Bio-inspired topologies to enhance the mechanical strength of co-cured Ti-CFRP hybrid joints
Creators: name: CASTRO CASTILLO, BREYNER ANDRES
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400102
subject: Aerospace structures

description: The main aim of this study is to investigate utilising root-like architectures to improve the interfacial strength of pin-integrated hybrid composite-metal structures. Joints are critical elements in aircraft structures and require special consideration due to their vital role in the structural integrity of airframes. Although bonded and bolted systems are used commonly in engineering applications, pinning technologies have shown tremendous potential for hybrid joints. Nonetheless, exploring complex pin geometries has been challenging when considering associated manufacturing techniques. Using pre-preg constrains the topography of the pin, while vacuum resin infusion offers more possibilities to manufacture complex hybrid joints. 

This study develops an experimental methodology to fabricate hybrid joints with arbitrary pin geometry using Selective Laser Melting (SLM) and resin infusion techniques. First, titanium parts with four inclination angles of 0°, 20°, 45° and 80° were fabricated using the SLM method, and their surface roughness was examined using a profilometer. Then, dry unidirectional carbon fabrics were stacked into pins in a hand layup process followed by epoxy infusion in a vacuum chamber. The cross-section of the fabricated joints was inspected employing an optical microscope, SEM and µCT-scanner. The pin angle and fibre alignment impacted the geometry and shape of the resin-rich area in the insertion region. Manufacturing defects such as pores were present in the composite with a high density closer to the titanium-composite bondline. Despite those setbacks, resin-infused composites were integrated into highly tilted pins such as 45° or 80°. This precedent demonstrated that root-like pins could also be co-cured with composites. 

The effect of pin angle on the mechanical performance of hybrid joints was experimentally investigated. The load-displacement response of various specimens was recorded while joints were subjected to pull-off tensile loading. Experimental results showed how the pin angle affects the stiffness, strength and strain energy of joints. Damage initiation and propagation in the hybrid joints were also investigated by examining the fracture surfaces and the relationship between damage mechanisms and pin angles. Significant matrix crushing was realised in joints with 45° and 80° inclined pins with pin rupture only observed in the latter. 

Furthermore, a numerical study was conducted to explore the response of multiple root-like pin geometries. A 2D model was developed and validated for parametric study based on the experimentally measured interfacial strength. This bidimensional model accurately captured the experimental load-displacement curves for joints under tensile load. After calibration, the model was used to study the effect of three branches geometrical features of a root-like pin on the mechanical behaviour of the hybrid joint. It was demonstrated that the diameter, length and location of side branches play a vital role in the failure behaviour of joints by affecting the stress distribution in composite adherend. 

Based on the findings from this study, it can be concluded that the integration of complex pin shapes and composite are likely using resin infusion techniques. Moreover, root-like pins with optimised topology can be incorporated into hybrid joints to enhance bonding strength and failure behaviour.