Fracture of Advanced Laminated Composites With Nanofiber Reinforced Interfaces
Xiang-fa Wu - Doctoral Dissertation Defense
Advisor: Dr. Yuris Dzenis
Date: Wednesday, July 2, 2003
Time: 7:30 a.m.
Place: W183 Nebraska Hall
Advanced polymer composites are finding increasing use in a wide variety of structural applications. One of the major problems with these materials is their poor resistance to delamination along ply interfaces. Various designs to improve delamination resistance resulted in considerable weight or cost penalties. A novel delamination suppression concept based on nanofiber reinforcement of interfaces between plies in advanced laminated composites has been recently developed by Dzenis and Reneker (patents awarded and pending). Static and fatigue fracture of these novel materials was analyzed earlier in the group and an indication of possible edge stresssuppression was achieved. The objectives of this dissertation were a comprehensive experimental and theoretical analysis of edge effects in these novel composites, analysis of rate effects on their interlaminar fracture, development of methods and experimental characterization of their dynamic impact fracture, and development of improved fracture models suitable for laminate analysis and optimization. Edge delamination in the novel materials was comprehensively studied on commercial graphite-epoxy composites nanoreinforced by high temperature continuous polymer nanofibers produced by electrospinning. Nanofiber diameter was around 300 nm. Laminate lay-up was designed to induce maximum interlaminar free-edge stresses using a modified numerical scheme based on semi-analytic Lekhnitski stress potentials and the principle of complementary strain energy. Specimens with and without nanofiber reinforcement at interfaces were manufactured and tested in static and fatigue tension. Statistically significant improvements in the edge delamination onset stress, ultimate tensile strength, and fatigue life of composites with nanoreinforced interfaces were demonstrated for the first time experimentally. Nano- and micro-mechanisms of these improvements were studied by SEM fractography and numerical analysis. Suppression of the interlaminar edge stresses with nanoreinforcement was demonstrated numerically for the first time. Rate effects on Mode-I and II interlaminar fracture of advanced composites were evaluated based on the double cantilever beam (DCB) and end notched flexure (ENF) tests in the range of loading rates and testing temperatures. A phenomenological fracture model based on thermal activation concept was developed for the rate-dependent fracture. Substantial improvements in the interlaminar fracture toughness were recorded as a result of nanoreinforcement in the entire range of loading rates studied. Novel test specimens and fixtures were designed for impact dynamic fracture testing of composites using dynamic finite element (FE) method. Impact fracture testing was performed utilizing split Hopkinson pressure bar (SHPB) with dynamic crack initiation evaluated by a crack detection gage. Calculations of dynamic stress intensity factors (SIF) were performed based on the calculated loading history in the SHPB and measured crack initiation time, using dynamic FE analysis. Results showed that nanofiber reinforcement led to substantial delays in crack initiation and increased both Mode I and II dynamic fracture toughness. Finally, several new closed-form SIF solutions were derived based on fracture mechanics. These solutions enhance the available library of elementary solutions suitable for laminate analysis. The results of this dissertation can be used for design and analysis of novel nanofiber-based structural nanocomposites and development of new supernanocomposites for ballistic and other dynamic applications.
