Edu Ruiz

Thermomechanical Properties during Cure of Glass-Polyester RTM Composites: Elastic and Viscoelastic Modeling

Resin transfer molding (RTM) is a widely used technique for the manufacturing of composite parts. A proper selection of process parameters is the key to yield successful molding results and obtain a good part. During composite consolidation, resin cure, also called chemical conversion, plays a decisive role on the final mechanical properties of the part. The modeling of resin kinetics and the evolution of composite properties during cure are crucial for process optimization. In this paper, the curing of a thermosetting polyester resin is studied by differential scanning calorimetry (DSC). A semiempirical autocatalytic model is developed to describe the kinetics of the chemical reaction. The model accounts for the maximum degree of polymerization as a function of cure temperature and induction time, i.e., the time required to attain total inhibitor degradation. The evolution of mechanical properties during resin cure for two glass-polyester composites is also studied with a dynamical mechanical thermal analyzer (DMTA) and a thermomechanical analyzer (TMA). Given that for a low chemical conversion, the elastic properties of the resin remain low, an initial degree of polymerization called after gel point (AGP) is introduced in the analysis of the mechanical properties during cure. A normalized elastic modulus is defined from the value at AGP, taken as a reference. The normalized elastic modulus is then compared to the polymerization degree. For pure resin samples, the logarithm of chemical conversion is found to be almost linearly related to the logarithm of the elastic modulus. Based on this comparison, a thermochemical model is proposed to describe the evolution of mechanical properties during the cure of composite samples with different fiber volume fractions. The viscoelastic behavior is also determined by performing stress relaxation tests with the DMTA. Resin specimens are tested for different cure states below the glass transition temperature, and master curves of stress relaxation during cure are constructed by applying the time-temperature superposition principle. The measurements depict the relaxation modulus of polyester resins as sharply affected by the degree of polymerization. Based on the experimental data, a relaxation modulus is modeled in a thermorheologically simple manner using exponential and power laws. Finally, a linear volume change model is constructed based on the TMA measurements of thermal expansion and resin shrinkage. The volume changes resulting from composite expansion-contraction and resin polymerization shrinkage are modeled as a function of temperature and degree of polymerization. The purpose of this work is to develop appropriate models of chemo- and thermomechanical behaviors of glass-polyester composites during cure. A resin cure kinetics model is developed by adding the glass transition effects to the J.L.B. model. For the mechanical properties, two new models are presented to account for the elastic and viscoelastic behaviors of the resin and the composite. Finally, the coefficients of the volume changes model are measured to account for the composite thermal expansion-contraction and resin chemical shrinkage. These models will be used in future investigations for thermal and curing optimization of composites processed by resin transfer molding.

A comparative study of dispersion techniques for nanocomposite made with nanoclays and an unsaturated polyester resin

Over the last few years, polymer/clay nanocomposites have been an area of intensive research due to their capacity to improve the properties of the polymer resin. These nanocharged polymers exhibit a complex rheological behavior due to their dispersed structure in the matrix. Thus, to gain fundamental understanding of nanocomposite dispersion, characterization of their internal structure and their rheological behavior is crucial. Such understanding is also key to determine the manufacturing conditions to produce these nanomaterials by liquid composite molding (LCM) process. This paper investigates the mix of nanoclays particles in an unsaturated polyester resin using three different dispersion techniques: manual mixing, sonication, and high shear mixing (HSM). This paper shows that the mixing method has a significant effect on the sample morphology. Rheology, scanning electron microscopy (SEM), and differential scanning calorimetry (DSC) characterization techniques were used to analyze the blends morphology and evaluate the nanoclays stacks/polymer matrix interaction. Several phenomena, such as shear thinning and premature polymer gelification, were notably observed.

