Exp7 Lab Manual- Charpy Impact Testing PDF

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METU Department of Metallurgical and Materials Engineering Met.E 206 Materials Laboratory Experiment 7 Prof. Dr. Tayfur Öztürk INTRODUCTION

Many solids can break in either ductile or brittle manner. In ductile fracture material is subject to extensive deformation before voids are formed and coalesced resulting in the final failure. In brittle fracture, the material display very little resistance to crack propagation. The energy required to break the material is therefore quite low.

A simple measure of toughness can be obtained from stress-strain curve derived from a tensile test, Fig.1. Here the area under the curve is a measure of the energy expanded in breaking the sample and therefore it can be treated as the toughness of the material. It should be noted that here the Material B is the toughest since it has a good combination of both strength and ductility (for details of tensile testing see Exp . I). There are several factors that have a profound influence on the toughness of a material. One of these is the so-called notch effect. If a notch of certain depth is machined around a round bar and tested in tension, the stress –strain behaviour may be little affected. If this is the case the material is notch-tough. Some materials are however is very sensitive to such notches and their stress-strain behaviour is affected drastically. They then fail in brittle manner and are said to be notch- brittle. An example of this is railroad rails. The rails normally manufactured from high carbon steel perform exceptionally well in heavy loads, but if a flaw or crack is present they fail in a brittle manner at a fraction of the same load (The rails therefore should be inspected quite regularly for cracks and defects and defective rails should be replaced). Notch brittlness is extremely important since flaws or defects are always present in the surface of engineering materials. High strength materials, i.e. strength > E/150 have such lowtoughness that brittle fracture can occur while the material in elastic range at all temperatures. Most ceramics and thermoplastic polymers are notch- brittle at temperatures below 0.5-0.7 of the absolute melting point, Tm. There is usually a transition from notch brittle-to notch- tough behaviour with increasing temperature. In metals,this transition occurs at 0.1 to 0.2 of Tm. 1

Rate of loading is another factor that influences if the material’s behaviour. Ductile-to-Brittle Transition Temperature is affected by the rate of laoding. Under static laoding in tensile testing, a medium C-steel may fail in ductile manner. But the same steel may fail in brittle manner under impact . There are several standard types of toughness test that generate data for specific loading conditions and/or component design approaches. Here we shall consider Charpy Impact Testing, since it is easy to carry out and can be used to assess the relative toughness of different materials, e.g. steel , aluminum and PVC .

Fig.1. Stres-strain curve of high carbon, medium carbon and low carbon steel.Area skectched under the curve is a measure of the materials toughness. (http://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/Mechanical/Toughness.htm)

CHARPY IMPACT TESTING Charpy Impact Testing was first proposed around 1900 and has become a standart test in 1940’s. Main drive for implementation of impact testing testing in fabrication standards and material specifications came as a result of the large number of ship failures that occurred during World War II ( Liberty ships). The test involves the use of a sample in the form of rectangular beam notched in the center. This sample is placed between two anvils, and is broken by a swinging pendulum, i.e. under impact. By measuring the height to which the pendulum rises after impact compared to the height from which it was dropped, one can find the total energy absorbed by the sample. Fig. 2. The energy measured by the Charpy test is the work done to fracture the specimen. On impact, the specimen deforms elastically until yielding takes place, and a plastic zone develops at the notch. As the test specimen continues to be deformed by the impact, the 2

plastic zone work hardens. This increases the stress and strain in the plastic zone until the specimen fractures.

The Charpy impact energy therefore is made up of three parts: (i) the elastic strain energy, (ii) the plastic work done during yielding and plastic deformation and (iii) the work done to create the fracture surface . For a brittle material, the total energy is dominated by the elastic energy and the surface energy. In ductile material, on the otherhand, in addition to these, there is a large contribution of the plastic work. The impact energy is then quite high. The total impact energy depends on the size of the test specimen, and on the depth and root curvature of the notch. Therefore, a standard specimen size and notch geometry is used to allow comparison between different materials (see Fig. 2).

