Abstract: A structural component having a stiffness which can be actively varied to control vibrations, for example in a helicopter rotor, an aircraft wing or tail, or in a robotic manipulator. The component has at least one recess or notch with opposed walls, which walls, in the absence of any intervening member, tend to move relative to each other when the component is stressed. A stiffness control element is situated in the recess or notch, this element comprising electrically or magnetically expandable material capable of being rapidly expanded from a first condition in which the element is out of contact with one of the walls, to an expanded condition in which the element extends across the recess or notch and transmits forces from one of the walls to the other, thus altering the stiffness of the component. The preferred expandable material is a stack of piezoelectric crystals.