Experimental investigation on stiffness and strength of single-lap z-pinned joints in a laminated CFRP stress-ribbon strip

Algirdas Juozapaitis,Aleksandr K. Arnautov,Viktor Gribniak,Janis Andersons,Vladimir Kulakov

doi:10.3846/bjrbe.2016.14

Abstract

Carbon fiber-reinforced polymer (carbon-polymer) is an advanced lightweight composite material with high strength and excellent resistance to corrosion and fatigue. Over the past decades, application of fiber-reinforced polymers has been spread from the aerospace to other branches of industry such as automobile and civil engineering. Unidirectional carbon-polymers have a high potential for replacing steel in tensile members. Recently, the first carbonpolymer stress-ribbon bridge has been constructed in Germany. The non-laminated strip-loop carbon-polymer thin strips were used as the load bearing components in this bridge. In comparison with the laminated components, the applied cables are characterized by a more uniform strain distribution though reduced structural integrity. Alternative jointing technologies of carbon-polymer laminates are considered in this paper with an intention to increase the structural integrity and reliability of the production. Tensile behavior of the single-lap joints was investigated experimentally. Three types of the joints were considered. Adhesive joint was set as the reference. The overlap region of the mechanically fastened joints was produced using 9, 25, or 36 steel needles (z-pins) of 1 mm diameter. The proposed hybrid joints were additionally connected with adhesive increasing the load-bearing capacity of the reference joint up to 230%. Concerning the brittle fracture of the adhesive counterparts, the extended progressive failure process within the hybrid joints is responsible for the improvement.

Highlights

Over the past decades, application of fiber reinforced polymer (FRP) composites has been spread from the aerospace to other branches of industry such as automobile and civil engineering
This paper introduces a solution for application of carbon fiber reinforced polymer (CFRP) laminates as the stress-ribbon-strips that simplifies construction of the bridges
The shear stiffness of the adhesive joint is much higher than that obtained for the pinned joints, the load-bearing capacity of the mechanically bonded joints increases with the number of z-pins (Fig. 6a)

Summary

Introduction

Application of fiber reinforced polymer (FRP) composites has been spread from the aerospace to other branches of industry such as automobile and civil engineering. Chang et al (2006) obtained 40% increase in strength of the single-lap joint of carbon fiber reinforced epoxy cross-ply laminate by inserting the fibrous z-pins in the overlap region though evident resin-rich zones at the pin locations. The main disadvantages of the stitched laminates can be related to the reductions in in-plane mechanical properties due to a failure and misalignment of the fibers and increased concentration of the resin (polymer) around the needles (Aymerich et al 2005). The CFRP stress-ribbon bridge constructed in Germany can be mentioned as another example of application of GFRP strips (Fig. 1) In this project, the non-laminated strip-loop cables were used as the load bearing components – stress-ribbon strips (Schlaich, Bleicher 2007). The proposed hybrid joints were connected with adhesive for improving the toughness and ultimate strength

Motivation of the research

Materials and test setup

Results and discussion

Conclusions