Adaptive structures require the ability to reconcile conflicting demands of high load-bearing capacity,

low mass, and structural compliance. To address this challenge, this paper presents an actively tunable

variable stiffness composite achieved through thermoplastic interleaving and electro-thermal activation.

Carbon fibre-reinforced polymer (CFRP) laminates were interleaved with polyethylene terephthalate

(PET) layers to enable stiffness modulation via thermally induced shear decoupling. The effect of various

design parameters including location, number and thickness of thermoplastic interlayers and loading

configuration (i.e. span to thickness ratio of the beam under 3-point bending) were investigated. A key

advancement of this work is the development of interleaved composites, incorporating embedded copper

wires within PET interlayers, that soften with localised heating. Unlike bulk heating, localised Joule

heating enables targeted, rapid, and energy-efficient stiffness control. The results show that activation

of multiple interlayers can produce reversible flexural stiffness reduction of more than 50% within 60

seconds at 9A.

The proposed architecture enables controllable, reversible, and spatially selective stiffness tuning

in CFRP laminates without significant compromise of room-temperature load-bearing capability. This

approach provides a scalable framework for adaptive lightweight structures with potential applications

in morphing aerospace components, soft robotics, and smart structural systems.