Recent advancements in smart variable stiffness structures have primarily focused on utilising thermoplastics for creating multi-layered structures capable of reversible stiffness alteration while maintaining material integrity and load-bearing capability. However, despite extensive research on ac-tive heating thermo-softening using metallic heaters, understanding the influence of factors like system length scale and heating element characteristics on the response time and de-stiffening remains limited. This thesis aims to address these gaps by providing a comprehensive investigation into these aspects. The thesis starts with a literature review followed by detailed analyses divided into three parts.

In the first part (Chapter 3), the response time and de-stiffening of multi-layered structures with linear heating elements are studied. Factors such as system length scale, current input, heating element spacing, size, and shape are analysed. While reducing the system length scale and increasing current input can decrease response time, practical limitations such as material melting are identified.

The second part (Chapters 4 and 5) explores the impact of patterned metallic mesh heating ele-ments on response time and de-stiffening. Different mesh patterns are studied numerically, with exper-imental trials conducted on rectangular pattern meshes. Mesh heating elements are found to achieve faster heating rates compared to linear elements.

In the third part (Chapter 6), a random metallic mesh is utilised for de-stiffening. An image pro-cessing algorithm is developed to simplify the mesh geometry and reduce the computational cost of modelling while maintaining precision. Random mesh heating elements demonstrate the highest heating rate and de-stiffening compared to the other heating elements, both experimentally and numerically.

Overall, the thesis provides insights into the behaviour of variable stiffness structures, emphasis-ing the influence of heating element design and system scale on the response time and attainable de-stiffening. Conclusions and suggestions for future research are outlined in Chapter 7.