Micromechanics Study On Damage And Interlaminar Short Fiber Toughening Of Composite Laminates

Composites have been applied more and more in aviation-space and other structures. Continuous fiber reinforced polymer laminates are the main of advanced composites for structures, and many attentions focus on the materials properties and damage/failure mechanism of laminates. Recently, both theoretical and experimental research show that the interlaminar weakness and relative damage/failure behavior of laminated composites can seriously affect the loading capacity of the materials and structures, and so effective improving of interlaminar properties, specially, fracture toughness, is very important to theory analysis and applications as an active area in composite research. It has been shown by Hu and his cooperator’s test that interlaminar short fibers (ISFs) with better toughness can significantly increase the resistance to delamination of laminates. But up to now the relative theoretical and experimental researches are not sufficient, particularly the research at micromechanics level for interlaminar properties, damage and toughening.This paper studies on the interlaminar properties, stress transfer, short fiber toughening and its influence factors of fiber reinforced polymer laminates by tests, analytical method and numerical simulation based on micromechanics.In the test aspect, compression-shearing,4 and 3-point bending were conducted out for the specimens of carbon fiber/resin laminates with and without ISFs for comparison (in Chapter 2 and 4). Firstly, according to the existing test methods and by an improved test set, compression-shear tests and a converse analysis were used for specimens with double notches to determine the in situ interlaminar shear strength and modulus of laminates with and without Zylon ISFs (as interleaves), and to observe and analyze the failure feature and process of the interleaves. Secondly, a mini-set for bending test was designed for microscope real-time observation of fracture. The set was used in 3 or 4 point bending of 900 layer-up laminates to investigate transverse matrix cracking and the effect of ISFs on matrix crack growth. The mechanical behavior and damage of laminates are quite complex, and so the research at micromechanics level may be necessary. In this paper the elastic modulus of no-damaged ISF interleaves was predicted by Mori-Tanaka method and finite element analysis for RVE. In addition, considering the disarray distribution of ISFs, a refined equivalent approach to interface debonding (by Fitoussi et al 1990) was used to analyze the damage and stress transfer in ISF interleaves and to predict the elastic modulus and damage evolution of damaged interleaves with comparison to the test results (in Chapter 3).The previous tests have showed that the toughening from ISFs mainly results from the energy dissipation caused by fiber bridging. Since the pullout of randomly distributed fibers is very complex to analyze hardly, based on a quantitative degree analysis, a simplified model and analytical method were presented and used with an improved Leung’s matrix spalling criterion to simulate the whole process of quasi-brittle fiber pullout from brittle matrix. The simulation indicates that inclining angle, length and interface friction stress of fibers are the main parameters to affect pullout and toughening and there is an optimal combination of fiber length and interface friction stress for toughening (in Chapter 5).In order to investigate the interaction among damages and the in situ properties of damaged laminates, a 2D FE analysis for RVE in cross-ply laminates was conducted on, in which matrix crack, interlaminar cracking and fiber fracture were considered and in-plane and out of plane loading combination was applied. From the modeling the damage evolution rule and the effect of damage on effective stiffness were discussed (in Chapter 6)Finally, using and extending the obtained micromodeling results of damage, a typical stiffened panel under low-speed impact was analyzed by 3D EF analysis, in which damage evolution and interaction were considered and an interlaminar strain failure criterion was used. The numerical results show that ISF toughening is effective to increase the resistance to impact damage (in Chapter 7).