Analytical formulae for electromechanical effective properties of 3-1 longitudinally porous piezoelectric materials - Bravo-Castillero, Julián; Rodríguez-Ramos, Reinaldo; Guinovart-Díaz, Raúl; Sabina, Federico J.; Aguiar, Adair Roberto; Silva, Uziel P.; Gómez-Muñoz, José Luis (Acta Materialia, 2009)