$($i$-.$ What is fatigue failure in engineering metals and alloys? Explain concisely.

$($ii$-$ How can one increase the fatigue resistance and$/$or strength of a given engineering material? Provide a concise yet rigorous qualitative discussion and use sketches and such in your answer;

$($iii$-.$ What is the fatigue limit of the materials shown on the S$/$N diagram in Figure $2?$

$($iv$--$ How many cycles would the materials shown in Figure $2$ last at a stress amplitude of $150$ MPa $?$