Notch bar impact test - Lab text PDF

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IMPACT TEST ON POLYMERS 1.

Introduction The goal of this experiment is to get acquainted with a common mechanical test method: the IZOD impact test. It is also the goal to get an idea of the toughness and the corresponding impact resistance of a number of widely used polymers. We also investigate the influence of the presence of notches and the dependency of this property on temperature.

2.

Theory Read the relevant chapters in the lecture notes as well (or e.g. Shackelford, 8e ed., ch. 8.1). The toughness of a polymer is described as the ability to resist fracture by the absorption of energy, and therefore is an important material property. Toughness can be defined as the area under the stress-strain-diagram. It is a measure of the energy a material can absorb before it fractures.

Fig. 1. In the impact test, we measure the impact resistance of a material, that means its ability to resist a shock load. This is usually done using an instrument with a pendulum hammer that provides for the shock load. The most common methods are shown below. When determining impact value, a notch is made in the test sample to initiate fracture at this position.

Lab materials science – impact test on polymers 2018

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Fig. 2: the notch bar impact tester

A pendulum hammer with mass m is raised to a height h. When we release it, its potential energy is transformed into kinetic energy. A normalized notched test sample is broken by the impact of the hammer at the lowest point of the trajectory. Part of the energy is used to break the test sample, the other part causes the mass to swing through up to a certain height h’. This means that the energy absorbed by the test sample is mg(h – h’). With A the area of the fracture surface, we can define the energy absorbed per unit area as the Notch Bar Impact Value K: K = mg(h – h’) / A

(units: J/cm²)

For a tough material, this value is large, for a brittle material it is small. Unfortunately, the value determined in this way is not a material property. It also depends on: - the size and form of the sample and the notch, - the temperature, - the velocity of the hammer when it hits the test sample. This means that the test apparatus and the test samples have to be normalized in order to be able to compare the results of different tests. Lab materials science – impact test on polymers 2018

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Many of the commonly used polymers have an impact value smaller than 5 kJ/m2. A designer using polymers is therefore bound to pay al lot of attention to the possible effects of sharp corners, screw-threads, scratches and other places where stress concentrations may occur. When unnotched, many polymers have an excellent impact value. The influence of temperature is very important. Even tough materials turn out to behave brittle when subject to shock loads at low temperatures. When preparing for this experiment, read the corresponding theory in the course text, and also re-read the part about determination of the statistical error in the introduction of the first year lab text on error theory.

3.

Experiment

3.1.

Method of measurement

3.1.1.

Preparation of the test samples. You have a number of bars of PS (polystyrene) and PVC (polyvinylchloride) at your disposal. From these bars, you cut 35 samples of PS and 5 samples of PVC, each with a length of 80 mm. You can do that with the cutter on the workbench. Please wear safety gloves and safety glasses. Each student measures width and thickness of 2 PS samples using a Vernier caliper. Use the average value for your calculations. Repeat for the PVC samples. Mount a saw blade between the jaws of the vice on the workbench with the teeth pointing upward. It must be positioned horizontally and should protrude 2 mm above the top surface of the jaws. Now cut a notch in the thin long side of 10 of the PS samples by moving them back and forth across the teeth of the saw blade. The notch musty be in the middle of each sample. Try to keep the samples perpendicular to the blade during cutting!

Do the same for all 5 PVC samples. Switch on the oven and put 5 of the notched PS samples inside. The temperature is set to 80 or 90 °C. Each student measures the depth of 2 notches. Continue with the average value of all these measurements. Repeat the procedure to cut a notch of 1 mm deep in five of the remaining PS samples, and a notch of 0.5 mm deep in another 5 samples. Again each student of the team measures the depth of these notches 2 times, and you will use the average value in your calculations.

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In yet another series of 5 samples, make one single scratch with a scratch stylus. 5 more PS samples are left without scratch or notch. Finally, sand the thin faces of the last 5 samples slightly with fine sand paper in the length direction until all cutting marks have disappeared and the surface feels smooth.

3.1.2.

Izod measurements Open the safety screen at the left side of the impact tester. The on/off switch is at the top left side of the instrument. Switch it on. Normally, the apparatus is adjusted correctly. So you do not need to use any of the keys on the front panel, not even the reset-key!

Fig. 3.

Fig. 4.

