Venkata Bheemreddy

Evaluation of skin-core adhesion bond of out-of-autoclave honeycomb sandwich structures

Composite sandwich structures offer several advantages over conventional structural materials such as lightweight, high bending and torsional stiffness, superior thermal insulation and excellent acoustic damping. One failure mechanism in a composite sandwich structure is the debonding of the composite facesheets from the core structure. A well-formed adhesive fillet at the interface of the honeycomb core cell walls and the laminate is a significant factor in preventing bond failure. In the present work, honeycomb composite sandwich panels are manufactured using a low-cost vacuum-bag-pressure-only out-of-autoclave manufacturing process. CYCOM®5320 out-of autoclave prepreg is used for the facesheet laminates and FM® 300-2U film adhesive is used for the facesheet-to-core bond. The manufactured composite sandwich panels are of aerospace quality with a facesheet laminate void content of around 1%. In this study, adhesive fillet formation and adhesive mechanical strength are evaluated as a function of several different sandwich construction design variables. Both aluminum and aramid Nomex® honeycomb core materials are considered to study the effect of core cell size and core material. The effect of film adhesive thickness is studied. A process for reticulation of the adhesive is applied and studied. A quantitative investigation of the adhesive fillet geometry is carried out for all the panels. Manufactured panels are evaluated for flatwise tensile strength in accordance with test method ASTM C297. Optimized combinations of core material, core density, cell size and adhesive thickness are identified. Results show that the reticulation process improves fillet formation and increases flatwise tensile properties.

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).

Characterization of polyurethane composites manufactured using vacuum assisted resin transfer molding

Glass fiber-reinforced polymer composites have promising applications in infrastructure, marine, and automotive industries due to their low cost, high specific stiffness/strength, durability, and corrosion resistance. Polyurethane (PU) resin system is widely used as matrix material in glass fiber-reinforced composites due to their superior mechanical behavior and higher impact strength. Glass fiber-reinforced PU composites are often manufactured using pultrusion process, due to shorter pot life of PU resin system. In this study, E-glass/PU composites are manufactured using a low-cost vacuum-assisted resin transfer molding process. A novel, one-part PU thermoset resin system with a longer pot life is adopted in this study. Tensile, flexure, and impact tests are conducted on both the thermoset PU neat resin system and E-glass/PU composites. A three-dimensional finite element model is developed in a commercial finite element code to simulate the impact behavior of E-glass/PU composite for three different energy levels. Finite element model is validated by comparing it with experimental results.

Computational study of micromechanical damage behavior in continuous fiber-reinforced ceramic composites

A comprehensive numerical analysis of micromechanical damage behavior in a continuous fiber-reinforced ceramic composite is presented. A three-dimensional micromechanical finite element modeling procedure is developed for effective elastic property estimation and damage evaluation by the example of a composite consisting of a silicon carbide matrix unidirectionally reinforced with silicon carbide fiber (SiC/SiCf). The effect of a fiber/matrix interface on predicted elastic properties of the SiC/SiCf composite is considered. Representative volume element (RVE) models are developed for an SiC/SiCf composite with damageable interfaces. Statistically equivalent RVE models with randomly distributed fibers are generated using a developed algorithm. The statistical variability of fiber and matrix strengths is considered in developing RVE models and assumed to follow a Weibull probability law. A user-material subroutine with an adaptive material constitutive law is developed to predict damage behavior in the RVE. The predicted uniaxial stress versus strain behavior and damage in the composite are discussed.

Evaluation of Low-Velocity Impact Response of Honeycomb Sandwich Structures Using Factorial-based Design of Experiments

A response surface analysis of data from factorial experiments is used to determine the effect of different design factors on the low-velocity impact response of the honeycomb sandwich structures. A low-cost, out-of-autoclave manufacturing process is utilized to manufacture aerospace quality honeycomb sandwich panels. CYCOM® 5320 out-of-autoclave prepreg and FM® 300-2M are used as facesheet and adhesive, respectively. The out-of-autoclave process uses the vacuum bag pressure, thus, avoiding costly tooling and making the process more economical. A completely randomized design is used while manufacturing and testing the samples. Three design factors: angle difference between successive prepreg layers, number of prepreg layers, and number of adhesive layers, are selected as experimental variables. Each variable is considered at three levels, yielding a 33 factorial design. To investigate the effect of the experimental variables on the three response variables: energy absorbed (X); peak contact force (Y) and maximum deflection (Z), a low-velocity impact test is conducted. Analysis of variance and response surface techniques are used to analyze the data. The significant design factors for each response variable are identified using analysis of variance. Response surface analysis is carried out and the resulting regression models are employed to quantify the behavior of each of the response variables in response to changes in the design factors. The regression models are verified with the confirmation test results and both are in close agreement.

Three-Dimensional Micromechanical Modeling of Continuous Fiber Reinforced Ceramic Composites With Interfaces

Continuous fiber reinforced ceramic composites (CFCCs) are widely used in high performance and high temperature applications. The behavior of CFCCs under various conditions is not easily predicted. Micromechanical modeling of CFCCs using a representative volume element (RVE) approach provides useful prediction of the composite behavior. Conventionally, the effect of fiber-matrix interface on effective property prediction of the CFCCs is not considered in the micromechanical modeling approach. In the current work, a comprehensive three-dimensional micromechanical modeling procedure is proposed for effective elastic behavior estimation of CFCCs. Application of the micromechanical model for various interfaces has been considered to evaluate the effect of different interfaces and highlight the applicability of current model. Cohesive damage modeling approach is used to model the crack growth along with fiber bridging. The finite element model is validated by comparing with available data in the literature.© 2012 ASME