Tackling wrinkling in advanced textile composites

August 6th, 2018 / By: , / Featured

New research shows breakthroughs in solving a persistent challenge in textile composite manufacturing.

Image 1: The fabrication of textile composites with geometrical complexity, such as in doubly curved parts, are difficult to manufacture reliably. Photo: Composites Research Network at the University of British Columbia (UBC).

The birth of the modern composites industry dates back to the introduction of high performance continuous fibers, including aramid and carbon, in the 1960s. Over the years, the increasing availability of such fibers has allowed the rapid development of a range of advanced engineering structures by utilizing unprecedented high specific strength and stiffness properties, especially in aerospace and automotive applications.

Continuous fiber-reinforced composites based on different matrix systems (usually polymeric) exhibit great adaptability for the structural properties to be tuned towards each specific design requirement, processing condition and fabrication method. For instance, a custom design of polymer-matrix composite component based on carbon fibers can attain weight reductions in the range of 30 percent compared to aluminum, or even 70 percent in comparison with steel.

Yet, there are a range of difficulties in reaching high-quality, reliable manufacturing of fiber composite structures, which is directly associated with the continuous nature of the reinforcement, particularly when they are in textile form, and it becomes more obvious when structures of certain geometrical complexity, such as doubly curved parts (See image 1), need to be fabricated.

These manufacturing issues have led to the ongoing need for research on the deformation and forming mechanisms of both dry (uncured) reinforcements as well as their cured counterparts. This effort was largely initiated in the late 1970s and to date continues in different forms, as new, automated, forming processes are being developed—and are becoming increasingly complicated to analyze.

Challenges in complex-shaped textile composites

Upon success of their uni-directional counterparts, multi-directional textile fiber composites are now changing the way products are designed faster, stronger and at lower cost. The technologies used currently for the fabrication of textile fibrous materials and preforms (near-net shape and dry fabric structures) and their exact positioning within the molds, are typically related to high-tech applications, where low tolerances are needed for parts to pass the quality tests.

On the other hand, the wide range of available textile fiber types and architectures, production technologies and their combinations, has obscured a universal design strategy for optimizing their composite-forming processes. The following challenges are currently deemed to be most relevant to industries towards efficient and reproducible manufacture of textile composite products and preforms:

  • Mitigation of defects and therefore reduction of production costs by correct selection of fabric materials and their lay-ups, optimum set-up of manufacturing steps, and waste management, e.g. by using near-net shape production methods and defect-free processing of dry and pre-impregnated fabrics;
  • Implementation of high-fidelity simulation tools, especially for forming parts with complex 3D shapes.
Image 2: An example of a wrinkled composite sandwich panel with carbon fibers; multiple wrinkles have been indicated by red arrows. Photo: Composites Research Network, UBC.

One of the main difficulties associated with the manufacture of textile composites is wrinkling (See image 2), which is regarded as a critical quality issue by designers, as it can contribute to the reduced mechanical properties and life span of the final structures by 80 percent, potentially causing a massive economic loss for companies. The severity of wrinkling can be affected by various manufacturing factors, including mold selection, material characteristics, layup configuration, tool-part interaction and quality of processing equipment, among others.

In essence, generation of excessive shear angles on sharp corners or tights edges (where more formability is required) can lead the interwoven fibers to buckle and reduce the inter-layer cohesion from the adjacent layers. Hence, a good understanding of each step of the manufacturing process and the forming behavior of the material system is a key to mitigate such wrinkles and control the properties of the final part.

Success with predictive simulation models

We are now very close to being able to predict the right amount of forces needed to be applied during manufacturing of textile composites to diminish the wrinkles in the final product. This can be achieved through applying appropriate boundary conditions enforced at fabric edges at the 2D level, and then correlating it to the actual 3D forming scenarios using blank holders, grips or thick membrane sheets.

Image 3: A new characterization methodology has been proposed at UBC to investigate the wrinkling/de-wrinkling behavior of textile composites. Graphic: Composites Research Network, UBC.

