IMPROVING THE DAMAGE TOLERANCE OF COMPOSITE JOINTS WITH TUFTING

Glass fibre T-stiffened skin-stringer joints were manufactured with through-thickness carbon fibre reinforcement using a one-sided stitching technique, known as tufting. Specimens were tested in four-point bending to identify the delamination trigger mechanisms and mitigate damage within the joint. An untufted control specimen showed that delamination initiates at two transitions within the joint: where the stringer flange meets the skin and where the flange transitions into the web. Continued loading of the untufted joint results in stringer separation from the skin. Tufting successfully increased the initiation failure load by 8% when tufts were inserted into the flange to skin transition, and by 16% with additional tufting in the skin at the stringer tip. Delamination between the skin and flange occurred outside the tufted region, but separation of these elements was mitigated, increasing the stiffness and damage tolerance of the joint. Unfortunately, the failure mode changed from skin-stringer peeling to web-splitting. Additional tufts may be required in the flange to web transition to further improve the onset of joint delamination.