For many years engineers have been trying to harness mechanical work from thermal energy by taking advantage of the crystallographic phase change of shape memory alloys (SMA’s). SMA’s can exhibit strains of up to 8% actuated by a transformation of the internal crystal structure from martensite to austenite as the metal is heated. This solid state phase change causes a shearing of the internal structure that deforms the material. By introducing additional internal stresses the alloy can be “trained” to transition between two states by applying temperature changes. One of the most well-known projects of the past is the Smart Aircraft and Marine Propulsion System demonstration (SAMPSON), intending to demonstrate the potential of SMA’s in tailoring the geometry of jet-propulsion systems through a series of experiments.
One experiment investigated the utilisation of bending actuation of SMA’s to optimise the compromise between noise-mitigation at take-off and landing (noise levels are strictly regulated by civil agencies), and maximum thrust at cruise altitude. To achieve this Boeing formed the trailing edge of the exhaust nozzles on commercial turbo-fat jet engines in a triangular “chevron” shape (Figure 1) designed to be reconfigurable by actuation of embedded SMA beam components. The “Variable Geometry Chevrons” (Figure 2) feature NiTi (60% Ni and 40% Ti by weight) SMA beam elements encased in the composite chevrons in a complex 3-D configuration to induce the necessary bending moments to force the chevrons inwards into the bypass flow at low altitudes and low speeds where the engine temperature is high. The intruding chevrons cause a disturbance in the bypass flow, inducing a broader diffusion and mixing of the hot exhaust gases with the cooler bypass flow. Thereby the shear stress between the two different-velocity flows is decreased leading to a reduction in the noise level.
At higher altitudes and high speeds where the engine temperature is low, the chevrons relax and straighten-out. This guarantees a smooth exit flow that decreases the pressure difference between the inlet and exit of the engine and thus increases the engine thrust. In the original work published by Mabe et al. (2005) the system is designed for both autonomous operation as well as controlled actuation using heaters installed in the engine casing with a closed loop controller to maintain optimum in-flight tip immersions. A parametric study showed that during cruise marginal immersion helped to reduce shock cell noise with negligible thrust penalty.
NASA developed an active bending chevron system by embedding tensile pre-stressed NiTinol SMA strips on one side of the neutral axis of the composite laminate. Actively controlled thermal excitation thus causes the SMA actuators to attempt recovery of the pre-strain constrained by the bond to the host material. The resulting asymmetry in thermal stress causes a moment that deflects the structure. The aerodynamic load due to engine flow and the strain energy stored in the deformed host composite are used to restore the structure to the un-actuated configuration.
The simple design appeals by its lightweight construction with low part count and opportunity to be fully integrated into an autonomous morphing system. The “Variable Geometry Chevron” demonstrates the excellent potential of SMA’s to be integrated in composite laminates to provide internal actuation for smart structures.
(1) DJ Hartl & DC Lagoudas (2007). Aerospace applications of shape memory alloys. Proc. IMechE Vol. 221 Part G: J. Aerospace Engineering
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