Fatigue of Composite Scarf Joints in Wind Energy Rotor Blades and in Spar Beams for Light Aircraft

Abstract

The paper presents results of fatigue tests on scarf joints in Glass Fiber Reinforced Plastic (GFRP)-components of wind energy rotor blades and in Carbon Fiber Reinforced Plastic (CFRP)-spar beams for light aircraft wings.  

The GFRP-component tests were carried out in the Fifth-framework EU Project “OptiMat Blades” which will be introduced in this paper.1  The tests were aimed at repair procedures for load carrying parts of rotor blades.  If repairs are not accepted as a standard procedure, even localized deficiencies may require the destruction of the whole blade, which is crucial for larger blades.  The typical repair procedure is surveyed step by step and adapted for the special needs for application on rotor blades.  Practical stress calculation procedures are selected from a literature review and proposed for the evaluation of the restored strength during the repair design.  Applications are experimentally evaluated through tensile testing both under static and fatigue loading of repaired coupons.  Test results also are used to compare the repair methods applied.  Among the evaluated techniques, the most promising method was the scarf repair with a slope of 1:50.  Test data reveal that a strength restoration of over 80% can be achieved.

The CFRP-spar beams were tested within a program on the applicability of analytical and experimental certification procedures on the service life of light aircraft structures.2  The 2 m long structures were loaded transversely at one end.  In the load introduction part of the other end, the thickness of the strengthened spar caps was reduced with a splice ratio of 1:40.  By means of thermography and ultrasonic inspections, applied during one-step fatigue tests, an increasing delamination was observed in the splice area.  Therefore the splice ratio of subsequent test beams was redesigned to 1:100, which performed well during the fatigue tests.  The structure of the spar beams, the fatigue tests and the NDT-inspections, as well as the stiffness behavior are discussed in this paper.