Konstantine A. Fetfatsidis

A Friction-Test Benchmark with Twintex PP

This paper presents an update on a friction benchmark, that was proposed during the 13th ESAFORM conference. The goal is to compare different friction test set-ups [1–4] by determining the coefficient of friction (CoF) for Twintex® PP. The benchmark instructions are based on the ASTM standard D1894 [5] but also account for different friction velocities, pressures and temperatures. At the time of writing five research groups contributed to the benchmark, each with a custom designed test set-up, differing in size, mechanism, force control and temperature regulation. All tests will be conducted with woven glass reinforced polypropylene, from the same Twintex® batch. Conclusions will be drawn about the comparability of different testing methods by recognizing and analyzing systematically deviating results.

Digital Method of Analyzing the Bending Stiffness of Non‐Crimp Fabrics

A digital‐analytical method for characterizing the bending behavior of NCFs (Non‐Crimp Fabrics) is developed. The study is based on a hanging fabric loaded to a known displacement. The image of the deformed fabric is captured digitally, and then analyzed to describe the deformed shape of the beam using x‐y coordinates. The bending stiffness of the fabric is then determined through an iterative method using a finite element method (ABAQUS). This effective bending stiffness is of importance in the formation of wave defects in NCFs during manufacturing processes such as thermoforming, vacuum assisted resin transfer molding, and compression molding.

Fabric thermostamping in polymer matrix composites

Abstract: Thermostamping is a low-cycle, high-volume manufacturing process for continuous fabric-reinforced composites. To ensure high-quality parts are manufactured during the thermostamping process, the mechanical behaviors of the fabric reinforcements and polymer matrix and the critical manufacturing process parameters must be thoroughly understood. Various mechanical tests are conducted to characterize the fabric-reinforcement and polymer-matrix mechanical behaviors, and analytical models (i.e., finite element models) are developed to predict part quality. This chapter describes the thermostamping process and provides examples of typical experimental and analytical methods used to create high-quality composite parts using the thermostamping process.

Simulation of the manufacturing process and subsequent structural stiffness of a composite wind turbine blade with and without defects

Traditional ply-based and zone-based models are limited in their ability to account for the fiber directions resulting from the forming of fabric-reinforced composite wind turbine blades. Compounding the problem is the presence of defects such as resin-rich pockets of the polymer matrix due to out-of-plane and in-plane waves resulting from the manufacturing process. As a result, blades are typically overdesigned, unnecessarily increasing weight and material costs. In the current research, a methodology is presented for simulating the manufacturing process for fabric-reinforced composite wind turbine blades using ABAQUS/Explicit. The methodology captures the evolution of the yarn directions during the forming process thereby allowing for a map of the fiber orientations throughout the blade. A hybrid approach using conventional beam and shell elements is used to model the various fabric layers. Using experimental shear, tensile, bending, and friction data to characterize the mechanical behavior of the fabric layers, the model captures in-plane yarn waviness and changes in the in-plane yarn orientations as they conform to the shape of the mold, as well as out-of-plane wave defects as a result of the manufacturing process. Subsequently, after the fabric layers have been laid into the mold and the final yarn orientations are known, the structural stiffness of the blade resulting from the resin-infused fabrics can be calculated. The methodology can thereby link the resulting bending and torsional stiffnesses of the blade back to the manufacturing process. This paper discusses the methodology for determining the material properties of the beam and shell elements in their final orientations in the cured composite to predict the structural stiffness of a wind turbine blade.

Characterization and Finite Element Modeling of Unidirectional Non-Crimp Fabric for Composite Manufacturing

A hybrid finite element discrete mesoscopic approach is proposed to model the forming of composite parts using a unidirectional glass prepreg non-crimp fabric (NCF). The tensile behavior of the fabric is represented using 1-D beam elements, and the shearing behavior is captured using 2-D shell elements. The material is characterized using tensile and shear frame tests. These properties are then incorporated into an ABAQUS/Explicit finite element model via user-defined material subroutines. The shear frame characterization test is simulated using a finite element model of the fabric, and the finite element results are compared to experimental data as a validation of the methodology. The thermostamping of a double-dome geometry, which has been used in an international benchmarking program, is modeled as a demonstration of the capabilities of the proposed methodology.

