For plane strain fracture toughness (\(K_{Ic}\)) testing of a Maraging steel specimen, find the minimum thickness needed for a valid \(K_{Ic}\) measurement (answer as an integer, in mm).
Given: \(K_{Ic} = 90\ \text{MPa}\sqrt{\text{m}}\) and yield stress \(\sigma_y = 900\ \text{MPa}\).