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.
Mechanical and Physical Properties of Advanced Composites
The mechanical and physical properties of graphite, glass, PRD-49, and boron epoxy composites are compared with today's commonly used industrial materials to illustrate the many unique design alternatives available with advanced composite materials. Mechanical properties and the fatigue, creep, impact, thermal expansion, thermal conductivity, specific heat, electrical properties, wear, and friction characteristics of the materials are summarized. The purpose of each comparison is to show where performance improvements can be achieved by replacing present materials with advanced composite materials. Cost advantages in the use of these materials are briefly discussed. Increased performance, increased productivity, longer service life, and less component maintenance should outweigh today's higher material costs of advanced composites.
Study of the effect of substrate roughness on adhesive joints by SEM image analysis
The effect of adherend surface roughness on epoxy bonded aluminium joints has been studied. Several epoxy adhesives were tested to evaluate the influence of adhesive nature on the roughness effect. The aluminium pre-treatments applied were abrasion and impression processes, generating different texture levels. The abrasion with grinding papers of different grain sizes provides surfaces with high density of low summits. In contrast, the surfaces subjected to impression process present low density of very deep valleys. The roughness measurements were made by image analysis of micrographs obtained by Environmental Scanning Electron Microscopy (ESEM). In addition to the average roughness, other surface descriptors were determined to characterise completely the surfaces, allowing to evaluate the influence on the adhesive strength of different texture variables, such as the height, shape and density of the peaks and valleys which constitute the roughness profile. An optimum value of surface roughness was found for the joint strength measured by the lap shear tensile test. An increase of adherend roughness causes an increase of effective area but, at the same time, decreases the ability for adhesive penetration. It has also been shown that the joint strength depends on the main characteristics of the adherend surface, such as the density and depth of the protuberances. Finally, it has been found that the roughness effect seems be influenced by the adhesive nature. Different epoxy adhesives with similar mechanical properties present different joint strengths due to their different penetration abilities.