Scale Dependence of Material Response at Extreme Incident Radiative Heat Flux

摘要

The thermal environment generated during an intense radiation event like a nuclear weapon airburst, lightning strike, or directed energy weaponry has a devastating effect on many exposed materials. Natural and engineered materials can be damaged and ignite from the intense thermal radiation, potentially resulting in sustained fires. Understanding material behavior in such an event is essential for mitigating the damage to a variety of defense systems, such as aircraft and weaponry. Flammability and ignition studies in this regime (very high heat flux, short duration) are less plentiful than in the heat flux regimes representative of typical fires. The flammability and ignition behavior of a material may differ at extreme heat flux due to the balance of the heat conduction into the material compared to other processes. Length scale effects may also be important in flammability and ignition behavior, especially in the high heat flux regime. A variety of materials have recently been subjected to intense thermal loads (～100-1000 kW/m~2) in testing at both the Solar Furnace and the Solar Tower at the National Solar Thermal Test Facility at Sandia National Laboratories. The Solar Furnace, operating at a smaller scale (≈30 cm~2 area), provides the ability to test a wide range of materials under controlled radiative flux conditions. The Solar Tower exposes objects and materials to the same flux on a much larger scale (≈4 m~2 area), integrating complex geometry and scale effects. Results for a variety of materials tested in both facilities are presented and compared. Material response often differs depending on scale, suggesting a significant scale effect. Mass loss per unit energy tends to go down as scale increases, and ignition probability tends to increase with scale.