Characteristics and Automated Control of a Dual-Frequency Servohydraulic Test System

The objective of the present research is to create a servohydraulic test system that can apply two independent excitations, simultaneously, to a specimen. The motivation for this objective is provided by a range of engineering situations in which such excitations are encountered, and the opportunity for monitoring specimen integrity by dynamic analysis during a "standard" fatigue test. In the first case, bi-frequency excitation is common to such engineering components as combustor buckets in jet engines that undergo low-frequency structural fatigue and high-frequency (low amplitude) fatigue loading having to do with the pulsation of the burning process. Other situations of that type include a variety of acoustic/structural excitations and other combinations typically encountered in transportation and propulsion systems. The second opportunity, to use such a capability to monitor the integrity of a specimen that is being characterized under high-amplitude cyclic loading by conducting dynamic analysis on the same specimen with a low-amplitude high frequency (superimposed) excitation, is an idea that is related to the well-known "dynamic mechanical analysis" that is widely used for elastomeric material characterization. The system described in the present paper was designed and built by a cooperative effort between the Materials Response Group at Virginia Tech and MTS Systems Corporation. The system consists of two separate actuators, in series, that are capable of applying high-frequency (up to about 1500 Hz) excitation with amplitudes up to about 2000 lb (8800N) beyond standard high-load, low frequency excitations. The secondary high-frequency actuator is isolated from the primary actuator in a manner that makes the control of the system by a computer interface possible. Other features of the system include a tube-testing grip arrangement, high-temperature capability, and internal pressurization capability for tube specimens. The present paper will describe the attributes of the system, and present the characteristic response of the system under limiting conditions. Data will be presented to demonstrate the utility and capabilities of the system, using several types of specimens and materials. Control of the system will be discussed, especially as it is influenced by the stiffness of the specimen under test, and the nature of the superimposed test frequencies.