Hongbin Shen

Mechanical characterization of short fiber reinforced phenolic foam

The mechanical performance of fiber-reinforced phenolic foam is characterized and compared with unreinforced foam in terms of friability, compression and shear properties, and flexural behavior of simple sandwich beams. Compared with conventional phenolic foam, foam reinforced with aramid fibers exhibits significantly lower friability, higher resistance to cracking, and more isotropic behavior, while glass fiber-reinforced foam is significantly stiffer and stronger. Sandwich structures with reinforced phenolic foam cores show unique failure behavior, in which catastrophic collapse of the structure is not only delayed, but avoided altogether. The findings in this paper, coupled with earlier results, demonstrate the potential of reinforced phenolic foam as a fire-resistant, tough and low-cost engineering material.

Experimental and analytical study of nonlinear bending response of sandwich beams

The practical value of the geometrically nonlinear higher-order theory is demonstrated using four-point bend tests carried out on sandwich beam specimens comprised of aluminum face sheets and a PVC foam core. The experimental results were compared with the predictions of classical sandwich theory, and with linear and geometrically nonlinear higher-order sandwich panel theory. The analytical predictions based on the higher-order theory are in excellent agreement with the experimental results. Response parameters show fundamentally distinct behavior with increasing external load, both in the particular section and along the span. Considering the longitudinal displacements, there is a significant geometrically nonlinear stage of the response that precedes the appearance of the material nonlinearity. The peeling stresses also exhibit significant geometrical nonlinearity in the vicinity of the internal supports. The linear higher-order theory can be used efficiently to estimate the vertical displacements of the soft-core sandwich beams up to high load levels with a great accuracy. Premature failure of sandwich beam specimens with weak adhesive layers is caused by high peeling stresses in the upper interface layer at the ends of the specimen, and the loading capacity decreases by more than 40%.

Enhanced peel resistance of fiber reinforced phenolic foams

Abstract Short-fiber reinforced phenolic foams were synthesized and characterized by climbing drum peel tests and tensile tests. Significant improvements in mechanical properties were realized, including a multiple-fold increase in peel strength. Because peel strength is closely linked to the fracture toughness of foams, particular attention was focused on the mechanism responsible for the enhanced peel resistance. Scanning electron microscope observations revealed that phenolic foams reinforced with aramid fibers exhibited a unique ‘micro-peel’ process. This process was caused by a moderately weak interface between the flexible aramid fibers and the surrounding phenolic matrix, resulting in higher toughness relative to similar foams reinforced with stiffer glass fibers. In addition, a design-of-experiments analysis supported the expectation that fiber loading and length were primary factors contributing to the improved peel strength.

Mechanical Behavior of Hybrid Composite Phenolic Foam

Hybrid composite phenolic foams are reinforced with chopped glass and aramid fibers in varied proportions. The mechanical properties are measured and compared with those of foams reinforced with only aramid and glass fibers. The compression and shear properties of the hybrid reinforced foams are also compared with those of commercial polyurethane foams. The reinforced hybrid phenolic foams exhibit greater resistance to cracking and are significantly stiffer and stronger than foams with only glass and Nomex® fibers. In general, the mechanical properties of reinforced hybrid phenolic foams are comparable to that of commercial polyurethane foam of equivalent density. The experimentally observed compressive properties (compression modulus) of reinforced phenolic foam with different fiber loading have been compared with existing theories of reinforcement. Composite models such as parallel, series, Halpin—Tsai, and the Hirsch model have been evaluated to fit the experimental data. The findings presented here, coupled with earlier results, demonstrate the potential use of hybrid composite foams as a low-cost engineering material that is tough, strong, and fire retardant.

Accurate predictions of bending deflections for soft-core sandwich beams subject to concentrated loads

Although computationally simple and physically perceptible, the formula for bending deflections used by the classical sandwich theory is inaccurate when applied to sandwich beams with a transversely flexible (soft) core. Two correction factors to the classical deflection formula for sandwich beams are proposed based on the higher-order theory (HSAPT) approach. The influence of changes in the geometry and mechanical properties of soft-core sandwich beams on the proposed correction factors are studied numerically. The use of the correction factors reported in the present work offers a simple and accurate way of calculating the bending deflections in soft-core sandwich beams subject to quasi-static concentrated loads, and as such can provide a valuable tool for the designer.The proposed approach also can be used for accurate determination of the core shear modulus from the measurement of compliance in short beam shear tests of sandwich beams.

Tenacious Composite Foams as Cryogenic Tank Insulation

Reinforcing polymer foams with short aramid fibers has the potential to greatly improve the reliability of the thermal protection systems applied to cryogenic propellant tanks. The critical failure mode is peeling of the foam along the bondline with the tank. One of the worst structural foams, phenolic, exhibited great peel strength increase when reinforced with short aramid fibers, and the improvement was retained when tested at -320 degrees F. This is noteworthy because polymers lose toughness at cryogenic temperatures, and reinforcement with short fibers appears to be a promising method for circumventing this shortcoming. This paper shows some of the remarkable performance improvements aramid fiber filled foams exhibit, both at ambient and cryogenic temperatures.

Composite Foams for Sandwich Structures

Recent work at USC has focused on strategies to enhance the toughness and overall mechanical performance of polymer foams for use in lightweight sandwich structures. Both mechanical and chemical approaches have been employed with reasonable success. Fiber reinforcement, though difficult from a processing perspective, can lead to substantial enhancements in toughness and strength, while reducing friability. Chemical modifications are also challenging from a processing perspective, but can produce similar enhancements in performance. Efforts to enhance performance of phenolic foam and PVC foam through fiber reinforcement and chemical modification are described, along with the resulting enhancements in performance.