PILOTED IGNITION OF COMPOSITE MATERIALS UNDER EXTERNAL HEAT FLUX

摘要

The Forced-flow Ignition and flame-Spread Test (FIST) is a test method that introduces forced flow conditions into the established LIFT (ASTM E 1321-93) methodology. The FIST protocol is being used to examine the ignition behavior of various composite materials in an effort to extend the practical applications of this test method. Tested materials include epoxy/fiberglass (E/G) laminate, polypropylene/glass (PP/G), and high-density polyethylene/glass (HDPE/G) composites. Piloted ignition delay curves are obtained for each material for external heat fluxes ranging from 10 to 40 kW/m~2 with a forced air flow velocity of 1 m/s. Ignition delay times range from 45 to 1600 seconds. The ignition delay curves for the chopped fiber composites (PP/G and HDPE/G) are compared to curves generated with the corresponding pure polymer, which serves as the matrix material. Preliminary analyses reveal an increase in the respective ignition delay times and the critical heat flux of the composite materials as compared to the pure polymer. These results may be attributed to the higher heat capacity and conductivity of the glass whiskers contained in the composite material and the concomitant reduction in the polymer pyrolysis rate. The continuous fiber laminates exhibit vastly different behavior than the polymer-based composites as they approach the pyrolysis temperature. Delamination and emission of gas jets at the sample surface as well as non-isotropic mass transfer of combustible vapors are observed. Such observations suggest that composite materials, especially those that do not approximate a homogeneous structure can be expected to have flammability characteristics significantly different from those of their constituent elements. Further study of the effect of sample thickness and structure is needed to better define and understand these variations.