Increased Fracture Toughness of Graphite-Epoxy Composites through Intermittent Interlaminar Bonding
The objective of this study has been to increase the fracture toughness of multi-layer continuous-filament graphite-epoxy composites. The method used is intermittent interlaminar bonding, which can lead to a large in crease in the fracture surface area. In this study we achieved intermittent bonding through introduction of thin perforated Mylar between the layers of the composite. For the best optimum condition included in this study, fracture toughness was increased from about 100 kJ/m2 for untreated specimens to an average of about 500 kJ/m2, while tensile strength dropped from 500 MPa to 400 MPa, and elastic modulus remained the same at about 75 GPa. An approximate analysis is presented to explain the observed improvement in toughness.
Toughened graphite-epoxy composites exposed in near-Earth orbit for 5.8 years
This experiment was designed to measure the effect of near-Earth space exposure on three mechanical properties of specially toughened unidirectional T300/5208 graphite-epoxy cross-ply composite materials. The properties measured are fracture toughness, elastic modulus, and strength. Six toughness specimens and nine tensile specimens were mounted on an external frame during the 5.8-y orbit of the Long Duration Exposure Facility. Three identical sets of specimens were manufactured at the outset: the flight set, a zero-time nonflight set, and a total-time nonflight set. Results indicate that with proper protection from solar exposure and atomic-oxygen degradation, toughened graphite-epoxy composites can provide useful structural service in near-Earth orbit for at least five years.
Some Fundamental Fracture Mechanisms Applicable to Advanced Filament Reinforced Composites
The object of this study is to establish experimentally the critical fracture modes of an epoxy matrix in the vicinity of a break in a high modulus, high strength filament simulating the effect of tensile loading on unidirectional composites. Three distinct failure modes were observed to occur and the nature of these three modes is explained through an analysis of the stress state in the matrix. A single filament embedded in an epoxy novolac was used to examine the fracture process. The advantage of using this type of resin arises from its ability to be modified to varying degrees of crack sensitivity. The effects of varying gross strain rate were evaluated and the study was then extended to specimen configurations containing more than one filament. This approach permits the examination of the interaction of nearby filaments with localized fracture processes. Filaments of boron on tungsten (B/W) boron on Silica (B/SiO 2), boron carbide on B/W and tungsten wire were used with epoxy novolac (DEN 438).