Several techniques are available for joining materials together in various engineering applications. However, adhesive joints have recently attracted significant attention of researchers owing to their excellent strength to weight ratio property. Unfortunately, adhesive joints are susceptible to different failures due to loading conditions. To this note, researchers have been looking for various alternative methods to effectively predict joint behaviors under various loading and environmental conditions, so as to promote their use in critical structures. Thus, the finite element method has been identified as a promising solution.
Presently, the cohesive zone model is widely used for finite element analysis. It has been adopted by adhesive joints to model adhesive layers as it relates the material stress with the corresponding displacement between adjacent nodes. Besides, it is easy to implement and produces accurate results. Consequently, environmental factors such as humidity in a joint significantly influence the material parameters which may result in degradation. Humidity degraded joints can be modelled through cohesive zone model taking into consideration the traction-separation law.
Generally, fatigue analysis using cohesive zone model involves progressive damage application to the element. A formulation based on the experimental details is used to mimic the fatigue degradation. The damage variable is considered as a function for numerous fatigue cycles. Just like the initial methods, the main challenge remains controlling the fatigue damage parameters leading to less accurate results. In addition, the cycle-to-cycle approach involves complex and time-consuming computations thus limiting the accuracy of the obtained results.
Recently University of Porto scientists: Dr. Marcelo Costa, Dr. Lucas F M da Silva, Dr. Guilherme Viana and Dr. Raul D S G Campilho developed a finite element method based on the cohesive zone model for modelling adhesively bonded joints. The joints were subjected to fatigue and degradation by humidity. Consequently, the authors incorporated different types of traction-separation laws including exponential and trapezoidal. Furthermore, humidity degraded joints were modelled using a general law for property degradation as a function of the humidity content. On the other hand, modelling fatigue was based on experimental data relationship to ensuring reproducibility of the results. The authors aimed at simplifying the whole modelling systems to ensure more accurate results. Their work is published in the journal, International Journal of Fatigue.
The research team observed that the trapezoidal separation law was the most suitable approach for modeling experimental data for the aged and unaged samples. The number of cycles to the failure of the specimens were correctly predicted using fatigue degradation method. Also, both the numerical and experimental results were similar. This was attributed to the possibility of implementing different types of traction-separation law such as exponential, trapezoidal and triangular as well as a full cohesive zone model. However, the trapezoidal cohesive zone model resulted in more accurate values closer to the experimental results. Furthermore, MATLAB interface proved to be effective for speeding up the simulation process, extraction and visualization of the results.
The study by Dr. Marcelo Costa and his colleagues at University of Porto will advance modeling of joints subjected to different degradation and fatigue conditions thus an effective way of minimizing joints and structural failures. It will also enable design of specific joints with desired requirements and precision. The proposed technique can also be used for modelling an array of experimental conditions as well as a platform for including additional loading modes and further fatigue considerations.
Costa, M., Viana, G., Créac’hcadec, R., da Silva, L., & Campilho, R. (2018). A cohesive zone element for mode I modelling of adhesives degraded by humidity and fatigue. International Journal of Fatigue, 112, 173-182.Go To International Journal of Fatigue