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Chopped strand mat/polyester resin laminates are regarded as plane isotropic. However, under load the aligned fibers provide reinforcement, whereas the transverse fibers initiate progressive damage by filament debonding and resin cracking. This transverse damage rapidly involves the aligned fibers, particularly under repeated loading. Static and fatigue strength data have been developed for recognizable states of damage as well as final separation of the specimens. The effect of mean load on stress amplitude has been studied, together with cumulative damage and the effect of stress concentrators. Failure theories for biaxial stress conditions have been examined using thin-walled tubes. The biaxial tensile condition is particularly damaging.
There appear to be three main difficulties in applying conventional data to design. The S-N curves appear to be straight lines and there is difficulty in extrapolating to long lives; there is a marked adverse size effect on strength; and designing to avoid incipient damage at stress concentrators gives very uncompetitive designs. Fracture toughness and macroscopic crack growth studies appear to offer an alternative approach to the prediction of strength.
composite materials, glass fiber, chopped strand mat, polyester, fatigue, Goodman diagram, scatter, fracture toughness, crack propagation, damage stress rupture
University of Nottingham,