  This paper presents a Material Mask Overlay topology optimization approach
with the improved material assignment at the element level for achieving the
desired discreteness of the optimized designs for pressure-loaded problems.
Hexagonal elements are employed to parametrize the design domain. Such elements
provide nonsingular local connectivity; thus, checkerboard patterns and point
connections inherently get subdued. Elliptical negative masks are used to find
the optimized material layout. Each mask is represented via seven parameters
that describe the location, shape, orientation, material dilation, and erosion
variables of the mask. The latter two variables are systematically varied in
conjunction with a grayscale measure constraint to achieve the solutions'
sought 0-1 nature. Darcy's law with a drainage term is used to model the
pressure load. The obtained pressure field is converted into the consistent
nodal forces using Wachspress shape functions. Sensitivities of the objective
and pressure load are evaluated using the adjoint-variable method. The efficacy
and robustness of the approach are demonstrated by solving various
pressure-loaded structures and pressure-driven compliant mechanisms. Compliance
is minimized for loadbearing structures, whereas a multicriteria objective is
minimized for mechanism designs. The boundary smoothing scheme is implemented
within each optimization iteration to subdue the designs' undulated boundaries.
