Thermal creak induced dynamics of space structures

Abstract

Space structures may be subjected to a continually changing thermal environment due to Earth eclipse transients and changes in the spacecraft orientation. During the transient thermal state, components in a structure may experience different amounts of thermal strain due to temperature gradients or coefficient of thermal expansion (CTE) mismatches. Such differential thermal strain can result in stress build-up, especially in statically indeterminate structures. If a nonlinear element, such as a friction dependent joint, is present, stress in the element builds up until the maximum load that can be sustained by friction is reached, at which point the element slips and releases some of the stored elastic energy. Such a nonlinear release mechanism will induce impulsive broadband and possibly high frequency loading to the system, in response to low frequency thermal excitations. This phenomenon is referred to as thermal creak. Nonlinear joints with freeplay, tensioning cables and pulleys, and other structural components that depend on friction and allow relative motion are all examples of potential creak elements that are common in space structures. An analytical and experimental investigation of the thermal creak phenomenon is presented. A generic model of a thermal creak element is developed to understand the mechanism and to identify the key parameters. The model captures the thermoelastic response, the friction behavior, and the dynamic response of a system. Key parameters that govern the response and quantify the parameters correlated with the energy storage, energy release and energy propagation are identified. The dynamic response is parametrically studied to qualitatively understand the range of behaviors. Two laboratory experiments were conducted to demonstrate thermal creak and to correlate with the model behavior. The first experiment, a joint characterization, focused on the local thermal creak response and the friction behavior. The model is shown to capture the nonlinear creak response over a range of loading conditions and trends seen in the experiment. The second experiment, a set of thermal tests on a representative deployable structure, investigated the structural response due to thermal creak. Thermal creak events were observed and the resulting dynamics were characterized. The results from the ground experiments and an on-orbit flight experiment conducted by Jet Propulsion Laboratory are used to assess the model and its applicability. The developed model and the experimental results provide a tool for developing thermal creak analysis techniques and mitigation strategies.