An investigation of brittle fracture in extruded beryllium tubes is described following an unusual brittle fracture of a beryllium clad fuel element at plastic strain of about 0.15% produced by differential thermal contraction. A characteristic spiral form of fracture is shown to be caused by cleavage failure of the highly textured tube along (1120) planes u n d e r biaxial stress generated by the contraction of the can onto the pellets, but the crystallography of the fracture does not reveal cause of the unexpectedly low plasticity.
Mechanical testing of the tubes revealed circumferential ductilities of about 1% which is in agreement with values expected from the high degree of texture of the tube; axial ductilities were anomalously very low at about 0 . 2 0 . Less textured materials gave longitudinal ductilities of several percent, expected from the texture.
Thin filaments cut from the walls of double extruded material gaye much greater elongations than the whole tube, showing that the material was not inherently brittle and tests on filaments of varying arc widths suggested that progressively increasing the width of curved specimens was accompanied by decreasing ductility. Some evidence is presented supporting the theory that the brittleness is caused by premature failure following the action of a secondary deformation process activated by transverse loop stress arising from the plastic anisotropy of the material and the tube curvature. It is suggested that this process is twinning which is known to lead to non-ductile failure.