To do so, a multi-step test has been recently designed at the Composites Research Network’s lab at the University of British Columbia’s (UBC) Okanagan campus. With the help of a biaxial fixture frame, the test can assess the magnitude of the required forces needed to smooth out wrinkles of different sizes that were formed at different shear angles (See image 3).

Specialized image processing and 3D scanning made it possible to analyze the required forces and their impact on the wrinkling and de-wrinkling behaviour of the material. “Our analysis of wrinkling and de-wrinkling of woven fabrics uncovered new insights on how various parameters, such as yarn geometry or uncontrolled sources of variability during manufacturing,  can affect the wrinkling behavior” says Armin Rashidi, Ph.D. student working on the project.

Due to the trend of de-wrinkling forces, a correlation could be established between 2D characterization tests and the actual 3D forming operation of woven fabrics, which can then be implemented in numerical simulations to devise precise de-wrinkling strategies based upon the blank holder pressure, modification of the mould, or blank holder geometry.

Predictive numerical models have proven useful to reduce trial and error during experimental optimization of the forming processes. A reliable numerical model, however, requires an advanced geometrical mapping model for fiber tracking and multi-scale material models describing coupled behaviors of fibers and matrix.

Image 4: The “Digital twin” of a composite that’s forming accurately predicting the wrinkles size and shape. Images: Composites Research Network, UBC.

Implementation of the proposed de-wrinkling approach via numerical simulations is now being researched by multiple groups in the composites world. In particular, researchers at UBC have recently introduced a hybrid modeling framework to predict the forming pattern of woven fabrics over 3D surfaces with an unmatched time efficiency. Under this hybrid framework, they have implemented a multi-step design optimization methodology where a kinematic-based simulation can initially estimate the part regions prone to wrinkles, along with corresponding shear angle ranges, and then the full finite element models can further capture the exact location and size of wrinkles (See image 4).

The next milestone in this research has involved conducting tension-assisted forming trials with a modified blank holder geometry concept, to impose the optimum amount of boundary forces during forming, and hence assisting the manufactures to mitigate the wrinkles ‘passively’ at minimal costs.

The outlook and future trends

Composite textiles are changing the way products are designed and built in advanced manufacturing sectors. The trends in fabrication of lightweight composite structures, along with their automated manufacturing processes, point to an increased demand for these types of materials as an important basis in resource efficiency, CO2 emission reduction and development of components able to meet consumer demands with absolutely novel ideas. However, manufacturers using textile composites are increasingly looking for new ways to ensure defect-free final products, particularly when it comes to the wrinkling as it can significantly reduce the effective mechanical properties.

Currently, research capabilities to simulate reasonably well controlled forming systems with well defined boundary conditions have been improved with the help of customized testing equipment and characterization fixtures, along with the development of high fidelity simulation models. As innovation continues in this field  to include more polymer resin and fabric reinforcement options, along with new forming set-ups, further research will need to carry into providing up-to-date analysis for manufacturers, especially when concerning forming of multi-layer and hybrid fabric composites under varying processing cycles.

Also, the ongoing development of sensor-embedded characterization and draping methods are deemed to be significantly useful to add to our fundamental understanding of underlying forming mechanisms at the meso/micro levels, and hence for arriving at reliable digital twins for forming processes that may be used, not in a distant future within the emerging realm of “Composites 4.0.

 

Armin Rashidi is a Ph.D. student at Composites Research Network’s Okanagan Laboratory, University of British Columbia (UBC), Canada.

Abbas Milani, Ph.D., PEng, is Professor of Mechanical Engineering at UBC, Principal Investigator of the Okanagan Node of the Composites Research Network (CRN) in Canada, and the Director of the Materials and Manufacturing Research Institute (MMRI) at UBC. (http://crno.ok.ubc.ca/, https://mmri.ubc.ca/)

Readers interested in an in-depth reading of this subject are invited to contact Janet Preus, senior editor, Advanced Textiles Source, to obtain a list of reference materials provided by the authors.