Enhancing the post-buckling response of composite plate/panel structures utilizing shape memory alloy actuators - a smart structural concept.

Abstract

The feasibility of enhancing the post-buckling load bearing capability of carbon/epoxy----- - composite plate/panel structures' utilizing embedded activated near equiatomic nickeltitanium, Ni-Ti, shape memory alloy, SMA, wire actuators has been investigated. Enhanced post-buckling is achieved through utilization of the unique shape memory phenomenon inherent within the Ni-Ti material. The unique phenomenon requiring a thermal stimulus. Within this investigation, such a thermal stimulus is provided for by an electrical current. Several host laminates, varying in lay-up architecture, have been considered. Two control strategies have been employed that utilize the unique SMA response at an elevated temperature. Control strategy 1 features embedded SMA actuators located within tubes that run along the specimens neutral plane. Here, the SMA's are constrained to external boundaries. Control strategy 2 also features embedded SMA actuators. For this control strategy, however, the actuators are partially constrained to the host laminate. For each strategy, upon SMA energization, shape memory constraint results with the formation of a recovery force within the SMA material. It is this recovery force that is employed to control the post-buckling response of the selected laminated specimens. A requirement for control strategy 2 is that the SMA/host interface must be of sufficient quality to sustain an elevated temperature as well as the imposed recovery force. Pertaining to control strategy 1, for the associated specimens, activation of constrained pre-strained SMA wire actuators can result with a significant specimen post-buckled deflection alleviation while under the influence of an external compressive load that is approximately three times the critical buckling value. While not as effective as control strategy 1, the concept behind control strategy 2 has been shown to work. Its efficient, or optimal, utilization, however, has yet to be demonstrated. For all the specimen configurations, the constrained SMA response act's to pull the specimens back to their flat configuration. This is true even when employing a low SMA volume fraction. Depending on the magnitude of the in-plane compressive load, however, this can result with post-buckled instability. SMA restoration recovery forces not only reduce the peak displacement amplitude, they also alleviate high stress levels, local to the boundary supports, that are typical to postbuckled plate/panel configurations. The tendency of adaptation is to redistribute the loading back towards the plates central region, such that, a more uniform stressed state exists. The stability of the adapted shape is dependent upon the laminate stacking sequence. Due to the elevated temperature required for SMA energization, the stacking sequence chosen should be such that temperature effects have minimal influence on the structural performance. SMA utilization would certainly be of benefit when such components are subjected a thermal environment by a means other than electrical energization. As an example, heating, associated with skin friction, may be sufficient to drive the actuators through their phase transition such that they exert stabilising recovery forces on the skin sections of high speed aircraft. The performance benefits of the SMA/carbon/epoxy composites materials, however, must carefully be assesseda gainst issueso f technical risk, producability, maintainability, reliability, and, of course, cost. The improved performance must be at an affordable price.