I have casted a rod of A356 alloy alloy and got the compression test samples fro the center of it. The elastic modulus was measured to be 9 GPa which is far from average which has been reported (72 GPa). Is it possible to be like this or it could be because of the problem in calibration of testing machine. I don't think that the microstructure and casting defects can cause such a high difference.
No of course, you have to check everything again and redo the tests!, otherwise, your samples have a considerable amount of porosity!
I think that we figured the problem out. One possibility could be the surface imperfection and the sample settlement. Since the elastic modulus is calculated from a very narrow strain range, a very small test error may result in huge error in the slope of the stress-strain curve in elastic area.
Generally, it is quite difficult (almost impossible) to get E-modulus data from ordinary compression tests. Your low E modulus is probably due to the method you are using. In order to evaluate this further, I need to know your specimens geometry, the compression test set-up (have you minimized friction at both ends?, And: is your strain measured from the press stroke displacement or a strain gauge?.
If you need exact measurements of the E- modulus, you could: (I) machine an axial fatigue specimen with a relative thick gauge-length , (ii) use a normal extensometer, (iii) make sure the specimen is perfectly aligned vertically and have a good grip at both ends.(iv) load carefully at low strain rate (say 1x10-4 per sec) in compression to a maximum strain of about 8x10-4 while recording simultaneously the strain and the stress. Then you unload - reload - unload 5-10 times.The linear part of the average stress-strain curves is your E-modulus in compression. Please note that the E-modulus in tension and compression might be slightly different in this alloy, but not as much as for a Mg alloy or pure Mg.
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