Fracture Behavior and Damage of Carbon Fiber Reinforced Polymer and Titanium Tubular Adhesive Lap-Joints at Elevated Temperatures

In this work, we investigate the mechanical behavior of carbon fiber reinforced polymer (CFRP) and titanium (Ti) tubular adhesive lap-joints (TLJs) at elevated temperature dry (ETD) and room-temperature dry (RTD) conditions. ABAQUS Non-Linear Finite Element Analysis (FEA) software paired with Cohesive Zone Modeling (CZM) is employed to investigate the damage mechanisms within the adhesive and the interlaminar shear behavior within the CFRP adherend. To understand the effects of temperature on the failure mechanisms of the joint, room-temperature dry (RTD) conditions are also studied. Furthermore, the relationship between the tensile load carrying capability of the adhesive TLJ and the adhesive bondline length (L_Ad) is studied.

We first present experimental results for the adhesive TLJ subject to tensile loading at RTD and ETD conditions for nominal bondline length (L_Ad). Our results show that the TLJ exhibits poor performance at ETD conditions when compared to RTD conditions. Furthermore, at RTD conditions, the TLJ failed due to interlaminar shear failure between the first and second ply of the CFRP, although slight yielding within the Ti adherend was observed as well. At ETD conditions, the failure mode was found to be interlaminar shear failure between the first and second ply of the CFRP adherend as well; however, less yielding within the Ti adherend was present, indicating the CFRP failed with less resistance than at RTD conditions. Since the ETD conditions showed the poorer performance, the bondline length analysis was conducted at this condition for design purposes. The experimental results showed a linear increase in strength for an increase in bondline length (L_Ad) for lengths less than 15% above the nominal. However, for bondline lengths (L_Ad) 15% greater than the nominal, the results showed a switch in failure mechanism as the failure mode was Ti net-section fracture.

We next implement ABAQUS Non-Linear FEA to study the damage progression and failure modes of the adhesive TLJ at both RTD and ETD conditions. Numerical results show correlation within 7% of the experimental failure load and predict the experimental failure modes. For bondline lengths (L_Ad) less than 15% above nominal, the results show interlaminar shear failure between the first and second ply of the CFRP adherend. The numerical results show that the delamination initiates on the inserted side of the CFRP adherend and propagates towards the outer end. As the bondline length (L_Ad) is increased past nominal, a more uniform stress distribution is achieved across L_Ad, which causes the failure mode to switch to Ti net-section fracture.