.  Above the
DBTT, much more energy is absorbed in plastic deformation and it is more difficult for cracks to grow.
Triaxial stress states, such as those that exist at the roots of a notch or at the tips of a crack, also increase the likelihood of brittle behavior.  “Notch
sensitive” materials are particularly susceptible to flaws, and such stress-raisers must be avoided in these materials.  The greater the yield strength and tensile
strength of a material, the more likely it is to be notch sensitive.  Also, the larger a component, the greater its notch sensitivity will be.
A third important variable in determining the fracture mode is loading rate.  The sudden application of force, which occurs in impulse, shock, or impact loading,
increases the likelihood of brittle behavior.
A variety of secondary factors can influence deformation.  Small hydrogen or phosphorous inclusions can embrittle steel.  If a sample’s grains are small, it will tend
to be more brittle than a specimen with large grains.  The segregation of brittle components within a material, which can occur at grain boundaries in particular, can
lower ductility.  A high number of impurities in a material and neutron irradiation also increase the tendency for brittle fracture.
A material that shows ductility in a conventional uniaxial tensile test may nonetheless suffer brittle fracture in service due to the conditions listed above.  Many
tests have been proposed to predict failure behavior in service.  One of the earliest and most widely used is the Charpy impact test.  This requires a material
specimen with an artificial 450 notch, 2 mm deep, with a notch tip radius of 0.25 mm.  The specimen is subjected to a blow from a weighted, gravity driven pendulum.
The energy absorbed during the fracture process is found by measuring change in the height of the pendulum’s swing before and after its impact with the sample.  The
measurement must also be adjusted for friction losses based on a preliminary drop of the pendulum.
To determine the DBTT behavior, the energy loss is determined by graphing the impact energy dissipated vs. the temperature at which the specimen was treated.
For a given specimen, material, and condition, the usual procedure is to test samples heat treated at a variety of temperatures in order to locate the DBTT region.  It
should be noted that the DBTT can only be defined arbitrarily.  The transition from ductile to brittle behavior does not occur at one specific temperature; it
progresses across a range of temperatures that may be broad or narrow, depending on the material.
During heat treatment, specimens are placed in an oven and allowed to reach a thermal equilibrium.  The different temperatures that are tested appear in the first
column of Table I.  This table should be completed by students and a copy should be included in the lab report.
Table I: Table used to enter the data found in the DBTT lab.Temperature (°C) 1st Sample Impact Energy (ft-lbs) 2nd Sample Impact Energy (ft-lbs)0 15.15 17.325 25.23 28.1775 42.71 48.32100 67.30 72.15
The Chary impact machine, although simple to operate, can be EXTREMELY DANGEROUS.  Care should be taken to insure that all persons are clear of the path of the
pendulum before the test begins.

 

 

Procedure:
1. Make sure that the Fracta software is set as desired.2. Hold the latch prevent bar in the up position by pressing down on the short end of the bar.3. Place the operating lever in the latch position.4. Lift the pendulum counterclockwise slowly by hand until latch clicks. When lifting use caution not to lift the pendulum beyond the normal latch height. (Never
try to latch the pendulum while it is swinging. The machine has a safety latch designed to prevent latching the pendulum while it is swinging and it shouldn’t be
bypassed. If you attempt to latch while it is swinging you will damage the shaft and you may break the latch pin) 5. Insert Stop Pin into Hole 1 to prevent accidental release of pendulum.6. Put on your eye protection. Use the tong provided to place the specimen on the charpy impact machine. CAUTION: SAMPLES CAN BE HOT.  Position the sample in the
machine with the notch facing AWAY from the pendulum side.7. Set the pointer at the maximum dial value.8. Press the Pendulum Latched button to indicate the software that the pendulum is latched.9. For systems using the Fracta software package, check that the Fracta software is ready to collect data:a) Both the Current Angle and Pendulum Latched fields should be green to indicate a valid test state.b) The next test field should display the correct test number for the test. If it is desired to start a new test number sequence, enter a new number into next
test field.10. Stand behind the yellow tape surrounding the machine. Move the Stop Pin from Hole 1 to Hole 2 to prevent brake application during the test. Move the operating
lever to release the pendulum.11. Once the pendulum has completed its initial swing, remove the Stop Pin from Hole 2 and place the operating lever in the brake position. The pendulum will not
stop immediately; it takes approximately 12 swings before the pendulum will come to a stop.12. Once the pendulum has stopped swinging, move the operating lever to the Release position and place the Stop Pin back in Hole 2.13. Repeat for all samples.
Lab #2 Report Guidelines – Determination of Ductile-Brittle Transition Temperature
Theory:1. What is toughness?2. Explain brittle and ductile crack propagation.  Explain how cracks can be identified as brittle or ductile.3. Explain the theory of toughness testing (Charpy impact).4. What is DBTT and how is it useful?  There are many variables that affect the geometries and mechanical properties of material fracture: discuss them.5. What is the effect of increasing the temperature far above the DBTT?6. What is the effect of decreasing the temperature far below the DBTT?
Analysis:1. Graph the impact energy (y-axis) vs. heat treatment temperature (x-axis) for each sample. (Always graph the independent variable on the x-axis.)2. If a drastic change in impact properties was observed in either of the graphs, at what temperature did it occur?  What does this change imply?3. Were the results obtained in the lab consistent with the expected (theoretical) results?
Requirements:1. Follow the format guidelines given in the manual.2. A completed Table I is given in the “Results section” that lists the data from the experiment..

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