Lab Report Assignment - Google Docs

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Lab 3: Impact Testing Date: March 17, 2009

Lab 3: Impact TestingDepartment of Mechanical Engineering

By: Nickell Aktarian and Andrew Berry

March 17, 2009

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Table of Contents

Summary1 Introduction

2 Theory

2.1 Toughness

2.2 Impact Testing

2.3 Types of Fracture

3 Procedure

4 Results

4.1 Raw Data

4.2 Experimental Results

5 Discussion5.1 Effect of Temperature on Cv

5.2 Relationship Between VHN and Cv

5.3 Nature of Fractures Obtained

5.4 Sources of Error

6 Conclusions

Recommendations

References

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Summary

This lab report details the theory, observations and discussions of impact tests performed on

steel samples (carbon and mild). The temperature of the samples is varied, and the effect of the

temperature changes on the samples fracture energy and physical appearance of the fractures

are discussed. Mild steel is found to be more ductile than carbon steel, a fact that is also

supported by the Vickers hardness number being higher for mild steel then for carbon steel.

Also supported by the VHN is the increase in energy absorbed by the mild steel over the carbon

steel. Both samples exhibited more brittle fractures as temperatures of the samples decreased,

and more ductile fractures when the temperatures increased.

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1 Introduction

Understanding when and how something will fail is fundamental to engineering. The behaviour

of mild steel and carbon steel under different temperature conditions are observed in this

experiment. The brittle nature of fractures that occur at low temperatures and ductile fractures athigh temperatures are compared to the amount of energy absorbed in the break. To observe

and understand these behaviours and how they are related to Vickers hardness, energy and

types of steel are the objectives of this experiment.

2 Theory

2.1 Toughness

The toughness of a material is defined as the amount of energy it can absorb during plastic

deformation prior to fracture. Toughness can be interpreted as the area under the curve of

strain-stress graph, and is expressed in units of energy per unit volume (see figure below).

Although hard alloy steels may have very high yield stresses, they are not very ductile and do

not possess the same ability to deform and dissipate applied energy as mild steels.

The toughness of a material may depend on several factors, such as rate of loading,

temperature, and characteristics acquired due to manufacturing methods such as heat-treating

and control of grain growth.

2.2 Impact Testing

The two most commonly performed impact tests are the Charpy and Izod tests. In both

cases, notched bar samples are contacted by a strategically positioned striker. In the Charpytest, the sample is rested between two fixed supports located equidistant either side of the notch

and the striker contacts the face of the specimen obverse to the notch (see Figure 2, left). In the

Izod test, one end of the sample is clamped firmly such that the notch is aligned with the edge of

the clamp (Figure 2, right); the striker contacts the specimen on the notched face 22 mm above

the edge of the clamp.

The striker in this experiment consists of a heavy pendulum, as seen in Figure 3. Due to the

large impact velocity (about 4.5 m/s) and large mass of the pendulum, the sample is deformed at

a high strain rate of about 105 s-1, causing rapid fracture. The absorbed energy can be

calculated as the difference in potential energy of the pendulum at the top of its swing before and

after impact.

2.3 Types of Fracture

Depending on whether a material is ductile or brittle, different surface qualities may be visible

at the site of fracture. Hence, the shape of the fracture may be used to estimate the ductility of a

material: a fibrous surface may indicate failure due to shear forces (ductile failure), or a granular

cleavage may indicate failure from normal forces (brittle failure). The following figure shows

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varying degrees of ductile and brittle fracture characteristics for A36 steel:

In a ductile failure, the shear forces cause a type of plastic deformation known as necking

(a decrease in cross-sectional area) and a slight elongation of the sample if a tensile force is

applied axially. Necking reduces the cross-sectional area until the normal stress at the thinnest

point has been concentrated to the point that it has exceeded the ultimate stress of the materialand the sample fails. The ductility of a fracture can be then quantified by computing the percent

shear lip:

sheerlip 00%% = ABAB xy

1

whereAand Bare the dimensions of the cross-section of the undeformed specimen, andxand

yare the cross-sectional dimensions of the fractured surface. Alternatively, the sample ductility

can be approximated by visually matching the fracture area to a reference chart:

As stated before, the toughness of a material can depend on several factors such as

temperature. The ductility of a material is closely related to the toughness of a material, as ahighly ductile material can plastically deform in order to dissipate the energy applied through

loading. Like toughness, the ductility of a material is dependent on temperature this is due to

the fact that molecules have more energy at high temperature and can be interact more freely

and form slip planes. Low temperature fractures patterns may appear brittle, whereas fractures

at higher temperatures will approach complete ductility.

3 Procedure

A Tinius Olsen testing machine consisting of a heavy pendulum and anvil-style sample

supports was used to perform Charpy tests on a number of samples. Two types of steel were

tested in this experiment: low carbon steel and an unidentified hard steel alloy. A sample of each

was fractured using the impact testing machine and the absorbed energy was recorded.

