A nonconstraining templated powder grid for measurement of strain

Abstract

A method was developed and validated for mounting grid patterns with pitches on the order of tens to hundreds of micrometres onto specimens for the purposes of mechanical characterisation and fracture analysis. The method uses a low stiffness, low‐toughness photopolymer to create a textured pattern of pillars on the sample, and the spaces between these pillars are then filled in with an opaque powder to provide optical contrast. This type of grid is desirable both because of its relatively quick preparation time and because of its minimal influence on the mechanical properties of the sample. The method for mounting this powder‐based grid onto samples is discussed. Then, the accuracy of this new type of grid is validated against current state‐of‐the‐art epoxy‐mounted lithography grids as well as digitally fabricated sinusoidal grids using synthetic digital manipulation. Finally, the experimental performance of the powder‐based grid is assessed in both uniaxial tension on a poly(methyl methacrylate) specimen with a stress concentrator and fracture of stereolithographically printed photopolymer specimens. Synthetic testing shows that the powder grids are of comparable accuracy to lithography grids, but additional processing variability leads to strain errors of about 3% in powder grids compared with the 2.1% seen in lithography grids. In experiments, the powder‐based grids produce deformation patterns comparable with those expected from analytical results, and fracture toughness values for homogeneous stereolithographically printed specimens are uniform and consistent with expected values.

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