Mechanical Failure: Brittle Fracture and Impact Testing

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  • Topic: Fracture, Fracture mechanics, Charpy impact test
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  • Published : May 9, 2013
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FAKULTI TEKNOLOGI KEJURUTERAAN
UNIVERSITI TEKNIKAL MALAYSIA MELAKA
|
ENGINEERING MATERIALS|
BKTP 1313| SEMESTER 2| SESI 2012/2013|
LAB 4: IMPACT TEST |
NAME | |
MATRIX NUMBER| |
COURSE| |
DATE| 02 / 05 /2013|
NAME OF INSTRUCTOR| 1..|
| 2.|
EXAMINER’S COMMENT| VERIFICATION STAMP |
| TOTAL MARKSProcedure: /15Data recording: /15 Results: /15Discussion: /15Questions: /30Conclusion: /10 |

OBJECTIVE
1. To study the principle of impact testing by understanding the resistance of a material to crack propagation can be determined using impact test. 2. To characterize the impact fracture behavior of different temperature. 3. To determine the energy absorbed during fracture.

4. To understand the difference between brittle and ductile fracture and the energy absorbed in each condition. 5. To interpret the obtained experimental data for the selection of engineering materials.

EQUIPMENT AND APPARATUS

1. Impact testing machine
2. Copper alloy rod (D 6mm)
3. Aluminum alloy rod (D 6mm)
4. Teflon rod (D 6mm)
5. Handsaw
6. Verniercaliper
7. Beakers
8. Oven
9. Gloves
10. Tongs
11. Thermometer
12. Ice cube
13. NaCl (salt)
14. Water
15. Bunsen burner

SYNOPSIS ADN THEORY
Notched-bar impact test of metals provides information on failure mode under high velocity loading conditions leading sudden fracture where a sharp stress raiser (notch) is present. The energy absorbed at fracture is generally related to the area under the stress-strain curve which is termed as toughness in some references. Brittle materials have a small area under the stress-strain curve (due to its limited toughness) and as a result, little energy is absorbed during impact failure. As plastic deformation capability of the materials (ductility) increases, the area under the curve also increases and absorbed energy and respectively toughness increase. Similar characteristics can be seen on the fracture surfaces of broken specimens. The fracture surfaces for low energy impact failures, indicating brittle behavior, are relatively smooth and have crystalline appearance in the metals. On the contrary, those for high energy fractures have regions of shear where the fracture surface is inclined about 45° to the tensile stress, and have rougher and more highly deformed appearance, called fibrous fracture. Although two standardized tests, the Charpy and Izod, were designed and used extensively to measure the impact energy, Charpy v-notched impact tests are more common in practice. The apparatus for performing impact tests is illustrated schematically in Figure-I. The load is applied as an impact blow from a weighted pendulum hammer that is released from a position at a fixed height h. The specimen is positioned at the base and with the release of pendulum, which has a knife edge, strikes and fractures the specimen at the notch. The pendulum continues its swing, rising a maximum height h ' which should be lower than h naturally. The energy absorbed at fracture E can be obtained by simply calculating the difference in potential energy of the pendulum before and after the test such as, E = m.g.(h-h ')

where m is the mass of pendulum and g is the gravitational acceleration. The impact test results are affected by the lattice type of materials, testing temperature, thermo-mechanical history, chemical composition of materials, degree of strain hardening, etc.

Ductile to Brittle Transition Curve (DBTT)
III. Fracture Mechanisms
At higher temperatures the yield strength is lowered and the fracture is more ductile in nature. On the opposite end, at lower temperatures the yield strength is greater and the fracture is more brittle in nature. This relationship with temperature has to do with atom vibrations. As...
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