3D printed auxetic stiffener as a new concept for lightweight high strength and toughness metal-CFRP T-joint
Topic(s) : Special Sessions
Co-authors :
Ahmed AHMED WAGIH (SAUDI ARABIA), Hassan HASSAN MAHMOUD (SAUDI ARABIA), Gilles LUBINEAU (SAUDI ARABIA)Abstract :
Modern lightweight thin-walled structures are currently built by deploying a network of stiffeners with different geometry and materials to support the lightweight skin, which is typically made of polymer composite. A typical example is represented by T-joints, which is used in several applications, such as wind turbine blades, automotive, and airframes [1,2]. A main limitation of most of joining techniques is that it requires some modification (in most cases making holes) in the skin, which creates zones with high stress concentration and thus reducing the in-plane strength of the skin. On the other hand, adhesive-bonded joints can mitigate this limitation, however, the strength and toughness of the joint are relatively low compared to other joining techniques.
This work presents a new design of hybrid steel/CFRP adhesive bonded joints by creating auxetic structures in the stiffener that allows the redistribution of the stresses inside the adhesive layer and at the stiffener and hence delay the damage initiation and propagation. Steel stiffeners with the designed auxetic re-entrant structures were printed using 3D printer, bonded to CFRP substrate and tested under pull-off loading condition. We considered two configurations, one row and two rows re-entrant stiffener.
We developed a high-fidelity 2D FE model to predict the response of hybrid adhesive bonded T-joint with CFRP skin and auxetic stiffener. The model simulates the progressive damage inside the adhesive layer. The stiffener was defined using an elastic-plastic isotropic hardening model. The adhesive layer was defined with the same model, while the CFRP skin was defined using orthotropic material. The material properties of the adhesive and the CFRP used are described in our previous work [3]. Cohesive law was used to describe the contact between skin - adhesive layer and stiffener - adhesive layer. The evolution of the damage and strength and toughness was evaluated using the FE model and validated using experimental results.
We demonstrated a large enhancement on the maximum load (strength) of the joints with auxetic stiffener. The enhancement reached 3.5 times for the joint with one row of auxetic structures. Also, these joints showed larger enhancement of the displacement at failure (failure strain) compared to the baseline model, which suggests a huge improvement of the toughness.
In the base model, the delamination initiates at the skin/adhesive interface at the end of the bond line, which is typical for such joints. However, for the joints with auxetic stiffener, the damage initiates from the middle of the bondline under the first re-entrant structure. The change in the damage initiation mode is associated with the presence of the re-entrant structure, where large deformations inside the adhesive occur under the first re-entrant structure that reduces the stresses at the end of the bond line and thus inhibit the growth of the delamination at this area.
This work presents a new design of hybrid steel/CFRP adhesive bonded joints by creating auxetic structures in the stiffener that allows the redistribution of the stresses inside the adhesive layer and at the stiffener and hence delay the damage initiation and propagation. Steel stiffeners with the designed auxetic re-entrant structures were printed using 3D printer, bonded to CFRP substrate and tested under pull-off loading condition. We considered two configurations, one row and two rows re-entrant stiffener.
We developed a high-fidelity 2D FE model to predict the response of hybrid adhesive bonded T-joint with CFRP skin and auxetic stiffener. The model simulates the progressive damage inside the adhesive layer. The stiffener was defined using an elastic-plastic isotropic hardening model. The adhesive layer was defined with the same model, while the CFRP skin was defined using orthotropic material. The material properties of the adhesive and the CFRP used are described in our previous work [3]. Cohesive law was used to describe the contact between skin - adhesive layer and stiffener - adhesive layer. The evolution of the damage and strength and toughness was evaluated using the FE model and validated using experimental results.
We demonstrated a large enhancement on the maximum load (strength) of the joints with auxetic stiffener. The enhancement reached 3.5 times for the joint with one row of auxetic structures. Also, these joints showed larger enhancement of the displacement at failure (failure strain) compared to the baseline model, which suggests a huge improvement of the toughness.
In the base model, the delamination initiates at the skin/adhesive interface at the end of the bond line, which is typical for such joints. However, for the joints with auxetic stiffener, the damage initiates from the middle of the bondline under the first re-entrant structure. The change in the damage initiation mode is associated with the presence of the re-entrant structure, where large deformations inside the adhesive occur under the first re-entrant structure that reduces the stresses at the end of the bond line and thus inhibit the growth of the delamination at this area.