Exploring Debonding Technology

Carbon fiber veils are thin non-woven materials that enable debonding of adhesively bonded composite joints. This study examines the effects of three different carbon fiber veils on the mechanical, thermal, and electrical characteristics of epoxy adhesive systems sandwiched between glass fiber reinforced polymer (GFRP) layers.

Compared to neat epoxy configurations, interleaving with carbon fiber veils enhances storage modulus, thermal diffusivity, and lap shear strength (LSS) of the adhesive joints while lowering specific heat capacity (Cp) and glass transition temperature (Tg). Fourier-transform infrared spectroscopy (FTIR) analysis revealed that heated epoxy samples and composite samples made from interleaving carbon fiber veil sandwiched between two epoxy film adhesive layers at 100 °C for 1 min did not exhibit any detectable change in their chemical structures.

Surface roughness and water contact angle measurements were conducted to investigate the wettability of the GFRP adherends. Finite element coupled thermal-electric simulations and machine learning-based solutions displayed good agreement with Joule heating experiments. Thermomechanical debonding via Joule heating demonstrated effective debonding characteristics such as low force and time requirements, no fiber-tearing on the surface of the adherends, and selective heating of the bonded region of the joints.

Industrial Applications and Benefits

Adhesive bonding has gained significant attention in industrial applications such as aerospace, automotive, construction, and sports equipment due to its lightweight functionality, versatility, uniform stress distribution, corrosion resistance, and cost-effectiveness. However, adhesively bonded joints are sensitive to temperature and humidity, which can reduce their durability.

Adhesively bonded joints are also becoming increasingly important in the structural applications of fiber-reinforced polymer composites. The aerospace industry prioritizes composite materials because these lightweight polymer composites improve economic returns and provide sustainable solutions by reducing fuel consumption and CO2 emissions.

Additionally, there is a growing need to recycle glass fiber reinforced polymer (GFRP) and carbon fiber reinforced polymer matrix (CFRP) composites. International legislation on End-of-Life Vehicles (ELV) is an important initiative for increasing recycling, recovery, and reuse rates of composites, necessitating damage-free debonding of adherend materials. Consequently, there is an increasing trend in the development of debonding-on-demand adhesive technologies, as current debonding technologies based on mechanical separation are laborious, costly, and risk damaging the adherend materials.

The developed debonding technique will be useful for on-demand debonding of adhesively bonded composite joints or metal-composite hybrid joints in aerospace, wind energy, automotive, shipbuilding, and many other industries.

Innovative Heating Methods

Adhesive debonding technologies employ various heating methods such as oven, selective, and induction heating. Joule heating (i.e., resistance and Ohmic heating) is a promising method in composite manufacturing used for controlled heating of the bondline, adhesive bonding, and evaluation of debonding in CFRP-epoxy adhesive single-lap joints. Reports indicate that thermoset adhesive cured by Joule heating consumed 4.5 kJ at 4 kW, while a similar sample required 3 MJ at 800 W during oven curing.

Adhesive systems can be functionalized by non-woven veils for fabricating, producing electrothermal materials with rapid responsiveness, composite laminate manufacturing by Joule heating process, damage detection, and monitoring in composite materials and adhesively bonded joints.

Bridging the Knowledge Gap

This work aims to address the following gaps in the existing literature: (i) developing an effective debonding technique for structural adhesively bonded GFRP adherends while protecting them from the negative effects of thermomechanical debonding, and (ii) evaluating Joule heating as an energy-efficient heating method for debonding the joints.

The present study adopts a unique approach of utilizing the Joule heating method to debond joint configurations made from carbon fiber veils interleaved with epoxy. The investigations include: (i) surface characteristics of GFRP after surface treatment, (ii) the influence of interleaving different carbon fiber veils into epoxy adhesive joints on their thermal and mechanical properties, (iii) the Joule heating characteristics of different carbon fiber veil configurations, and (iv) the comparison of Joule heating tests with finite element-based coupled thermal-electric simulation results and machine learning-based solution results.

Methodology

Materials and Specimen Preparation:
Three types of carbon fiber veils were selected for this study, each with varying fiber diameters and areal densities. The veils were interleaved with epoxy adhesive systems and sandwiched between GFRP adherends. Specimens were prepared following standard procedures for adhesive bonding, ensuring consistent adhesive layer thickness and alignment of the GFRP layers.

Mechanical Testing:
Lap shear strength (LSS) tests were conducted to evaluate the mechanical performance of the bonded joints. The tests were performed at room temperature, and the results were compared to neat epoxy configurations. Additional mechanical properties, such as storage modulus, were measured using dynamic mechanical analysis (DMA).

Thermal and Electrical Characterization:
Thermal diffusivity and specific heat capacity (Cp) were measured using differential scanning calorimetry (DSC). The glass transition temperature (Tg) was also determined. Electrical conductivity measurements were performed to assess the Joule heating capabilities of the carbon fiber veil interleaved joints.

FTIR Analysis:
Fourier-transform infrared spectroscopy (FTIR) was used to analyze the chemical structures of heated epoxy samples and composite samples made from interleaving carbon fiber veil. Samples were heated at 100 °C for 1 min to observe any potential chemical changes.

Surface Roughness and Wettability:
Surface roughness measurements were conducted using a profilometer to assess the surface characteristics of the GFRP adherends. Water contact angle measurements were performed to evaluate the wettability of the treated surfaces.

Finite Element Simulations and Machine Learning:
Finite element simulations were carried out to model the coupled thermal-electric behavior of the bonded joints during Joule heating. A machine learning model was also developed to predict the Joule heating temperature based on the input parameters. The simulation and ML results were compared with experimental data to validate the models.

Joule Heating Experiments:
Joule heating experiments were performed to evaluate the debonding process. The bonded joints were subjected to electrical current, and the temperature profile was monitored. Debonding characteristics such as force, time requirements, and fiber-tearing on the adherend surfaces were recorded.

Results and Discussion

The interleaving of carbon fiber veils significantly improved the mechanical and thermal properties of the adhesive joints. The LSS of the joints increased, indicating enhanced bonding strength. The storage modulus and thermal diffusivity also showed improvements, while Cp and Tg decreased, suggesting better thermal management capabilities.

FTIR analysis confirmed that there were no significant chemical changes in the heated samples, indicating that the interleaving process did not alter the adhesive's chemical structure. Surface roughness and wettability measurements revealed improved surface characteristics, contributing to better adhesion.

Finite element simulations and machine learning models showed good agreement with experimental results, validating the accuracy of the predictive models. Joule heating experiments demonstrated efficient debonding with minimal force and time requirements, and no fiber-tearing on the adherend surfaces.

This study demonstrates the effectiveness of using carbon fiber veils interleaved with epoxy adhesive systems for debonding adhesively bonded GFRP joints via Joule heating. The interleaving process improves the mechanical and thermal properties of the joints, and Joule heating provides an energy-efficient and effective debonding method. The combined use of finite element simulations and machine learning models offers accurate predictions of the Joule heating behavior, making this approach a promising solution for on-demand debonding in various industrial applications.