The use of composite materials for various industrial applications has soared. The attractiveness of these materials can be attributed to their excellent mechanical properties, high corrosion resistance and flexible design. In most marine applications, the materials absorb water when in direct contact with seawater. Due to the potential impact of the absorbed water on the material’’ properties and functionality, considerable research has been conducted to explore the moisture transportation in composite materials. Moisture transportation is a two-stage process in which the first stage involves diffusion control while the second stage involves matrix relaxation control. Additionally, the moisture distribution in these materials is anisotropic, and the moisture diffusion parameters can be evaluated and characterized through gravimetric study and edge correction factors, respectively.
When the composite structures are exposed to water on one side, they experience slow moisture diffusion. Also, multidirectional composite laminates exposed to similar conditions experience asymmetric moisture content along the laminate thickness. The resulting distribution of the residual stress has been extensively investigated. The most notable difference was the uniform moisture distribution at equilibrium state and non-uniform distribution at transient state. Nevertheless, most of these studies focus on the distribution of non-mechanical stress and strain with little attention to the effect of such hygroscopic non-uniformity on the mechanical behaviour of the laminates, especially when the material property degrades with moisture diffusion. This degradation can be described as functions of immersion time or absorbed moisture content. Moreover, the moisture asymmetry also results in material property asymmetry in the composite laminates. Therefore, it is important to understand the effects of moisture-induced symmetry on the bending response of composite materials.
On this account, Dr. Rajaram Attukur Nandagopal, Professor Srikanth Narasimalu, and Professor Gin Boay Chai investigated the effect of moisture-induced asymmetry on the bending behaviour of multi-directional composite laminates. In particular, due to the inherent uncertainties of the mechanical properties of the composites, the authors used a probabilistic approach to compare the ply stresses induced at different aging periods in the laminates. The original research article is currently published in the journal, Composite Structures.
In their approach, the laminate bending response was numerically simulated using the first-order shear deformation theory. Next, a surrogate model was used in the probabilistic simulations of the bending response based on both Monte Carlo simulation and Polynomial Chaos Expansion Method. Also, a Fick diffusion model was used to predict the moisture diffusion in the laminates and determine moisture asymmetry for different ageing periods. The factors affecting the material properties degradation were discussed in detail. Finally, the bending response was compared for aged and dry conditions, and the obtained results were validated experimentally.
The authors observed that an increase in the ageing changed the moisture distribution from a non-uniform state during the initial days to uniform upon reaching the equilibrium state. The moisture asymmetry induced unwanted bending-extension coupling, which was observed to disappear as the laminate approached equilibrium state. The deterministic analysis was characterized with a change in the bending response attributed to ageing while for statistical analysis, the ageing-induced change in the bending response was not obvious. Furthermore, the resulting material property asymmetry exhibited remarkable effects on the ply stresses.
In summary, the authors reported a numerical simulation of the probabilistic bending behaviour of composite laminates subjected to dry and aged material conditions. The simulations provided a thorough and in depth understanding of the degradation of the material properties due to moisture asymmetry. Based on the results, the ply stresses are dependent on the moisture diffusion rate, moisture content, loading condition, the ply position, and the stress redistribution. Consequently, a probabilistic simulation was of great importance in drawing a statistically significant conclusion regarding the bending behaviour due to dry and aged conditions. In a statement to Advances in Engineering, the authors explained the study will facilitate designing high-performance composite materials for marine applications.
Attukur Nandagopal, R., Chai, G., & Narasimalu, S. (2020). Probabilistic bending behaviour of a symmetric multi-directional composite laminate subjected to moisture induced material property asymmetry. Composite Structures, 254, 112845.