impact testing

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Leticia Garreto
1234043
ENMP211-15A
Materials 1
Experiment 6
Impact testing
Lab stream: Wednesday 10 – 1
Date of experiment: 22/04/2015
Due Date: 29/04/2015
Abstract
Impact testing is used to determine fracture characteristics of materials. It can be Charpy or Izod: both tests are done in the same way, only changing the position that sample is tested. In this experiment, aluminium and low carbon steel were tested under very low and high temperatures, aiming to analyse the effect of temperature in their mechanical behaviour. It was observed that aluminium did not show considerable changes in the amount of energy absorbed, whereas the impact results for low carbon steel fluctuated. As low carbon has a BCC crystal structure, it displayed brittle behaviour at lower temperatures and ductile behaviour at higher temperatures. Aluminium has a FCC crystal structure, so it behaved as ductile. 
Introduction
Impact testing is used to access the fracture characteristics of materials. The conditions chosen to perform an impact test represent the most severe conditions that leads to fracture: deformation at relatively low temperatures, high strain rate, and triaxial stress state (introduced by the presence of notches). 
There are two impact tests used to measure the impact energy: Charpy and Izod. For both tests, the sample is in the shape of a bar of square cross section, with a V-notch in one of the surfaces. The specimen is positioned at the base of the tester (figure 1), and is hit by a weighted pendulum hammer which is released from a cocked position. This causes the fracture of the specimen at the notch, which acts as a point of stress concentration for high-velocity blow impact. The main difference between Charpy and Izod is the way the specimen is placed in the base of tester (figure 1). 
Figure 1 - Representation of impact test
In general, materials can fail in a ductile or brittle manner. Ductile materials neck down to a point of fracture, showing reduction in cross sectional area, causing the “cup and cone” failure shape. At the microscope, it is possible to see numerous spherical “dimples”. Each dimple is one half of microvoid that formed and then separated during the fracture process. Brittle fracture takes place without plastic deformation, and by rapid crack propagation. It results a flat surface, and with naked eye it is possible to see V-shaped “chevron” markings or lines radiating from the origin of the crack in a fanlike pattern. Also, the propagation of the crack can be transgranular (cracks pass through the grains) or intergranular (cracks propagate along the grain boundaries).
One of the main functions of Charpy and Izod tests is to determine if the material experiences a ductile-to-brittle transition when temperature is decreased. The amount of impact energy absorbed (if it is changing or not) revels a possible temperature dependence of the material. For some materials, at higher temperatures the Charpy V-notch (CVN) energy is relatively large, in correlation with a ductile mode of failure. Analogously, the energy absorbed drops at lower temperatures, representing a brittle mode of failure. This behaviour is related to the crystal structure of the material.
Face-centred cubic (FCC) and hexagonal close packed (HCP) metals do not experience ductile-to-brittle transition, and remain impact energies (i.e. remain ductile) with decreasing temperature. Examples are some aluminium and copper alloys. Body-centred cubic (BCC) metals will decrease the amount of impact energy absorbed at lower temperatures, and increase the energy absorbed at higher temperatures, so that displaying ductile-to-brittle transition. An example is low carbon steel. 
Experimental
A Charpy impact tester was used.
Two metals were subjected to an impact test: low carbon steel and aluminium.
The samples were tested under six temperatures, and for each temperature, three samples of each metal were tested.
Two fractured samples of low carbon steel at -196°C and 100°C were collected for microscopic analysis.
Experiment was done quickly to avoid change in temperature of samples.
Impact values were recorded and summarized in table 1.
Table 1 - Impact results for aluminium and low carbon steel
	Temperature (°C)
	Energy absorbed (J)
	
	Aluminium
	Low carbon steel
	-196
	23
	1
	
	22
	2
	
	24
	2
	-20
	27
	5
	
	23
	8
	
	23
	11
	0
	29
	10
	
	23
	12
	
	24
	8
	25
	22
	90
	
	21
	110
	
	21
	102
	60
	22
	99
	
	23
	100
	
	23
	100
	100
	20
	101
	
	21
	94
	
	22
	103
Refer to ENMP211-15A lab manual for more details.
Results and Discussion
From the results reported (table 1), an impact values vs temperature graph was plotted (figure 2).
Figure 2 - Energy absorbed vs temperature graph
In the graph, it is possible to see that aluminium remained the amount of energy absorbed nearly the same regardless the temperature, whereas low carbon steel showed a lot of variation. As the temperature was increasing, low carbon steel was changing from a brittle to a ductile behaviour. This is due to its body-centred cubic (BCC) structure. BCC is characterized by the presence of lots of slip systems (which are a combination of close packed planes and close packed direction) that are temperature dependent. That is, when the temperature is raised, low carbon steel shows a ductile behaviour; when temperature is decreased, it behaves as brittle. Aluminium has a face-centred cubic (FCC) structure, having a lot of slip systems that are not temperature dependent, so it shows a ductile behaviour regardless the change in temperature. 
The change in the behaviour of low carbon steel can be observed in the samples collected for analysis. The sample exposed to -196°C showed a flatter surface than the one exposed to 100°C, which displayed almost a “cup and cone” fracture. Flat surfaces are a feature found in brittle fractures, while “cup and cone” can be found in ductile fracture. 
At the microscope, it could be observed an extensive tearing in the sample subjected to 100°C, which is another key feature of ductile fracture (figure 3). 
Figure 3 - a) low carbon steel at - 196 °C b) low carbon steel at 100 °C
The type of crack propagation observed in the cold sample was intragranular or transgranular (figure 4a), at which the crack propagates through the grains. The fracture surface may have a grainy texture. Crack propagation occurred in the hot sample was intergranular, that is, crack propagated along the grain boundaries (figure 4b).
Figure 4 - Crack propagation a) intragranular b) intergranular
The temperature dependence should be taken in consideration when choosing low carbon steel for some applications. Knowing at which temperature the material starts to behave as brittle can avoid catastrophic failure. However, it is difficult to specify a single ductile-to-brittle transition temperature. The most conservative transition temperature is that at which the fracture surface becomes 100% fibrous. 
Conclusion 
Some materials can change their behaviour by changing the external temperature, showing a ductile-to-brittle transition. At low temperatures, the material behaves as brittle and at higher temperatures, the material behaves as ductile. Knowing the ductile-to-brittle transition temperature can be crucial to avoid catastrophic failures of the material.
References 
Callister, W. D., Materials science and engineering: An introduction, 7th Edition, John Wiley & Sons: New York, N.Y., 2007
ENMP211-15A Laboratory manual; University of Waikato: Hamilton, New Zealand, 2015
Appendix 
Figure 1 – Retrieved from Callister, W. D., Materials science and engineering: An introduction, 7th Edition, John Wiley & Sons: New York, N.Y., 2007, p. 224
Figure 4 – Retrieved from Callister, W. D., Materials science and engineering: An introduction, 7th Edition,John Wiley & Sons: New York, N.Y., 2007, p. 213, 214