Question

i.What is strain hardening phenomenon? Explain briefly with an appropriate drawing.

i.List two examples where fatigue failure is of primary concern.

ii.With appropriate drawings explain the difference between Fatigue limit and Endurance limit.

i.Give two examples of strengthening mechanisms.

ii.Give examples of the products that can be realized based on these mechanisms.

Answer 1

(i)Strain hardening is generally defined as heating at a relatively low temperature after cold-working. During strain hardening the strenth of the metal is increased and ductility decreased.

To go a step further in explaining, if a low-carbon steel is cold-worked, or strained passed the yield point, then aged for several days at room temperature, it will have a higher yield stress after the aging. This happens because during the aging carbon or nitrogen atoms diffuse to dislocations, reanchoring them.

Also, not everything can be strain aged, or recovered at low temperatures. The low carbon steel is just an example. Different materials will show different behaviors during recovery.

(i)An example of fatigue failure was the airline crashes of the De Havilland Comets in 1954. Three of these passenger jets broke up mid-air and crashed within a single year. Sharp corners around the plane’s window openings were found to have acted as stress concentrators which initiated cracks.

Pressurisation of the aircraft during each flight created stress cycles in the fuselage that propagated the cracks over time. At some point a critical crack length was reached and fast fracture of the fuselage shell then occurred.

(iii) These strengthening mechanisms give engineers the ability to tailor the mechanical properties of materials to suit a variety of different applications. For example, the favorable properties of steel result from interstitial incorporation of carbon into the iron lattice. Brass, a binary alloy of copper and zinc, has superior mechanical properties compared to its constituent metals due to solution strengthening. Work hardening (such as beating a red-hot piece of metal on anvil) has also been used for centuries by blacksmiths to introduce dislocations into materials, increasing their yield strengths.