mml2014 ga.r.g sreekar repcompression

8/11/2019 MML2014 GA.R.G Sreekar REPCompression

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A.R.G Sreeka

MM12B002

(Total pages -5


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To calculate the Yield stress, Ductility at different strain rates and also Youngs modulus and strain rate sensitivity of Pure

(99.9%) Aluminium.

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ZWICK/ROELL Machine of maximum capacity 10KN, Digital Vernier calipers, aluminium samples.

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Aluminium alloys are important technological materials primarily due to their advantageous

strength to weight ratio. They are used in diverse applications ranging from packaging to aeronautic industry. The

compression test is performed on the aluminium at different strain rates to find out how the material behaves in

such conditions. Increase in the strain rate generally leads to increase in dislocation density, so, the dislocations

will interact with each other and may form lomer-lock which hinders the movement of dislocation and thus results

to an increase in the amount of stress required to move the dislocations. The energy that has to be provided fordislocations to overcome the barriers they encounter during slip determines the dependence of the flow stress on

Temperature and applied strain rate. Flow stress is the amount of stress required to move the dislocation. If the

energy barriers are sufficiently small for thermal energy (~kT) to be significant, thermal vibrations of the crystal

atoms may assist dislocations to overcome obstacles at lower values of applied stress than that required at 0 K.

Under such conditions, an increase in temperature, or a reduction in applied strain rate, will reduce the flow stress.

To the overcome the barrier either thermal has to be supplied or mechanical work must be done . The relation

between strain rate and flow stress is reported by HULL[1].

The relation states strain rate is directly proportional to dislocation density. Stress dependency

on the strain rate () is Stress = A*(strain rate) ^m; where A is a material constant, m is strain rate sensitivity, which

is material dependent. So the flow stress should decrease with a decrease in the strain rate, but there are some

exceptions. The experiment was performed under room temperature. The sample used in the test should satisfy

the condition (D/L)


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Youngs Modulus of the pure Aluminium is obtained from the Fig-1 by finding out the slope between the two

points in the elastic region.

E = (193.9-140.6)/(0.03462-0.02733) = 7.3 GPa.

Yield stress is the point of intersection of the straight line and the curve in Fig-1. So, the yield stress at a strain

rate of 1 mm/min is found to be 266.9 MPa.

Ductility is defined as the maximum elongation the sample has undergone before fracture. So the ductility at a

strain rate of 1 mm/min was found to be 25%. Caution, here ductility is measured at a maximum force of 8KN

and for different strain rates.

Strain rate sensitivity (m) is found out from the Fig-2 at a true strain of 0.05.

m= log[

1/2]/log[

1/2

]

=log(262.7/251.9)/log(0.1/0.0001).

= 0.0060.

Youngs Modulus of pure aluminium (E) = 7.3 GPa.

Yield stress at strain rate 1 mm/min = 266.9 MPa.

Ductility at strain rate 1 mm/min = 25%.

Strain rate sensitivity (m) = 0.0060.


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Yield stress is decreasing with decreasing in the strain rates ranging from 0.1 mm/min to 0.0001mm/min from

Fig-3 but deviates when the strain rate is 1 mm/min, may be due to the generation of thermal energy at that

strain rate.

Work hardening rate appears to be same at all strain rates. Ductility is decreasing with the decrease in the strain rates.

Youngs modulus calculated from the experimental data is far less compared to the value calculated by

Tadanobu Inoue[2].

The image of the sample after the compression test is sheared, which should not happen. There is a frictional

force acting on the sample at the contacts during compression.

There is an error in the experimental data recorded.

:

1. D. Hull and D.J Bacon : Introduction to Dislocations fourth edition. Page 195.

2.

Tadanobu Inoue, Zenji Horita, Hidetoshi Somekawa and Fuxing Yin : Distributions of Hardnessand Strain during Compression in Pure Aluminum Processed with Equal-Channel Angular Pressing andSubsequent Annealing

mml2014 ga.r.g sreekar repcompression

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