$($a$)$ Briefly describe the respective mechanisms for dislocation creep, Nabarro$-$Herring creep and Coble creep.

$(5$ marks$)$

$($b$)$ Cite three metallurgical$/$processing techniques that are employed to enhance the creep resistance of high temperature alloys.

$(5$ marks$)$

$($c$)$ A low carbon steel which is ductile becomes very brittle, after electro$-$plating. Its ductility can be recovered after baking at $200C$ for $24$ hours. What is the cause for the embrittlement?

$(4$ marks$)$

$($d$)$ The wide panel as shown in Figure Q$1($a$)$ contains a $1\mathrm{.}3$ cm diameter hole with a crack $0\mathrm{.}8$ cm long which is perpendicular to the load axis. As shown in Figure Q$1($b$),$ a $10\mathrm{.}2$ cm wide edge$-$cracked specimen made from an identical sheet material fractures at a stress of $48\mathrm{.}6$ MPa $,$ when the crack length is $2\mathrm{.}5$ cm $.$ Both sheets are $1$ cm thick and the material has $450$ MPa yield strength. Determine the residual strength of the panel with the cracked hole.

The stress intensity factor K for Figure $1($b$)$ is given by$,$

$k=\sqrt[\phantom{2}]{a}[1\mathrm{.}12-0\mathrm{.}231(\frac{a}{w})+10\mathrm{.}55(\frac{a}{w}{)}^2-21\mathrm{.}72(\frac{a}{w}{)}^3+30\mathrm{.}39(\frac{a}{w}{)}^4]$

where a and w are the crack length and sample width respectively. The stress intensity factor for a cracked hole in Figure Q$2($a$)$ is given by

$k=\sqrt[\phantom{2}]{a}(\frac{0\mathrm{.}8733}{0\mathrm{.}3245+\frac{a}{R}}+0\mathrm{.}6762)$

where a and R are the crack length and the radius of the hole respectively.

$(7$ marks$)$

$($e$)$ Discuss, from the engineering design point of view, what we could learn from the accidents of: $1)$ the collapse of the World Trade Center Twin Towers about $45$ minutes after being struck by commercial airplanes in $2001$; and $2)$ emergency landing by a commercial airplane in the Hudson River after its both engine were hit by wild geese in $2009.$

$(4$ marks$)$