The mechanism of fiber waviness formation and its effect on material properties in CFRTP laminates
Topic(s) : Special Sessions
Co-authors :
Takayu NISHIOKA (JAPAN), Ryo HIGUCHI (JAPAN), Tomohiro YOKOZEKI (JAPAN)Abstract :
In recent years, the application of Carbon Fiber Reinforced Thermoplastic (CFRTP) has gained attention, especially in the field of aerospace engineering, due to its high productivity and recyclability. However, fiber waviness, one of the typical defects in CFRTP, is known to reduce the longitudinal compressive strength of the unidirectional composite structure. Nevertheless, its generation mechanism and effect on those material properties remain unclear, and a quantitative predictable scheme is needed to assess these effects.
In previous studies [1,2], many numerical analysis methods have been proposed to assess the effect of fiber waviness, with most of them focusing on the geometric shape of fiber waviness as the metric. However, in semi-crystalline resins commonly used in CFRTP, it is necessary to consider relatively large thermal strains, compared to thermosetting resins, resulting from the change of mechanical properties due to crystallized behavior. Therefore, it is necessary to consider the effect of resin crystallinity, which depends on forming conditions, during the fiber waviness formation process. This study aims to investigate the effect of fiber waviness on mechanical properties in unidirectional composite laminates, including the fiber waviness formation process.
Based on the previous study [3] on fiber waviness formation, it is assumed that the geometric shape of fiber waviness is determined by the difference of thermal strains of the material and mold among the maximum temperature and the crystallization onset temperature. Below the crystallization onset temperature, the Nakamura-Ziabicki model is introduced for non-isothermal crystallization behavior. The crystallinity calculated by the model is used to determine the resin's stiffness linearly and the crystallization strain based on the specific volume of the crystal phase and amorphous phase. Then, considering the fibers, those homogenized values are applied for each element in the finite element method analysis. These crystallization parameters of thermoplastic resins are known to depend on the cooling rate. This study sets the cooling rate as a metric for the molding conditions. The strength analysis uses the same analytical model as the author’s previous study [4].
The material assumed for the analysis is MCP1223. The results of the longitudinal compressive (bending) strength analysis are as shown in Figure 1. From these results, it is observed that increasing the cooling rate leads to a reduction in the longitudinal compressive strength. The author believes that the proposed scheme in this study is beneficial not only for predicting the strength of thermoplastic laminates with fiber waviness but also for the application to the optimization of CFRTP molding conditions.
In previous studies [1,2], many numerical analysis methods have been proposed to assess the effect of fiber waviness, with most of them focusing on the geometric shape of fiber waviness as the metric. However, in semi-crystalline resins commonly used in CFRTP, it is necessary to consider relatively large thermal strains, compared to thermosetting resins, resulting from the change of mechanical properties due to crystallized behavior. Therefore, it is necessary to consider the effect of resin crystallinity, which depends on forming conditions, during the fiber waviness formation process. This study aims to investigate the effect of fiber waviness on mechanical properties in unidirectional composite laminates, including the fiber waviness formation process.
Based on the previous study [3] on fiber waviness formation, it is assumed that the geometric shape of fiber waviness is determined by the difference of thermal strains of the material and mold among the maximum temperature and the crystallization onset temperature. Below the crystallization onset temperature, the Nakamura-Ziabicki model is introduced for non-isothermal crystallization behavior. The crystallinity calculated by the model is used to determine the resin's stiffness linearly and the crystallization strain based on the specific volume of the crystal phase and amorphous phase. Then, considering the fibers, those homogenized values are applied for each element in the finite element method analysis. These crystallization parameters of thermoplastic resins are known to depend on the cooling rate. This study sets the cooling rate as a metric for the molding conditions. The strength analysis uses the same analytical model as the author’s previous study [4].
The material assumed for the analysis is MCP1223. The results of the longitudinal compressive (bending) strength analysis are as shown in Figure 1. From these results, it is observed that increasing the cooling rate leads to a reduction in the longitudinal compressive strength. The author believes that the proposed scheme in this study is beneficial not only for predicting the strength of thermoplastic laminates with fiber waviness but also for the application to the optimization of CFRTP molding conditions.