CFRP/Titanium Hybrid Material Improving Composite Structure Coupling

One of the most important challenges during the development of composite structures is found in the design of structural interconnections. Although composite technology offers the advantage of reducing structural coupling by means of integral design and manufacturing techniques, joining parts to subassemblies remains unavoidable due to inspection, repair, material limitation, production methods and transportation requirements. One of the main methods currently used for joining components is mechanical fastening, which has the advantage of no special surface preparation requirements, easy disassembly and inspection and represents a reliable wellestablished and well-known method from its origin in the design of metallic structures. To realize the great potential of composites in lightweight aircraft structures, it is absolutely necessary to ensure that the joints do not reduce structural efficiency. This challenge is much more severe with composite materials than metals due to the notch sensitivity of composite materials, their brittleness and their reduced bearing strength and shear strength at highly anisotropic behaviour. That means, material advantages of high directional stiffness and strength cannot be exploited without disadvantages in weight: stress concentrations at holes require a local buildup thicker laminate which excites further local stresses due to eccentricities. This aspect is aggravated by the fact that joint efficiency decreases with increasing directional structure performance. Several design approaches have been developed in the past in view of optimising mechanical fastening of composite structures. One method of increasing mechanical joint efficiency consists in reinforcing the joining area with thin metal laminates, which provide a higher bearing strength and shear strength of the joint in comparison to pure composite material. Thus, both material performance and joint efficiency can be kept high. The suitability of Titanium as reinforcing material was researched at the Institute of structural mechanics of the German Aerospace Center (DLR). Extensive research work dealt with the compatibility of titanium and carbon composite material, some plies being replaced by thin titanium sheets, and was mainly focused in the investigation of the strength increase of bolted joints. Experimental results showed a significant gain of bearing strength and bolted joint strength, whereas interlaminar shear strength featured only a minimal decrease. A higher specific strength could also be achieved by relatively low titanium rates. Furthermore, an efficient design of the transition from pure composite to hybrid titanium composite material was developed, which enables the local use of hybrid titanium composite material in structure interconnections and force transmission points of composite structures