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In Callister it says, "two modes of failure are possible ductile and brittle." Why not malleable?Furthermore why is uniaxial tensile test chosen for predicting failure?

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Ductile ("able to be drawn out into a thin wire") makes sense as the failure method, because the test is stretching the sample. Malleable ("able to be hammered or pressed into shape without breaking or cracking") doesn't make sense here, since the sample is not under compression! You could describe a three-point bend failure as Malleable, however. When @blacksmith37 states that "Malleable is the same as ductile", this is what he's referring to. They are somewhat analogous, where "Ductile" is in tension, and "Malleable" in compression. It's important to note, however, that they are not the same physical property. For example, Zinc and Lead both exhibit much higher malleability than they do ductility.

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  • $\begingroup$ Lead elongation is 30 to 50 % depending on thermo-mechanical history. Zinc elongation is 40 to 60 % . Metallurgists would generally consider these materials to be ductile , or malleable if you prefer. $\endgroup$ – blacksmith37 Feb 20 '18 at 2:14
  • $\begingroup$ Ductility and malleability are not always coextensive – for instance, while gold has high ductility and malleability, lead has low ductility but high malleability.2 The word ductility is sometimes used to encompass both types of plasticity.3 - Via Wikipedia page on Ductility. $\endgroup$ – Jonathan R Swift Feb 20 '18 at 9:36
  • $\begingroup$ N.B. I did not state that the ductility of lead was low, but I did say that its malleability was much higher than its ductility, in a way that isn't true for many other metals. Consequently, stating that ductility and malleability are the same thing could be considered misleading. $\endgroup$ – Jonathan R Swift Feb 20 '18 at 9:38
  • $\begingroup$ Ignoring my previous comment : Ductility refers to the ability to be drawn into a wire. Malleability refers to the ability to be rolled into a sheet. Although similar , not identical. $\endgroup$ – blacksmith37 Feb 20 '18 at 13:33
  • $\begingroup$ Plastic is probably the appropriate term we want to use here instead of malleable or ductile. It doesn't necessarily feel right intuitively; but plasticity covers malleability and ductility. $\endgroup$ – JMac Feb 20 '18 at 16:12
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Malleable is the same as ductile. The uniaxial tensile test provides a uniform method to measure the mechanical properties of materials. It only predicts failure of uniaxial test bars. There are a variety of techniques / methods to predict failures or structures ; Such as L E F M , linear elastic fracture mechanics.

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Ductile: The capacity of materials to allow large deformations or large extension without failure is termed as ductility. The materials with high ductility are termed as ductile materials. Normally if the elongation exceeds 15% the material is ductile. Brittle: A brittle material is one which exhibit a relatively small extensions or deformations to fracture and failure occur suddenly without any warning. If the elongation is less than 5% the material is considered to be brittle. Malleable:It is a special case of ductility where it can be rolled into thin sheets but it is not necessary to be so strong.

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