A Study of Nanoclay Reinforcement of Biocomposites Made by Liquid Composite Molding

Liquid composite molding (LCM) processes are widely used to manufacture composite parts for the automotive industry. An appropriate selection of the materials and proper optimization of the manufacturing parameters are keys to produce parts with improved mechanical properties. This paper reports on a study of biobased composites reinforced with nanoclay particles. A soy-based unsaturated polyester resin was used as synthetic matrix, and glass and flax fiber fabrics were used as reinforcement. This paper aims to improve mechanical and flammability properties of reinforced composites by introducing nanoclay particles in the unsaturated polyester resin. Four different mixing techniques were investigated to improve the dispersion of nanoclay particles in the bioresin in order to obtain intercalated or exfoliated structures. An experimental study was carried out to define the adequate parameter combinations between vacuum pressure, filling time, and resin viscosity. Two manufacturing methods were investigated and compared: RTM and SCRIMP. Mechanical properties, such as flexural modulus and ultimate strength, were evaluated and compared for conventional glass fiber composites (GFC) and flax fiber biocomposites (GFBiores-C). Finally, smoke density analysis was performed to demonstrate the effects and advantages of using an environment-friendly resin combined with nanoclay particles.

Lightweight damping of composite sandwich beams: Experimental analysis

The purpose of this article is to experimentally study the damping of composite sandwich beams with lightweight honeycomb core. The top and bottom facesheets are made of carbon/epoxy layers with partial interleaved viscoelastic layers. A new damping approach consisting of selectively targeting the inflection points of the bending mode shapes is proposed. At the nodes, the shearing deformation in the beam is maximal, and so is the strain in the viscoelastic layers. The experimental investigation of damping is made by means of standard impact tests using an instrumented hammer performed on beam specimens. The nodes are determined experimentally by moving a small accelerometer along the beam axis and by measuring the amplitude of the acceleration at each point. This novel damping approach keeps the damping ratio as high as the ratio obtained with standard (full coverage) surface damping treatment while reducing the added mass by almost 50%. A comparison of the results obtained in this study with experimental and numerical results found in the literature leads to the conclusion that the most efficient way of damping this type of sandwich structure is to modify and/or improve the viscoelastic properties of the core.

Re-meshing algorithms applied to mould filling Simulations in resin transfer moulding

In injection moulding processes such as Resin Transfer Moulding (RTM) for example, numerical simulations are usually performed with a fixed mesh, on which the displacement of the flow front is predicted by the numerical algorithm. During the injection, special physical phenomena occur on the front, such as capillary effects inside the fibre tows or heat transfer when the fluid is injected at a different temperature than the mould. In order to approximate these phenomena accurately, it is always better to adapt the mesh to the shape of the flow front. This can be achieved by implementing re-meshing algorithms, which will provide not only more accurate solutions, but also faster calculations. In order to represent precisely the shape of the saturated domain in the cavity, the mesh needs to be non-isotropic in the vicinity of the flow front. The size of the elements along the front is connected to the overall accuracy needed for the simulation; the size in the perpendicular direction governs the accuracy on the position of the moving boundary in time. Since these two constraints on element size are not related, the need for non-isotropic mesh refinement is crucial. In the approach proposed here, the mesh is changed at each time step from a background isotropic mesh used as starting point in the refinement algorithm. The solution needs to be projected on the new mesh after each re-meshing. This amounts to adopting a new filling algorithm, which will be validated by comparison to a standard simulation (without re-meshing) and with experimental data.

Adaptive mesh generation for mould filling problems in resin transfer moulding

In injection moulding processes such as Resin Transfer Moulding (RTM) for example, numerical simulations are usually performed on a fixed mesh, on which the numerical algorithm predict the displacement of the flow front. Error estimations can be used in the numerical algorithm to optimise the mesh for the finite element analysis. The mesh can be also adapted during mould filling to follow the shape of the moving boundary. However, in order to minimize computer time, it is preferable to optimise the mesh before carrying out the filling calculation. In this paper, these ideas are adapted to 3D shells, which represent the most common type of composite parts manufactured by RTM. An error estimator generally used in planar or solid geometries is extended for curved 3D surfaces in the specific case of RTM calculations. The extension consists of a projection of the solution field in the tangent plane to avoid problems related to the curvature of the part. Some other issues specific to shell geometries are pointed out and the results of a filling simulation made on a real part are presented. Non-isothermal filling simulations are also carried out in a rectangular mould to illustrate the stability conditions that arise from the convective heat transfer problem. Finally, an analytical study of radial injections is carried out to illustrate issues related to four types of different mesh refinement procedures: (1) a constant time step, (2) constant radial density (to allow a regular progression of the flow front at each time step), (3) a constant Courant number (to ensure stable thermal simulations); and (4) finally, a constant interpolation error.

