I noticed 10 times reduction in percentage of elongation for magnesium alloy while sample size was reduced by a factor of two. The initial dimension was 25*6*6 mm and I got 10% elongation. Percentage of elongation was only 1% [calculated from the machine generated data] after reducing the size to 12.5*3*3 mm. Does it an experimental error? Load vs displacement curve of smaller sample showed an extension of 1.2 mm before fracture. According to that percentage of elongation is 10% at breaking point. I found that slimness ratio has inversely proportional relation with elongation but in this case gauge length also reduced by a same factor.
Yes, this is definitely be the experimental error or the smaller sample might have some issue (defect etc). The stress-strain curve is independent of the size of the sample. Although we sometimes assume that by changing the dimension the amount of defects in the sample can be changed. In very thin samples (
https://www.sciencedirect.com/search?qs=elongation%20of%20tensile%20sample%20and%20size&show=25&sortBy=relevance
It is highly dependent on the tensile specimen size due to occurrence of different state of stress and possible constrains which are mainly applied by grip sides. Lower l/w (gauge length/ width of reduced section) results in lower total elongation due to occurrence of plane strain condition.
What was the initial condition (whether as-cast) and grain size of the sample ? If grain size is of the order of the minimum dimension that was used for testing then that causes problems in scaling properties.
yes there is a "size effect" for elongation. Not only depends on the gauge length, but also when smaller, the volume fraction of defects on the cross section may raise up that in return reduces the elongation.
Yes, elongation of tensile sample is not only depend upon size of the sample, but also dependent upon grain size, size of defect in material.
Hi
There was a very interesting question! I believe that by reducing sample sizes, the role of defects can be prominent
and this is very effective for materials with low SFE and for this materials, defects is determined elongation behavior of them under tensile test.
Yes, it is depenedet on tensile sample size due to gauge length changing.
Yes the size matters for the percentage of elongation in Tensile test.Experiments have shown that as the slimness ratio decreases the measured Elongation increases. Holding the slimness ratio, K, constant will minimize the effect of changes in specimen dimensions on Elongation.The formula for slimness ratio is (𝑺𝒍𝒊𝒎𝒏𝒆𝒔𝒔 𝑹𝒂𝒕𝒊𝒐 = 𝑳𝟎 √𝑨o).
(In tensile test using different geometry of test piece , Reduction of Area is normally measured only on round test specimens because the shape of the reduced section in the neck remains essentially circular throughout the test. With rectangular test pieces, the corners prevent uniform flow from occurring, and consequently the shape in the neck is no longer rectangular)
Dear Sourav Dutta
Absolutely size plays a major role in the percentage of elongation in a tensile test.
Please refer the attachment below.
https://www.admet.com/effect-specimen-geometry-tensile-testing-results/
Best regards,
Balasaravanan K.
Not only size, but the conditions under which the sample was made. Granted, this is probably more obvious with plastics (my current platform) than metals. By changing various parameters in the process we can affect the tensile strength and elongation of monofilament without affecting its size. This is why we always make a fresh control before making test samples - we find that comparing to previous data is a bit like comparing Macintosh apples to Gala apples. While it is still all apples, there is enough variation to skew results.
This is a very strange result. Usually the ultimate deformation decreases with the increase of the sample size in accordance with the scale effect. In your case, it is either the instability of the technology or the experiment error
Do you have statistics on the repeatability of the results? It can have such an effect if your samples have a non-homogeneous composition in length.
@Petr @Plamen The size of the filament is determined by extruder flow rate, die holes, and process exit rate. After the size is set, the tensile strength and elongation properties can be manipulated within quite a large range by adjusting the various heat and draw stages, affecting how the long polymer chains interact with one another. Homogeneous for thousands of miles worth of filaments – the customers would be unhappy elsewise. In prior experience variances in the mix ratio of 2 part RTVs or epoxies would affect these properties as well, which is what made it a good quality check for the mix process.
That depends on which types of specimens you adopted in the experiment. As recommended by EN ISO 6892-1, a proportional specimen is suggested with a standardized correlation between gauge length and cross-sectional area. In simple terms, gauge length is equal to 5-times of its diameter for a cylindrical coupon. As such, all test results are comparable under the same definitions of all quantities.
You may refer the following papers for determination of true stress-strain characteristics:
[1] Ho HC, Liu X, Chung KF, Xiao M. Tensile tests on high strength steel materials with high precision measurements. In: Fifteenth East Asia-Pacific Conference on Structural Engineering and Construction (EASEC-15), 11 ∼ 13 October 2017, Xi'an, China. (Paper No.: P1404); 2017.
[2] Ho HC, Liu X, and Chung KF, Xiao M, Yam MCH and Nethercot DA. (2018). True stress-strain characteristics of high strength S690 steels. International Conference on Engineering Research and Practice for Steel Construction 2018, Hong Kong, pp 160-167.
[3] Ho HC, Chung KF, Liu X, Xiao M and DA. Nethercot. (2019). Modelling tensile tests on high strength S690 steel materials undergoing large deformations. Engineering Structures, 192:305-322.
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