Sudharshan Anandan

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.

Embeddable fiber optic strain sensor for structural monitoring

An extrinsic Fabry-Perot interferometric (EFPI) fiber optic sensor is presented for measurement of strain at high ambient temperatures. The sensor is fabricated using a femto-second (fs) laser. The EFPI sensor is fabricated by micromachining a cavity on the tip of a standard single-mode fiber and is then self-enclosed by fusion splicing another piece of singlemode fiber. The fs-laser based fabrication makes the sensor thermally stable to sustain temperatures as high as 800 °C. The sensor is relatively insensitive towards the temperature as compared to its response towards the applied strain. The sensor can be embedded in Carbon fiber/Bismaleimide (BMI) composite laminates for strain monitoring at high ambient temperatures.

Failure In metal honeycombs manufactured by selective laser melting of 304 L stainless steel under compression

ABSTRACT Cellular structures, specifically honeycombs, are commonly used as core materials in sandwich structures. This is especially true in aerospace applications where high bending and out-of-plane compressive stiffness coupled with low component weight is required. Additive manufacturing techniques are well suited for the manufacture of such cellular structures in a cost-effective manner. The current work focuses on honeycombs using selective laser melting of 304 L stainless steel. The mechanical behaviour of honeycombs was evaluated using out-of-plane compression tests. A numerical model was built to describe failure of the additively manufactured honeycombs. Compression tests were performed, on cylindrical samples to build the nonlinear material model. The material behaviour was found to be dependent on the build direction. Results of experiments and simulation show that failure occurs through a plastic buckling mechanism.

Strain monitoring using distributed fiber optic sensors embedded in carbon fiber composites

A distributed fiber optic strain sensor based on Rayleigh backscattering, embedded in a fiber-reinforced polymer composite, has been demonstrated. The optical frequency domain reflectometry (OFDR) technique was used to analyze the backscattered signal. The shift in the Rayleigh backscattered spectrum (RBS) was observed to be linear to the change in strain of the composite material. The sensor (standard single-mode fiber) was embedded between the layers of the composite laminate. A series of tensile loads were applied to the laminate using an Instron testing machine, and the corresponding strain distribution of the laminate was measured. The results show a linear response indicating a seamless integration of the optic fiber in the composite material and a good correlation with the electrical-resistance strain gauge results. In this study, distributed strain measurements in a composite laminate were successfully obtained using an embedded fiber optic sensor.

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 sandwich composite failure under three-point bending: Simulation and experimental validation

A sandwich structure consists of a two thin and strong facesheets, bonded to a thick lightweight core material. The mechanical response of a sandwich structure depends on the properties of its constituents. A numerical model and experimental validation of the three-point bending test of sandwich composites are presented in this study. The core material is aluminum honeycomb. The facesheets are made of IM7/Cycom5320-1, which is a carbon fiber/epoxy prepreg system. A comprehensive model of the failure under flexural loading was developed. Facesheet failure was modeled using Hashin’s failure criteria. A detailed meso-scale model of the honeycomb core was included in the model. The experiments indicated that failure initiation was due to local buckling in the honeycomb core. Failure propagation was in the form of core failure, facesheet compressive failure, and interlaminar failure. The developed meso-scale model was able to accurately simulate failure initiation and propagation in the composite sandwich structure. The effect of elevated temperature on the three-point bending behavior was studied numerically as well as experimentally. An increase in test temperature to 100°C resulted in a drop of 9.2% in flexural strength, which was also predicted by the numerical model.

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.

Monitoring cure properties of out-of-autoclave BMI composites using IFPI sensor

A non-destructive technique for inspection of a Bismaleimide (BMI) composite is presented using an optical fiber sensor. High performance BMI composites are used for Aerospace application for their mechanical strength. They are also used as an alternative to toughened epoxy resins. A femtosecond-laser-inscribed Intrinsic Fabry-Perot Interferometer (IFPI) sensor is used to perform real time cure monitoring of a BMI composite. The composite is cured using the out-of-autoclave (OOA) process. The IFPI sensor was used for in-situ monitoring; different curing stages are analyzed throughout the curing process. Temperature-induced-strain was measured to analyze the cure properties. The IFPI structure comprises of two reflecting mirrors inscribed on the core of the fiber using a femtosecond-laser manufacturing process. The manufacturing process makes the sensor thermally stable and robust for embedded applications. The sensor can withstand very high temperatures of up to 850 °C. The temperature and strain sensitivities of embedded IFPI sensor were measured to be 1.4 pm/μepsilon and 0.6 pm/μepsilon respectively.

Strain monitoring of bismaleimide composites using embedded microcavity sensor

Abstract. A type of extrinsic Fabry–Perot interferometer (EFPI) fiber optic sensor, i.e., the microcavity strain sensor, is demonstrated for embedded, high-temperature applications. The sensor is fabricated using a femtosecond (fs) laser. The fs-laser-based fabrication makes the sensor thermally stable to sustain operating temperatures as high as 800°C. The sensor has low sensitivity toward the temperature as compared to its response toward the applied strain. The performance of the EFPI sensor is tested in an embedded application. The host material is carbon fiber/bismaleimide (BMI) composite laminate that offer thermally stable characteristics at high ambient temperatures. The sensor exhibits highly linear response toward the temperature and strain. Analytical work done with embedded optical-fiber sensors using the out-of-autoclave BMI laminate was limited until now. The work presented in this paper offers an insight into the strain and temperature interactions of the embedded sensors with the BMI composites.

Monitoring of out-of-autoclave BMI composites using fiber optic sensors

Bismaleimide (BMI) composites are used in applications that require good mechanical properties at high temperatures. In this paper, a Non-destructive inspection technique for BMI composites which can be used at high temperatures is presented. Cavity based External Fabry-Perot Interferometer (EFPI) optical sensors have been developed and embedded in the laminates. These sensors are capable of operating in temperatures up to 800°C. The embedded sensors are used to perform real time cure monitoring of a BMI composite. The composite is cured using an out-of-autoclave (OOA) process. Once the composite is cured, the same sensors are used to measure mechanical performance of the laminate. The performance of the embedded sensor is investigated under tensile loading at room temperature as well as elevated temperatures.