Question

Hw 3: Material Properties

1. You take an unknown opaque material and leave it in a mild acid solution for 3 days. After that time, the sample has lost no mass and there is minimal change in the chemical structure. You put the sample into an oven and set is as hot as it will go. After 30 minutes under high heat the sample shows no obvious changes. Is it more likely that this material is a metal, ceramic, or polymer and why? (10pts)

2. You take the material from the previous question and measure its electrical impedance. You record a value of 40μΩ*m. Is this value surprising why or why not? (15pts)

3. You perform a cyclic fatigue test on a structural steel sample and find that it lasts 50000 cycles at 300MPa. If you submerge the sample in warm saltwater and repeat the test at the same stress amplitude, what result would you expect and why? (10pts)

Answer 1

1)

Case1 : Reaction towards mild acid

From the given statement it can be seen that the material had no to negligible effect towards mild acid. Polymers are organic compounds made up of monomers. They contain chain, which readily loose their covalent bond and react to acid. Hence the material could not be a polymer. Metals combine with acid to form salt and release hydrogen gas, since its a mild acid the changes could be negligible. Whereas ceramics are more resilient towards acid. Hence the material could be a metal or a ceramic.

Case 2 : reaction towards heating in oven

Metals have a lower melting point compared to ceramics and since the maximum temp inside oven has not been mentioned, it can be assumed to be at least 500 degree Celsius and since the material is kept in it for 30 minutes, low melting point metals like aluminium could have melted , or at least started to deform due to thermal expansion. But ceramic have a very high melting point of >2000degree Celsius.Hence from the above to cases it could be safe to assume that the material is a Ceramic.

Case3: electrical impedance of 40ohm*m.

Electrical impedance is the amount of opposition/restriction that the circuit provides towards the flow of current. This value is shocking because, ceramics are insulators with low conductivity, hence should have a high impedance value opposite to a low value of 40ohm*m.

2)

Though stainless steel is said to be corrosion resistant, it is only up to a certain limit. when kept in corrosive medium like salt water and made to undergo cyclic loads, it is under the influence of both corrosion and fatigue. Corrosion fatigue results in retarding of the SN curve and in some cases even shift the curve to the lift. This means that the overall fatigue strength of the material is reduced with elimination of fatigue limit. Hence the stainless steel will run for a lower number of cycles at 300Mpa, if submerged in salt water.

note: refer wikipedia page on corrosion fatigue, for further in depth information.