Simulation of the Manufacturing of Non‐Crimp Fabric‐Reinforced Composite Wind Turbine Blades to Predict the Formation of Wave Defects

NCFs (Non‐Crimp Fabrics) are commonly used in the design of wind turbine blades and other complex systems due to their ability to conform to complex shapes without the wrinkling that is typically experienced with woven fabrics or prepreg tapes. In the current research, a form of vacuum assisted resin transfer molding known as SCRIMP® is used to manufacture wind turbine blades. Often, during the compacting of the fabric layers by the vacuum pressure, several plies may bunch together out‐of‐plane and form wave defects. When the resin is infused, the areas beneath the waves become resin rich and can compromise the structural integrity of the blade. A reliable simulation tool is valuable to help predict where waves and other defects may appear as a result of the manufacturing process. Forming simulations often focus on the in‐plane shearing and tensile behavior of fabrics and do not necessarily consider the bending stiffness of the fabrics, which is important to predict the formation of wrinkles and/or waves....

Friction properties of reinforcements in composites

Abstract: The use of binders during the forming of fabric-reinforced composites by thermostamping induces in-plane tensile forces as a result of friction between the fabric and the tooling and adjacent layers of fabric. These in-plane forces reduce the potential for the development of defects in the form of fabric wrinkling. However, if in-plane forces are too high, then fabric tearing can occur. Thus, it is critical to understand the relationship between the effective friction, the forming rate and other processing parameters to produce quality parts using this high-volume low-cost manufacturing process. This chapter summarizes recent research on the friction at the tool/fabric and fabric/fabric interfaces.

Characterization of Cured Composite Materials for Wind Turbine Blades

NCFs (Non-Crimp Fabrics) infused with epoxy resin are popular in the design of wind turbine blades and other complex systems due to their ability to conform to complex shapes. Past work in the development of a combination beam-shell modeling approach to simulate the forming of NCF composites has been demonstrated to capture the change in the orientations of the yarns during a forming process. The structural performance of these manufactured blades is often analyzed using finite element simulations that consider the material properties of the fibers and of the resin based on the rule of mixtures and orthotropic shells where the model is sectioned into zones that account for changes in the material properties due to variations in the orientations of the lamina and number of layers. With the availability of the beam-shell model, the use of zones can be removed if the individual contributions of the yarns (beam elements) and resin (shell elements) can be characterized and the orientations of the yarns resulting from a forming simulation can be used to account for the variations in the material properties of the composite throughout the blade. This research uses a combination of static flexure tests and impact modal tests to ascertain the material properties of the fibers and resin in a unidirectional and biaxial non-crimp fabric laminate plates. The material properties are used in a finite element model of the plate and the model is analyzed in flexure and in a free-free modal configuration to compare to experimental results. Two different approaches are used in the commercially available software Abaqus to model the plate. One approach uses a combination of beam and shell elements to represent the fibers and the resin, respectively. The other approach uses orthotropic shell elements to capture the unbalanced behavior of the fiber/resin composite. The beam/shell modeling approach better represents the overall behavior of a single-layer plate and can be extended to consider multiple plies.

Finite Element Modeling of Unidirectional Non-Crimp Fabric for Manufacturing of Composites with Embedded Cabling

A hybrid finite element discrete mesoscopic approach is used to model the forming of composite parts using a unidirectional glass prepreg non-crimp fabric (NCF). The tensile behavior of the fabric is represented using 1-D beam elements, and the shearing behavior is captured using 2-D shell elements into an ABAQUS/Explicit finite element model via a user-defined material subroutine. The forming of a hemisphere is simulated using a finite element model of the fabric, and the results are compared to a thermostamped part as a demonstration of the capabilities of the used methodology. Forming simulations using a double-dome geometry, which has been used in an international benchmarking program, were then performed with the validated finite element model to explore the ability of the unidirectional fabric to accommodate the presence of interlaminate cabling.