Composites based on carbon fiber reinforced polymer matrix (CFRP) have become one of the important structural materials for the aerospace industry due to their excellent mechanical properties. CFRP composites will be affected by coupling factors such as ambient temperature, self-curing reaction, and change in characteristics during the curing molding process, which will lead to uneven residual stress within the material, which will affect the subsequent behavior of the composite structure in service. . To solve this problem, Hui Xinyu, Xu Yingjie, Zhang Weihong and their team from Northwestern Polytechnic University published an article titled "Integrated Simulation of Curing and Cross-Tension Process of Unidirectional CFRP Composites" in the "Composite Structures" section. In this article, mesoscopic residual stresses generated during the curing process and their effect on the behavior of composites under load failure are investigated using a multiscale modeling approach.
1. Sample preparation for testing composite materials
The composite sample used for the study was autoclaved from a T800/AC531 unidirectional prepreg. Figure 1 shows the molding temperature and pressure curves.
Fig. 1. T800/AC531 Composite Molding Temperature and Pressure Curve
2. Theoretical modeling
2.1 Multi-scale modeling of residual stress during curing
Fig. 2 Multiscale modeling of residual stress during curing
In order to solve the problem of internal residual stress of the composite material during the curing process, this study first determined the curing response and heat transfer characteristics of the composite material through experiments, including thermal conductivity, specific heat capacity, and glass fibers associated with curing. transition temperature. Heat transfer analysis of composite materials takes into account external heat transfer and heat generation from internal curing reactions, and 3D macroscale thermochemical interaction analysis is performed to calculate temperature and degree of cure. during the curing of the composite material; Thermomechanical interaction of materials (fiber, interface, matrix) and curing shrinkage of the resin, the obtained temperature and curing rate data are fed into the established RVE mesoscale model to solve the mesoscopic thermomechanical bond residual stress, deformation.
2.2 Damage Rejection Model
Because fiber failure does not affect lateral damage to unidirectional composites, matrix and interface failure is mainly studied. The resin matrix consistently exhibits elasticity, ductility, and damage under load, and the extended linear Drucker-Prager characteristic is used to determine its flow behavior, and the viscous viscosity criterion is used to describe its damage behavior. The interface is modeled by connected elements of zero thickness and is characterized by a bilinear constitutive model.
Figure 3. Matrix equation
Fig. 4. Basic interface equation
2.3 Integrated Process Corruption Recovery Analysis Framework
Through multiscale analysis of thermochemical force interaction of composite materials, the mesoscopic residual stress during the curing process was obtained. In order to further study the effect of mesoscopic residual stress on the mechanical behavior of composite materials, the residual stress was used as the initial stress. A certain field is introduced into the composite material to predict damage during transverse loading, and an integrated fracture analysis structure during curing is established as shown below.
Fig. 5. Integrated analysis framework for the elimination of failures caused by process damage
3. Simulation results
Fig.6. Changes in temperature and degree of curing of the center point of the composite material during curing
Fig.7. Residual stress distribution of the hardened composite
Fig.7 Stress-strain curve under transverse tensile load
Fig. 8. Morphology of damage during loading
This study used a multi-scale method to predict mesoscopic residual stress during the curing and molding process of composite materials, and studied the effect of residual stress on subsequent performance, and established the integration of the curing and failure process. analysis systems for unidirectional carbon fiber composites.
The agreement of the predicted results with the experimental results confirms the effectiveness of the proposed method. Compared to the results of calculations without taking into account the influence of residual stresses during vulcanization, the presence of residual stresses during vulcanization changes the initial location of the damage and the path of damage propagation. , which agrees with the experimental results. The results are more consistent. Since the residual stress during curing in the matrix is in a state of compression, interfacial separation and cracking of the matrix are delayed, which increases the strength of the composite under transverse tensile load.