An apparatus for determining the strength of composite sandwich beam in which an enclosure and insertable bladder is provided. The apparatus comprises positioning a bladder within the enclosure. The beam to be tested is slid through a cutout or aperture of the enclosure with support feet holding the beam in place within the enclosure. The enclosure supports the bladder sides with one face of the bladder pressuring the composite sandwich beam. For testing, water is pumped under pressure through a fitting into the bladder. The tolerances between the beam, enclosure and an extrusion seal do not allow the bladder to squeeze outward with the result being measurable testing pressure on the face of the composite beam.