THERMOSTRUCTURAL TESTING


THERMOSTRUCTURAL TESTING

        All high Mach number vehicles experience severe kinetic heating. The heat flux values beyond one W/mm2 are very common. The surface temperature may reach up to 2000 K affecting the integrity of the airframe and transmission of heat to the critical electronic packages. Thermostructural tests are therefore mandatory for improving the confidence level in design of airframe structures working in high temperature conditions.

       DRDO has established a thermostructural testing facility for its various ambitious projects. The details are:

Heating Systems
       The radiant heaters typically consist of three basic elements–infrared heating lamps, reflectors and cooling system housed in the form of a cassette. The heaters are custom built to the geometry and heating requirements of each test. Quartz lamps are instant on and off type and offer extremely high heat flux density. Gold plated reflector with 95 per cent reflectivity is used with water circulation for cooling.

       Three-phase electrical power has been made available from the substation to the test area. In order to control the IR heating panels, thyristor-based power controllers have been used. Data acquisition system with 128-channel signal conditioning unit can sample all channels at a rate of 1 kHz. Single-loop PID controller has been deployed to work in manual (open)/automatic (closed loop) control modes.


IR heater models. L-R : Conical type, low watt density, high watt density

Power Controllers
        Power is controlled in the set up using a PID controller and a thyristor. Feedback is taken from measured temperature or heat flux. A temperature or flux time history has been programmed into the controller, and proper PID parameters are selected for control. IR heaters take severe high in rush current when first powered up, tripping breaker and blowing fuses because of low cold resistance. To overcome this, a control circuit has been designed to perform soft start whenever the power is put on or whenever it changes.


Test Instrumentation


Heat Flux Sensor

        Thermocouples are the principal sensors used to measure temperature. For fast surface temperature measurement applications, quick acting foil type (0.0005") thermocouples are being used. K-type thermocouples (Chrome-Alumel) with a range from -270 °C to 1372 °C are being used for the tests.
Heat flux sensors have also been developed in-house.