A system comprising: a fan blade configured to rotate about an axis; a fan case circumferentially surrounding the fan blade and having an interior surface ; and an abradable material having: an outer surface coupled to the interior surface such that the abradable material is positioned circumferentially about the axis; and an inner surface , characterised in that : the inner surface defines a circumferential ridge (307,309,311;603) and a circumferential groove ; an axial cross section of the inner surface defines a rounded wave; and the circumferential ridge (307,309,311;603) and the circumferential groove (303;305) are designed such that, in response to contact between the fan blade and the abradable material , an angle of incidence between an engine central longitudinal axis (A-A') and a line tangential to a forward side of the circumferential ridge (307,309,311;603) in the axial cross section at a point of contact between the fan blade and the abradable material is less than forty-five degrees, wherein the circumferential ridge (307,309,311;603) is not symmetric about an apex of the at least one circumferential ridge (307,309,311;603). a fan blade configured to rotate about an axis; a fan case circumferentially surrounding the fan blade and having an interior surface ; and an abradable material having: an outer surface coupled to the interior surface such that the abradable material is positioned circumferentially about the axis; and an inner surface , characterised in that : the inner surface defines a circumferential ridge (307,309,311;603) and a circumferential groove ; an axial cross section of the inner surface defines a rounded wave; and the circumferential ridge (307,309,311;603) and the circumferential groove (303;305) are designed such that, in response to contact between the fan blade and the abradable material , an angle of incidence between an engine central longitudinal axis (A-A') and a line tangential to a forward side of the circumferential an outer surface coupled to the interior surface such that the abradable material is positioned circumferentially about the axis; and an inner surface , characterised in that : the inner surface defines a circumferential ridge (307,309,311;603) and a circumferential groove ; an axial cross section of the inner surface defines a rounded wave; and the circumferential ridge (307,309,311;603) and the circumferential groove (303;305) are designed such that, in response to contact between the fan blade and the abradable material , an angle of incidence between an engine central longitudinal axis (A-A') and a line tangential to a forward side of the circumferential the inner surface defines a circumferential ridge (307,309,311;603) and a circumferential groove ; an axial cross section of the inner surface defines a rounded wave; and the circumferential ridge (307,309,311;603) and the circumferential groove (303;305) are designed such that, in response to contact between the fan blade and the abradable material , an angle of incidence between an engine central longitudinal axis (A-A') and a line tangential to a forward side of the circumferential ridge (307,309,311;603) in the axial cross section at a point of contact between the fan blade and the abradable material is less than forty-five degrees, wherein the circumferential ridge (307,309,311;603) is not symmetric about an apex of the at least one circumferential ridge (307,309,311;603). <EOS>