It is well known that brittle materials absorb lesser energy to fracture than the ductile materials. Does this imply that the brittle materials have lower tensile strength than the ductile materials? Its baffling in the sense that if brittle materials have lower tensile strength than the ductile counterparts, then the hardness (which is in direct proportion to tensile strength) must be more in case of ductile materials. But we all know it is the other way round. Where is the justification going wrong?
It is easiest to think of this question in terms of metals and ceramics, metals are ductile, have the (relatively) lower strength, but the (relatively) higher toughness or energy to failure because of greater ductility/post yield deformation. Ceramics on the other hand have the significantly higher strength, but because they are brittle, have much lesser energy to failure and ductility.
The other fallacy is that hardness is directly proportional to tensile strength - it is actually directly proportional to uniaxial yield strength. Metals have a higher tensile strength than yield, but in ceramics, the two are very similar.
Hope that helps a little.
It is easiest to think of this question in terms of metals and ceramics, metals are ductile, have the (relatively) lower strength, but the (relatively) higher toughness or energy to failure because of greater ductility/post yield deformation. Ceramics on the other hand have the significantly higher strength, but because they are brittle, have much lesser energy to failure and ductility.
The other fallacy is that hardness is directly proportional to tensile strength - it is actually directly proportional to uniaxial yield strength. Metals have a higher tensile strength than yield, but in ceramics, the two are very similar.
Hope that helps a little.
Strength and toughness of naturally occurring materials are balanced - brittle materials generally are of higher strength but lower toughness that their ductile counterparts.
Hardness are comparative measure of strength - thus higher strength materials usually display higher hardness number.
well said teja guda. I would like to add some more detail to it. Its well established that the hardness and UTS are directly related. Hence the hard materials possess higher strength as well. But brittle materials severely lack in toughness. In most practical applications strength without sufficient toughness is useless. Hence brittle materials even though may possess high strength, in the absence of tougness have a limited use.
I agree Mr. Arshad Siddiquee. I read it in texts that Tensile Strength is Brinells hardness times some constant which varies with material chosen. And rightly said that its not the hardness/ strength alone which makes a material useable. That is why some processes, such as tempering, are done to induce that requisite toughness although on cost of hardness and strength. I had some confusions in interrelating these terms which however, Mr. Teja Guda cleared largely :)
Your justification goes wrong on the point that you assume that strength is the only parameter important for energy absorption. Actually, energy absorption (under tensile conditions) is (as a first approximation, depending on the actual application) given by the total surface under the tensile stress-strain curve. Brittle materials like ceramics (typically only for compressive loading) or high-strength steels have very high strength, but a very low failure strain. This strongly diminishes the total surface under the stress-strain curve, even if their tensile strength is very high.
Brittle materials have higher Y.P. and U.T.S. stresses and lower elongatio and reduction.
"brittle" and "ductile" have only relative meaning, taking into account the temperature effect. For the tensile strength, it also depends on the elastic modulus and the material's deformability. For the ceramic and intermetallic materials, below the "brittle-to-ductile-transition" temperature, the cracks prevail; above that temperature, the defects prevail and the ductility increases. For the metals, normally speaking, have the work-hardening ability which means its strength can be improved by deformation with the reduction in ductility. Hope these can help.
Mechanical properties in tensile testing are strength and plastic. Notwithstanding the tensile ductility at low there is a high brittleness.
So the answer to your question is "NO"
All issues regarding ductility are complex because a large number of factors are involved. For this reason, all what is said is right but only under specific circumstances. For instance, ceramics are in general brittle but elongations of mor than 2000% in tension are found at 1400ºC. Other example, an increase of ductility is found when temperature is increased. Yes indeed it is in general true but only at high absolute temperatures (about 0.3 Tm, Tm= melting temperature in K)
The fundamental contradiction in material science is the contradiction between strength and plasticity. Brittle materials are in the absence of plasticity. Hence, the strength of brittle materials is greater!
Dear Saransh,
Although the previous comments have given you definitely at least part of the answer, I still would like to add the following, specifically to this part of your question:
Its baffling in the sense that if brittle materials have lower tensile strength than the ductile counterparts, then the hardness (which is in direct proportion to tensile strength) must be more in case of ductile materials. But we all know it is the other way round. Where is the justification going wrong?
There is actually a hidden assumption in there, namely that the material tested behaves similarly in tensile (like during a classical tensile test) and in compresssive (as mostly during a hardness test) loading. This is through for isotropic materials with a symmetrical yield locus (as assumed using the Von Mises and Tresca criteria, and relatively valid for metal materials) and symmetrical 'failure' (although this is normally not the case). So the link between a hardness test, and a tensile test, can only be reasonable be made for metallic materials (e.g. based on cavity expansion analysis or an empirical relationship). For other materials, e.g. ceramics, the asymmetry in their resistance make it near to impossible to find such a clear correlation.
I go with Mr. N. Tontchev , Brittle materials have higher strength than ductile.
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