A joined structure includes a first component, a second component, and an insert. The first component is formed of a fiber-reinforced material and has a hole defining an axis. The second component is adjacent the first component and has a hole aligned with the hole of the first component. The insert is positioned in the hole of the first component. The insert includes an inner layer having a tubular shape about the axis, a middle layer concentrically adjacent to the inner layer about the axis, and an outer layer concentrically adjacent to the middle layer about the axis. The layers are formed of fiber-reinforced materials. One of a boundary between the inner and middle layers and a boundary between the middle and outer layers has a nonlinear profile in a direction parallel to the axis.