Lift the pendulum hammer by hand (turn counterclockwise) until it rests on the hook. Now pull down lever 1 (“hendel 1” in fig. 4) and put the sample in the sample holder. It must be positioned vertically in the centre of the holder, the notch pointing towards the hammer and the centre of the notch coinciding with the upper edge of the holder, as in the top left drawing of fig. 2. Then push handle 1 upward again so that the sample is fixed.

Fig. 5. Lab materials science – impact test on polymers 2018

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Now pull handles 2 and 3 at the same time, using both hands. The hammer drops and fractures the specimen. Brake the pendulum by pushing down the large lever on top of the instrument. Read the energy absorbed by the test sample on the screen. Collect both halves of the sample. One flies into the screened space behand the instrument, the other will fall into the small drawer at the front, after you pull down lever1 again. Do not try to reset the instrument for the next measurement.

3.1.3.

Safety prescriptions. Wear safety goggles and safety gloves while using the cutter. Be very careful and never bring your fingers in the vicinity of the knife edge. Wear safety goggles while executing the impact test. Avoid burns and use heat resistant gloves and pliers while manipulating warm samples.

3.2.

Measurements First measure friction losses by repeating the test 5 times without a sample. Then measure the energy absorption successively for: - the 5 PVC samples; - the 5 PS samples with a notch of 2 mm; - the PS samples with a notch of 1 mm; - the PS samples with a notch of 0.5 mm; - the PS samples with the scratch. - the unnotched PS samples. Take care that the centre of the sample coincides with the top edge of the sample holder. You can indicate this centre with a felt tip pen, but not with a scratch stylus! - the grinded PS samples. Finally measure the energy absorption for each of the PS samples with a notch of 2 mm heated to (approximately) 80°C. About the measurements at higher temperature: Turn on the polymer oven in time. It is set at 80°C. However, the temperature dependence of the impact value will only be studied qualitatively, because it is impossible to measure the temperature of the samples properly with the given set-up. Place the 5 rods of the last group in the oven. Use gloves and pliers. After 20 minutes you take one rod out of the oven. Clamp it in the IZOD device and measure the impact energy measurement immediately. Work as quickly as possible to prevent the rod from cooling down too much. Repeat for the other rods. Then switch off the oven! Lab materials science – impact test on polymers 2018

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REPORT IMPACT TEST

SCORE

Group: Team: Names:

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Date: Teacher:

_____________________________________________ 1.

Measuring results

1.1.

Frictional losses

measurement

Energy loss (

)

1 2 3 4 5 Average value

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1.2.

Polyvinylchloride (2mm notch) State the measuring error for all dimensions! Sample thickness:

Sample width:

Notch depth:

Fracture area:

Meas.

Impact energy (

Corrected impact energy

)

(

)

Impact value K (

)

1 2 3 4 5

Complete example calculation of area and impact value:

Average impact value : Standard deviation : Statistical error : Conclusion: KPVC = Lab materials science – impact test on polymers 2018

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1.3.

Polystyrene: influence of notch depth State the measuring error for all dimensions! Sample thickness: Sample width: Uncorrected measured impact energy W in Joule Notch depth (mm)

W2

W1

W3

W4

W5

Wgem

σW

scratched unnotched grinded

Results: Notch depth (mm)

Wgem

Fracture area A

(

)

(

)

Corrected value

Impact value

Wc

K

(

)

(

)

Statistical error

(

)

scratched unnotched grinded The statistical error of K is calculated as 2

Lab materials science – impact test on polymers 2018

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=

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1.5.

Influence of temperature (polystyrene)

Meas.

Impact energy (

)

Corrected impact energy (

)

Impact value (

)

1 2 3 4 5

Average value: Standard deviation : Statistical error : Conclusion: KPVC, Thigh =

2.

Conclusions 1. Compare the impact behavior of polystyrene and polyvinylchloride.

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2. What influence does the notch depth have on the impact value of polystyrene? Does this match your expectations?

3. How does the temperature influence the impact value of polystyrene? Does this match your expectations? Can you explain this?

3.

Questions 1. Which losses occur when the test is executed without a test sample? Which type of error can be associated with these losses (random or systematic)?

2. State some possible causes of error for these measurements.

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3. Could you also determine the impact value of steel alloys with this instrument? Explain.

4. In reality you will seldom find notches like the ones in the measuring samples. Give some examples of notches that do occur in engineering practice.

5. Give two examples from daily life where the impact value of the polymer used has been decisive for het material choice of the product.

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