Abstract: Method and apparatus for heat-welding two members by applying radiation, such as infra-red radiation, through one of the members to enter layer(s) interposed between the two members. Method relies on inclusions through thickness of the layer(s) to be responsive to said radiation to raise temperature through thickness and promote bonds at inter-faces with surfaces of said members. The bonds are assessed using thermal imaging camera detecting a near visible radiation arising from the layer(s), which radiation is different from infra-red used for heating.