Samples of each steel heated in boiling water (100C) and cooled in dry ice (about -75C) were

then tested and the fracture patterns were observed.

4 Results

4.1 Raw Data

The absorbed energy from each test was read directly off the dial gauge on the apparatus;

the data is tabulated below:

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Temperature Low Carbon Steel Alloy Steel

-75 20 100

3 16 37

2.5 3.5 3.5

4.2 Experimental Results

The energy absorbed (Cv) was plotted as a function of sample temperature:

5 Discussion

5.1 Effect of Temperature on Cv

From Figure 6, it can be seen that the Cv value for the low carbon steel increases

dramatically with temperature, whereas the Cv for the alloy steel increases only slightly. The

physical interpretation of this is that, although both steels are relatively brittle at low temperature,the low carbon steel become highly ductile with an increase in temperature.

5.2 Relationship Between VHN and Cv

The Vickers hardness number (VHN) is a measure of a materials resistance to deformation,

particularly due to indentation. The published VHNs for low carbon steel vary between 10.0 and

64.0 [2]. The VHN values for low alloy steel vary between 36.0 and 848 [3] and range between

219 and 661 for medium alloy steels [4]. It is quite clear from this data that alloy steels are much

harder than low carbon steels.

Although both steel samples gave similar Cv values for the dry ice test, the low carbon steel

is radically more ductile at higher temperatures. It may be interesting to note that publishedvalues from Charpy tests dont explicitly support or detract from this claim, as the Cv values of

6.00 to 250 ft-lb are expected for low carbon steels [2] and values of 11.1 to 250 ft-lb are

expected for low alloy steel [3].

Using the published VHNs of the samples and the experimentally determined Cv values, it

may be conjectured that VHN and Cv are inversely proportional that is, a hard material is not

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as tough as a soft, ductile material.

5.3 Nature of Fractures Obtained

The types of fractures obtained for each temperature for each type of steel is pictured in

figures 7 and 8.

As the temperature in increased for the carbon steel the facture observed showed more and

more distortion, this observation supports the data recorded. More energy is needed to distort

the hotter, more ductile metal resulting in a higher energy reading from the pendulum.

The mild steel is very ductile at all temperatures tested, but did show more distortion in the

higher temperature sample compared to the coldest sample. The center of the lowest

temperature sample showed brittle fracture and therefore required less energy to break.

5.4 Sources of Error

Possible sources of error for this experiment include inhomogeneous or defective samples,

inconsistencies in the notches, and differences in testing conditions such as materialtemperature and striking location. There is also significant room for error in the testing apparatus

due to bearing friction and air resistance of the pendulum, as well as a small amount of energy

lost during impact in the form of wave propagation.

Evidence of error in the data collected is given by the significant disparity between a typical

Cv value for mild steel at room temperature, about 60 ft-lb (given by lab manual), and the

experimentally determined Cv of 16 ft-lb. The percent error for these two values was computed

to be 73.3 % using the following formula:

6 ConclusionsTemperature can change the behaviour of steel from brittle to ductile with the addition of heat

or from ductile to brittle with the removal of heat. The amount of energy required to break a

ductile material is greater than for brittle material. Vickers Hardness is related to the energy

absorbed in the fracture of a material. The energy required to break the carbon steel was always

less than for the mild steel, supporting the evidence that carbon steel is more brittle and has a

lower VHN. When compared to accepted values and known characteristics, these hypotheses

are found to be correct. The increase in ductility with the addition of heat is also publicly

supported.

Recommendations

To improve the learning experience of this lab some calculations would be helpful. Calculating

the VHN or modulus of elasticity for the samples would be more interactive for students.

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References

1. Anonymous, Mech 320 Solid Mechanics II: Lab #2, Combined Bending and Torsion Lab

Manual, online document, accessed March 6, 2009 [updated Spring 2009], available at:

http://www.me.uvic.ca/~mech320/MECH320-Lab2-CBT-Manual-2009.pdf2. MatWeb materials database, Overview of materials for low carbon steel, online

document, accessed March 18, 2009, available at:

http://www.matweb.com/search/DataSheet.aspx?MatGUID=034970339dd14349a8297d

2c83134649

3. MatWeb materials database, Overview of materials for low alloy steel, online document,

accessed March 18, 2009, available at:

http://www.matweb.com/search/DataSheet.aspx?MatGUID=d1bdbccde4da4da4a9dbb8

918d783b29

4. MatWeb materials database, Overview of materials for medium alloy steel, online

document, accessed March 18, 2009, available at:http://www.matweb.com/search/DataSheet.aspx?MatGUID=f7666326ceb3482f87a9f41ace1

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