The above answer does not seem satisfactory. Indeed, strain hardening, which is due to an increase of the dislocation, will occur whatever the loading direction is (i.e. tension or compression).

In my opinion, the most likely explanation for this asymmetry is the impact of processing. Indeed, during the fabrication of your alloy, a significant amount of plastic deformation has been used for forming (e.g. extrusion, rolling). Because this plastic deformation result in strain hardening, the yield surface of your alloy has significantly been altered by processing. Strain hardening is usually directional (i.e. kinematic hardening), which means that the evolution of the yield strength is direction dependent. The consequence is that, because of processing-induced hardening, the compression and tensile yield strength are different.

No, the Yield strength for compression test and tensile test will be different for AA-7075. The possible reason for this is .. when we compress the material the dislocation density increases which leads to increase in strain hardening of the materials results in higher strength.

The above answer does not seem satisfactory. Indeed, strain hardening, which is due to an increase of the dislocation, will occur whatever the loading direction is (i.e. tension or compression).

In my opinion, the most likely explanation for this asymmetry is the impact of processing. Indeed, during the fabrication of your alloy, a significant amount of plastic deformation has been used for forming (e.g. extrusion, rolling). Because this plastic deformation result in strain hardening, the yield surface of your alloy has significantly been altered by processing. Strain hardening is usually directional (i.e. kinematic hardening), which means that the evolution of the yield strength is direction dependent. The consequence is that, because of processing-induced hardening, the compression and tensile yield strength are different.

No,the yield strength is different in tensile and compression test.

No,it is not the same value

This may result because of Bauschinger effect. For more information about this effect please refer to the following link :

https://en.m.wikipedia.org/wiki/Bauschinger_effect

Yield strength cannot be same for the compression test and tensile test of a metallic material and it is more pronounced in case of dductile materials. It may be largely understood in terms of the dynamics of delay in defects creation under this two different kinds of stressing pattern in the matrix.

@Charles Mareau, Thanks for your information and I am partially agreed with you. However, there is still different on the yield strength even on the as-cast aluminum alloys, such as Al-Si 356 or 319 alloy, in which the processing is not involved.

@Kun Liu, Yes, the above explanation does not apply to as-cast aluminium alloys (e.g. Al-Si alloys) for which the reason for the asymmetric yielding is different. My knowledge of these alloys is limited but my understanding is that the aluminium-rich phase and the silicon-rich phase have quite different thermal expansion coefficients so that significant internal stresses are produced during cooling. As a consequence, even when no macroscopic stress is applied, the soft aluminium-rich phase, which governs the development of plasticity, is already stressed and the compression and tension yield strengths are not the same.

Yes. The dynamics of defects creation in the matrix of a cast and wrought material are different under test loading. This is primarily because the Cast matrix is having lots of different kinds of heterogeneity that hinders the dislocation and diffusion characteristics.

Внутренные структурные свойства исходного материала.

Sir Zaza Taliashvili ,

Need English version.

Dear Chakraborty,

The yield strength in compression and tensile tests could be the same only when the materials are perfect ( e.g. no inclusions, no voids). Because your material is not perfect, definitely you will observe a different in the yield strength. Generally, the tensile test has a very high sensitive to the defects, unlike the compression test, and every defect in the material will act as a stress raiser that will reach the test into the plasticity region at lower stress level when compared to the compression test.

Firstly, what kind of material are you testing? However, yield strength obtained under compression and tensile strength are not the same as expected because they are both different failure mode of material. Compression gives failure of material under compressive force while tensile is failure of material under tensile loading so they cannot have the same yield strength.

Dear Mr. Chakraborty:

No, these values will be diffeent, by about 7-10 MPa as AA7075 is a heavily alloyed wrought aluminum material. For pure aluminum, the effect will be ~0.3%.

What this means is that a given material does not obey the Schmid law. This is not surprising for bcc materials such as ferritic steels. Indeed, dislocation cores on the primary slip system {110} ) for Fe have an asymmetric atomic structure, which reacts differently to tension, compression, and hydrostatic pressure loading. Please see the classical works of Spitzig and Richmond on this subject: Richmond, O. and Spitzig, W., Pressure Dependence and Dilatancy of Plastic Flow. Int. Union of Theoretical and Applied Mechanics, 1980; and Spitzig, W. A. and Richmond, O., Acta metall., 1984, 32, 457.

Even fcc aluminum displays this effect (dependence of plastic flow upon applied hydrostatic pressure) but as I noted above, it is considerably less pronounced. In our work AN ATOMISTIC DISLOCATION MECHANISM OF PRESSURE-DEPENDENT PLASTIC FLOW IN ALUMINUM (Acta Mater. Vol. 47, No. 12, pp. 3507-3514, 1999 by Bulatov, Richmond, and Glazov), we tried to understand the nature of this asymmetry for pure aluminum using atomistic simulations and came to the conclusion that it was the result of interaction of the transient activation dilatancy of moving dislocations with external pressure, and was not associated with the permanent dilatation, as required by the normality flow rule.

I hope this helps. Sincerely,

Michael

Thanks for the information. However, I found that the tensile YS is higher than compression in some alloys while it is reversed in some other alloys. Sorry, I can't recall the series of aluminum alloys and I will add them later. Any reason for these different tendency on tensile/compression YS in different aluminum alloys? Thanks.

Hi, you can see this paper:

" Tensile and compressive deformation behavior of the Al–Si–Cu–Mg cast alloy with additions of Zr, V and Ti " Article Tensile and compressive deformation behavior of the Al-Si-Cu...

Thanks