Z. Huo

Experimentation and simulation of moisture diffusion in foam-cored polyurethane sandwich structure

The moisture diffusion behavior of two-part thermoset polyurethane neat resin, woven E-glass fiber-reinforced polyurethane face sheet, closed-cell rigid polyurethane foam core and their corresponding sandwich specimens was investigated in this study. The vacuum-assisted resin transfer molding process was used to manufacture the polyurethane sandwich panels. Open-edge moisture diffusion experiment was conducted for sandwich panel and its constituents by immersing each type of samples in distilled water at room temperature for nearly seven months. Moisture diffusivities and solubility for neat resin, face sheet and foam core specimens were characterized according to the experimental analysis. The moisture diffusion behavior for closed-cell polyurethane foam was found to deviate significantly from classical Fick’s law, and a multi-stage diffusion model was thus proposed to explain this deviation using a time-dependent diffusivity scheme. A user-defined subroutine was developed to implement this scheme into the commercial finite element analysis code ABAQUS. A three-dimensional dynamic finite element model was developed to predict the moisture diffusion behavior in neat resin, face sheet, foam core and sandwich specimens. This finite element model was then validated by comparing simulation results with experimental findings.

Modelling of concentration-dependent moisture diffusion in hybrid fibre-reinforced polymer composites

Hybrid fibre-reinforced polymer composites have extensive applications due to their high strength, cost effectiveness, improved product performance, low maintenance and design flexibility. However, moisture absorbed by composite components plays a detrimental role in both the integrity and durability of hybrid structure because it can degrade the mechanical properties and induce interfacial delamination failures. In this study, the moisture diffusion characteristics in two-phase hybrid composites using moisture concentration-dependent diffusion method have been investigated. The two phases are unidirectional S-glass fibre-reinforced epoxy matrix and unidirectional graphite fibre-reinforced epoxy matrix. In the moisture concentration-dependent diffusion method, the diffusion coefficients are not only dependent on the environmental temperature but also dependent on the nodal moisture concentration due to the internal swelling stress built during the diffusion process. A user-defined subroutine was developed to implement this method into commercial finite element code. Three-dimensional finite element models were developed to investigate the moisture diffusion in hybrid composites. A normalization approach was also integrated in the model to remove the moisture concentration discontinuity at the interface of different material components. The moisture diffusion in the three-layer hybrid composite exposed to 45℃/84% relative humidity for 70 days was simulated and validated by comparing the simulation results with experimental findings. The developed model was extended to simulate the moisture diffusion behaviour in an adhesive-bonded four-layer thick hybrid composite exposed to 45℃/84% relative humidity for 1.5 years. The results indicated that thin adhesive layers (0.12-mm thick) did not significantly affect the overall moisture uptake as compared with thick adhesive layers (0.76-mm thick).

Curing of Thick Thermoset Composite Laminates: Multiphysics Modeling and Experiments

Fiber reinforced polymer composites are used in high-performance aerospace applications as they are resistant to fatigue, corrosion free and possess high specific strength. The mechanical properties of these composite components depend on the degree of cure and residual stresses developed during the curing process. While these parameters are difficult to determine experimentally in large and complex parts, they can be simulated using numerical models in a cost-effective manner. These simulations can be used to develop cure cycles and change processing parameters to obtain high-quality parts. In the current work, a numerical model was built in Comsol MultiPhysics to simulate the cure behavior of a carbon/epoxy prepreg system (IM7/Cycom 5320–1). A thermal spike was observed in thick laminates when the recommended cure cycle was used. The cure cycle was modified to reduce the thermal spike and maintain the degree of cure at the laminate center. A parametric study was performed to evaluate the effect of air flow in the oven, post cure cycles and cure temperatures on the thermal spike and the resultant degree of cure in the laminate.

Manufacturing and evaluation of polyurethane composite structural insulated panels

Composite materials are increasingly used in applications of civil infrastructure and building materials. The new generations of two-part thermoset polyurethane resin systems are desirable materials for infrastructure applications. This is due to high impact resistance, superior mechanical properties, and reduced volatile organic compounds when compared to the conventionally used resin systems such as vinyl ester and polyester. Glass fiber-reinforced two-part polyurethane composites and low-density polyurethane foam are used to design and manufacture composite structural insulation panels using vacuum assisted resin transfer molding process for temporary housing applications. Using these types of composite panels in building construction will result in cost-efficient, high-performance products due to inherent advantages in design flexibility. Use of core-filled composite structures offers additional benefits such as high strength, stiffness, lower structural weight, ease of installation and structure replacement, and higher buckling resistance than the conventional panels. Energy efficiency is known to be inherently better with the core-filled composite panel than in a metallic material. The panels can be designed to resist the required loads, and the study aims to evaluate the ability of lab scale tests and models to predict part quality in full-scale parts. Furthermore, it discusses the manufacturing challenges. Flexural tests and energy consumption evaluations were performed on these structural components. Finite element simulation results were used to validate the flexural experiment findings.

Effect of salt water exposure on foam-cored polyurethane sandwich composites

This study investigated the effect of moisture absorption on the mechanical performance of polyurethane sandwich composites. The core material was a closed cell polyurethane foam. Face sheets were made of E-glass/polyurethane composite laminates. Vacuum-assisted resin transfer molding process was used to manufacture specimens for testing. The foam core, laminates, and sandwich composites were submerged in salt water for prolonged periods of time. Mechanical property degradation due to moisture absorption for each constituent was evaluated. Compression test was performed on the foam core samples. Laminates were evaluated by three-point bending tests. The interfacial bond strength in the sandwich structure was evaluated by double cantilever beam mode-I interfacial fracture test. The testing results revealed that the effect of salt water exposure on the compressive properties of the foam core is insignificant. The flexural modulus of polyurethane laminates degraded 8.9% and flexural strength degraded 13.0% after 166 days in 50% salinity salt water at 34°C conditioning. The interfacial fracture toughness of polyurethane sandwich composites degraded 22.4% after 166 days in 50% salinity salt water at 34°C conditioning.

Investigation of three-dimensional moisture diffusion modeling and mechanical degradation of carbon/bismaleimide composites under seawater conditioning:

The effect of moisture diffusion on the mechanical properties of carbon/bismaleimide composites exposed to seawater conditioning at elevated temperatures was investigated in this study. Carbon/bismaleimide composites with two stacking sequences (unidirectional and cross-ply) were fabricated using out-of-autoclave process. Testing coupons were immersed in the seawater at two elevated temperatures (50℃ and 90℃) for approximately 3 months. Moisture diffusivities and solubility for each type of carbon/bismaleimide specimen were characterized using the experimental data. A three-dimensional dynamic finite element model was developed using these parameters to predict the moisture diffusion behavior in the carbon/bismaleimide laminates. The degradation of mechanical properties due to hygrothermal aging was assessed by short-beam shear and three-point bending tests. It was found that flexural strength and interlaminar shear strength reductions are higher at 90℃ aging than that at 50℃ aging. The reduction in mechanical properties for bismaleimide laminates can be attributed to the fiber/matrix interfacial cracks observed by scanning electron microscopy.