Thermoelastic Wave Propagation in Functionally Graded Sandwich Plates: Impact of Porosity and Thermal Effects on Structural Behavior

Wave behavior in functionally graded materials (FGMs) has become a fascinating area of research especially for industries where materials have to handle intense physical stress and big temperature shifts. FGMs aren’t like traditional composite materials where different layers have distinct properties. Instead, FGMs are crafted so that their material properties gradually change across the thickness. This gradual shift helps to ease stress points and adds resilience which make them useful in aerospace, automotive engineering, and large-scale construction applications. Yet, even with these benefits, predicting how FGMs will hold up in real-world conditions remains tricky. A big issue lies in dealing with porosity—the tiny air pockets that naturally form within the material. These voids can mess with how waves move through the material, especially when the material faces temperature swings or mechanical loads. Porosity can reduce strength, change how the material reacts under pressure and make its performance harder to rely on. Traditional models often oversimplify these factors and leave out important details about porosity’s impact on FGMs’ performance. Then there’s the temperature aspect: when the temperature shifts, the material’s properties—like stiffness and wave speed—can change as well, impacting how stress is distributed. Most conventional models assume these properties stay mostly constant, which doesn’t line up with what actually happens in harsh environments. This is where a new study, published in Thin-Walled Structures, makes a significant contribution. Led by Professor C.W. Lim, with PhD candidates Chen Liang and Guifeng Wang from the Department of Architecture and Civil Engineering at the City University of Hong Kong, along with Associate Professor Zhenyu Chen from Southeast University, the researchers developed a more refined model for FGM sandwich structures. They focused specifically on sandwich rectangular plates, a type of layered material commonly used in construction due to its mix of stiffness and lightweight properties. By factoring in both temperature sensitivity and porosity, their model provides a more realistic prediction of how FGMs behave.

To do this, the team built a detailed model focusing on three main setups: one with FGM layers on the outside and a hard core, another with FGM layers and a softer core, and a third that used a solid outer layer with an FGM core. By working through each setup, they aimed to capture a realistic view of how these different structures would hold up under varying thermal and physical stresses. One of the key elements in their model was accounting for porosity, those tiny pockets of air scattered throughout the material, which can really change how waves travel through it. Including a function to simulate different porosity levels, they could see how these air gaps affect the stiffness (technically known as Young’s modulus) across the plate’s thickness. The authors showed that as the amount of porosity increased, the stiffness dropped, which made the material weaker overall. This result made it clear that porosity isn’t just a small detail; it has a big impact on how fast waves move through the material and on the structure’s durability under dynamic forces. The researchers looked closely at how temperature changes affect the behavior of waves in these specialized materials. They set up their model to capture what happens as temperatures rise and fall, and they found that when temperatures go up, stiffness in the material drops quite a bit, slowing down the speed of the waves. This softening effect was most noticeable in plates with a softer core—when the heat increased, these plates became more flexible and less capable of handling heavy loads. These findings highlight why it’s essential to account for temperature swings in FGM designs, especially for materials used in places where temperatures can fluctuate widely.

Another key area they studied carefully was how the ratio between the core and the thickness, known as the core-to-thickness ratio (CTR), influences wave frequencies and speed. Interestingly, they noticed that CTR had different effects depending on the core type. In structures with a hard core, increasing the CTR led to higher wave frequencies, which means the structure was stiffer. On the other hand, in softer-core setups, increasing the CTR caused the wave frequencies to drop which make the structure more flexible. For configurations with an FGM core, CTR changes had a balanced effect, without causing major shifts in how the waves moved. These results suggest that adjusting the CTR gives engineers a way to fine-tune the material’s properties, depending on whether stiffness or flexibility is needed for a particular application. The team also looked at something called the power-law exponent (PLE), which describes how the material properties change from one layer to another in FGMs. When the PLE was low, meaning there was a sharp shift from ceramic to metal, the wave frequencies were higher, and the waves moved faster. In contrast, a higher PLE, which represents a smoother transition, led to lower frequencies and slower waves. To check the accuracy of their model, the researchers compared their findings with results from finite element method (FEM) simulations. The two approaches matched closely, especially for lower wave frequencies, although there were slight differences at higher frequencies due to variations in how each method approaches the calculations. This close match confirmed that the model is a reliable tool for understanding how FGM sandwich plates work. Lastly, the researchers explored how different levels of porosity affected wave behavior across various setups. They found that in plates with hard cores, porosity had a strong impact on wave movement, while in FGM-core configurations, it mattered less. This variation shows that the influence of porosity depends heavily on the type of structure, underlining the importance of tailoring designs based on both the material’s structure and its intended use.

In conclusion, the significance of Professor C.W. Lim and his team’s study is in the advancements it brings to the modeling of FGM sandwich plates. The theoretical expressions characterizing temperature-dependent material properties maintain continuity amidst the effects of the generation of porosity, fluctuations in porosity volume fraction, and variations in temperature. It indicates that a large interfacial shear stress concentration can be alleviated effectively based on the present mixture rules of FGM sandwich structures. With this improved model, it’s now possible to design FGM sandwich plates with tailored structural and mechanical properties, opening up even more possibilities for use in various industries. We believe the implications of this research go even further, especially in terms of improving the durability and reliability of advanced composite materials in challenging environments. For example, understanding how different core-to-thickness ratios influence wave behavior can help engineers choose configurations that offer the right balance of stiffness or flexibility for specific applications. Additionally, being able to accurately model and manage porosity and its distribution across the material helps reduce stress concentrations which are usually the starting point for fatigue or failure. Additionally, the findings into how temperature affects wave propagation also make it easier to select the right materials for environments with extreme or changing temperatures, like in high-altitude aerospace applications or high-performance car parts. With this framework, engineers now have a powerful tool to create designs that aren’t just resilient but also adaptable and efficient enough to meet the demands of modern engineering.