Cracking Stopping in the Bondline of Adhesively Bonded Composite Adherents by Means of a Mechanical Fastener: Numerical and Experimental Investigation

摘要

The use of composite materials in aircraft manufactures increases more and more with the need of light weight and efficient airplanes. Combining composite materials with an appropriate joining method is one of the primordial ways of exploiting its light weight potential. Since the widely-established mechanical fastening, which originally, was developed for metallic materials, is not a suitable joining method for composite materials because of its low bearing strength, the adhesively bonding technology might be an appropriate alternative. However, adhesively bonding in the aircraft manufacturing, especially for joining of primary structures is liable to certification requirements, such as testing of every bond up to limit load before the operation begins or non-destructive testing of every bond before the operation begins as proof of the joint characteristics, which cannot be fulfilled with the current state of the art. Combining adhesively bonding with mechanical fastening as a so-called "bonded/bolted hybrid joint" is one of the possible solutions to this problem. The mechanical fastener also called Disbond Stopping Feature (DSF) shall guarantee the structural integrity of the aircraft so that in case of cracking in the bondline, limit load can still be stood. To achieve this goal, it should be able to stop or reduce the cracking speed so that within a maintenance interval the structure integrity is still unharmed. It is therefore necessary to understand the bondline cracking behavior of an adhesively bonded joint and the influence of the mechanical fastener on it. This paper deals therefore with the investigation of cracking in the bondline of an adhesively bonded joint and the influence of the mechanical fastener by means of the cracked lap shear (CLS) specimen. The short term cracking behaviour has been investigated using numerical simulation method, while the long term behaviour has been investigated experimentally. The CLS specimen consists of adherents made of the composite materials IM7-8552. The fastener considered is specified by the standards ASNA2042/ASNA2045. The bondline consists of a non-traded adhesive which is a mixture made up of 80% of Hysol EA9395 and 20% of Hysol EA9396. As for the investigation of the short term behaviour, a simulation model has been developed and validated by comparing the simulation results with experiments regarding the adherents' strain, the force-displacement curve, the crack length-force curve and the shape of the crack front. The fastener installation takes place in a fully automated C-frame riveting machine with all-electric drilling and riveting operations aptitude.