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.
Advanced Fibre Composites with Engineered Interfaces
The damage tolerance of advanced fibre composite laminates can be significantly improved by smartly engineering the physical, chemical and mechanical properties of the interfaces between fibre and matrix and between individual plies. It is shown that the most effective method for unidirectional fibre composites is to apply an appropriate thermoplastic surface coating to the fibres, either intermittently or fully. Transverse impact fracture toughness can be enhanced by several times without little loss of strength. Micromechanics analysis is given to explain these toughness results. The effects of the coating on the stress distributions on the fibre-matrix interface are analysed using finite element method. For angle ply composites, interleaving between plies with perforated thin plastic films has proven to be a very successful technique in improving the damage resistance against edge fraying and delamination around discontinuities. Examples on boron, carbon and Kevlar fibre composites are given to demonstrate the applicability of these toughening techniques.
Influence of filler addition, storage medium and evaluation time on biaxial flexure strength and modulus of adhesive systems
Abstract Objective. This study evaluated the effects of filler addition, storage medium and time on biaxial flexural strength and flexural modulus of six adhesive systems. Materials and methods. The adhesives were either unfilled resins: Single Bond, Prime&Bond 2.1 and One-Step; or filled resins: Single Bond Plus, Prime&Bond NT and One-Step Plus. Resin discs of each product (0.5 mm thick × 6.0 mm diameter) were prepared using silicon molds (n = 10). The discs were stored dry, in water, mineral oil or ethanol for 1 week or 3 months before biaxial flexural testing. Data were statistically analyzed by 3-way ANOVA and Tukey's post-hoc test (α = 0.05). The specimens were tested in a universal testing machine at 1.27 mm/min until failure occurred. Results. In general, the storage in ethanol led to significantly lower flexural strength and flexural modulus values than the other storage conditions for all adhesives in both storage periods. Filler addition increased flexural strength and flexural modulus for Prime&Bond NT, when it was stored dry or in water for 1 week. For Single Bond Plus, the filler addition resulted in higher flexural modulus, when it was stored for 1 week in oil and in ethanol. No significant differences in flexural modulus were observed between One-Step and One-Step Plus in any storage medium. Conclusions. Results suggested that filler addition does not necessarily have to increase the flexural strength and flexural modulus. Ethanol and oil storages tended to yield opposite effects.