Sample Positioning

Sample

Charpy Impact tester Fig.2 Charpy impact testing (W.D. Callister Materials Science and Engineering 7th Edition John Wiley and Sons , NewYork 2007)

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The geometry of notch is particularly important since it acts as a stress concentrator in the specimen. It greatly amplify the stress around the notch for a uniformly applied load. The stress concentration of the notch causes plastic deformation to occur locally at the notch area. A plastic hinge can develop at the notch, which reduces the total amount of plastic deformation in the test specimen. This reduces the work done by plastic deformation before fracture. In addition, the constraint exersized by the notch increases the tensile stress in the plastic zone. The degree of constraint depends on the severity of the notch (depth and sharpness or root curvature). The increased tensile stress encourages fracture and reduces the work done by plastic deformation before fracture occurs. A standard notch tip radius and notch depth are therefore necessary to enable comparison between different materials.

Fig.3 Scanning Electron Microscope (SEM) images of fracture surface (http://www.coe.uncc.edu/~qwei/Teaching.htm/MEGR%203152/Chapters/)

(a)Micro-void coalescence (ductile, fracture consumes a large amount of energy)

(b) Cleavage (brittle, much less energy consumed)

When the testpiece fails in ductile manner, the fracture usually occurs by microvoid coalescence. These micro-voids grow and link up until final failure occurs, Fig. 3(a). In BCC metals, failure can also occur by cleavage along the {001} crystal planes at a critical tensile stress. As the yield strength of the metal is increased, the tensile stress in the plastic zone can become sufficiently high for cleavage to occur. The fracture mechanism in a ferritic carbon steel therefore changes from micro-void coalescence to cleavage as the yield strength increases. This can also be caused by an increase in strain rate or a decrease in temperature. The work of fracture of cleavage is much less than the work of fracture of micro-void coalescence since it involves much less plastic deformation, Fig. 3(b). The change in fracture mechanism therefore causes a sharp ductile to brittle transition in the Charpy impact energy. The ductile to brittle transition (DBTT) is usually observed in most BCC metals and 4

alloys. But the exact temperature is a strong function of the type of material, and for a given metal, a strong function of the purity level. Most FCC metals do now show the DBTT behavior.

Ductile to Brittle Transition Temperature The Charpy impact test is used to determine the ductile to brittle transition behavior of a material. The test involves the measurement of impact energy at various temperatures. The curve constructed then shows the effect of temperature on the fracture energy. The impact energy generally decreases with decreasing temperature as the yield strength increases and the ductility decreases. A sharp transition, where the energy changes by a large amount for a small temperature changes, can occur when there is a change in the fracture mechanism.

Fig.4 A graph of the temperature dependence on the Charpy V-notch impact energy (From Callister W.D. Jr., Materials Science and Engineering : An Introduction , Pg. 225 Figure 8.13, John Wiley and Sons, Inc.)

If the material has a sharp ductile to brittle transition, then a transition temperature can be defined below which the material has poor toughness. This can be used as a guideline for the minimum service temperature. It is less easy to do this in materials with a smooth transition from ductile to brittle behavior. The transition temperature may be defined using the mean impact energy between the highest and lowest values.

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% shear lip = (shear area/ total area)X100

LateralExpansionW= Wf-Wi

Fig.5 . Measurement of lateral expansion and percent shear iıp in Charpy impact testpieces.

The transition temperature can also be defined in terms of the lateral expansion of the specimen (a measure of the amount of plastic deformation), or changes in the fracture surface appearance, see Hertzberg .

Experiment

In this experiement you will carry out Charpy impact testing on three materials. Two of these are metals; carbon steel, commercial purity aluminium and the third is a polymer PVC (polyvinyl chloride). The test will be conducted at i) 100 oC , ii) Room Temperature and iii) 21 oC . Three samples will be used for each test.

EQUIPMENT: 1. Charpy Testing Machine ( 358J) 2. Charpy Testing Machine (15J) 3. Sample tongs 4. Tray for ice-salt mixture for low temperature 5. Tray for hot(boiling) water 6. 2 Beakers with ice 7. Salt 8. Digital thermometers 9. Vernier calipers

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Samples 10. 9 Standard Charpy samples of C steel 11. 9 standart Charpy Samples of Commercial Purity Aluminium 12. 9 Standart Charpy Sample of PVC

Before the Lab 1- Read the relevant chapter in ref 1.

