Numerical modeling of compaction induced defects in thick 2D textile composites

The past few decades have witnessed an incredible advance in the use of composite structures in engineering and construction applications. Fiber reinforced composite materials have undoubtedly revolutionized traditional design concepts in engineering circles by creating new and exciting possibilities for high performance applications. In determining the performance and feasibility of composite structures, the quality of the final part is always critical.

Manufacturing of composite parts involves two main steps: forming and compaction. The forming process ensures that the material conforms to the desired shape, while the compaction is responsible for attaining the needed thickness and fiber volume fraction. Both steps require the material to deform, however, if the deformations exceed the limits of the material, defects (commonly wrinkles) may occur. Process modelling helps understand the limits of the material in respect to the manufacturing process, providing a powerful tool for eliminating unwanted deformations when integrated in the early stages of the design process.

Process modeling of textile composites has primarily focused on the forming of planar textiles into three-dimensional shapes, aiming to capture defects resulting from the large shear strains caused by the process. Although compaction induced defects are smaller in magnitude, their impact on the parts performance is equally as severe and their presence is often less noticeable, therefore developing effective modelling tools for predicting these defects is a key research priority. On this account, University of Bristol researchers: Dr Adam Thompson, Joseph McFarlane, Dr Jonathan Belnoue, and Professor Stephen Hallett developed a new modeling approach for predicting compaction induced defects in thick textile composites. The work is currently published in the journal, Materials and Design.

Briefly, the authors extended the conventional finite element forming models to include the through-thickness compliance of the textiles. The textiles were modelled in a continuous manner using mutually constrained membrane and shell elements to accommodate different stiffness forms such as low out-of-plane bending, high tensile, and non-linear shear stiffnesses. Instead of using material behaviors to capture the through-thickness compliance of the material, the new model used a penalty contact relationship. Simulation of the compaction of stacked textile layers was conducted to analyze their deformation behaviors. Finally, the simulation results were compared to the experimental data to validate the feasibility of the model and determine the relationship between the defect formation and the design and manufacturing parameters.

Results demonstrated an effective simulation of the compaction process of thick two-dimensional textile components. By introducing the compliant penalty contact, the authors accurately determined the mechanical response during the compaction process through surface interactions rather than through material behavior. The formation of the compaction-induced wrinkle defects as well as their magnitude, location, and shape were predicted. Furthermore, the design parameters such as lay-up and part design, as well as manufacturing process i.e. vacuum bagging procedure, exhibited a significant influence on the formation and magnitude of the wrinkles.

In summary, University of Bristol scientists successfully developed a new numerical modeling approach that takes into consideration the through-thickness compliance of the textile material during forming simulations. The model was experimentally validated, and the influence of design and manufacturing parameters on the formation of defects was established. The key parameters included the configuration of the vacuum bags, fiber orientation and tool profile. The method offers a new approach for modeling and predicting compaction-induced defects. In a statement to Advances in Engineering, the authors explained that the main benefit of this approach is the ability to simulate a wide range of conditions, making it possible to examine the effect that different parameters within the design space and manufacturing capabilities have on the final part. Through this, a better understanding of the mechanisms behind compaction induced defects can be gained and used to help inform part design and manufacturing process.