Coupled Non-Conforming Finite Element and Finite Difference Approximation Based on Laminate Extrapolation to Simulate Liquid Composite Molding Processes. Part I: Isothermal Flow

In composite manufacturing by resin injection through a fibrous reinforcement, several phenomena occur involving the flow of resin and heat exchanges by the resin with the fiber bed and the mold. During processing, through-thickness flows commonly appear when the fibrous preform is made out of a stack of plies of different permeabilities. In many situations, simulation of the mold filling in Liquid Composite Molding (LCM) requires performance of a full threedimensional analysis. In the pre-processing stage, the construction of three-dimensional finite element meshes of complex parts made out of components of small thickness with respect to their length is very tedious. These parts are typically composed of an assembly of shell and flat panels. In such parts, a good aspect ratio of the finite elements must be respected to ensure appropriate simulation results. These conditions result in meshes with a very large number of degrees of freedom, which translates into a too high computational burden. For these reasons, a new numerical approach is presented in this paper for accurate and faster simulation of the filling phase in LCM. Based on the fabric reinforcement layup, a new non-conforming finite element is developed to quickly evaluate the throughthickness flow. Starting with a spatial triangular mesh as input geometry and based on the stacking sequence of the preform, an extrapolation algorithm is used to extrude the mesh in the thickness direction and generate the 3D non-conforming finite elements. To further evaluate the validity of the three-dimensional model, an experimental verification was performed for a typical through-thickness flow. Then, a comparative study is conducted to demonstrate the advantages of the proposed methodology in terms of accuracy. The results are compared with 2D (triangles) and 3D (tetrahedrons) finite element solutions. Finally, the performance of the model is assessed in terms of computer time.

High-frequency vibrations on the compaction of dry fibrous reinforcements

Liquid Composite Molding (LCM) is a composite manufacturing technique, in which a dry fibrous reinforcement is placed inside a mold, impregnated with a liquid resin and cured. During the fabrication process, the fibrous preform is subject to through-thickness and in-plane deformations. These deformations may affect the quality of the components and their mechanical performance. Hence, the compaction behavior of the reinforcement is crucial in order to predict its deformation during processing. The present work deals with the first stage of LCM manufacturing, during which the dry preform is draped into the rigid base mold and then compressed during closure of the upper mold. Under traditional manufacturing conditions, the preform is subjected to a single static compressive load. In the current study, controlled vibrations are applied to the preform before static compaction. These vibrations have a strong impact on nesting and on the further static compaction behavior of the reinforcement. The scope of this investigation is to study the influence of vibration parameters such as amplitude and frequency on the compaction of continuous fiber beds used to reinforce high-performance composites. Mechanical tests were performed using a special DMA instrument that allows characterizing the dynamic compaction of fibrous reinforcements at high frequency.

Viscoelastic behavior of an epoxy resin during cure below the glass transition temperature: Characterization and modeling:

This paper presents a viscoelastic temperature- and degree-of-cure-dependent constitutive model for an epoxy resin. Multi-temperature relaxation tests on fully and partially cured rectangular epoxy specimens were conducted in a dynamic mechanical analysis apparatus with a three-point bending clamp. Master curves were constructed from the relaxation test results based on the time–temperature superposition hypothesis. The influence of the degree of cure was included through the cure-dependent glass transition temperature which was used as reference temperature for the shift factors. The model parameters were optimized by minimization of the differences between the model predictions and the experimental data. The model predictions were successfully validated against an independent creep-like strain history over which the temperature varied.

Optimization of Alumina Slurry for Oxide-Oxide Ceramic Composites Manufactured by Injection Molding

This paper focuses on the rheological study of an alumina suspension intended for the manufacturing of oxide-oxide composites by flexible injection. Given the production constraints, it is required to have stable suspension with low viscosity and a Newtonian behavior. This is achieved with a concentration of nitric acid between 0.08 wt% and 0.2 wt% and amount of 3 wt% of PVA binder. The maximum loading of the suspension of 47 vol% suggests that there is no structure development within the suspension with optimized concentration of acid and PVA.