For titanium alloy, at the quasi static condition strain rate is 0.01s-1, are the yield limits of the flow curves of compression and tensile tests same?
normally for a isotropic material it does but since the TI alloys are not that isotropic, there may be some differences in the tension and compression.
Isotropy of limit behaviour, say stress limit in tensile and compressive range, is related to ductile behaviour (say metallic materials). Converseley, fragile material shows les tensile strenght than compressive such as ceramics. I guess that titalium alloy could be considered to belong to ductile materials, al least in ordinary structural analysis, except than for specific fracture induced failure.
It we go to more detailed scale, failure happens when dislocations pile up in certain area and start to cause micro voids, that then grow to cracks and finally compete failure. Materials does not exhibit same failure in tensile and compressive side. Usually tensile stresses cause failure much quicker, but still everything depends on activated slip systems in strained material.
No, yielding in tension and compression is not the same. Titanium is a good example of a material showing strength differencial effects, i.e., yielding in tension is different of yielding in compression. Please remark that this has nothing to do with anisotropy. A given material can be isotropic and, at the same time, display tension-compression asymmetry. Remark that the material is said to be isotropic if the their answer is the same whatever the loading direction AND for the same loading path. Tension and compression are different loading paths.
If the material even, then the tensile and compression properties and behavior is the same. On the other hand, if the material is uneven , it will have different compressive and tensile properties. Casts are good example of uneven materials. Of course, isotropy can be considered, but even if the material is isotropic, i.e. no preferred orientation, it may be even or uneven.
Thank you for your answer, and I am sorry for the question, because I am not skilled at the material forming and the structure of the material.
In my knowledge, it is nearly impossible to eliminate basal texture in titanium. I totally agree with previous comments. however, it should be kept in mind that prismatic slip is more important in Ti contrary to the case of Mg. This is because c/a ratio of Ti (1.588) is much lower than that of Mg (1.624)..
Yes, quite right. Due to the reason, principal slip system of Zr and Hf is prismatic too not basal. Basal is much more important in Cd, Zn, Mg, Co where c/a ratio is over 1.62.
No, yielding in tension and compression is not the same for the material, it is all to do with the microstructure of the materials. Steels for example have higher compressive yield stress say 3 times than the tensile yield stress. However, concrete for example have higher compressive yield stress say 20 times than the tensile yield stress.
No they are not the same because the material behaviour is different, and the easy way to check that by looking to the mechanical properties tables for different materials.
From the stress – strain diagram for compression, if we load a crystalline material sample in compression, the force displacement curve and the stress-strain curve is simply the reverse of that for loading in tension at small strains in the elastic region. But the tension and compression curves are different at larger strains i.e., the compression specimen is squashed and the tension specimen enters the plastic region.
As somebody who has only worked on polycrystaline, isotropic FCC stainless steels have found this an interesting discussion and would like to single out J.L. Alves, Jeoung Han Kim and Luv Sharma for their contributions.
No, because for brittle material the compressive yielding is more than for tensile yielding such as cast iron and the concrete. However, for ductile material such as steel may be the yield for tension and compression is the same. However, it is prefer to conduct the tensile test for ductile material because it is easy and no any problem such as instability and the buckling occurred for compression test.
Most T/C yield asymmetry is due to twinning, seems clear on that.
If, there is a big if, we can totally suppress twinning, could we weaken or even eliminate the asymmetry?
For ductile materials the tensile yield is equal to the compression yield
Interesting discussion, useful answers. Another possibly complicating factor is Poisson's ratio. https://en.wikipedia.org/wiki/Poisson%27s_ratio
At the very least it means one needs--in actual tests--to decide where and at what level of deformation to measure the cross sectional area, in order to compute yield strength. I remember that Poisson's ratio for titanium is ca. 0.3. I don't know if it is the same in tension and compression, but from the crystal structure discussion above, I suspect not. Also, you mentioned Ti alloys, not pure Ti. Alloying elements seem likely complicate yield behavior.
The yield behaviour for titanium alloy at compression test is different in tensile test because the slipping mechanism and twinning mechanism are different.
Normally the metallic materials have same yields limits in tension, compression or torsion, if isotropic and Von-Mises hypothesis are used for computation of true stress and true elasto-plastic strains. In this cases all the true stress - true strain curves must to be similar.
If observed differences concerning behaviour curves, you must to see in the following priority:
1° decomposition of elastic and plastic strain part. Elastic behaviour is generally similar and symmetric.
2° computation method of the stress and of the strains: in tension you have striction phenomenon and in compression you have friction influence. For torsion you have radial variation of all mechanical and thermal variables.
3° strain rate sensitivity of the material. In tension and compression the strain rate is not the same.
4° analysis the hypothesis of isotropic criterion (Von-Mises) or if it necessary to use a, anisotropic one. It is the necessary to compute correctly the stress for each strain at the same strain rate value.
5° analysis of plastic mechanisms via microscopy or metallographic studies: dislocation's glides, twinning or phase change, only if an important plastic strain occurs during the experimental test.
6° influence of thermal softening using corrections necessary to transform the anisothermal stress into a isothermal one.
For materials with Bauschinger effect the yield limits is not the same in compression and tensile test as the hardneing law is different. But for these materials hypothesis of classical von-Mises criterion is not valid. A kinematics hardening criteria must be add. Furthermore it is necessary to regard the rigor of the experimental measurements because many errors can leads to different yield limits in compression and tensile caused firstly by different influences of test machine rigidity and in a second point of view by used exprimental techniques to measure the strain toghether with used hypothesis to compute the Cauchy stress (Never we can not measure directly the stress - only by photoelasticimetric or laser techniques - and from several times many peoples compute wrong the true stress).
No, they are not the same, and the reason is because of the failure mode and the failure mechanism as result to the external stress imposed. To understand this easily try to think about it in the grain/ Crestal level. In the comparison the grains need high level of stress in order to start sliding on each other (plasticly or elastically) as result the strong bond + friction force between the grains, especially when the shear stress is very limited i.e. (the applied force and the constraint force and the specimen are all in perfect alignment.
In the tensile stress all these factors are still apply however, there is no Friction force involve
Only for ductile and homogeneous materials without Bauschinger effect and any other anisotropic effects, same yield stress are obtained from compression and tensile tests.
- Badges
- Science topic