A Small-Scale, Contactless, Pure Bending Device for In-situ Testing

The introduction of flexible electronics calls for new ways of measuring the mechanical limits and failure mechanisms resulting from (large) bending loads during manufacturing and application. Therefore, an autonomous, miniaturized, pure bending test apparatus was developed to investigate material systems at the microscopic level. Improvements to conventional bending test methods are: (1) well-defined pure moment application without friction or local pressure contacts, through active feedback control of parasitic axial and transverse forces at the specimen edges, (2) continuous reversibility of the loading direction within an angular range of −114∘≤𝜃≤114∘ (minimum radius of 2 mm), (3) no view-obstruction from both the thickness and in-plane perspective, enabling in-situ optical and scanning electron microscopic studies of failure mechanisms under constant field-of-view. The pure bending test setup was validated by moment-curvature measurements of monocrystalline silicon. The setup’s continuous load-reversibility and strain determination were validated by cyclic tests and in-situ digital image correlation, respectively, on polyethylene napthalate specimens. Furthermore, in-situ microscopic failure analysis was demonstrated on multilayered flexible electronics, revealing fracture, delamination and buckling.