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Impact Testing

Objective

The main objective of this lab is to perform tests on Charpy impacts where 1081 Steel and 6061 Aluminium are examined in order to determine the transition temperatures from ductile to brittle under various temperatures. The Charpy Impact test is used to measure notch toughness or the impact energy. The two materials underwent 8 tests at different temperatures which were obtained by mainly having the Charpy endure different conditions. Among the conditions included: liquid nitrogen bath, boiling water, the box furnace twice at different temperatures, room temperature, dry ice bath, ice water bath and ice-brine. The reason for performing this particular test is to duplicate a life situation where such a material may be put in. The lateral expansion length was also measured in order to obtain the cross section of the V notch. The cross sections were later used where they were compared with the initial cross sections to determine the percentage shear failure caused by temperature. All the tests were conducted uniformly with the material temperature changing so as to give best results.

Theory

Toughness is a measurement of the amount of energy, which is exerted, a material absorbs before leading to its fracture. For engineers, toughness is highly important because it is used in selecting materials that they are to use in their projects. In selecting the materials, there are three conditions that are looked at and they are: a high strain rate, a tri-axial stress state and deformation at a relatively low temperature. The determination of a material’s toughness and the impact energy is tested under two standardised tests, the Izod and Charpy test. The Charpy test is the most commonly used test today which is also referred to as CVN (Charpy V-notch).

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The figure above is used to show the Impact Sample Placement.

The specimen is fractured once the pendulum hammer, which is at a fixed height, is released striking it at the notch as shown in the above figure. The test apparatus will then display the amount of energy that is applied. This shows different amounts of energy under different temperatures. Transition from brittle to ductile state occurs over the ranging temperatures where there are several procedures that help in determining the actual transition temperature. In this lab, the tested temperatures form a curve that is shown below which shows the Charpy Impact Test results.

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The transition temperature is when the surface which is fractured is 50% ductile. The way the transition temperature is usually determined is the temperature where the fracture energy is actually half way between pure ductile and pure brittle.

Experimental procedure and apparatus

The apparatus that was used in this lab was a standard V-notch test.

First, the pendulum hummer is unchained and locked into a position behind the fence. The specimen is later placed in the anvil with the use of a pair of tongs. Then, the joule gauge is set to the maximum value while the safety latch is unlatched. Once everything is set, the hammer is released and when the hammer breaks through the specimen, the energy value is recorded. The hammer is later re-engaged in the same locked position as before as the safety latch is put in place. The broken specimens are collected and the lateral expansion is measured. On completion, the broken specimens are labelled.

Results and discussions

The objective for carrying out this lab was to see the effect temperature has on different materials. With the use of the Charpy test, the impact behaviour was recorded and is as follows in the figure below:

|Temperature(°C) |Energy(J) |Energy(J) |
| |Aluminum |Steel |
|-196 |26.66666667 |0 |
|0 |20.66667 |71.66667 |
|20 |22 |88 |
|100 |19.5 |83.66667 |
|250 |19.66667 |92.33333 |

From the general observation, temperature minimally affects the aluminium material and affects the impact behaviour of steel. More energy was required for the breaking of the steel V-notch as the temperature rose.

Impact behaviour of aluminium shows how temperature had minimum effect on the material. This is shown in the graph presented below:

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The samples of aluminium show no transition to brittle even when subdued to nitrogen’s temperature. The difference in crystal structures is the main reason for this in the metal. Steel usually has cubic crystal structure while aluminium has a cubic crystal structure which tends to experience less transition in any temperature range.

Results on the failed cross sections of each of the materials are shown in the table below:

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The results mainly help in comparing with the original cross sections. With the values, percentage shear failure could be determined at each of the varying temperatures. Steel’s cross section area is steady while that of aluminium varies. Steel’s percentage shear failure falls with a range of 6-8 percent which is normal. On the other hand, aluminium’s percentage shear failure at boiling water temperature shows that it has a stronger resistance to fail in shear.

Conclusion

This lab shows us how temperature affects the behaviour of aluminium and steel. For steel, it broke because the temperature was below its brittle temperature. Due to different crystal structures, this lab shows the difference between steel and aluminium. For prospecting engineers, this lab shows the major importance of knowing how different materials behave under different temperatures and conditions.