Techniques for noise and signature reduction applications

Abstract

In recent years, DMA test systems have become commercially available. Prior to this, workers in this field developed equipment to meet their own specific needs. Current commercial equipment is aimed at the thermal analysis market and is seen as complimentary to established techniques, such as DSC and TMA. Great emphasis is placed on temperature control, but, in general, less attention is paid to frequency range and mechanical design. Upper frequency limits are usually around 100 Hz. While low frequencies are ideal for the study of molecular relaxation processes and for routine material characterization, they have little immediate application in the field of noise and signature reduction. Recent work at ARE has resulted in the development of computerized WLF technique; this method is based on a reference frequency as well as a reference temperature. In general (though not essential), the reference point selected is the temperature of peak tan at a frequency of 1 Hz. Where multiplexed frequency data are available, it is a simple matter to verify that an unknown material will fit the standard equation by comparing measured shifts in peak tan with computed values based on the standard constants. Signature reduction applications require systems that, in some cases, work at frequencies up to 1 MHz. Computerized predictions based on WLF data, shifted from low‐frequency measured data, show very good agreement with high‐frequency measured acoustic data, but it would be reassuring to be able to measure dynamic mechanical properties at kilohertz frequencies. Recent improvements to an inertial mass, air‐bearing supported system, developed at ARE some 10 years ago, have resulted in a system capable of yielding complex shear modulus data at frequencies up to 10 kHz.