An Investigation of Necking Instability in Fiber Reinforced Polypropylene

Thermoplastic composite materials hold considerable promise for increased use in low cost high volume applications because of the potential for processing by solid phase forming. Unfortunately, because of the wide variety of such materials, inherent variability in properties, and complex temperature and strain rate dependence, large strain behavior of these materials has not been well characterized. Of particular importance is failure during processing due to localized necking instability, and it is this phenomenon that is the primary focus of this study. Large strain behavior under uniaxial tension is characterized over a range of temperatures and strain rates, and a simple linear viscoelastic model is fit to the observed data. The strain rate and temperature dependence is then used to predict limiting tensile strains, based on Marciniak imperfection theory. Excellent correlation was obtained between theory and experiment, and the results are summarized in maps of forming limit as a function of temperature and strain rate. These results suggest that excellent solid phase formability may be obtained for the materials tested under properly chosen conditions.