Question

help with question 5 please.

there are two samples given with two different pictures of each.

A material may fail in many ways; in a ductile or brittle mode; by creep or fatigue; by corrosion or stress corrosion; by hydrogen or liquid metal embrittlement; by slow tensile overload or by impact. An examination of the failure may tell you a lot about the material and the failure mode. In this course, you have learned (we trust) the correlations between structure-properties, - applications. This leads to material selection. However, the engineer has to consider one further aspect, the possibility of failure and the consequences thereof. In this assignment, listen first to the TA describing some failure examples to you and then you examine the three unknown specimens provided. Write paragraphs in response to the questions on the backside for any TWO of the specimens. The following comments are general guidelines only. Material By weight/color and general appearance is it ferrous or non-ferrous? If ferrous, is it steel or cast iron? Remember that stainless steels are not necessarily magnetic (if they have the FCC structure). A cast iron could be gray or white; here the fracture appearance should be a giveaway. Note presence or absence of any scale; the color may help; thus nickel oxide is green; a plain carbon steel rusts easier than a low alloy steel. Is the fracture surface mildly or heavily tarnished, suggesting sample broke a while ago, or at an elevated temperature? Ductile Failures There is evidence of plastic deformation, but not necessarily on a macroscopic scale. The stronger the alloy the less likely is extensive visible plastic deformation, but the failure can still be ductile. Remember the relative necking in 1018 and 4340 steel? Remember the small amount of necking in some high strength aluminum alloys? Brittle Failure Cleavage occurs on particular crystal planes, therefore it looks, as it were, micro-crystalline, and may appear shiny as the flat surfaces of grains reflect the light. A prerequisite for brittle failure may be a lack of plastic deformation, e.g., grey cast iron. Look for notches! Creep Failure This is intergranular and approximately normal to the stress axis. There may be many, many cracks. Corrosion/Stress Corrosion The evidence should be visible. Stress corrosion may reveal a multitude of cracks, it may be intergranular, but should be easily recognizable from a creep failure by the corrosion and directional pattern. Do not get misled by corrosion that occurred in lab afterwards. Fatigue There should be a 2 zone appearance corresponding to the gradual crack propagation stage and to the abrupt fracture of the remaining cross section. The proportions of the two should suggest whether the stress level was high or low, as should whether there seems to have been one or many cracks (high 6). The location of the abrupt fracture region should reveal fatigue mode, e.g., in center for rotating bending, at one side for one way reversed bending. Look, too, for point(s) of origin! References ASM Metals Handbook 10th edn., Failure Analysis and Prevention, Vol. 11. ASM Metals Handbook 10th edn., Fractography, Vol. 12. ASM Metals Handbook 10th edn., Corrosion, Vol. 13. ASM Metals Handbook 10th edn., Fatigue and Fraction, Vol. 19.

all of the information to solve is given

Answer 1

Soln.

Sample-1

It is a brittle failure but not fresh. The sample is not fit for the applications and required to be redesigned by altering the material composition or process of manufacturing.

Sample-2

It is a ductile failure as clearly visible. The sample is not properly designed for the application. It is to be redesigned by changing the manufacturing process including heat treatment and/or material composition.