VARTM Process Research Articles

Stochastic modeling of preform heterogeneity to address dry spots formation in the VARTM Process

Dry spot formation during the filling process was investigated for a system consisting of a heterogeneous porous medium coupled with a homogeneous porous layer. This system is representative of a composite manufacturing process in which the resin is introduced through an injection gate into a mold containing stacks of woven fabrics and a vacuum is drawn at the downstream end to impregnate the fibers. Dry spots are the regions that are not impregnated by the resin when the resin reaches the vent. The heterogeneity of the porous medium was quantified by modeling it with by-pass paths. They represent the gaps at the intersections of fiber tows in woven fabrics and have a larger permeability than the bulk porous medium permeability. When these fabrics are stacked to form a preform before injecting the resin, it is probable that some of these gaps across the thickness for all the layers would be inline creating the by-pass path for the resin. Hence, the need for stochastic modeling and random distribution of these by-pass paths. A log–normal distribution, K bp ∼ LN ( μ ,σ K ) was chosen to describe their formation. A five-parameter space was defined to describe the stochastic system. Numerical simulations of resin flow in this space for discrete combinations of the parameters were performed and the dry spot content ( c DR ) was obtained. The average dry spot content ( c ¯ DR ) and its standard deviation ( σ DR ) were calculated by conducting sufficient number of simulations with random placement of by-pass paths for each set of parameters. It was found that ( c ¯ DR ) and ( σ DR ) depends on ( σ K ), but the influence of ( σ K ) on ( c ¯ DR ) and ( σ DR ) can be altered by adjusting other parameters.

Use of Resin Transfer Molding Simulation to Predict Flow, Saturation, and Compaction in the VARTM Process

The present paper examines the analysis and simulation of the vacuum assisted resin transfer molding process (VARTM). VARTM differs from the conventional resin transfer molding (RTM) in that the thickness of the preform varies during injection affecting permeability and fill time. First, a governing equation for VARTM is analytically developed from the fundamental continuity condition, and used to show the relation between parameters in VARTM. This analytical work is followed by the development of a numerical 1-D/2-D solution, based on the flow simulation software LIMS, which can be used to predict flow and time dependent thickness of the preform by introducing models for compaction and permeability. Finally, the results of a VARTM experimental plan, focusing on the study of the influence of outlet pressure on compaction and fill time, are correlated with both the analytical and the numerical work. The present work also proposes an explanation for the similarities between VARTM and RTM and shows when modeling VARTM and RTM can result in an oversimplification.

Experimental behavior of monolithic composite cuff connections for fiber reinforced plastic box sections

A series of laboratory experiments investigated the structural performance of monolithic composite cuff connections for attaching pultruded fiber reinforced plastic (FRP) box sections. Cuff connections of three thicknesses were produced using the VARTM process, with fiber orientation selected to favor the mechanical response for this application. Test frames were assembled with the cuffs connecting box beams and columns (of pultruded FRP or steel), using adhesive bonding. Twelve frames were tested, each subjected to either cyclic or monotonic loading until failure; frame behavior was evaluated from the standpoints of stiffness and strength. Monolithic cuff connections of moderate thickness were capable of developing the flexural capacity of a pultruded FRP box beam (with proper detailing), and the cuffs themselves exhibited somewhat ductile failure modes upon reaching their maximum load. Cuff connection flexibility was a relatively minor contributor to overall frame displacement. The pultruded FRP frames were stronger and stiffer than previous generation approximations of an ideal cuff connection for similar box beam and column members.

223 FRP構造体のVaRTM成形における樹脂含浸挙動(OS 高分子/高分子基複合材料(3))

223 FRP構造体のVaRTM成形における樹脂含浸挙動(OS 高分子/高分子基複合材料(3))

Resin infusion of triaxially braided preforms with through‐the‐thickness reinforcement

AbstractVacuum assisted resin transfer molding (VARTM) has shown potential to significantly reduce the manufacturing cost of high‐performance aerospace composite structures. In this investigation, high fiber volume fraction, triaxially braided preforms with through‐the‐thickness stitching were successfully resin infiltrated by the VARTM process. The preforms, resin infiltrated with three different resin systems, produced cured composites that were fully wet‐out and void free. A three‐dimensional finite element model was used to simulate resin infusion into the preforms. The predicted flow patterns agreed well with the flow patterns observed during the infiltration process. The total infiltration times calculated using the model compared well with the measured times.

A Closed Form Solution for Flow During the Vacuum Assisted Resin Transfer Molding Process

A closed form solution to the flow of resin in vacuum assisted resin transfer molding process (VARTM) has been derived. VARTM is used extensively for affordable manufacturing of large composite structures. During the VARTM process, a highly permeable distribution medium is incorporated into the preform as a surface layer. During infusion, the resin flows preferentially across the surface and simultaneously through the preform giving rise to a complex flow front. The analytical solution presented here provides insight into the scaling laws governing fill times and resin inlet placement as a function of the properties of the preform, distribution media and resin. The formulation assumes that the flow is fully developed and is divided into two regimes: a saturated region with no crossflow and a flow front region where the resin is infiltrating into the preform from the distribution medium. The flow front region moves with a uniform velocity. The law of conservation of mass and Darcy’s Law for flow through porous media are applied in each region. The resulting equations are nondimensionalized and are solved to yield the flow front shape and the development of the saturated region. It is found that the flow front is parabolic in shape and the length of the saturated region is proportional to the square root of the time elapsed. The results thus obtained are compared to data from full scale simulations and an error analysis of the solution was carried out. It was found that the time to fill is determined with a high degree of accuracy while the error in estimating the flow front length, d, increases with a dimensionless parameter ε=K2xxh22/K2yyd2. The solution allows greater insight into the process physics, enables parametric and optimization studies and can reduce the computational cost of full-scale 3-dimensional simulations. A parametric study is conducted to establish the sensitivity of flow front velocity to the distribution media/preform thickness ratio and permeabilities and preform porosity. The results provide insight into the scaling laws for manufacturing of large scale structures by VARTM. [S1087-1357(00)02002-5]