Brian Raji

Fracture Mechanics Based Total Fatigue Life Behavior for Stitch-Bonded Polymer Matrix Composites

Stitch-bonded biaxial polymer matrix composite is a compromise between unidirectional composite and woven composite materials. In this paper, fatigue delamination behavior of stitch-bonded biaxial polymer matrix composites was investigated. The delamination growth rate in three domains of subcritical, linear, and unstable growth rate were studied. The effects of increased fracture resistance due to fiber bridging, multiple cracks, separation, and matrix cracking were considered in fatigue behavior of the composite. The fatigue onset life test was conducted to determine the threshold energy release rate. The total life delamination growth rate of stitch-bonded polymer matrix composites was characterized and presented.Copyright

Environmental effects on flexural over-strength factor of epoxy resin materials

Polymer composite structures are usually subjected to large flexural loadings during their life-span of the structures, so the flexural behavior of these structures and their constituents in different environmental conditions are critical to their use. A novel analytical approach for epoxy resin semi-brittle materials with strain softening model in tension and compression has been developed to investigate the flexural behavior of these materials. The value of the flexural over-strength factor which is the ratio of the flexural strength to the strength obtained from tension and compression stress strain models depends on stress gradient, size and loading system and it has already been evaluated at the laboratory condition. The mechanical properties of epoxy resin materials are sensitive to environmental effects at which they are loaded. The influence of temperature 60°C and humidity 90% Rh on tension, compression and flexural behavior of epoxy resin polymeric materials PRI and PR 520 have been investigated. Digital image correlation system was used for material characterization. The effects of heat and humidity on softening localization in flexural response were considered. The influence of heat and humidity on the flexural over-strength factor was evaluated.Copyright

Effects of curing temperature and environmental conditions on mode I fracture behavior for a laminated biaxial carbon/epoxy composite

Fracture mechanic behavior of a type of stitch-bonded biaxial carbon epoxy composite is presented. The influence of damage modes such as separation, tow splitting, multiple cracks, and fiber bridging on the energy release rate initiation and resistance of thin and thick laminates is investigated. The possible effects of manufacturing and in-service environmental conditions on energy release rate initiation and propagation have been presented. Details of loading speed, crack velocity, plasticity zone, curing and environmental conditions for smooth and nonsmooth crack growth have been discussed. The curing and in-service conditions have been found to influence the mode I fracture behavior of composite laminates. The results showed that different propagation curves should be included in the design guidelines for stitch-bonded composites.