Fracture surface after Uni-axial tensile testing of pure aluminium sub-size tensile specimen shows the presence of elongated dimples instead of equi-axed dimples. What could be the reason behind this? Does it imply that the material failed by shear?
Dear Mr. Akash Mukhopadhyay
Due to the incompatibility of the Al-matrix and intermetallic particles with different properties, the voids are initiated or nucleated primarily at the grain boundaries once plastic deformation occurs. These characteristics are convenient for exploring the mechanisms of damage evolution and ductile fracture. As can be seen from situ tensile tests of the Al-alloy, dimple-dominant fracture usually occurs under the tension-stress-dominant condition for ductile materials. Under tension-dominant deformation, a higher triaxiality accelerates void initiation and propagation, which results in a large number of dimples and cracks perpendicular to the maximum tensile stress.
best regards...
Akash:
It depends on the size and geometry of your test specimens.
Elongated dimples which form in shear are very common in uniaxial tensile test fracture surfaces. The classical example is the "cup-and-cone" failure mode. In the center of the samples - the "cup" - where the stress-state is dominated by the applied tensile stresses, the process of ductile dimple formation occurs by microvoid coalescence, i.e., voids nucleate at particles (either by the particle cracking or separation at the particle/matrix interfaces); these voids then grow, primarily driven by the triaxial stresses at the center of the sample, until they coalesce or more likely the separating ligaments between the voids "neck down" due to plastic instability. Nominally the same process occurs near the surface of the sample - the "cone" - but now the process is dominated by the near-surface shear stresses. The result is the coalescence of voids formed around particles in shear, which naturally results in elongated dimples.
As you describe your aluminum sample as "sub-size", it is highly likely that the process of microvoid coalescence is dominated by the near-surface shear stresses, in which case elongation dimples would be the result.
ROR
P.S. It is interesting to note here that the formation of dimples both in tension and in shear can even occur at the center of a uniaxial tensile sample in the "cup" region. In many low-alloy steels, the initial voids responsible for their microvoid coalescence ductile fracture are formed at inclusions, e.g., MnS inclusions which readily debond from the matrix. These voids then grow under the triaxial stresses, as described above, but the necking down between these larger voids by a plastic instability can occur by shear-induced microvoid coalescence of cracks in the much smaller carbide particles. The latter is know as a void sheet instability.
Dear Akash Mukhopadhyay ,
As Robert O Ritchie mentions, it depends on size and geometry, and precisely that phenomenon is very common due to the triaxial efforts involved when entering the state of plasticity in the tensile test, after the point of instability precisely. It is common to occur in inclusions or small-angle grain limits producing elongated dimples.
Hi
I recommend u to see the following link
http://www.phase-trans.msm.cam.ac.uk/2008/weld/weld.html
All the best@
Hi
I addition to professor Robert Richie;
the movement of dislocations during the uniaxial tensile tests and the pile up of the dislocations at either the grain boundaries, in the case of pure aluminum, or at the interfaces between the matrix and the second phase particles, in the case of aluminum alloys, would lead to the micro-voids nucleation in the center of the aluminum sample. Then, the micro-voids grow by coalescing to form voids. The present of voids in the center of the sample will trigger the failure of the aluminum by localized/unstable shear stresses when the cross area of the sample is no longer can sustain the uniaxial load. As a result, the aluminum sample will fail leaving eventually a fracture surface of a shear-dimples, i.e., cup and cone, feature.
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