Investigation on automated fiber placement (AFP) for efficient manufacturing of advanced composites

• Main Author: Eldho, Mathew
• Other Authors: Sunil Chandrakant Joshi
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Engineering::Materials::Composite materials; Engineering::Materials::Material testing and characterization
• Online Access: https://hdl.handle.net/10356/166576
• Institution: Nanyang Technological University

Composites can be manufactured in numerous ways. Among these available methods, AFP is the most advanced and latest technology being utilized by companies for current and future aerospace projects. Although it offers many benefits, it also has unique manufacturing challenges and quality issues. Presence of tow placement defects like tow gaps, tow overlaps, twisted tows, and missing tows, associated with the AFP process is a cause for concern as these directly lead to a decrease in the mechanical performance of the fabricated parts. Although it is not possible to completely avoid the occurrence of these defects, optimizing key process parameters is a possible way to minimize it. In this work, effects of compaction roller pressure and the presence of base prepreg layer were studied. Test layups were made to get a preliminary understanding of their effects. Tensile and bending tests were carried out on laminates of different configurations fabricated using AFP technique to study the effects of these process parameters on the layup defects. From the test results, it is found that using a compaction roller pressure of 3.5 bar and a base prepreg layer of the same material as the towpreg corresponds to minimum tow defects, and hence, leads to the best tensile and bending properties. Even after optimizing process parameters, tow defects still occur, albeit much fewer in number. A novel technique which can minimize the effects of these defects and improve the integrity of the AFP laminate is proposed. The proposed method is to selectively interleave the AFP layup using micro thin films made from a suitable thermoplastic material. Two case studies using interleaved Elvax (EVA copolymer) films in Glass/BMI prepreg layups and Carbon/Epoxy towpreg layups were carried out to validate the effectiveness of the proposed method. In the first case study using prepreg system, fracture toughness tests in Mode I (opening mode), Mode II (sliding mode) and Mixed Mode I/II are conducted respectively using DCB, ENF, and MMB test specimens. Interleaving thermoplastic film was found to have a positive influence on both the Mode I as well as Mixed Mode I/II IFT. In the case of Mode II, the interleaving film led to a decrease in the IFT. In the second case study using towpreg system, tensile and bending tests were carried out to check the effectiveness of the interleaved Elvax (EVA copolymer) film. Test specimens with the interleaved thin film showcased the best tensile and bending properties. A technique for fabricating the required thin thermoplastic films was also customized in detail. Rule of mixtures can be used to accurately calculate the mechanical properties like Young’s moduli of laminates with a constant fiber orientation within each lamina, but it cannot be directly applied in cases where the fiber orientations change within a lamina. With the advancements in ATL and AFP techniques, it is easier to lay curved towpregs which have constantly varying fiber orientations. A modified rule of mixtures approach is taken to design a novel model and formulate equations which can predict the mechanical properties of laminates with varying fiber orientations within a lamina accurately. A case study using variable stiffness laminate was conducted to verify the formulated equations. In laminates with a combination of continuous and discontinuous layers, the load transfer in the latter will be incomplete because of the breaks at the section boundaries. A novel method to estimate Young’s modulus in the longitudinal direction of these type of laminates is proposed using a factor, and the effectiveness was checked by comparing with test results. Tension-compression (T-C) fatigue response is one of the important design criteria for CFRP material, as well as stress concentration. Hence, a study to investigate and quantify the stress concentration in CFRP dog-bone specimens due to T-C quasi-static and fatigue loadings (with anti-buckling fixtures) was carried out. Dog-bone specimens with a [(0/90),(45/-45)4]s layup were fabricated using woven CFRP prepregs and their low-cycle fatigue behaviour was studied at two stress ratios (-0.1 and -0.5) and two frequencies (3 Hz and 5 Hz). During testing, strain gauges were mounted at the centre and edge regions of the dog-bone specimens to obtain accurate, real-time strain measurements. The corresponding stresses were calculated using Young’s moduli. The stress concentration at the specimen edges, due to quasi-static tension, was significant compared to quasi-static compression loads. Furthermore, the stress concentration increased with the quasi-static loading within the elastic limit. Similarly, the stress concentration at the specimen edges, due to tensile fatigue loads, was more significant and consistent than due to compressive fatigue loads. Finally, the effects of the stress ratio and loading frequency on the stress concentration were noted to be negligible.