35 Design D3 has the same arrow-head reinforcement but instead of the un-toughened RTM6 the toughened epoxy resin system Cytec Prism ® EP2400 was used in order to improve the fracture toughness. This toughened epoxy resin is used for all further specimens D4–D7, too. Design D4 is comparable to D3 but has additional pins in the center region in order to investigate their influence on failure initiation and energy absorption. An optimization of the reinforcement architecture or pin lay-out was attempted in design D5 . This optimized design is characterized by a maximum pin density in order to investigate, if more shorter pins are superior to fewer longer pins. All pins have an equal length of 2 mm and cannot be positioned any closer than in the selected design in order to prevent undulation problems and excessive resin-rich zones. Design D6 is similar to D3 but uses a different filler material in the T-joint center. Instead of a carbon fiber braid like in all other designs, a milled thermoplastic filler made of polyetherimide (PEI) was used, which enables higher elasticity and a more precise ply path of the spar laminate plies and avoids resin-rich zones at the corners. In design D7 the influence of additional thermoplastic co-polyamide (PA) binder fleece layers between all skin and spar plies was assessed, which is supposed to further increase the fracture toughness and damage resistance. The pin reinforcement pattern is similar to design D5 with the high pin density. 4.3 Experimental Characterization The experimental test series was divided into three categories. First of all, standard coupon tests with the composite laminate and steel material of the pin sheets were performed in order to obtain mechanical properties for the numerical material modeling. These involved tensile, compression and shear tests as well as double cantilever beam (DCB) and end-notched flexure (ENF) tests for delamination properties characterization. Design Illustration Details D1 HTS-CF/RTM6; no metal pins D2 HTS-CF/RTM6; metal pins D3 HTS-CF/EP2400; metal pins D4 HTS-CF/EP2400; metal pins + additional center pins D5 HTS-CF/EP2400; metal pins of maximum density D6 HTS-CF/EP2400; metal pins + thermoplastic filler D7 HTS-CF/EP2400; metal pins of max. density + thermoplastic binder Fig. 4.3 Overview of composite T-joint designs investigated in this study 4 Investigation and Improvement of Composite T-Joints with Metallic Arrow-Pin Reinforcement
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