hello all,
i have another question:
if there are some voids and stress concentrations sites in my nanocomposites....which one is affected more by these defects, flexural properties or tensile properties?which one is more controlled by these defects?
please kindly attach some papers for me, send some papers, and write your nice comments....
thank you so much
The flexural strength would be the same as the tensile strength if the material were homogeneous. In fact, most materials have small or large defects in them which act to concentrate the stresses locally, effectively causing a localized weakness. When a material is bent only the extreme fibers are at the largest stress so, if those fibers are free from defects, the flexural strength will be controlled by the strength of those intact 'fibers'. However, if the same material was subjected to only tensile forces then all the fibers in the material are at the same stress and failure will initiate when the weakest fiber reaches its limiting tensile stress. Therefore, it is common for flexural strengths to be higher than tensile strengths for the same material. Conversely, a homogeneous material with defects only on its surfaces (e.g., due to scratches) might have a higher tensile strength than flexural strength.
At the bending, the stress concentration on defects which placed near "stretched" surface, evidently, play a great role, because they stimulate crack propagation.
This factor may reduce the flexural strength rather than at tension. The defects near central (neutral) part and compressed side of the bending sample do not "operate" as stress concentrators. So, the localisation of defects determines the flexural strength. This rule is a common for classic structural materials. The tension strength, as noted collegue Morteza Amjadi, really less depends on this factor. Best regards.
Dear Elmira,
Formation of voids due to outgasing low molecular weight gaseous species result in the matrix embrittlement (reduction in material toughness). As result, fracture toughness properties are markedly reduced by the brittleness in the matrix.
In case you don't have any alternative except tensile or flexural test, results depend on the amount of void concentration, size and curing stage of your composites. To obtain an accurate result, you initially need to check whether the curing process was completed ( via DSC test). If it is, you can choose tensile test and measure the toughness value through the area under Stress-Strain curve. However, there is numerous literature which compared the tensile test and fracture toughness test in order to a proper indicator for the brittleness in polymer matrix composites. They suggested that fracture toughness properties are so sensitive to brittleness in the matrix.
In addition, in aerospace martial testing procedure which performed by aeronautical manufacturers, flexural (SBS) test is much more interested as an indicator for measuring material strength while containing some defects caused by processing.
In conclusion:
1). If you are doing academic research, kindly perform fracture toughness test or any mechanical test for measuring toughness value and SEM, make sure curing process has been completed.
2). If you intend to measure the material strength while carrying some defects, first measure the amount of voids and their average size, then perform SBS and analyse the fracture cross-section by SEM.
Kind regards,
Amin
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