Tensile Strength of Bonded Lap-mitered Butt-Joints between Layered CFRP Bands : -In collaboration with RUAG Space AB

University essay from KTH/Lättkonstruktioner


Joints in structures always cause strength reduction. The percentage of strength reduction depends upon the selection of several factors such as: type of joint (i.e. adhesive or mechanical), technique of joint (i.e. lap joint, butt joint etc.), geometry of joint, mode of load application etc. Here in this research, the strength of adhesively bonded butt joints with several geometries, later referred as joint angles, is investigated under uniaxial tension loading.

Adhesively bonded simple butt joints, where joints are placed perpendicular to the loading direction are in common practice mainly because of ease in manufacturing process. But when the joint is fabricated with an angle respective to the loading direction, the geometry of the joint itself affects the strength of the joint significantly. Without going too deep into other factors that affects the joint strength such as manufacturing techniques, manufacturing defects, material behavior etc. only the geometry of the joint is considered and it is evaluated whether it is worth to change the joint geometry or not.

The significant issue in adhesive joint technology is the prediction of joint strength. However, an approach similar to plastic yield criterion later referred as elastic limit offset method (attempted for 0.025% offset) is considered to estimate the linear elastic limit. Since RUAG Space AB (the industry for which this project is performed) is only interested in the linear elastic regime of the stress-strain curve, therefore the elastic limit offset method is considered to be the suitable one.

The present work is concerned with the study of adhesively bonded angled butt joint vs. strength behavior. The strength of adhesively bonded butt joints is examined for several butt joint angles under uniaxial tensile loading. The employed butt joint angles are: 0°, 30°, 45°, 60° and 75°. The main objective of the current investigation is to find the joint angle that has the highest strength or the highest capability of load transfer.

In addition to the above, the influence of the joint on the stress field, joint strength and type of failure is also evaluated using DSP (Digital Speckle Photography) technique and simulated using well known finite element tool, ABAQUS. It is observed that specific strength of the joint is greatly influenced with joint angle. The 45° joint showed the highest elasticity and failed like ductile behavior whereas 75° joint showed the lowest elasticity and failure was purely brittle. Moreover, post-failure inspection of fractured surfaces showed cohesive failure (failure within adhesive layers) for 0°, 30°, 45° and 60°whereas 75° showed composite or adherend failure.

The simulation is performed for each joint angle. However to validate the model only 45° and 75° joints results are compared with experimental results and plotted in the report. The simulation results of these angles showed good agreement with the experimental ones. Moreover, the stress fields for each joint angle are captured (from ABAQUS), showing that all joints are susceptible to inter-laminar shear. Besides, the relative slip between the top and middle adherends is also calculated, the results show that, the 45° joint has higher tendency of relative slip than others.

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