With standard equipment and analysis methods you cannot reliably measure an elastic modulus from an ordinary tensile test (instead, what is measured here is a much lower system stiffness value which involves many other factors).

Elastic modulus probing must be better done by ultrasonic excitation for instance or by small dynamical loading cycling etc. Even then you need very detailed error estimates and data analysis methods. There is good dedicated literature available for that, see e.g. national labs and professional testing labs etc.

When you try to extract the material's modulus from a tensile test you have a number of effects which can substantially alter (more specific: substantially lower) the result, such as for instance:

(i) the relative motion between the sample and the grips,

(ii) the non-negligible stiffness of the entire machine and particularly of the sample grip / holding set up,

(iii) the way how the strain is actually measured (one should try to do that for instance by laser methods or by digital image correlation directly on the sample etc (best in the center region, and only during the very early straining stage; by no means near the yield strength), i.e. do not measure strains by tracking the motion of the sample holding devices),

(iv) Microplasticity: The very early onset of microplastic inelastic deformation far below the yield strength ( this is simply due to the fact what we call yield strength is actually only a signal for the large-scale percolative motion of huge numbers of dislocations while below the yield strength there is also substantial plasticity (which we call microplasticity) which adds to the apparent elastic stiffness of the system - this is however a result of microplasticity. The reason for that is that also for stress levels below the yield strength many grown-in / inherited dislocations have very large pinning point spacings so that their bow-out stress is actually much smaller than the yield strength, i.e. they are active at very low strain, they just usually do not percolate the glide plane from GB to GB (see image).

If u do have to do it on a standard test than better make sure to only use the force – diplacmt data from the early onset of elastic yielding; the lower the stress the better, but better use more advanced test protocols.

Good luck.

Probably experimental error due to slippage.

Prof. Dierk Raabe has already explained the reasons in detail.

"Standard" tension tests are NOT the best way to obtain the Young's Modulus. If you really want to obtain the elastic modulus from tension test, it is recommended to perform repetative loading and releasing between 20% to 80% of yield strength (to minimize the effect of possible slippage of grips), use high precision extansometers (clip-on, laser or DIC), use well calibrated load cell and also take the stiffness of your setup into account.

You can check Chapter 3 of the following thesis:

https://tez.yok.gov.tr/UlusalTezMerkezi/TezGoster?key=sY7m19PfcL6F1NUw-cr80GGQkg91BgKsb9-mODhF_3mY2m5cngZ8Rt5eqksCMNc5

for comparison of the techniques (repetative tension, ultrasound velocity measurement, RFDA, 3 or 4 point bending).

Also note that "National Physics Laboratory" recommends RFDA (resonance frequency and damping analysis) method (per ASTM E1876) for measurement of elastic properties. This method can also be used to determine the elastic properties as a function of temperature.

This is caused by sliping of sample in the grips and deformation of equipment (beams, jaws etc.)

Raabe is correct, Stiffness of the tensile tester masks true value of modulus. Ultrasonic test is best.

The stiffness of universal testing machine and relative slippage of grips and specimen can affect the measured young modulus.

Clip on extensometers with accuracy grade of 0.5 according to ISO may help you.

Well! Slippage can be one possibility! But first let me know whether you have used extensometer to measure the strain?

I would carryout a calibration of the load cell. Also check the capacity of the machine for the geometric size of the specimen being tested.

Several mention equipment related sources of error. Measuring E from the slope of the ASTM E8 stress strain curve is not the best approach.

I recommend ASTM E111 to measure precision modulus. Measure Dynamic Modulus via ASTM E1875 or 1876. The downside of measuring the dynamic modulus is that you cannot separate the tension from the compression moduli. You will also get a slightly higher number from the dynamic modulus compared to the static modulus.

Consider the application. Airframe designers prefer the static modulus while engine designers use dynamic modulus because these different measures are applicable to their different scenarios.

Have you taken the reduction in the cross-sectional area into consideration?