Airfoil containment structure including a notched and tapered inner shell

A fan blade containment structure for a rotatable machine including a bladed rotor, said containment structure comprising:

• a double-walled inner shell having a forward end, an aft end, and a substantially cylindrical body extending therebetween, said inner shell including:
• an inner wall at least partially surrounding a bladed portion of said bladed rotor, said inner wall including a first circumferential notch;
• and
• an outer portion that branches radially outward from said inner wall;
• and
• a substantially cylindrical back sheet coupled to said outer portion and disposed radially outward of said inner wall.