Before the experiment 1- Label each sample and measure the lateral dimension of the each sample. 2- Prepare tray with ice -23.3 wt% salt mixture. 3- place 3 samples from each material into the bath 4- Prepare tray with boiling water 5- place 3 samples from each material into the bath

Procedure 1- Review safety considerations THE IMPACT MACHINE COULD BE DANGEROUS AND CAN CAUSE SERIOUS INJURY. THE INSTRUCTOR WILL SHOW YOU HOW TO OPERATE IT. BROKEN SAMPLES DO FLY AWAY. YOU SHOULD THEREFORE NEVER CROSS BEYOND THE LINE MARKED ON THE FLOOR . SAMPLES TAKEN FROM BOILING WATER ARE HOT AND COULD BURN YOUR FINGERS. SO PLEASE HANDLE SUCH SAMPLES WITH SAMPLE TONGS.

. 2.1 Carry out Room Temperature Testing impact testing for carbon steel and aluminum (Use 358 Jmachine) a. Raise the hammer to a prescribed position b. place the sample at its location .( For consistent results, it is important that all specimens be positioned identically in the anvil; use the special tongs provided to correctly position the specimen.) c. Release the hammer d. Observe the final position of the hammer after impact and record the energy indicated by the dial ( this is the absrobed energy ). e. Collect the pieces and cellotape them together .

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2.2 Carry out impact testing at -21 oC for carbon steel and aluminum ( Use 358 J machine). ( caution, the test must be performed within 5 seconds after the sample has been removed from the bath) 2.3 Carry out impact testing at 100 oC for carbon steel and aluminum ( Use 358 J machine) 3.1 Carry out Room Temperature impact testing for PVC ( Use 15 J machine) 3.2 Carry out impact testing at -21 oC for PVC ( Use 15 J machine) 3.3 Carry out impact testing at 100 oC for PVC ( Use 15 J machine)

After the experiment Observe the nature of the fracture surface. The fracture specimens should all be carefully examined and particular attention should be paid to the type of fracture that is obtained in each particular case. Try to relate the type of fracture to the energy absorbed by the metal being fractured. A magnifying glass is available to study these fractures carefully. Measure lateral dimensions after impact.

Assignment 1- Tabulate the data using Excel ; graphs should be overlayed for the C steel, aluminium. Graph for PVC should be separate; a. Impact Energies vs temperature b. Lateral expansion vs temperature 2- Calculate the average farcture energy versus temperature for steel, aluminium and PVC. Report Format 1- Introduction 2- Objective 3- Experiemntal procedure a. Specimen shape, dimension b. Test equipment c. Explain how you conducted the experiment

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4- Results and Discussion ; a. Present the data in both tables and plots. b. Discuss the results in terms of the factors that affect the fracture toughness of materials . 5- Conclusions 6- References

References 1-W.D. Callister Materials Science and Engineering 7th Edition John Wiley and Sons , NewYork 2007 2-George E. Dieter “Mechnaical Metallurgy”,McGraw-Hill, London, 1988 3-R.W. Hertzberg “Deformation and Fracture of Engineering Materials” , John Wiley and Sons, New York, 1989 4-http://www.tms.org/pubs/journals/jom/9801/felkins-9801.html ( accessed on March14th 2010)

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Sample # 1

Material

Initial width Wi (mm)

Aluminium

Target Temp (ºC) 100

2

Aluminium

100

3

Aluminium

100

4

Aluminium

20

5

Aluminium

20

6

Aluminium

20

7

Aluminium

-21

8

Aluminium

-21

9

Aluminium

-21

10

C-Steel

100

11

C-Steel

100

12

C-Steel

100

13

C-Steel

20

14

C-Steel

20

15

C-Steel

20

16

C-Steel

-21

17

C-Steel

-21

18

C-Steel

-21

19

PVC

100

20

PVC

100

21

PVC

100

22

PVC

20

23

PVC

20

24

PVC

20

25

PVC

-21

26

PVC

-21

27

PVC

-21

10

Temp. (ºC)

Fracture Energy (J)

Final width Wf (mm)...