Mechanical Testing of Interlocking Microfabricated Metamaterial Surfaces

Abstract

This thesis reports testing methods for microfabricated arrays of interlocking structures designed for mechanical joining for electronics packaging applications. The scope of this research also explores the design and testing of millimeter-scale patterns with potential applications in automated manufacturing.Testing of the microfabricated structures involved identifying relevant methods and developing the necessary equipment to obtain repeatable and accurate data through those tests. This included developing methodology and equipment to image and record the microfabricated arrays during mechanical testing, as well as designing a way to manipulate the specimens to control the relative alignment between their parallel surfaces. Specific details of these methods can be found in the Appendices. Tests started with initial contact, then slowed through the interlocking phase, and ended on detection of a sudden change in force. Testing revealed that initial microfabricated prototypes need further development to achieve mechanical lock-in performance. The nature of the test failures suggested that an important pathway for improvement is the reinforcement of the microstructure pillars with more material to avoid reaching their proportional limit stress prior to lock-in. Further investigation into the physics of thin-shelled buckling demonstrated that the low cross-sectional area of the pillars relative to their height placed them in an intermediate classification of columns that have buckling loads significantly lower than model predictions. The development of millimeter-scale patterns, referred to as macroscale patterns to distinguish them from the microstructures, was pursued as a pathway towards faster shape iteration in design, with additional potential applications in assembly of aerospace frames and components. These patterns drew inspiration from existing mechanical fastener designs to create patterns that could work as a surface array to lock-in separately manufactured parts for efficient assembly with minimal to no downtime between assembly and operation. Three-dimensional arrays of structures were drafted and 3D printed, then tested to determine their capabilities. Patterns with non-flat cantilever designs had the best force asymmetry ratio when compared to patterns involving pillars with features that lock in to corresponding chambers, or patterns of pillars with solid caps. It was found that further iterations and adjustments to the dimensions of the patterns could yield more favorable results, since the modifications required to achieve the initial lock-in of patterns involved an iterative process of dimensional refinements.