Fracture of Laminated Composites With Interfaces Modified by Thermoplastic Nanofibers
Dynamic Tribometry of Fracture Surfaces
Rate Effects on Interlaminar Fracture of Polymer Composites
Date: Tuesday, December 5, 2000
Time: 3:30 p.m.
Place: W128 Nebraska Hall
Fracture of Laminated Composites- With Interfaces Modified by Thermoplastic Nanofibers
Emrah Gokdag
Department of Engineering Mechanics
University of Nebraska, Lincoln, NE 68588-0526
Advisor: Dr. Yuris Dzenis
Advanced composites are extensively used in high technology applications such as aerospace and defense structures because of their high modulus and strength, good fatigue durability, and excellent corrosion resistance. However, delamination often limits their applications as this damage mode causes serious reduction in stiffness and can lead to catastrophic failure. There are different approaches to suppress delamination and to improve interlaminar fracture toughness of composites. One of them, pursued in our group, involves toughening the interfaces with polymer nanofibers. The objective of this work was experimental analysis of laminated composites with interfaces reinforced with thermoplastic nanofibers. Interlaminar Mode I and Mode II fracture of composites with layers of PAN nanofibers of various thickness was studied. Test results were presented and discussed. The directions of future work were outlined. Preliminary analysis of specimen size effect on Mode I fracture was also performed utilizing new experimental method based on a DMTA apparatus.
Dynamic Tribometry of Fracture Surfaces
Hongfa Huang
Department of Engineering Mechanics
University of Nebraska, Lincoln, NE 68588-0526
Advisor: Dr. Ruqiang Feng
A good understanding of the frictional response of closed fracture surfaces of microcracks in brittle solids is important for predicting the ballistic performance of these materials. To this end, a dynamic tribometer has been developed utilizing a modified Kolsky torsion bar device. This new technique enables time-resolved frictional measurements at sliding velocities up to 10 m/s and for contact forces up to 30 kN. An optical surface profilometer is used to correlate the surface topography of tribo-pairs with the resulting frictional response and to identify the underlying tribological mechanism. Preliminary experiments have been carried out on flat-flat and as-fractured tribo-pairs of 7075-T6 aluminum. The results show that the frictional coefficient of the flat-flat tribo-pairs is independent of the sliding velocity. However, the frictional coefficient of the as-fractured tribo-pairs increases with contact pressure, indicating strongly nonlinear behavior. The significance of these findings and future work will be discussed.
Rate Effects on Interlaminar Fracture of Polymer Composites
Xiangfa Wu
Department of Engineering Mechanics
University of Nebraska, Lincoln, NE 68588-0526
Advisor: Dr. Yuris Dzenis
This work is part of the larger project aimed at the development of novel polymer composites with improved impact resistance. It is well-known that impact failure of advanced composites is closely linked to their delamination fracture. Interlaminar fracture of advanced composites under high loading rates has not been extensively studied. In the first part of this research, the rate and temperature effects on the Mode I and II fracture of an advanced graphite-epoxy composite were studied experimentally in the medium rate range. A phenomenological model was developed and applied for data description. In the second part of the research, specimens were designed for interlaminar fracture analysis under high-speed impact loadings. Dynamic FEM models of Mode I and II specimens were developed and used to calculate the dynamic stress intensity factors. These models are being currently used for experimental specimen optimization.
